1 Network Working Group Jon Callas 2 Internet-Draft PGP Corporation 3 Intended status: Standards Track 4 Expires October 2007 Lutz Donnerhacke 5 Apr 2007 6 7 Obsoletes: 1991, 2440 Hal Finney 8 PGP Corporation 9 10 David Shaw 11 12 Rodney Thayer 13 14 OpenPGP Message Format 15 draft-ietf-openpgp-rfc2440bis-22 16 17 18 Status of this Memo 19 20 By submitting this Internet-Draft, each author represents that any 21 applicable patent or other IPR claims of which he or she is aware 22 have been or will be disclosed, and any of which he or she becomes 23 aware will be disclosed, in accordance with Section 6 of BCP 79. 24 25 Internet-Drafts are working documents of the Internet Engineering 26 Task Force (IETF), its areas, and its working groups. Note that 27 other groups may also distribute working documents as 28 Internet-Drafts. 29 30 Internet-Drafts are draft documents valid for a maximum of six 31 months and may be updated, replaced, or obsoleted by other documents 32 at any time. It is inappropriate to use Internet-Drafts as reference 33 material or to cite them other than as "work in progress." 34 35 The list of current Internet-Drafts can be accessed at 36 http://www.ietf.org/1id-abstracts.html 37 38 The list of Internet-Draft Shadow Directories can be accessed at 39 http://www.ietf.org/shadow.html 40 41 Copyright Notice 42 43 Copyright (C) The IETF Trust (2007). 44 45 Abstract 46 47 This document is maintained in order to publish all necessary 48 information needed to develop interoperable applications based on 49 the OpenPGP format. It is not a step-by-step cookbook for writing an 50 application. It describes only the format and methods needed to 51 read, check, generate, and write conforming packets crossing any 52 network. It does not deal with storage and implementation questions. 53 It does, however, discuss implementation issues necessary to avoid 54 security flaws. 55 56 Callas, et al. Expires Oct 24, 2007 [Page 1] 57 INTERNET-DRAFT OpenPGP Message Format Apr 24, 2007 59 60 OpenPGP software uses a combination of strong public-key and 61 symmetric cryptography to provide security services for electronic 62 communications and data storage. These services include 63 confidentiality, key management, authentication, and digital 64 signatures. This document specifies the message formats used in 65 OpenPGP. 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 Callas, et al. Expires Oct 24, 2007 [Page 2] 114 INTERNET-DRAFT OpenPGP Message Format Apr 24, 2007 116 117 Table of Contents 118 119 Status of this Memo 1 120 Copyright Notice 1 121 Abstract 1 122 Table of Contents 3 123 1. Introduction 7 124 1.1. Terms 7 125 2. General functions 7 126 2.1. Confidentiality via Encryption 8 127 2.2. Authentication via Digital signature 9 128 2.3. Compression 9 129 2.4. Conversion to Radix-64 9 130 2.5. Signature-Only Applications 10 131 3. Data Element Formats 10 132 3.1. Scalar numbers 10 133 3.2. Multiprecision Integers 10 134 3.3. Key IDs 11 135 3.4. Text 11 136 3.5. Time fields 11 137 3.6. Keyrings 11 138 3.7. String-to-key (S2K) specifiers 11 139 3.7.1. String-to-key (S2K) specifier types 11 140 3.7.1.1. Simple S2K 12 141 3.7.1.2. Salted S2K 12 142 3.7.1.3. Iterated and Salted S2K 12 143 3.7.2. String-to-key usage 13 144 3.7.2.1. Secret key encryption 13 145 3.7.2.2. Symmetric-key message encryption 14 146 4. Packet Syntax 14 147 4.1. Overview 14 148 4.2. Packet Headers 14 149 4.2.1. Old-Format Packet Lengths 15 150 4.2.2. New-Format Packet Lengths 15 151 4.2.2.1. One-Octet Lengths 16 152 4.2.2.2. Two-Octet Lengths 16 153 4.2.2.3. Five-Octet Lengths 16 154 4.2.2.4. Partial Body Lengths 16 155 4.2.3. Packet Length Examples 17 156 4.3. Packet Tags 17 157 5. Packet Types 18 158 5.1. Public-Key Encrypted Session Key Packets (Tag 1) 18 159 5.2. Signature Packet (Tag 2) 19 160 5.2.1. Signature Types 20 161 5.2.2. Version 3 Signature Packet Format 22 162 5.2.3. Version 4 Signature Packet Format 24 163 5.2.3.1. Signature Subpacket Specification 25 164 5.2.3.2. Signature Subpacket Types 27 165 5.2.3.3. Notes on Self-Signatures 27 166 5.2.3.4. Signature creation time 28 167 5.2.3.5. Issuer 28 168 5.2.3.6. Key expiration time 28 169 170 Callas, et al. Expires Oct 24, 2007 [Page 3] 171 INTERNET-DRAFT OpenPGP Message Format Apr 24, 2007 173 174 5.2.3.7. Preferred symmetric algorithms 28 175 5.2.3.8. Preferred hash algorithms 29 176 5.2.3.9. Preferred compression algorithms 29 177 5.2.3.10.Signature expiration time 29 178 5.2.3.11.Exportable Certification 29 179 5.2.3.12.Revocable 30 180 5.2.3.13.Trust signature 30 181 5.2.3.14.Regular expression 30 182 5.2.3.15.Revocation key 31 183 5.2.3.16.Notation Data 31 184 5.2.3.17.Key server preferences 32 185 5.2.3.18.Preferred key server 32 186 5.2.3.19.Primary User ID 32 187 5.2.3.20.Policy URI 33 188 5.2.3.21.Key Flags 33 189 5.2.3.22.Signer's User ID 34 190 5.2.3.23.Reason for Revocation 34 191 5.2.3.24.Features 35 192 5.2.3.25.Signature Target 35 193 5.2.3.26.Embedded Signature 36 194 5.2.4. Computing Signatures 36 195 5.2.4.1. Subpacket Hints 37 196 5.3. Symmetric-Key Encrypted Session Key Packets (Tag 3) 37 197 5.4. One-Pass Signature Packets (Tag 4) 38 198 5.5. Key Material Packet 39 199 5.5.1. Key Packet Variants 39 200 5.5.1.1. Public Key Packet (Tag 6) 39 201 5.5.1.2. Public Subkey Packet (Tag 14) 39 202 5.5.1.3. Secret Key Packet (Tag 5) 39 203 5.5.1.4. Secret Subkey Packet (Tag 7) 40 204 5.5.2. Public Key Packet Formats 40 205 5.5.3. Secret Key Packet Formats 41 206 5.6. Compressed Data Packet (Tag 8) 43 207 5.7. Symmetrically Encrypted Data Packet (Tag 9) 44 208 5.8. Marker Packet (Obsolete Literal Packet) (Tag 10) 44 209 5.9. Literal Data Packet (Tag 11) 45 210 5.10. Trust Packet (Tag 12) 46 211 5.11. User ID Packet (Tag 13) 46 212 5.12. User Attribute Packet (Tag 17) 46 213 5.12.1. The Image Attribute Subpacket 47 214 5.13. Sym. Encrypted Integrity Protected Data Packet (Tag 18) 47 215 5.14. Modification Detection Code Packet (Tag 19) 50 216 6. Radix-64 Conversions 51 217 6.1. An Implementation of the CRC-24 in "C" 51 218 6.2. Forming ASCII Armor 52 219 6.3. Encoding Binary in Radix-64 54 220 6.4. Decoding Radix-64 55 221 6.5. Examples of Radix-64 56 222 6.6. Example of an ASCII Armored Message 56 223 7. Cleartext signature framework 56 224 7.1. Dash-Escaped Text 57 225 8. Regular Expressions 58 226 227 Callas, et al. Expires Oct 24, 2007 [Page 4] 228 INTERNET-DRAFT OpenPGP Message Format Apr 24, 2007 230 231 9. Constants 58 232 9.1. Public Key Algorithms 59 233 9.2. Symmetric Key Algorithms 59 234 9.3. Compression Algorithms 60 235 9.4. Hash Algorithms 60 236 10. IANA Considerations 60 237 10.1. New String-to-Key specifier types 60 238 10.2. New Packets 61 239 10.2.1. User Attribute Types 61 240 10.2.1.1.Image Format Subpacket Types 61 241 10.2.2. New Signature Subpackets 61 242 10.2.2.1.Signature Notation Data Subpackets 61 243 10.2.2.2.Key Server Preference Extensions 62 244 10.2.2.3.Key Flags Extensions 62 245 10.2.2.4.Reason For Revocation Extensions 62 246 10.2.2.5.Implementation Features 62 247 10.2.3. New Packet Versions 62 248 10.3. New Algorithms 63 249 10.3.1. Public Key Algorithms 63 250 10.3.2. Symmetric Key Algorithms 63 251 10.3.3. Hash Algorithms 63 252 10.3.4. Compression Algorithms 64 253 11. Packet Composition 64 254 11.1. Transferable Public Keys 64 255 11.2. Transferable Secret Keys 65 256 11.3. OpenPGP Messages 65 257 11.4. Detached Signatures 66 258 12. Enhanced Key Formats 66 259 12.1. Key Structures 66 260 12.2. Key IDs and Fingerprints 67 261 13. Notes on Algorithms 68 262 13.1. PKCS#1 Encoding In OpenPGP 68 263 13.1.1. EME-PKCS1-v1_5-ENCODE 69 264 13.1.2. EME-PKCS1-v1_5-DECODE 69 265 13.1.3. EMSA-PKCS1-v1_5 70 266 13.2. Symmetric Algorithm Preferences 71 267 13.3. Other Algorithm Preferences 71 268 13.3.1. Compression Preferences 71 269 13.3.2. Hash Algorithm Preferences 72 270 13.4. Plaintext 72 271 13.5. RSA 72 272 13.6. DSA 73 273 13.7. Elgamal 73 274 13.8. Reserved Algorithm Numbers 73 275 13.9. OpenPGP CFB mode 74 276 13.10. Private or Experimental Parameters 75 277 13.11. Extension of the MDC System 75 278 13.12. Meta-Considerations for Expansion 76 279 14. Security Considerations 76 280 15. Implementation Nits 79 281 16. Authors' Addresses 80 282 17. References (Normative) 81 283 284 Callas, et al. Expires Oct 24, 2007 [Page 5] 285 INTERNET-DRAFT OpenPGP Message Format Apr 24, 2007 287 288 18. References (Informative) 83 289 19. Full Copyright Statement 84 290 20. Intellectual Property 84 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 Callas, et al. Expires Oct 24, 2007 [Page 6] 342 INTERNET-DRAFT OpenPGP Message Format Apr 24, 2007 344 345 1. Introduction 346 347 This document provides information on the message-exchange packet 348 formats used by OpenPGP to provide encryption, decryption, signing, 349 and key management functions. It is a revision of RFC 2440, "OpenPGP 350 Message Format", which itself replaces RFC 1991, "PGP Message 351 Exchange Formats." [RFC1991] [RFC2440] 352 353 1.1. Terms 354 355 * OpenPGP - This is a definition for security software that uses 356 PGP 5.x as a basis, formalized in RFC 2440 and this document. 357 358 * PGP - Pretty Good Privacy. PGP is a family of software systems 359 developed by Philip R. Zimmermann from which OpenPGP is based. 360 361 * PGP 2.6.x - This version of PGP has many variants, hence the 362 term PGP 2.6.x. It used only RSA, MD5, and IDEA for its 363 cryptographic transforms. An informational RFC, RFC 1991, was 364 written describing this version of PGP. 365 366 * PGP 5.x - This version of PGP is formerly known as "PGP 3" in 367 the community and also in the predecessor of this document, RFC 368 1991. It has new formats and corrects a number of problems in 369 the PGP 2.6.x design. It is referred to here as PGP 5.x because 370 that software was the first release of the "PGP 3" code base. 371 372 * GnuPG - GNU Privacy Guard, also called GPG. GnuPG is an OpenPGP 373 implementation that avoids all encumbered algorithms. 374 Consequently, early versions of GnuPG did not include RSA public 375 keys. GnuPG may or may not have (depending on version) support 376 for IDEA or other encumbered algorithms. 377 378 "PGP", "Pretty Good", and "Pretty Good Privacy" are trademarks of 379 PGP Corporation and are used with permission. The term "OpenPGP" 380 refers to the protocol described in this and related documents. 381 382 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 383 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 384 document are to be interpreted as described in RFC 2119. 385 386 The key words "PRIVATE USE", "HIERARCHICAL ALLOCATION", "FIRST COME 387 FIRST SERVED", "EXPERT REVIEW", "SPECIFICATION REQUIRED", "IESG 388 APPROVAL", "IETF CONSENSUS", and "STANDARDS ACTION" that appear in 389 this document when used to describe namespace allocation are to be 390 interpreted as described in RFC 2434. 391 392 2. General functions 393 394 OpenPGP provides data integrity services for messages and data files 395 by using these core technologies: 396 397 398 Callas, et al. Expires Oct 24, 2007 [Page 7] 399 INTERNET-DRAFT OpenPGP Message Format Apr 24, 2007 401 402 - digital signatures 403 404 - encryption 405 406 - compression 407 408 - radix-64 conversion 409 410 In addition, OpenPGP provides key management and certificate 411 services, but many of these are beyond the scope of this document. 412 413 2.1. Confidentiality via Encryption 414 415 OpenPGP combines symmetric-key encryption and public key encryption 416 to provide confidentiality. When made confidential, first the object 417 is encrypted using a symmetric encryption algorithm. Each symmetric 418 key is used only once, for a single object. A new "session key" is 419 generated as a random number for each object (sometimes referred to 420 as a session). Since it is used only once, the session key is bound 421 to the message and transmitted with it. To protect the key, it is 422 encrypted with the receiver's public key. The sequence is as 423 follows: 424 425 1. The sender creates a message. 426 427 2. The sending OpenPGP generates a random number to be used as a 428 session key for this message only. 429 430 3. The session key is encrypted using each recipient's public key. 431 These "encrypted session keys" start the message. 432 433 4. The sending OpenPGP encrypts the message using the session key, 434 which forms the remainder of the message. Note that the message 435 is also usually compressed. 436 437 5. The receiving OpenPGP decrypts the session key using the 438 recipient's private key. 439 440 6. The receiving OpenPGP decrypts the message using the session 441 key. If the message was compressed, it will be decompressed. 442 443 With symmetric-key encryption, an object may be encrypted with a 444 symmetric key derived from a passphrase (or other shared secret), or 445 a two-stage mechanism similar to the public-key method described 446 above in which a session key is itself encrypted with a symmetric 447 algorithm keyed from a shared secret. 448 449 Both digital signature and confidentiality services may be applied 450 to the same message. First, a signature is generated for the message 451 and attached to the message. Then, the message plus signature is 452 encrypted using a symmetric session key. Finally, the session key is 453 encrypted using public-key encryption and prefixed to the encrypted 454 455 Callas, et al. Expires Oct 24, 2007 [Page 8] 456 INTERNET-DRAFT OpenPGP Message Format Apr 24, 2007 458 459 block. 460 461 2.2. Authentication via Digital signature 462 463 The digital signature uses a hash code or message digest algorithm, 464 and a public-key signature algorithm. The sequence is as follows: 465 466 1. The sender creates a message. 467 468 2. The sending software generates a hash code of the message. 469 470 3. The sending software generates a signature from the hash code 471 using the sender's private key. 472 473 4. The binary signature is attached to the message. 474 475 5. The receiving software keeps a copy of the message signature. 476 477 6. The receiving software generates a new hash code for the 478 received message and verifies it using the message's signature. 479 If the verification is successful, the message is accepted as 480 authentic. 481 482 2.3. Compression 483 484 OpenPGP implementations SHOULD compress the message after applying 485 the signature but before encryption. 486 487 If an implementation does not implement compression, its authors 488 should be aware that most OpenPGP messages in the world are 489 compressed. Thus, it may even be wise for a space-constrained 490 implementation to implement decompression, but not compression. 491 492 Furthermore, compression has the added side-effect that some types 493 of attacks can be thwarted by the fact that slightly altered, 494 compressed data rarely uncompresses without severe errors. This is 495 hardly rigorous, but it is operationally useful. These attacks can 496 be rigorously prevented by implementing and using Modification 497 Detection Codes as described in sections following. 498 499 2.4. Conversion to Radix-64 500 501 OpenPGP's underlying native representation for encrypted messages, 502 signature certificates, and keys is a stream of arbitrary octets. 503 Some systems only permit the use of blocks consisting of seven-bit, 504 printable text. For transporting OpenPGP's native raw binary octets 505 through channels that are not safe to raw binary data, a printable 506 encoding of these binary octets is needed. OpenPGP provides the 507 service of converting the raw 8-bit binary octet stream to a stream 508 of printable ASCII characters, called Radix-64 encoding or ASCII 509 Armor. 510 511 512 Callas, et al. Expires Oct 24, 2007 [Page 9] 513 INTERNET-DRAFT OpenPGP Message Format Apr 24, 2007 515 516 Implementations SHOULD provide Radix-64 conversions. 517 518 2.5. Signature-Only Applications 519 520 OpenPGP is designed for applications that use both encryption and 521 signatures, but there are a number of problems that are solved by a 522 signature-only implementation. Although this specification requires 523 both encryption and signatures, it is reasonable for there to be 524 subset implementations that are non-conformant only in that they 525 omit encryption. 526 527 3. Data Element Formats 528 529 This section describes the data elements used by OpenPGP. 530 531 3.1. Scalar numbers 532 533 Scalar numbers are unsigned, and are always stored in big-endian 534 format. Using n[k] to refer to the kth octet being interpreted, the 535 value of a two-octet scalar is ((n[0] << 8) + n[1]). The value of a 536 four-octet scalar is ((n[0] << 24) + (n[1] << 16) + (n[2] << 8) + 537 n[3]). 538 539 3.2. Multiprecision Integers 540 541 Multiprecision Integers (also called MPIs) are unsigned integers 542 used to hold large integers such as the ones used in cryptographic 543 calculations. 544 545 An MPI consists of two pieces: a two-octet scalar that is the length 546 of the MPI in bits followed by a string of octets that contain the 547 actual integer. 548 549 These octets form a big-endian number; a big-endian number can be 550 made into an MPI by prefixing it with the appropriate length. 551 552 Examples: 553 554 (all numbers are in hexadecimal) 555 556 The string of octets [00 01 01] forms an MPI with the value 1. The 557 string [00 09 01 FF] forms an MPI with the value of 511. 558 559 Additional rules: 560 561 The size of an MPI is ((MPI.length + 7) / 8) + 2 octets. 562 563 The length field of an MPI describes the length starting from its 564 most significant non-zero bit. Thus, the MPI [00 02 01] is not 565 formed correctly. It should be [00 01 01]. 566 567 568 569 Callas, et al. Expires Oct 24, 2007 [Page 10] 570 INTERNET-DRAFT OpenPGP Message Format Apr 24, 2007 572 573 Unused bits of an MPI MUST be zero. 574 575 Also note that when an MPI is encrypted, the length refers to the 576 plaintext MPI. It may be ill-formed in its ciphertext. 577 578 3.3. Key IDs 579 580 A Key ID is an eight-octet scalar that identifies a key. 581 Implementations SHOULD NOT assume that Key IDs are unique. The 582 section, "Enhanced Key Formats" below describes how Key IDs are 583 formed. 584 585 3.4. Text 586 587 Unless otherwise specified, the character set for text is the UTF-8 588 [RFC3629] encoding of Unicode [ISO10646]. 589 590 3.5. Time fields 591 592 A time field is an unsigned four-octet number containing the number 593 of seconds elapsed since midnight, 1 January 1970 UTC. 594 595 3.6. Keyrings 596 597 A keyring is a collection of one or more keys in a file or database. 598 Traditionally, a keyring is simply a sequential list of keys, but 599 may be any suitable database. It is beyond the scope of this 600 standard to discuss the details of keyrings or other databases. 601 602 3.7. String-to-key (S2K) specifiers 603 604 String-to-key (S2K) specifiers are used to convert passphrase 605 strings into symmetric-key encryption/decryption keys. They are used 606 in two places, currently: to encrypt the secret part of private keys 607 in the private keyring, and to convert passphrases to encryption 608 keys for symmetrically encrypted messages. 609 610 3.7.1. String-to-key (S2K) specifier types 611 612 There are three types of S2K specifiers currently supported, and 613 some reserved values: 614 615 ID S2K Type 616 -- --- ---- 617 0 Simple S2K 618 1 Salted S2K 619 2 Reserved value 620 3 Iterated and Salted S2K 621 100 to 110 Private/Experimental S2K 622 623 624 625 626 Callas, et al. Expires Oct 24, 2007 [Page 11] 627 INTERNET-DRAFT OpenPGP Message Format Apr 24, 2007 629 630 These are described as follows: 631 632 3.7.1.1. Simple S2K 633 634 This directly hashes the string to produce the key data. See below 635 for how this hashing is done. 636 637 Octet 0: 0x00 638 Octet 1: hash algorithm 639 640 Simple S2K hashes the passphrase to produce the session key. The 641 manner in which this is done depends on the size of the session key 642 (which will depend on the cipher used) and the size of the hash 643 algorithm's output. If the hash size is greater than the session key 644 size, the high-order (leftmost) octets of the hash are used as the 645 key. 646 647 If the hash size is less than the key size, multiple instances of 648 the hash context are created -- enough to produce the required key 649 data. These instances are preloaded with 0, 1, 2, ... octets of 650 zeros (that is to say, the first instance has no preloading, the 651 second gets preloaded with 1 octet of zero, the third is preloaded 652 with two octets of zeros, and so forth). 653 654 As the data is hashed, it is given independently to each hash 655 context. Since the contexts have been initialized differently, they 656 will each produce different hash output. Once the passphrase is 657 hashed, the output data from the multiple hashes is concatenated, 658 first hash leftmost, to produce the key data, with any excess octets 659 on the right discarded. 660 661 3.7.1.2. Salted S2K 662 663 This includes a "salt" value in the S2K specifier -- some arbitrary 664 data -- that gets hashed along with the passphrase string, to help 665 prevent dictionary attacks. 666 667 Octet 0: 0x01 668 Octet 1: hash algorithm 669 Octets 2-9: 8-octet salt value 670 671 Salted S2K is exactly like Simple S2K, except that the input to the 672 hash function(s) consists of the 8 octets of salt from the S2K 673 specifier, followed by the passphrase. 674 675 3.7.1.3. Iterated and Salted S2K 676 677 This includes both a salt and an octet count. The salt is combined 678 with the passphrase and the resulting value is hashed repeatedly. 679 This further increases the amount of work an attacker must do to try 680 dictionary attacks. 681 682 683 Callas, et al. Expires Oct 24, 2007 [Page 12] 684 INTERNET-DRAFT OpenPGP Message Format Apr 24, 2007 686 687 Octet 0: 0x03 688 Octet 1: hash algorithm 689 Octets 2-9: 8-octet salt value 690 Octet 10: count, a one-octet, coded value 691 692 The count is coded into a one-octet number using the following 693 formula: 694 695 #define EXPBIAS 6 696 count = ((Int32)16 + (c & 15)) << ((c >> 4) + EXPBIAS); 697 698 The above formula is in C, where "Int32" is a type for a 32-bit 699 integer, and the variable "c" is the coded count, Octet 10. 700 701 Iterated-Salted S2K hashes the passphrase and salt data multiple 702 times. The total number of octets to be hashed is specified in the 703 encoded count in the S2K specifier. Note that the resulting count 704 value is an octet count of how many octets will be hashed, not an 705 iteration count. 706 707 Initially, one or more hash contexts are set up as with the other 708 S2K algorithms, depending on how many octets of key data are needed. 709 Then the salt, followed by the passphrase data is repeatedly hashed 710 until the number of octets specified by the octet count has been 711 hashed. The one exception is that if the octet count is less than 712 the size of the salt plus passphrase, the full salt plus passphrase 713 will be hashed even though that is greater than the octet count. 714 After the hashing is done the data is unloaded from the hash 715 context(s) as with the other S2K algorithms. 716 717 3.7.2. String-to-key usage 718 719 Implementations SHOULD use salted or iterated-and-salted S2K 720 specifiers, as simple S2K specifiers are more vulnerable to 721 dictionary attacks. 722 723 3.7.2.1. Secret key encryption 724 725 An S2K specifier can be stored in the secret keyring to specify how 726 to convert the passphrase to a key that unlocks the secret data. 727 Older versions of PGP just stored a cipher algorithm octet preceding 728 the secret data or a zero to indicate that the secret data was 729 unencrypted. The MD5 hash function was always used to convert the 730 passphrase to a key for the specified cipher algorithm. 731 732 For compatibility, when an S2K specifier is used, the special value 733 254 or 255 is stored in the position where the hash algorithm octet 734 would have been in the old data structure. This is then followed 735 immediately by a one-octet algorithm identifier, and then by the S2K 736 specifier as encoded above. 737 738 739 740 Callas, et al. Expires Oct 24, 2007 [Page 13] 741 INTERNET-DRAFT OpenPGP Message Format Apr 24, 2007 743 744 Therefore, preceding the secret data there will be one of these 745 possibilities: 746 747 0: secret data is unencrypted (no passphrase) 748 255 or 254: followed by algorithm octet and S2K specifier 749 Cipher alg: use Simple S2K algorithm using MD5 hash 750 751 This last possibility, the cipher algorithm number with an implicit 752 use of MD5 and IDEA, is provided for backward compatibility; it MAY 753 be understood, but SHOULD NOT be generated, and is deprecated. 754 755 These are followed by an Initial Vector of the same length as the 756 block size of the cipher for the decryption of the secret values, if 757 they are encrypted, and then the secret key values themselves. 758 759 3.7.2.2. Symmetric-key message encryption 760 761 OpenPGP can create a Symmetric-key Encrypted Session Key (ESK) 762 packet at the front of a message. This is used to allow S2K 763 specifiers to be used for the passphrase conversion or to create 764 messages with a mix of symmetric-key ESKs and public-key ESKs. This 765 allows a message to be decrypted either with a passphrase or a 766 public key pair. 767 768 PGP 2.X always used IDEA with Simple string-to-key conversion when 769 encrypting a message with a symmetric algorithm. This is deprecated, 770 but MAY be used for backward-compatibility. 771 772 4. Packet Syntax 773 774 This section describes the packets used by OpenPGP. 775 776 4.1. Overview 777 778 An OpenPGP message is constructed from a number of records that are 779 traditionally called packets. A packet is a chunk of data that has a 780 tag specifying its meaning. An OpenPGP message, keyring, 781 certificate, and so forth consists of a number of packets. Some of 782 those packets may contain other OpenPGP packets (for example, a 783 compressed data packet, when uncompressed, contains OpenPGP 784 packets). 785 786 Each packet consists of a packet header, followed by the packet 787 body. The packet header is of variable length. 788 789 4.2. Packet Headers 790 791 The first octet of the packet header is called the "Packet Tag." It 792 determines the format of the header and denotes the packet contents. 793 The remainder of the packet header is the length of the packet. 794 795 796 797 Callas, et al. Expires Oct 24, 2007 [Page 14] 798 INTERNET-DRAFT OpenPGP Message Format Apr 24, 2007 800 801 Note that the most significant bit is the left-most bit, called bit 802 7. A mask for this bit is 0x80 in hexadecimal. 803 804 +---------------+ 805 PTag |7 6 5 4 3 2 1 0| 806 +---------------+ 807 Bit 7 -- Always one 808 Bit 6 -- New packet format if set 809 810 PGP 2.6.x only uses old format packets. Thus, software that 811 interoperates with those versions of PGP must only use old format 812 packets. If interoperability is not an issue, the new packet format 813 is RECOMMENDED. Note that old format packets have four bits of 814 packet tags, and new format packets have six; some features cannot 815 be used and still be backward-compatible. 816 817 Also note that packets with a tag greater than or equal to 16 MUST 818 use new format packets. The old format packets can only express tags 819 less than or equal to 15. 820 821 Old format packets contain: 822 823 Bits 5-2 -- packet tag 824 Bits 1-0 - length-type 825 826 New format packets contain: 827 828 Bits 5-0 -- packet tag 829 830 4.2.1. Old-Format Packet Lengths 831 832 The meaning of the length-type in old-format packets is: 833 834 0 - The packet has a one-octet length. The header is 2 octets long. 835 836 1 - The packet has a two-octet length. The header is 3 octets long. 837 838 2 - The packet has a four-octet length. The header is 5 octets long. 839 840 3 - The packet is of indeterminate length. The header is 1 octet 841 long, and the implementation must determine how long the packet 842 is. If the packet is in a file, this means that the packet 843 extends until the end of the file. In general, an implementation 844 SHOULD NOT use indeterminate length packets except where the end 845 of the data will be clear from the context, and even then it is 846 better to use a definite length, or a new-format header. The 847 new-format headers described below have a mechanism for 848 precisely encoding data of indeterminate length. 849 850 4.2.2. New-Format Packet Lengths 851 852 New format packets have four possible ways of encoding length: 853 854 Callas, et al. Expires Oct 24, 2007 [Page 15] 855 INTERNET-DRAFT OpenPGP Message Format Apr 24, 2007 857 858 1. A one-octet Body Length header encodes packet lengths of up to 859 191 octets. 860 861 2. A two-octet Body Length header encodes packet lengths of 192 to 862 8383 octets. 863 864 3. A five-octet Body Length header encodes packet lengths of up to 865 4,294,967,295 (0xFFFFFFFF) octets in length. (This actually 866 encodes a four-octet scalar number.) 867 868 4. When the length of the packet body is not known in advance by 869 the issuer, Partial Body Length headers encode a packet of 870 indeterminate length, effectively making it a stream. 871 872 4.2.2.1. One-Octet Lengths 873 874 A one-octet Body Length header encodes a length of from 0 to 191 875 octets. This type of length header is recognized because the one 876 octet value is less than 192. The body length is equal to: 877 878 bodyLen = 1st_octet; 879 880 4.2.2.2. Two-Octet Lengths 881 882 A two-octet Body Length header encodes a length of from 192 to 8383 883 octets. It is recognized because its first octet is in the range 192 884 to 223. The body length is equal to: 885 886 bodyLen = ((1st_octet - 192) << 8) + (2nd_octet) + 192 887 888 4.2.2.3. Five-Octet Lengths 889 890 A five-octet Body Length header consists of a single octet holding 891 the value 255, followed by a four-octet scalar. The body length is 892 equal to: 893 894 bodyLen = (2nd_octet << 24) | (3rd_octet << 16) | 895 (4th_octet << 8) | 5th_octet 896 897 This basic set of one, two, and five-octet lengths is also used 898 internally to some packets. 899 900 4.2.2.4. Partial Body Lengths 901 902 A Partial Body Length header is one octet long and encodes the 903 length of only part of the data packet. This length is a power of 2, 904 from 1 to 1,073,741,824 (2 to the 30th power). It is recognized by 905 its one octet value that is greater than or equal to 224, and less 906 than 255. The partial body length is equal to: 907 908 909 910 911 Callas, et al. Expires Oct 24, 2007 [Page 16] 912 INTERNET-DRAFT OpenPGP Message Format Apr 24, 2007 914 915 partialBodyLen = 1 << (1st_octet & 0x1f); 916 917 Each Partial Body Length header is followed by a portion of the 918 packet body data. The Partial Body Length header specifies this 919 portion's length. Another length header (one octet, two-octet, 920 five-octet, or partial) follows that portion. The last length header 921 in the packet MUST NOT be a partial Body Length header. Partial Body 922 Length headers may only be used for the non-final parts of the 923 packet. 924 925 Note also that the last Body Length header can be a zero-length 926 header. 927 928 An implementation MAY use Partial Body Lengths for data packets, be 929 they literal, compressed, or encrypted. The first partial length 930 MUST be at least 512 octets long. Partial Body Lengths MUST NOT be 931 used for any other packet types. 932 933 4.2.3. Packet Length Examples 934 935 These examples show ways that new-format packets might encode the 936 packet lengths. 937 938 A packet with length 100 may have its length encoded in one octet: 939 0x64. This is followed by 100 octets of data. 940 941 A packet with length 1723 may have its length coded in two octets: 942 0xC5, 0xFB. This header is followed by the 1723 octets of data. 943 944 A packet with length 100000 may have its length encoded in five 945 octets: 0xFF, 0x00, 0x01, 0x86, 0xA0. 946 947 It might also be encoded in the following octet stream: 0xEF, first 948 32768 octets of data; 0xE1, next two octets of data; 0xE0, next one 949 octet of data; 0xF0, next 65536 octets of data; 0xC5, 0xDD, last 950 1693 octets of data. This is just one possible encoding, and many 951 variations are possible on the size of the Partial Body Length 952 headers, as long as a regular Body Length header encodes the last 953 portion of the data. 954 955 Please note that in all of these explanations, the total length of 956 the packet is the length of the header(s) plus the length of the 957 body. 958 959 4.3. Packet Tags 960 961 The packet tag denotes what type of packet the body holds. Note that 962 old format headers can only have tags less than 16, whereas new 963 format headers can have tags as great as 63. The defined tags (in 964 decimal) are: 965 966 967 968 Callas, et al. Expires Oct 24, 2007 [Page 17] 969 INTERNET-DRAFT OpenPGP Message Format Apr 24, 2007 971 972 0 -- Reserved - a packet tag MUST NOT have this value 973 1 -- Public-Key Encrypted Session Key Packet 974 2 -- Signature Packet 975 3 -- Symmetric-Key Encrypted Session Key Packet 976 4 -- One-Pass Signature Packet 977 5 -- Secret Key Packet 978 6 -- Public Key Packet 979 7 -- Secret Subkey Packet 980 8 -- Compressed Data Packet 981 9 -- Symmetrically Encrypted Data Packet 982 10 -- Marker Packet 983 11 -- Literal Data Packet 984 12 -- Trust Packet 985 13 -- User ID Packet 986 14 -- Public Subkey Packet 987 17 -- User Attribute Packet 988 18 -- Sym. Encrypted and Integrity Protected Data Packet 989 19 -- Modification Detection Code Packet 990 60 to 63 -- Private or Experimental Values 991 992 5. Packet Types 993 994 5.1. Public-Key Encrypted Session Key Packets (Tag 1) 995 996 A Public-Key Encrypted Session Key packet holds the session key used 997 to encrypt a message. Zero or more Public-Key Encrypted Session Key 998 packets and/or Symmetric-Key Encrypted Session Key packets may 999 precede a Symmetrically Encrypted Data Packet, which holds an 1000 encrypted message. The message is encrypted with the session key, 1001 and the session key is itself encrypted and stored in the Encrypted 1002 Session Key packet(s). The Symmetrically Encrypted Data Packet is 1003 preceded by one Public-Key Encrypted Session Key packet for each 1004 OpenPGP key to which the message is encrypted. The recipient of the 1005 message finds a session key that is encrypted to their public key, 1006 decrypts the session key, and then uses the session key to decrypt 1007 the message. 1008 1009 The body of this packet consists of: 1010 1011 - A one-octet number giving the version number of the packet type. 1012 The currently defined value for packet version is 3. 1013 1014 - An eight-octet number that gives the key ID of the public key 1015 that the session key is encrypted to. If the session key is 1016 encrypted to a subkey then the key ID of this subkey is used 1017 here instead of the key ID of the primary key. 1018 1019 - A one-octet number giving the public key algorithm used. 1020 1021 - A string of octets that is the encrypted session key. This 1022 string takes up the remainder of the packet, and its contents 1023 are dependent on the public key algorithm used. 1024 1025 Callas, et al. Expires Oct 24, 2007 [Page 18] 1026 INTERNET-DRAFT OpenPGP Message Format Apr 24, 2007 1028 1029 Algorithm Specific Fields for RSA encryption 1030 1031 - multiprecision integer (MPI) of RSA encrypted value m**e mod n. 1032 1033 Algorithm Specific Fields for Elgamal encryption: 1034 1035 - MPI of Elgamal (Diffie-Hellman) value g**k mod p. 1036 1037 - MPI of Elgamal (Diffie-Hellman) value m * y**k mod p. 1038 1039 The value "m" in the above formulas is derived from the session key 1040 as follows. First the session key is prefixed with a one-octet 1041 algorithm identifier that specifies the symmetric encryption 1042 algorithm used to encrypt the following Symmetrically Encrypted Data 1043 Packet. Then a two-octet checksum is appended which is equal to the 1044 sum of the preceding session key octets, not including the algorithm 1045 identifier, modulo 65536. This value is then encoded as described in 1046 PKCS#1 block encoding EME-PKCS1-v1_5 in Section 12.1 of RFC 3447 to 1047 form the "m" value used in the formulas above. See Section 13.1 of 1048 this document for notes on OpenPGP's use of PKCS#1. 1049 1050 Note that when an implementation forms several PKESKs with one 1051 session key, forming a message that can be decrypted by several 1052 keys, the implementation MUST make a new PKCS#1 encoding for each 1053 key. 1054 1055 An implementation MAY accept or use a Key ID of zero as a "wild 1056 card" or "speculative" Key ID. In this case, the receiving 1057 implementation would try all available private keys, checking for a 1058 valid decrypted session key. This format helps reduce traffic 1059 analysis of messages. 1060 1061 5.2. Signature Packet (Tag 2) 1062 1063 A signature packet describes a binding between some public key and 1064 some data. The most common signatures are a signature of a file or a 1065 block of text, and a signature that is a certification of a User ID. 1066 1067 Two versions of signature packets are defined. Version 3 provides 1068 basic signature information, while version 4 provides an expandable 1069 format with subpackets that can specify more information about the 1070 signature. PGP 2.6.x only accepts version 3 signatures. 1071 1072 Implementations SHOULD accept V3 signatures. Implementations SHOULD 1073 generate V4 signatures. 1074 1075 Note that if an implementation is creating an encrypted and signed 1076 message that is encrypted to a V3 key, it is reasonable to create a 1077 V3 signature. 1078 1079 1080 1081 1082 Callas, et al. Expires Oct 24, 2007 [Page 19] 1083 INTERNET-DRAFT OpenPGP Message Format Apr 24, 2007 1085 1086 5.2.1. Signature Types 1087 1088 There are a number of possible meanings for a signature, which are 1089 indicated in a signature type octet in any given signature. Please 1090 note that the vagueness of these meanings is not a flaw, but a 1091 feature of the system. Because OpenPGP places final authority for 1092 validity upon the receiver of a signature, it may be that one 1093 signer's casual act might be more rigorous than some other 1094 authority's positive act. See section 5.2.4, "Computing Signatures," 1095 for detailed information on how to compute and verify signatures of 1096 each type. 1097 1098 These meanings are: 1099 1100 0x00: Signature of a binary document. 1101 This means the signer owns it, created it, or certifies that it 1102 has not been modified. 1103 1104 0x01: Signature of a canonical text document. 1105 This means the signer owns it, created it, or certifies that it 1106 has not been modified. The signature is calculated over the text 1107 data with its line endings converted to <CR><LF>. 1108 1109 0x02: Standalone signature. 1110 This signature is a signature of only its own subpacket 1111 contents. It is calculated identically to a signature over a 1112 zero-length binary document. Note that it doesn't make sense to 1113 have a V3 standalone signature. 1114 1115 0x10: Generic certification of a User ID and Public Key packet. 1116 The issuer of this certification does not make any particular 1117 assertion as to how well the certifier has checked that the 1118 owner of the key is in fact the person described by the User ID. 1119 1120 0x11: Persona certification of a User ID and Public Key packet. 1121 The issuer of this certification has not done any verification 1122 of the claim that the owner of this key is the User ID 1123 specified. 1124 1125 0x12: Casual certification of a User ID and Public Key packet. 1126 The issuer of this certification has done some casual 1127 verification of the claim of identity. 1128 1129 0x13: Positive certification of a User ID and Public Key packet. 1130 The issuer of this certification has done substantial 1131 verification of the claim of identity. 1132 1133 Most OpenPGP implementations make their "key signatures" as 0x10 1134 certifications. Some implementations can issue 0x11-0x13 1135 certifications, but few differentiate between the types. 1136 1137 1138 1139 Callas, et al. Expires Oct 24, 2007 [Page 20] 1140 INTERNET-DRAFT OpenPGP Message Format Apr 24, 2007 1142 1143 0x18: Subkey Binding Signature 1144 This signature is a statement by the top-level signing key that 1145 indicates that it owns the subkey. This signature is calculated 1146 directly on the primary key and subkey, and not on any User ID 1147 or other packets. A signature that binds a signing subkey MUST 1148 have an embedded signature subpacket in this binding signature 1149 which contains a 0x19 signature made by the signing subkey on 1150 the primary key and subkey. 1151 1152 0x19 Primary Key Binding Signature 1153 This signature is a statement by a signing subkey, indicating 1154 that it is owned by the primary key and subkey. This signature 1155 is calculated the same way as a 0x18 signature: directly on the 1156 primary key and subkey, and not on any User ID or other packets. 1157 1158 0x1F: Signature directly on a key 1159 This signature is calculated directly on a key. It binds the 1160 information in the signature subpackets to the key, and is 1161 appropriate to be used for subpackets that provide information 1162 about the key, such as the revocation key subpacket. It is also 1163 appropriate for statements that non-self certifiers want to make 1164 about the key itself, rather than the binding between a key and 1165 a name. 1166 1167 0x20: Key revocation signature 1168 The signature is calculated directly on the key being revoked. A 1169 revoked key is not to be used. Only revocation signatures by the 1170 key being revoked, or by an authorized revocation key, should be 1171 considered valid revocation signatures. 1172 1173 0x28: Subkey revocation signature 1174 The signature is calculated directly on the subkey being 1175 revoked. A revoked subkey is not to be used. Only revocation 1176 signatures by the top-level signature key that is bound to this 1177 subkey, or by an authorized revocation key, should be considered 1178 valid revocation signatures. 1179 1180 0x30: Certification revocation signature 1181 This signature revokes an earlier User ID certification 1182 signature (signature class 0x10 through 0x13) or direct-key 1183 signature (0x1F). It should be issued by the same key that 1184 issued the revoked signature or an authorized revocation key. 1185 The signature is computed over the same data as the certificate 1186 that it revokes, and should have a later creation date than that 1187 certificate. 1188 1189 0x40: Timestamp signature. 1190 This signature is only meaningful for the timestamp contained in 1191 it. 1192 1193 1194 1195 1196 Callas, et al. Expires Oct 24, 2007 [Page 21] 1197 INTERNET-DRAFT OpenPGP Message Format Apr 24, 2007 1199 1200 0x50: Third-Party Confirmation signature. 1201 This signature is a signature over some other OpenPGP signature 1202 packet(s). It is analogous to a notary seal on the signed data. 1203 A third-party signature SHOULD include Signature Target 1204 subpacket(s) to give easy identification. Note that we really do 1205 mean SHOULD. There are plausible uses for this (such as a blind 1206 party that only sees the signature, not the key nor source 1207 document) that cannot include a target subpacket. 1208 1209 5.2.2. Version 3 Signature Packet Format 1210 1211 The body of a version 3 Signature Packet contains: 1212 1213 - One-octet version number (3). 1214 1215 - One-octet length of following hashed material. MUST be 5. 1216 1217 - One-octet signature type. 1218 1219 - Four-octet creation time. 1220 1221 - Eight-octet key ID of signer. 1222 1223 - One-octet public key algorithm. 1224 1225 - One-octet hash algorithm. 1226 1227 - Two-octet field holding left 16 bits of signed hash value. 1228 1229 - One or more multiprecision integers comprising the signature. 1230 This portion is algorithm specific, as described below. 1231 1232 The concatenation of the data to be signed, the signature type and 1233 creation time from the signature packet (5 additional octets) is 1234 hashed. The resulting hash value is used in the signature algorithm. 1235 The high 16 bits (first two octets) of the hash are included in the 1236 signature packet to provide a quick test to reject some invalid 1237 signatures. 1238 1239 Algorithm Specific Fields for RSA signatures: 1240 1241 - multiprecision integer (MPI) of RSA signature value m**d mod n. 1242 1243 Algorithm Specific Fields for DSA signatures: 1244 1245 - MPI of DSA value r. 1246 1247 - MPI of DSA value s. 1248 1249 The signature calculation is based on a hash of the signed data, as 1250 described above. The details of the calculation are different for 1251 DSA signatures than for RSA signatures. 1252 1253 Callas, et al. Expires Oct 24, 2007 [Page 22] 1254 INTERNET-DRAFT OpenPGP Message Format Apr 24, 2007 1256 1257 With RSA signatures, the hash value is encoded as described in 1258 PKCS#1 section 9.2.1 of RFC 3447 encoded using PKCS#1 encoding type 1259 EMSA-PKCS1-v1_5 as described in section 12.1 of RFC 3447. This 1260 requires inserting the hash value as an octet string into an ASN.1 1261 structure. The object identifier for the type of hash being used is 1262 included in the structure. The hexadecimal representations for the 1263 currently defined hash algorithms are: 1264 1265 - MD5: 0x2A, 0x86, 0x48, 0x86, 0xF7, 0x0D, 0x02, 0x05 1266 1267 - RIPEMD-160: 0x2B, 0x24, 0x03, 0x02, 0x01 1268 1269 - SHA-1: 0x2B, 0x0E, 0x03, 0x02, 0x1A 1270 1271 - SHA224: 0x60, 0x86, 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x04 1272 1273 - SHA256: 0x60, 0x86, 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x01 1274 1275 - SHA384: 0x60, 0x86, 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x02 1276 1277 - SHA512: 0x60, 0x86, 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x03 1278 1279 The ASN.1 OIDs are: 1280 1281 - MD5: 1.2.840.113549.2.5 1282 1283 - RIPEMD-160: 1.3.36.3.2.1 1284 1285 - SHA-1: 1.3.14.3.2.26 1286 1287 - SHA224: 2.16.840.1.101.3.4.2.4 1288 1289 - SHA256: 2.16.840.1.101.3.4.2.1 1290 1291 - SHA384: 2.16.840.1.101.3.4.2.2 1292 1293 - SHA512: 2.16.840.1.101.3.4.2.3 1294 1295 The full hash prefixes for these are: 1296 1297 MD5: 0x30, 0x20, 0x30, 0x0C, 0x06, 0x08, 0x2A, 0x86, 1298 0x48, 0x86, 0xF7, 0x0D, 0x02, 0x05, 0x05, 0x00, 1299 0x04, 0x10 1300 1301 RIPEMD-160: 0x30, 0x21, 0x30, 0x09, 0x06, 0x05, 0x2B, 0x24, 1302 0x03, 0x02, 0x01, 0x05, 0x00, 0x04, 0x14 1303 1304 SHA-1: 0x30, 0x21, 0x30, 0x09, 0x06, 0x05, 0x2b, 0x0E, 1305 0x03, 0x02, 0x1A, 0x05, 0x00, 0x04, 0x14 1306 1307 1308 1309 1310 Callas, et al. Expires Oct 24, 2007 [Page 23] 1311 INTERNET-DRAFT OpenPGP Message Format Apr 24, 2007 1313 1314 SHA224: 0x30, 0x31, 0x30, 0x0d, 0x06, 0x09, 0x60, 0x86, 1315 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x04, 0x05, 1316 0x00, 0x04, 0x1C 1317 1318 SHA256: 0x30, 0x31, 0x30, 0x0d, 0x06, 0x09, 0x60, 0x86, 1319 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x01, 0x05, 1320 0x00, 0x04, 0x20 1321 1322 SHA384: 0x30, 0x41, 0x30, 0x0d, 0x06, 0x09, 0x60, 0x86, 1323 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x02, 0x05, 1324 0x00, 0x04, 0x30 1325 1326 SHA512: 0x30, 0x51, 0x30, 0x0d, 0x06, 0x09, 0x60, 0x86, 1327 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x03, 0x05, 1328 0x00, 0x04, 0x40 1329 1330 DSA signatures MUST use hashes that are equal in size to the number 1331 of bits of q, the group generated by the DSA key's generator value. 1332 If the output size of the chosen hash is larger than the number of 1333 bits of q, the hash result is truncated to fit by taking the number 1334 of leftmost bits equal to the number of bits of q. This (possibly 1335 truncated) hash function result is treated as a number and used 1336 directly in the DSA signature algorithm. 1337 1338 5.2.3. Version 4 Signature Packet Format 1339 1340 The body of a version 4 Signature Packet contains: 1341 1342 - One-octet version number (4). 1343 1344 - One-octet signature type. 1345 1346 - One-octet public key algorithm. 1347 1348 - One-octet hash algorithm. 1349 1350 - Two-octet scalar octet count for following hashed subpacket 1351 data. Note that this is the length in octets of all of the 1352 hashed subpackets; a pointer incremented by this number will 1353 skip over the hashed subpackets. 1354 1355 - Hashed subpacket data set. (zero or more subpackets) 1356 1357 - Two-octet scalar octet count for the following unhashed 1358 subpacket data. Note that this is the length in octets of all of 1359 the unhashed subpackets; a pointer incremented by this number 1360 will skip over the unhashed subpackets. 1361 1362 - Unhashed subpacket data set. (zero or more subpackets) 1363 1364 1365 1366 1367 Callas, et al. Expires Oct 24, 2007 [Page 24] 1368 INTERNET-DRAFT OpenPGP Message Format Apr 24, 2007 1370 1371 - Two-octet field holding the left 16 bits of the signed hash 1372 value. 1373 1374 - One or more multiprecision integers comprising the signature. 1375 This portion is algorithm specific, as described above. 1376 1377 The concatenation of the data being signed and the signature data 1378 from the version number through the hashed subpacket data 1379 (inclusive) is hashed. The resulting hash value is what is signed. 1380 The left 16 bits of the hash are included in the signature packet to 1381 provide a quick test to reject some invalid signatures. 1382 1383 There are two fields consisting of signature subpackets. The first 1384 field is hashed with the rest of the signature data, while the 1385 second is unhashed. The second set of subpackets is not 1386 cryptographically protected by the signature and should include only 1387 advisory information. 1388 1389 The algorithms for converting the hash function result to a 1390 signature are described in a section below. 1391 1392 5.2.3.1. Signature Subpacket Specification 1393 1394 A subpacket data set consists of zero or more signature subpackets. 1395 In signature packets the subpacket data set is preceded by a 1396 two-octet scalar count of the length in octets of all the 1397 subpackets. A pointer incremented by this number will skip over the 1398 subpacket data set. 1399 1400 Each subpacket consists of a subpacket header and a body. The header 1401 consists of: 1402 1403 - the subpacket length (1, 2, or 5 octets) 1404 1405 - the subpacket type (1 octet) 1406 1407 and is followed by the subpacket specific data. 1408 1409 The length includes the type octet but not this length. Its format 1410 is similar to the "new" format packet header lengths, but cannot 1411 have partial body lengths. That is: 1412 1413 if the 1st octet < 192, then 1414 lengthOfLength = 1 1415 subpacketLen = 1st_octet 1416 1417 if the 1st octet >= 192 and < 255, then 1418 lengthOfLength = 2 1419 subpacketLen = ((1st_octet - 192) << 8) + (2nd_octet) + 192 1420 1421 1422 1423 1424 Callas, et al. Expires Oct 24, 2007 [Page 25] 1425 INTERNET-DRAFT OpenPGP Message Format Apr 24, 2007 1427 1428 if the 1st octet = 255, then 1429 lengthOfLength = 5 1430 subpacket length = [four-octet scalar starting at 2nd_octet] 1431 1432 The value of the subpacket type octet may be: 1433 1434 0 = reserved 1435 1 = reserved 1436 2 = signature creation time 1437 3 = signature expiration time 1438 4 = exportable certification 1439 5 = trust signature 1440 6 = regular expression 1441 7 = revocable 1442 8 = reserved 1443 9 = key expiration time 1444 10 = placeholder for backward compatibility 1445 11 = preferred symmetric algorithms 1446 12 = revocation key 1447 13 = reserved 1448 14 = reserved 1449 15 = reserved 1450 16 = issuer key ID 1451 17 = reserved 1452 18 = reserved 1453 19 = reserved 1454 20 = notation data 1455 21 = preferred hash algorithms 1456 22 = preferred compression algorithms 1457 23 = key server preferences 1458 24 = preferred key server 1459 25 = primary User ID 1460 26 = policy URI 1461 27 = key flags 1462 28 = signer's User ID 1463 29 = reason for revocation 1464 30 = features 1465 31 = signature target 1466 32 = embedded signature 1467 1468 100 to 110 = private or experimental 1469 1470 An implementation SHOULD ignore any subpacket of a type that it does 1471 not recognize. 1472 1473 Bit 7 of the subpacket type is the "critical" bit. If set, it 1474 denotes that the subpacket is one that is critical for the evaluator 1475 of the signature to recognize. If a subpacket is encountered that is 1476 marked critical but is unknown to the evaluating software, the 1477 evaluator SHOULD consider the signature to be in error. 1478 1479 1480 1481 Callas, et al. Expires Oct 24, 2007 [Page 26] 1482 INTERNET-DRAFT OpenPGP Message Format Apr 24, 2007 1484 1485 An evaluator may "recognize" a subpacket, but not implement it. The 1486 purpose of the critical bit is to allow the signer to tell an 1487 evaluator that it would prefer a new, unknown feature to generate an 1488 error than be ignored. 1489 1490 Implementations SHOULD implement "preferences" and the "reason for 1491 revocation" subpackets. Note, however, that if an implementation 1492 chooses not to implement some of the preferences, it is required to 1493 behave in a polite manner to respect the wishes of those users who 1494 do implement these preferences. 1495 1496 5.2.3.2. Signature Subpacket Types 1497 1498 A number of subpackets are currently defined. Some subpackets apply 1499 to the signature itself and some are attributes of the key. 1500 Subpackets that are found on a self-signature are placed on a 1501 certification made by the key itself. Note that a key may have more 1502 than one User ID, and thus may have more than one self-signature, 1503 and differing subpackets. 1504 1505 A subpacket may be found either in the hashed or unhashed subpacket 1506 sections of a signature. If a subpacket is not hashed, then the 1507 information in it cannot be considered definitive because it is not 1508 part of the signature proper. 1509 1510 5.2.3.3. Notes on Self-Signatures 1511 1512 A self-signature is a binding signature made by the key the 1513 signature refers to. There are three types of self-signatures, the 1514 certification signatures (types 0x10-0x13), the direct-key signature 1515 (type 0x1f), and the subkey binding signature (type 0x18). For 1516 certification self-signatures, each User ID may have a 1517 self-signature, and thus different subpackets in those 1518 self-signatures. For subkey binding signatures, each subkey in fact 1519 has a self-signature. Subpackets that appear in a certification 1520 self-signature apply to the username, and subpackets that appear in 1521 the subkey self-signature apply to the subkey. Lastly, subpackets on 1522 the direct-key signature apply to the entire key. 1523 1524 Implementing software should interpret a self-signature's preference 1525 subpackets as narrowly as possible. For example, suppose a key has 1526 two usernames, Alice and Bob. Suppose that Alice prefers the 1527 symmetric algorithm CAST5, and Bob prefers IDEA or TripleDES. If the 1528 software locates this key via Alice's name, then the preferred 1529 algorithm is CAST5, if software locates the key via Bob's name, then 1530 the preferred algorithm is IDEA. If the key is located by key ID, 1531 the algorithm of the primary User ID of the key provides the 1532 preferred symmetric algorithm. 1533 1534 Revoking a self-signature or allowing it to expire has a semantic 1535 meaning that varies with the signature type. Revoking the 1536 self-signature on a User ID effectively retires that user name. The 1537 1538 Callas, et al. Expires Oct 24, 2007 [Page 27] 1539 INTERNET-DRAFT OpenPGP Message Format Apr 24, 2007 1541 1542 self-signature is a statement, "My name X is tied to my signing key 1543 K" and is corroborated by other users' certifications. If another 1544 user revokes their certification, they are effectively saying that 1545 they no longer believe that name and that key are tied together. 1546 Similarly, if the user themselves revokes their self-signature, it 1547 means the user no longer goes by that name, no longer has that email 1548 address, etc. Revoking a binding signature effectively retires that 1549 subkey. Revoking a direct-key signature cancels that signature. 1550 Please see the "Reason for Revocation" subpacket below for more 1551 relevant detail. 1552 1553 Since a self-signature contains important information about the 1554 key's use, an implementation SHOULD allow the user to rewrite the 1555 self-signature, and important information in it, such as preferences 1556 and key expiration. 1557 1558 It is good practice to verify that a self-signature imported into an 1559 implementation doesn't advertise features that the implementation 1560 doesn't support, rewriting the signature as appropriate. 1561 1562 An implementation that encounters multiple self-signatures on the 1563 same object may resolve the ambiguity in any way it sees fit, but it 1564 is RECOMMENDED that priority be given to the most recent 1565 self-signature. 1566 1567 5.2.3.4. Signature creation time 1568 1569 (4 octet time field) 1570 1571 The time the signature was made. 1572 1573 MUST be present in the hashed area. 1574 1575 5.2.3.5. Issuer 1576 1577 (8 octet key ID) 1578 1579 The OpenPGP key ID of the key issuing the signature. 1580 1581 5.2.3.6. Key expiration time 1582 1583 (4 octet time field) 1584 1585 The validity period of the key. This is the number of seconds after 1586 the key creation time that the key expires. If this is not present 1587 or has a value of zero, the key never expires. This is found only on 1588 a self-signature. 1589 1590 5.2.3.7. Preferred symmetric algorithms 1591 1592 (array of one-octet values) 1593 1594 1595 Callas, et al. Expires Oct 24, 2007 [Page 28] 1596 INTERNET-DRAFT OpenPGP Message Format Apr 24, 2007 1598 1599 Symmetric algorithm numbers that indicate which algorithms the key 1600 holder prefers to use. The subpacket body is an ordered list of 1601 octets with the most preferred listed first. It is assumed that only 1602 algorithms listed are supported by the recipient's software. 1603 Algorithm numbers are in section 9. This is only found on a 1604 self-signature. 1605 1606 5.2.3.8. Preferred hash algorithms 1607 1608 (array of one-octet values) 1609 1610 Message digest algorithm numbers that indicate which algorithms the 1611 key holder prefers to receive. Like the preferred symmetric 1612 algorithms, the list is ordered. Algorithm numbers are in section 9. 1613 This is only found on a self-signature. 1614 1615 5.2.3.9. Preferred compression algorithms 1616 1617 (array of one-octet values) 1618 1619 Compression algorithm numbers that indicate which algorithms the key 1620 holder prefers to use. Like the preferred symmetric algorithms, the 1621 list is ordered. Algorithm numbers are in section 9. If this 1622 subpacket is not included, ZIP is preferred. A zero denotes that 1623 uncompressed data is preferred; the key holder's software might have 1624 no compression software in that implementation. This is only found 1625 on a self-signature. 1626 1627 5.2.3.10. Signature expiration time 1628 1629 (4 octet time field) 1630 1631 The validity period of the signature. This is the number of seconds 1632 after the signature creation time that the signature expires. If 1633 this is not present or has a value of zero, it never expires. 1634 1635 5.2.3.11. Exportable Certification 1636 1637 (1 octet of exportability, 0 for not, 1 for exportable) 1638 1639 This subpacket denotes whether a certification signature is 1640 "exportable," to be used by other users than the signature's issuer. 1641 The packet body contains a Boolean flag indicating whether the 1642 signature is exportable. If this packet is not present, the 1643 certification is exportable; it is equivalent to a flag containing a 1644 1. 1645 1646 Non-exportable, or "local," certifications are signatures made by a 1647 user to mark a key as valid within that user's implementation only. 1648 Thus, when an implementation prepares a user's copy of a key for 1649 transport to another user (this is the process of "exporting" the 1650 key), any local certification signatures are deleted from the key. 1651 1652 Callas, et al. Expires Oct 24, 2007 [Page 29] 1653 INTERNET-DRAFT OpenPGP Message Format Apr 24, 2007 1655 1656 The receiver of a transported key "imports" it, and likewise trims 1657 any local certifications. In normal operation, there won't be any, 1658 assuming the import is performed on an exported key. However, there 1659 are instances where this can reasonably happen. For example, if an 1660 implementation allows keys to be imported from a key database in 1661 addition to an exported key, then this situation can arise. 1662 1663 Some implementations do not represent the interest of a single user 1664 (for example, a key server). Such implementations always trim local 1665 certifications from any key they handle. 1666 1667 5.2.3.12. Revocable 1668 1669 (1 octet of revocability, 0 for not, 1 for revocable) 1670 1671 Signature's revocability status. The packet body contains a Boolean 1672 flag indicating whether the signature is revocable. Signatures that 1673 are not revocable have any later revocation signatures ignored. They 1674 represent a commitment by the signer that he cannot revoke his 1675 signature for the life of his key. If this packet is not present, 1676 the signature is revocable. 1677 1678 5.2.3.13. Trust signature 1679 1680 (1 octet "level" (depth), 1 octet of trust amount) 1681 1682 Signer asserts that the key is not only valid, but also trustworthy, 1683 at the specified level. Level 0 has the same meaning as an ordinary 1684 validity signature. Level 1 means that the signed key is asserted to 1685 be a valid trusted introducer, with the 2nd octet of the body 1686 specifying the degree of trust. Level 2 means that the signed key is 1687 asserted to be trusted to issue level 1 trust signatures, i.e. that 1688 it is a "meta introducer". Generally, a level n trust signature 1689 asserts that a key is trusted to issue level n-1 trust signatures. 1690 The trust amount is in a range from 0-255, interpreted such that 1691 values less than 120 indicate partial trust and values of 120 or 1692 greater indicate complete trust. Implementations SHOULD emit values 1693 of 60 for partial trust and 120 for complete trust. 1694 1695 5.2.3.14. Regular expression 1696 1697 (null-terminated regular expression) 1698 1699 Used in conjunction with trust signature packets (of level > 0) to 1700 limit the scope of trust that is extended. Only signatures by the 1701 target key on User IDs that match the regular expression in the body 1702 of this packet have trust extended by the trust signature subpacket. 1703 The regular expression uses the same syntax as the Henry Spencer's 1704 "almost public domain" regular expression package. A description of 1705 the syntax is found in a section below. 1706 1707 1708 1709 Callas, et al. Expires Oct 24, 2007 [Page 30] 1710 INTERNET-DRAFT OpenPGP Message Format Apr 24, 2007 1712 1713 5.2.3.15. Revocation key 1714 1715 (1 octet of class, 1 octet of PK algorithm ID, 20 octets of 1716 fingerprint) 1717 1718 Authorizes the specified key to issue revocation signatures for this 1719 key. Class octet must have bit 0x80 set. If the bit 0x40 is set, 1720 then this means that the revocation information is sensitive. Other 1721 bits are for future expansion to other kinds of authorizations. This 1722 is found on a self-signature. 1723 1724 If the "sensitive" flag is set, the keyholder feels this subpacket 1725 contains private trust information that describes a real-world 1726 sensitive relationship. If this flag is set, implementations SHOULD 1727 NOT export this signature to other users except in cases where the 1728 data needs to be available: when the signature is being sent to the 1729 designated revoker, or when it is accompanied by a revocation 1730 signature from that revoker. Note that it may be appropriate to 1731 isolate this subpacket within a separate signature so that it is not 1732 combined with other subpackets that need to be exported. 1733 1734 5.2.3.16. Notation Data 1735 1736 (4 octets of flags, 2 octets of name length (M), 1737 2 octets of value length (N), 1738 M octets of name data, 1739 N octets of value data) 1740 1741 This subpacket describes a "notation" on the signature that the 1742 issuer wishes to make. The notation has a name and a value, each of 1743 which are strings of octets. There may be more than one notation in 1744 a signature. Notations can be used for any extension the issuer of 1745 the signature cares to make. The "flags" field holds four octets of 1746 flags. 1747 1748 All undefined flags MUST be zero. Defined flags are: 1749 1750 First octet: 0x80 = human-readable. This note value is text. 1751 Other octets: none. 1752 1753 Notation names are arbitrary strings encoded in UTF-8. They reside 1754 two name spaces: The IETF name space and the user name space. 1755 1756 The IETF name space is registered with IANA. These names MUST NOT 1757 contain the "@" character (0x40). This this is a tag for the user 1758 name space. 1759 1760 Names in the user name space consist of a UTF-8 string tag followed 1761 by "@" followed by a DNS domain name. Note that the tag MUST NOT 1762 contain an "@" character. For example, the "sample" tag used by 1763 Example Corporation could be "sample (a] example.com". 1764 1765 1766 Callas, et al. Expires Oct 24, 2007 [Page 31] 1767 INTERNET-DRAFT OpenPGP Message Format Apr 24, 2007 1769 1770 Names in a user space are owned and controlled by the owners of that 1771 domain. Obviously, it's of bad form to create a new name in a DNS 1772 space that you don't own. 1773 1774 Since the user name space is in the form of an email address, 1775 implementers MAY wish to arrange for that address to reach a person 1776 who can be consulted about the use of the named tag. Note that due 1777 to UTF-8 encoding, not all valid user space name tags are valid 1778 email addresses. 1779 1780 If there is a critical notation, the criticality applies to that 1781 specific notation and not to notations in general. 1782 1783 5.2.3.17. Key server preferences 1784 1785 (N octets of flags) 1786 1787 This is a list of one-bit flags that indicate preferences that the 1788 key holder has about how the key is handled on a key server. All 1789 undefined flags MUST be zero. 1790 1791 First octet: 0x80 = No-modify 1792 the key holder requests that this key only be modified or 1793 updated by the key holder or an administrator of the key server. 1794 1795 This is found only on a self-signature. 1796 1797 5.2.3.18. Preferred key server 1798 1799 (String) 1800 1801 This is a URI of a key server that the key holder prefers be used 1802 for updates. Note that keys with multiple User IDs can have a 1803 preferred key server for each User ID. Note also that since this is 1804 a URI, the key server can actually be a copy of the key retrieved by 1805 ftp, http, finger, etc. 1806 1807 5.2.3.19. Primary User ID 1808 1809 (1 octet, Boolean) 1810 1811 This is a flag in a User ID's self signature that states whether 1812 this User ID is the main User ID for this key. It is reasonable for 1813 an implementation to resolve ambiguities in preferences, etc. by 1814 referring to the primary User ID. If this flag is absent, its value 1815 is zero. If more than one User ID in a key is marked as primary, the 1816 implementation may resolve the ambiguity in any way it sees fit, but 1817 it is RECOMMENDED that priority be given to the User ID with the 1818 most recent self-signature. 1819 1820 1821 1822 1823 Callas, et al. Expires Oct 24, 2007 [Page 32] 1824 INTERNET-DRAFT OpenPGP Message Format Apr 24, 2007 1826 1827 When appearing on a self-signature on a User ID packet, this 1828 subpacket applies only to User ID packets. When appearing on a 1829 self-signature on a User Attribute packet, this subpacket applies 1830 only to User Attribute packets. That is to say, there are two 1831 different and independent "primaries" - one for User IDs, and one 1832 for User Attributes. 1833 1834 5.2.3.20. Policy URI 1835 1836 (String) 1837 1838 This subpacket contains a URI of a document that describes the 1839 policy that the signature was issued under. 1840 1841 5.2.3.21. Key Flags 1842 1843 (N octets of flags) 1844 1845 This subpacket contains a list of binary flags that hold information 1846 about a key. It is a string of octets, and an implementation MUST 1847 NOT assume a fixed size. This is so it can grow over time. If a list 1848 is shorter than an implementation expects, the unstated flags are 1849 considered to be zero. The defined flags are: 1850 1851 First octet: 1852 1853 0x01 - This key may be used to certify other keys. 1854 1855 0x02 - This key may be used to sign data. 1856 1857 0x04 - This key may be used to encrypt communications. 1858 1859 0x08 - This key may be used to encrypt storage. 1860 1861 0x10 - The private component of this key may have been split by 1862 a secret-sharing mechanism. 1863 1864 0x20 - This key may be used for authentication. 1865 1866 0x80 - The private component of this key may be in the 1867 possession of more than one person. 1868 1869 Usage notes: 1870 1871 The flags in this packet may appear in self-signatures or in 1872 certification signatures. They mean different things depending on 1873 who is making the statement -- for example, a certification 1874 signature that has the "sign data" flag is stating that the 1875 certification is for that use. On the other hand, the 1876 "communications encryption" flag in a self-signature is stating a 1877 preference that a given key be used for communications. Note 1878 however, that it is a thorny issue to determine what is 1879 1880 Callas, et al. Expires Oct 24, 2007 [Page 33] 1881 INTERNET-DRAFT OpenPGP Message Format Apr 24, 2007 1883 1884 "communications" and what is "storage." This decision is left wholly 1885 up to the implementation; the authors of this document do not claim 1886 any special wisdom on the issue, and realize that accepted opinion 1887 may change. 1888 1889 The "split key" (0x10) and "group key" (0x80) flags are placed on a 1890 self-signature only; they are meaningless on a certification 1891 signature. They SHOULD be placed only on a direct-key signature 1892 (type 0x1f) or a subkey signature (type 0x18), one that refers to 1893 the key the flag applies to. 1894 1895 5.2.3.22. Signer's User ID 1896 1897 (String) 1898 1899 This subpacket allows a keyholder to state which User ID is 1900 responsible for the signing. Many keyholders use a single key for 1901 different purposes, such as business communications as well as 1902 personal communications. This subpacket allows such a keyholder to 1903 state which of their roles is making a signature. 1904 1905 This subpacket is not appropriate to use to refer to a User 1906 Attribute packet. 1907 1908 5.2.3.23. Reason for Revocation 1909 1910 (1 octet of revocation code, N octets of reason string) 1911 1912 This subpacket is used only in key revocation and certification 1913 revocation signatures. It describes the reason why the key or 1914 certificate was revoked. 1915 1916 The first octet contains a machine-readable code that denotes the 1917 reason for the revocation: 1918 1919 0 - No reason specified (key revocations or cert revocations) 1920 1 - Key is superseded (key revocations) 1921 2 - Key material has been compromised (key revocations) 1922 3 - Key is retired and no longer used (key revocations) 1923 32 - User ID information is no longer valid (cert revocations) 1924 1925 Following the revocation code is a string of octets which gives 1926 information about the reason for revocation in human-readable form 1927 (UTF-8). The string may be null, that is, of zero length. The length 1928 of the subpacket is the length of the reason string plus one. 1929 1930 An implementation SHOULD implement this subpacket, include it in all 1931 revocation signatures, and interpret revocations appropriately. 1932 There are important semantic differences between the reasons, and 1933 there are thus important reasons for revoking signatures. 1934 1935 1936 1937 Callas, et al. Expires Oct 24, 2007 [Page 34] 1938 INTERNET-DRAFT OpenPGP Message Format Apr 24, 2007 1940 1941 If a key has been revoked because of a compromise, all signatures 1942 created by that key are suspect. However, if it was merely 1943 superseded or retired, old signatures are still valid. If the 1944 revoked signature is the self-signature for certifying a User ID, a 1945 revocation denotes that that user name is no longer in use. Such a 1946 revocation SHOULD include an 0x20 code. 1947 1948 Note that any signature may be revoked, including a certification on 1949 some other person's key. There are many good reasons for revoking a 1950 certification signature, such as the case where the keyholder leaves 1951 the employ of a business with an email address. A revoked 1952 certification is no longer a part of validity calculations. 1953 1954 5.2.3.24. Features 1955 1956 (N octets of flags) 1957 1958 The features subpacket denotes which advanced OpenPGP features a 1959 user's implementation supports. This is so that as features are 1960 added to OpenPGP that cannot be backwards-compatible, a user can 1961 state that they can use that feature. The flags are single bits that 1962 indicate that a given feature is supported. 1963 1964 This subpacket is similar to a preferences subpacket, and only 1965 appears in a self-signature. 1966 1967 An implementation SHOULD NOT use a feature listed when sending to a 1968 user who does not state that they can use it. 1969 1970 Defined features are: 1971 1972 First octet: 1973 1974 0x01 - Modification Detection (packets 18 and 19) 1975 1976 If an implementation implements any of the defined features, it 1977 SHOULD implement the features subpacket, too. 1978 1979 An implementation may freely infer features from other suitable 1980 implementation-dependent mechanisms. 1981 1982 5.2.3.25. Signature Target 1983 1984 (1 octet PK algorithm, 1 octet hash algorithm, N octets hash) 1985 1986 This subpacket identifies a specific target signature that a 1987 signature refers to. For revocation signatures, this subpacket 1988 provides explicit designation of which signature is being revoked. 1989 For a third-party or timestamp signature, this designates what 1990 signature is signed. All arguments are an identifier of that target 1991 signature. 1992 1993 1994 Callas, et al. Expires Oct 24, 2007 [Page 35] 1995 INTERNET-DRAFT OpenPGP Message Format Apr 24, 2007 1997 1998 The N octets of hash data MUST be the size of the hash of the 1999 signature. For example, a target signature with a SHA-1 hash MUST 2000 have 20 octets of hash data. 2001 2002 5.2.3.26. Embedded Signature 2003 2004 (1 signature packet body) 2005 2006 This subpacket contains a complete signature packet body as 2007 specified in section 5.2 above. It is useful when one signature 2008 needs to refer to, or be incorporated in, another signature. 2009 2010 5.2.4. Computing Signatures 2011 2012 All signatures are formed by producing a hash over the signature 2013 data, and then using the resulting hash in the signature algorithm. 2014 2015 For binary document signatures (type 0x00), the document data is 2016 hashed directly. For text document signatures (type 0x01), the 2017 document is canonicalized by converting line endings to <CR><LF>, 2018 and the resulting data is hashed. 2019 2020 When a signature is made over a key, the hash data starts with the 2021 octet 0x99, followed by a two-octet length of the key, and then body 2022 of the key packet. (Note that this is an old-style packet header for 2023 a key packet with two-octet length.) A subkey binding signature 2024 (type 0x18) or primary key binding signature (type 0x19) then hashes 2025 the subkey using the same format as the main key (also using 0x99 as 2026 the first octet). Key revocation signatures (types 0x20 and 0x28) 2027 hash only the key being revoked. 2028 2029 A certification signature (type 0x10 through 0x13) hashes the User 2030 ID being bound to the key into the hash context after the above 2031 data. A V3 certification hashes the contents of the User ID or 2032 attribute packet packet, without any header. A V4 certification 2033 hashes the constant 0xb4 for User ID certifications or the constant 2034 0xd1 for User Attribute certifications, followed by a four-octet 2035 number giving the length of the User ID or User Attribute data, and 2036 then the User ID or User Attribute data. 2037 2038 When a signature is made over a signature packet (type 0x50), the 2039 hash data starts with the octet 0x88, followed by the four-octet 2040 length of the signature, and then the body of the signature packet. 2041 (Note that this is an old-style packet header for a signature packet 2042 with the length-of-length set to zero). The unhashed subpacket data 2043 of the signature packet being hashed is not included in the hash and 2044 the unhashed subpacket data length value is set to zero. 2045 2046 Once the data body is hashed, then a trailer is hashed. A V3 2047 signature hashes five octets of the packet body, starting from the 2048 signature type field. This data is the signature type, followed by 2049 the four-octet signature time. A V4 signature hashes the packet body 2050 2051 Callas, et al. Expires Oct 24, 2007 [Page 36] 2052 INTERNET-DRAFT OpenPGP Message Format Apr 24, 2007 2054 2055 starting from its first field, the version number, through the end 2056 of the hashed subpacket data. Thus, the fields hashed are the 2057 signature version, the signature type, the public key algorithm, the 2058 hash algorithm, the hashed subpacket length, and the hashed 2059 subpacket body. 2060 2061 V4 signatures also hash in a final trailer of six octets: the 2062 version of the signature packet, i.e. 0x04; 0xFF; a four-octet, 2063 big-endian number that is the length of the hashed data from the 2064 signature packet (note that this number does not include these final 2065 six octets. 2066 2067 After all this has been hashed in a single hash context the 2068 resulting hash field is used in the signature algorithm, and placed 2069 at the end of the signature packet. 2070 2071 5.2.4.1. Subpacket Hints 2072 2073 It is certainly possible for a signature to contain conflicting 2074 information in subpackets. For example, a signature may contain 2075 multiple copies of a preference or multiple expiration times. In 2076 most cases, an implementation SHOULD use the last subpacket in the 2077 signature, but MAY use any conflict resolution scheme that makes 2078 more sense. Please note that we are intentionally leaving conflict 2079 resolution to the implementer; most conflicts are simply syntax 2080 errors, and the wishy-washy language here allows a receiver to be 2081 generous in what they accept, while putting pressure on a creator to 2082 be stingy in what they generate. 2083 2084 Some apparent conflicts may actually make sense -- for example, 2085 suppose a keyholder has an V3 key and a V4 key that share the same 2086 RSA key material. Either of these keys can verify a signature 2087 created by the other, and it may be reasonable for a signature to 2088 contain an issuer subpacket for each key, as a way of explicitly 2089 tying those keys to the signature. 2090 2091 5.3. Symmetric-Key Encrypted Session Key Packets (Tag 3) 2092 2093 The Symmetric-Key Encrypted Session Key packet holds the 2094 symmetric-key encryption of a session key used to encrypt a message. 2095 Zero or more Public-Key Encrypted Session Key packets and/or 2096 Symmetric-Key Encrypted Session Key packets may precede a 2097 Symmetrically Encrypted Data Packet that holds an encrypted message. 2098 The message is encrypted with a session key, and the session key is 2099 itself encrypted and stored in the Encrypted Session Key packet or 2100 the Symmetric-Key Encrypted Session Key packet. 2101 2102 If the Symmetrically Encrypted Data Packet is preceded by one or 2103 more Symmetric-Key Encrypted Session Key packets, each specifies a 2104 passphrase that may be used to decrypt the message. This allows a 2105 message to be encrypted to a number of public keys, and also to one 2106 or more passphrases. This packet type is new, and is not generated 2107 2108 Callas, et al. Expires Oct 24, 2007 [Page 37] 2109 INTERNET-DRAFT OpenPGP Message Format Apr 24, 2007 2111 2112 by PGP 2.x or PGP 5.0. 2113 2114 The body of this packet consists of: 2115 2116 - A one-octet version number. The only currently defined version 2117 is 4. 2118 2119 - A one-octet number describing the symmetric algorithm used. 2120 2121 - A string-to-key (S2K) specifier, length as defined above. 2122 2123 - Optionally, the encrypted session key itself, which is decrypted 2124 with the string-to-key object. 2125 2126 If the encrypted session key is not present (which can be detected 2127 on the basis of packet length and S2K specifier size), then the S2K 2128 algorithm applied to the passphrase produces the session key for 2129 decrypting the file, using the symmetric cipher algorithm from the 2130 Symmetric-Key Encrypted Session Key packet. 2131 2132 If the encrypted session key is present, the result of applying the 2133 S2K algorithm to the passphrase is used to decrypt just that 2134 encrypted session key field, using CFB mode with an IV of all zeros. 2135 The decryption result consists of a one-octet algorithm identifier 2136 that specifies the symmetric-key encryption algorithm used to 2137 encrypt the following Symmetrically Encrypted Data Packet, followed 2138 by the session key octets themselves. 2139 2140 Note: because an all-zero IV is used for this decryption, the S2K 2141 specifier MUST use a salt value, either a Salted S2K or an 2142 Iterated-Salted S2K. The salt value will insure that the decryption 2143 key is not repeated even if the passphrase is reused. 2144 2145 5.4. One-Pass Signature Packets (Tag 4) 2146 2147 The One-Pass Signature packet precedes the signed data and contains 2148 enough information to allow the receiver to begin calculating any 2149 hashes needed to verify the signature. It allows the Signature 2150 Packet to be placed at the end of the message, so that the signer 2151 can compute the entire signed message in one pass. 2152 2153 A One-Pass Signature does not interoperate with PGP 2.6.x or 2154 earlier. 2155 2156 The body of this packet consists of: 2157 2158 - A one-octet version number. The current version is 3. 2159 2160 - A one-octet signature type. Signature types are described in 2161 section 5.2.1. 2162 2163 2164 2165 Callas, et al. Expires Oct 24, 2007 [Page 38] 2166 INTERNET-DRAFT OpenPGP Message Format Apr 24, 2007 2168 2169 - A one-octet number describing the hash algorithm used. 2170 2171 - A one-octet number describing the public key algorithm used. 2172 2173 - An eight-octet number holding the key ID of the signing key. 2174 2175 - A one-octet number holding a flag showing whether the signature 2176 is nested. A zero value indicates that the next packet is 2177 another One-Pass Signature packet that describes another 2178 signature to be applied to the same message data. 2179 2180 Note that if a message contains more than one one-pass signature, 2181 then the signature packets bracket the message; that is, the first 2182 signature packet after the message corresponds to the last one-pass 2183 packet and the final signature packet corresponds to the first 2184 one-pass packet. 2185 2186 5.5. Key Material Packet 2187 2188 A key material packet contains all the information about a public or 2189 private key. There are four variants of this packet type, and two 2190 major versions. Consequently, this section is complex. 2191 2192 5.5.1. Key Packet Variants 2193 2194 5.5.1.1. Public Key Packet (Tag 6) 2195 2196 A Public Key packet starts a series of packets that forms an OpenPGP 2197 key (sometimes called an OpenPGP certificate). 2198 2199 5.5.1.2. Public Subkey Packet (Tag 14) 2200 2201 A Public Subkey packet (tag 14) has exactly the same format as a 2202 Public Key packet, but denotes a subkey. One or more subkeys may be 2203 associated with a top-level key. By convention, the top-level key 2204 provides signature services, and the subkeys provide encryption 2205 services. 2206 2207 Note: in PGP 2.6.x, tag 14 was intended to indicate a comment 2208 packet. This tag was selected for reuse because no previous version 2209 of PGP ever emitted comment packets but they did properly ignore 2210 them. Public Subkey packets are ignored by PGP 2.6.x and do not 2211 cause it to fail, providing a limited degree of backward 2212 compatibility. 2213 2214 5.5.1.3. Secret Key Packet (Tag 5) 2215 2216 A Secret Key packet contains all the information that is found in a 2217 Public Key packet, including the public key material, but also 2218 includes the secret key material after all the public key fields. 2219 2220 2221 2222 Callas, et al. Expires Oct 24, 2007 [Page 39] 2223 INTERNET-DRAFT OpenPGP Message Format Apr 24, 2007 2225 2226 5.5.1.4. Secret Subkey Packet (Tag 7) 2227 2228 A Secret Subkey packet (tag 7) is the subkey analog of the Secret 2229 Key packet, and has exactly the same format. 2230 2231 5.5.2. Public Key Packet Formats 2232 2233 There are two versions of key-material packets. Version 3 packets 2234 were first generated by PGP 2.6. Version 4 keys first appeared in 2235 PGP 5.0, and are the preferred key version for OpenPGP. 2236 2237 OpenPGP implementations MUST create keys with version 4 format. V3 2238 keys are deprecated; an implementation MUST NOT generate a V3 key, 2239 but MAY accept it. 2240 2241 A version 3 public key or public subkey packet contains: 2242 2243 - A one-octet version number (3). 2244 2245 - A four-octet number denoting the time that the key was created. 2246 2247 - A two-octet number denoting the time in days that this key is 2248 valid. If this number is zero, then it does not expire. 2249 2250 - A one-octet number denoting the public key algorithm of this key 2251 2252 - A series of multiprecision integers comprising the key material: 2253 2254 - a multiprecision integer (MPI) of RSA public modulus n; 2255 2256 - an MPI of RSA public encryption exponent e. 2257 2258 V3 keys are deprecated. They contain three weaknesses in them. 2259 First, it is relatively easy to construct a V3 key that has the same 2260 key ID as any other key because the key ID is simply the low 64 bits 2261 of the public modulus. Secondly, because the fingerprint of a V3 key 2262 hashes the key material, but not its length, there is an increased 2263 opportunity for fingerprint collisions. Third, there are weaknesses 2264 in the MD5 hash algorithm that make developers prefer other 2265 algorithms. See below for a fuller discussion of key IDs and 2266 fingerprints. 2267 2268 V2 keys are identical to the deprecated V3 keys except for the 2269 version number. An implementation MUST NOT generate them and MAY 2270 accept or reject them as it sees fit. 2271 2272 The version 4 format is similar to the version 3 format except for 2273 the absence of a validity period. This has been moved to the 2274 signature packet. In addition, fingerprints of version 4 keys are 2275 calculated differently from version 3 keys, as described in section 2276 "Enhanced Key Formats." 2277 2278 2279 Callas, et al. Expires Oct 24, 2007 [Page 40] 2280 INTERNET-DRAFT OpenPGP Message Format Apr 24, 2007 2282 2283 A version 4 packet contains: 2284 2285 - A one-octet version number (4). 2286 2287 - A four-octet number denoting the time that the key was created. 2288 2289 - A one-octet number denoting the public key algorithm of this key 2290 2291 - A series of multiprecision integers comprising the key material. 2292 This algorithm-specific portion is: 2293 2294 Algorithm Specific Fields for RSA public keys: 2295 2296 - multiprecision integer (MPI) of RSA public modulus n; 2297 2298 - MPI of RSA public encryption exponent e. 2299 2300 Algorithm Specific Fields for DSA public keys: 2301 2302 - MPI of DSA prime p; 2303 2304 - MPI of DSA group order q (q is a prime divisor of p-1); 2305 2306 - MPI of DSA group generator g; 2307 2308 - MPI of DSA public key value y (= g**x mod p where x is 2309 secret). 2310 2311 Algorithm Specific Fields for Elgamal public keys: 2312 2313 - MPI of Elgamal prime p; 2314 2315 - MPI of Elgamal group generator g; 2316 2317 - MPI of Elgamal public key value y (= g**x mod p where x is 2318 secret). 2319 2320 5.5.3. Secret Key Packet Formats 2321 2322 The Secret Key and Secret Subkey packets contain all the data of the 2323 Public Key and Public Subkey packets, with additional 2324 algorithm-specific secret key data appended, usually in encrypted 2325 form. 2326 2327 The packet contains: 2328 2329 - A Public Key or Public Subkey packet, as described above 2330 2331 - One octet indicating string-to-key usage conventions. Zero 2332 indicates that the secret key data is not encrypted. 255 or 254 2333 indicates that a string-to-key specifier is being given. Any 2334 other value is a symmetric-key encryption algorithm identifier. 2335 2336 Callas, et al. Expires Oct 24, 2007 [Page 41] 2337 INTERNET-DRAFT OpenPGP Message Format Apr 24, 2007 2339 2340 - [Optional] If string-to-key usage octet was 255 or 254, a 2341 one-octet symmetric encryption algorithm. 2342 2343 - [Optional] If string-to-key usage octet was 255 or 254, a 2344 string-to-key specifier. The length of the string-to-key 2345 specifier is implied by its type, as described above. 2346 2347 - [Optional] If secret data is encrypted (string-to-key usage 2348 octet not zero), an Initial Vector (IV) of the same length as 2349 the cipher's block size. 2350 2351 - Plain or encrypted multiprecision integers comprising the secret 2352 key data. These algorithm-specific fields are as described 2353 below. 2354 2355 - If the string-to-key usage octet is zero or 255, then a 2356 two-octet checksum of the plaintext of the algorithm-specific 2357 portion (sum of all octets, mod 65536). If the string-to-key 2358 usage octet was 254, then a 20-octet SHA-1 hash of the plaintext 2359 of the algorithm-specific portion. This checksum or hash is 2360 encrypted together with the algorithm-specific fields (if 2361 string-to-key usage octet is not zero). Note that for all other 2362 values, a two-octet checksum is required. 2363 2364 Algorithm Specific Fields for RSA secret keys: 2365 2366 - multiprecision integer (MPI) of RSA secret exponent d. 2367 2368 - MPI of RSA secret prime value p. 2369 2370 - MPI of RSA secret prime value q (p < q). 2371 2372 - MPI of u, the multiplicative inverse of p, mod q. 2373 2374 Algorithm Specific Fields for DSA secret keys: 2375 2376 - MPI of DSA secret exponent x. 2377 2378 Algorithm Specific Fields for Elgamal secret keys: 2379 2380 - MPI of Elgamal secret exponent x. 2381 2382 Secret MPI values can be encrypted using a passphrase. If a 2383 string-to-key specifier is given, that describes the algorithm for 2384 converting the passphrase to a key, else a simple MD5 hash of the 2385 passphrase is used. Implementations MUST use a string-to-key 2386 specifier; the simple hash is for backward compatibility and is 2387 deprecated, though implementations MAY continue to use existing 2388 private keys in the old format. The cipher for encrypting the MPIs 2389 is specified in the secret key packet. 2390 2391 2392 2393 Callas, et al. Expires Oct 24, 2007 [Page 42] 2394 INTERNET-DRAFT OpenPGP Message Format Apr 24, 2007 2396 2397 Encryption/decryption of the secret data is done in CFB mode using 2398 the key created from the passphrase and the Initial Vector from the 2399 packet. A different mode is used with V3 keys (which are only RSA) 2400 than with other key formats. With V3 keys, the MPI bit count prefix 2401 (i.e., the first two octets) is not encrypted. Only the MPI 2402 non-prefix data is encrypted. Furthermore, the CFB state is 2403 resynchronized at the beginning of each new MPI value, so that the 2404 CFB block boundary is aligned with the start of the MPI data. 2405 2406 With V4 keys, a simpler method is used. All secret MPI values are 2407 encrypted in CFB mode, including the MPI bitcount prefix. 2408 2409 The two-octet checksum that follows the algorithm-specific portion 2410 is the algebraic sum, mod 65536, of the plaintext of all the 2411 algorithm-specific octets (including MPI prefix and data). With V3 2412 keys, the checksum is stored in the clear. With V4 keys, the 2413 checksum is encrypted like the algorithm-specific data. This value 2414 is used to check that the passphrase was correct. However, this 2415 checksum is deprecated; an implementation SHOULD NOT use it, but 2416 should rather use the SHA-1 hash denoted with a usage octet of 254. 2417 The reason for this is that there are some attacks that involve 2418 undetectably modifying the secret key. 2419 2420 5.6. Compressed Data Packet (Tag 8) 2421 2422 The Compressed Data packet contains compressed data. Typically, this 2423 packet is found as the contents of an encrypted packet, or following 2424 a Signature or One-Pass Signature packet, and contains a literal 2425 data packet. 2426 2427 The body of this packet consists of: 2428 2429 - One octet that gives the algorithm used to compress the packet. 2430 2431 - The remainder of the packet is compressed data. 2432 2433 A Compressed Data Packet's body contains an block that compresses 2434 some set of packets. See section "Packet Composition" for details on 2435 how messages are formed. 2436 2437 ZIP-compressed packets are compressed with raw RFC 1951 DEFLATE 2438 blocks. Note that PGP V2.6 uses 13 bits of compression. If an 2439 implementation uses more bits of compression, PGP V2.6 cannot 2440 decompress it. 2441 2442 ZLIB-compressed packets are compressed with RFC 1950 ZLIB-style 2443 blocks. 2444 2445 BZip2-compressed packets are compressed using the BZip2 [BZ2] 2446 algorithm. 2447 2448 2449 2450 Callas, et al. Expires Oct 24, 2007 [Page 43] 2451 INTERNET-DRAFT OpenPGP Message Format Apr 24, 2007 2453 2454 5.7. Symmetrically Encrypted Data Packet (Tag 9) 2455 2456 The Symmetrically Encrypted Data packet contains data encrypted with 2457 a symmetric-key algorithm. When it has been decrypted, it contains 2458 other packets (usually a literal data packet or compressed data 2459 packet, but in theory other Symmetrically Encrypted Data Packets or 2460 sequences of packets that form whole OpenPGP messages). 2461 2462 The body of this packet consists of: 2463 2464 - Encrypted data, the output of the selected symmetric-key cipher 2465 operating in OpenPGP's variant of Cipher Feedback (CFB) mode. 2466 2467 The symmetric cipher used may be specified in an Public-Key or 2468 Symmetric-Key Encrypted Session Key packet that precedes the 2469 Symmetrically Encrypted Data Packet. In that case, the cipher 2470 algorithm octet is prefixed to the session key before it is 2471 encrypted. If no packets of these types precede the encrypted data, 2472 the IDEA algorithm is used with the session key calculated as the 2473 MD5 hash of the passphrase, though this use is deprecated. 2474 2475 The data is encrypted in CFB mode, with a CFB shift size equal to 2476 the cipher's block size. The Initial Vector (IV) is specified as all 2477 zeros. Instead of using an IV, OpenPGP prefixes a string of length 2478 equal to the block size of the cipher plus two to the data before it 2479 is encrypted. The first block-size octets (for example, 8 octets for 2480 a 64-bit block length) are random, and the following two octets are 2481 copies of the last two octets of the IV. For example, in an 8 octet 2482 block, octet 9 is a repeat of octet 7, and octet 10 is a repeat of 2483 octet 8. In a cipher of length 16, octet 17 is a repeat of octet 15 2484 and octet 18 is a repeat of octet 16. As a pedantic clarification, 2485 in both these examples, we consider the first octet to be numbered 2486 1. 2487 2488 After encrypting the first block-size-plus-two octets, the CFB state 2489 is resynchronized. The last block-size octets of ciphertext are 2490 passed through the cipher and the block boundary is reset. 2491 2492 The repetition of 16 bits in the random data prefixed to the message 2493 allows the receiver to immediately check whether the session key is 2494 incorrect. See the Security Considerations section for hints on the 2495 proper use of this "quick check." 2496 2497 5.8. Marker Packet (Obsolete Literal Packet) (Tag 10) 2498 2499 An experimental version of PGP used this packet as the Literal 2500 packet, but no released version of PGP generated Literal packets 2501 with this tag. With PGP 5.x, this packet has been re-assigned and is 2502 reserved for use as the Marker packet. 2503 2504 2505 2506 2507 Callas, et al. Expires Oct 24, 2007 [Page 44] 2508 INTERNET-DRAFT OpenPGP Message Format Apr 24, 2007 2510 2511 The body of this packet consists of: 2512 2513 - The three octets 0x50, 0x47, 0x50 (which spell "PGP" in UTF-8). 2514 2515 Such a packet MUST be ignored when received. It may be placed at the 2516 beginning of a message that uses features not available in PGP 2.6.x 2517 in order to cause that version to report that newer software is 2518 necessary to process the message. 2519 2520 5.9. Literal Data Packet (Tag 11) 2521 2522 A Literal Data packet contains the body of a message; data that is 2523 not to be further interpreted. 2524 2525 The body of this packet consists of: 2526 2527 - A one-octet field that describes how the data is formatted. 2528 2529 If it is a 'b' (0x62), then the literal packet contains binary data. 2530 If it is a 't' (0x74), then it contains text data, and thus may need 2531 line ends converted to local form, or other text-mode changes. The 2532 tag 'u' (0x75) means the same as 't', but also indicates that 2533 implementation believes that the literal data contains UTF-8 text. 2534 2535 Early versions of PGP also defined a value of 'l' as a 'local' mode 2536 for machine-local conversions. RFC 1991 incorrectly stated this 2537 local mode flag as '1' (ASCII numeral one). Both of these local 2538 modes are deprecated. 2539 2540 - File name as a string (one-octet length, followed by a file 2541 name). This may be a zero-length string. Commonly, if the source 2542 of the encrypted data is a file, this will be the name of the 2543 encrypted file. An implementation MAY consider the file name in 2544 the literal packet to be a more authoritative name than the 2545 actual file name. 2546 2547 If the special name "_CONSOLE" is used, the message is considered to 2548 be "for your eyes only". This advises that the message data is 2549 unusually sensitive, and the receiving program should process it 2550 more carefully, perhaps avoiding storing the received data to disk, 2551 for example. 2552 2553 - A four-octet number that indicates a date associated with the 2554 literal data. Commonly, the date might be the modification date 2555 of a file, or the time the packet was created, or a zero that 2556 indicates no specific time. 2557 2558 - The remainder of the packet is literal data. 2559 2560 Text data is stored with <CR><LF> text endings (i.e. network-normal 2561 line endings). These should be converted to native line endings by 2562 the receiving software. 2563 2564 Callas, et al. Expires Oct 24, 2007 [Page 45] 2565 INTERNET-DRAFT OpenPGP Message Format Apr 24, 2007 2567 2568 5.10. Trust Packet (Tag 12) 2569 2570 The Trust packet is used only within keyrings and is not normally 2571 exported. Trust packets contain data that record the user's 2572 specifications of which key holders are trustworthy introducers, 2573 along with other information that implementing software uses for 2574 trust information. The format of trust packets is defined by a given 2575 implementation. 2576 2577 Trust packets SHOULD NOT be emitted to output streams that are 2578 transferred to other users, and they SHOULD be ignored on any input 2579 other than local keyring files. 2580 2581 5.11. User ID Packet (Tag 13) 2582 2583 A User ID packet consists of UTF-8 text that is intended to 2584 represent the name and email address of the key holder. By 2585 convention, it includes an RFC 2822 mail name-addr, but there are no 2586 restrictions on its content. The packet length in the header 2587 specifies the length of the User ID. 2588 2589 5.12. User Attribute Packet (Tag 17) 2590 2591 The User Attribute packet is a variation of the User ID packet. It 2592 is capable of storing more types of data than the User ID packet 2593 which is limited to text. Like the User ID packet, a User Attribute 2594 packet may be certified by the key owner ("self-signed") or any 2595 other key owner who cares to certify it. Except as noted, a User 2596 Attribute packet may be used anywhere that a User ID packet may be 2597 used. 2598 2599 While User Attribute packets are not a required part of the OpenPGP 2600 standard, implementations SHOULD provide at least enough 2601 compatibility to properly handle a certification signature on the 2602 User Attribute packet. A simple way to do this is by treating the 2603 User Attribute packet as a User ID packet with opaque contents, but 2604 an implementation may use any method desired. 2605 2606 The User Attribute packet is made up of one or more attribute 2607 subpackets. Each subpacket consists of a subpacket header and a 2608 body. The header consists of: 2609 2610 - the subpacket length (1, 2, or 5 octets) 2611 2612 - the subpacket type (1 octet) 2613 2614 and is followed by the subpacket specific data. 2615 2616 The only currently defined subpacket type is 1, signifying an image. 2617 An implementation SHOULD ignore any subpacket of a type that it does 2618 not recognize. Subpacket types 100 through 110 are reserved for 2619 private or experimental use. 2620 2621 Callas, et al. Expires Oct 24, 2007 [Page 46] 2622 INTERNET-DRAFT OpenPGP Message Format Apr 24, 2007 2624 2625 5.12.1. The Image Attribute Subpacket 2626 2627 The image attribute subpacket is used to encode an image, presumably 2628 (but not required to be) that of the key owner. 2629 2630 The image attribute subpacket begins with an image header. The first 2631 two octets of the image header contain the length of the image 2632 header. Note that unlike other multi-octet numerical values in this 2633 document, due to an historical accident this value is encoded as a 2634 little-endian number. The image header length is followed by a 2635 single octet for the image header version. The only currently 2636 defined version of the image header is 1, which is a 16 octet image 2637 header. The first three octets of a version 1 image header are thus 2638 0x10 0x00 0x01. 2639 2640 The fourth octet of a version 1 image header designates the encoding 2641 format of the image. The only currently defined encoding format is 2642 the value 1 to indicate JPEG. Image format types 100 through 110 are 2643 reserved for private or experimental use. The rest of the version 1 2644 image header is made up of 12 reserved octets, all of which MUST be 2645 set to 0. 2646 2647 The rest of the image subpacket contains the image itself. As the 2648 only currently defined image type is JPEG, the image is encoded in 2649 the JPEG File Interchange Format (JFIF), a standard file format for 2650 JPEG images. [JFIF] 2651 2652 An implementation MAY try and determine the type of an image by 2653 examination of the image data if it is unable to handle a particular 2654 version of the image header or if a specified encoding format value 2655 is not recognized. 2656 2657 5.13. Sym. Encrypted Integrity Protected Data Packet (Tag 18) 2658 2659 The Symmetrically Encrypted Integrity Protected Data Packet is a 2660 variant of the Symmetrically Encrypted Data Packet. It is a new 2661 feature created for OpenPGP that addresses the problem of detecting 2662 a modification to encrypted data. It is used in combination with a 2663 Modification Detection Code Packet. 2664 2665 There is a corresponding feature in the features signature subpacket 2666 that denotes that an implementation can properly use this packet 2667 type. An implementation MUST support decrypting these packets and 2668 SHOULD prefer generating them to the older Symmetrically Encrypted 2669 Data Packet when possible. Since this data packet protects against 2670 modification attacks, this standard encourages its proliferation. 2671 While blanket adoption of this data packet would create 2672 interoperability problems, rapid adoption is nevertheless important. 2673 An implementation SHOULD specifically denote support for this 2674 packet, but it MAY infer it from other mechanisms. 2675 2676 2677 2678 Callas, et al. Expires Oct 24, 2007 [Page 47] 2679 INTERNET-DRAFT OpenPGP Message Format Apr 24, 2007 2681 2682 For example, an implementation might infer from the use of a cipher 2683 such as AES or Twofish that a user supports this feature. It might 2684 place in the unhashed portion of another user's key signature a 2685 features subpacket. It might also present a user with an opportunity 2686 to regenerate their own self-signature with a features subpacket. 2687 2688 This packet contains data encrypted with a symmetric-key algorithm 2689 and protected against modification by the SHA-1 hash algorithm. When 2690 it has been decrypted, it will typically contain other packets 2691 (often a literal data packet or compressed data packet). The last 2692 decrypted packet in this packet's payload MUST be a Modification 2693 Detection Code packet. 2694 2695 The body of this packet consists of: 2696 2697 - A one-octet version number. The only currently defined value is 2698 1. 2699 2700 - Encrypted data, the output of the selected symmetric-key cipher 2701 operating in Cipher Feedback mode with shift amount equal to the 2702 block size of the cipher (CFB-n where n is the block size). 2703 2704 The symmetric cipher used MUST be specified in a Public-Key or 2705 Symmetric-Key Encrypted Session Key packet that precedes the 2706 Symmetrically Encrypted Data Packet. In either case, the cipher 2707 algorithm octet is prefixed to the session key before it is 2708 encrypted. 2709 2710 The data is encrypted in CFB mode, with a CFB shift size equal to 2711 the cipher's block size. The Initial Vector (IV) is specified as all 2712 zeros. Instead of using an IV, OpenPGP prefixes an octet string to 2713 the data before it is encrypted. The length of the octet string 2714 equals the block size of the cipher in octets, plus two. The first 2715 octets in the group, of length equal to the block size of the 2716 cipher, are random; the last two octets are each copies of their 2nd 2717 preceding octet. For example, with a cipher whose block size is 128 2718 bits or 16 octets, the prefix data will contain 16 random octets, 2719 then two more octets, which are copies of the 15th and 16th octets, 2720 respectively. Unlike the Symmetrically Encrypted Data Packet, no 2721 special CFB resynchronization is done after encrypting this prefix 2722 data. See OpenPGP CFB Mode below for more details. 2723 2724 The repetition of 16 bits in the random data prefixed to the message 2725 allows the receiver to immediately check whether the session key is 2726 incorrect. 2727 2728 The plaintext of the data to be encrypted is passed through the 2729 SHA-1 hash function, and the result of the hash is appended to the 2730 plaintext in a Modification Detection Code packet. The input to the 2731 hash function includes the prefix data described above; it includes 2732 all of the plaintext, and then also includes two octets of values 2733 0xD3, 0x14. These represent the encoding of a Modification Detection 2734 2735 Callas, et al. Expires Oct 24, 2007 [Page 48] 2736 INTERNET-DRAFT OpenPGP Message Format Apr 24, 2007 2738 2739 Code packet tag and length field of 20 octets. 2740 2741 The resulting hash value is stored in a Modification Detection Code 2742 packet which MUST use the two octet encoding just given to represent 2743 its tag and length field. The body of the MDC packet is the 20 octet 2744 output of the SHA-1 hash. 2745 2746 The Modification Detection Code packet is appended to the plaintext 2747 and encrypted along with the plaintext using the same CFB context. 2748 2749 During decryption, the plaintext data should be hashed with SHA-1, 2750 including the prefix data as well as the packet tag and length field 2751 of the Modification Detection Code packet. The body of the MDC 2752 packet, upon decryption, is compared with the result of the SHA-1 2753 hash. 2754 2755 Any failure of the MDC indicates that the message has been modified 2756 and MUST be treated as a security problem. Failures include a 2757 difference in the hash values, but also the absence of an MDC 2758 packet, or an MDC packet in any position other than the end of the 2759 plaintext. Any failure SHOULD be reported to the user. 2760 2761 Note: future designs of new versions of this packet should consider 2762 rollback attacks since it will be possible for an attacker to change 2763 the version back to 1. 2764 2765 NON-NORMATIVE EXPLANATION 2766 2767 The MDC system, as packets 18 and 19 are called, were created to 2768 provide an integrity mechanism that is less strong than a 2769 signature, yet stronger than bare CFB encryption. 2770 2771 It is a limitation of CFB encryption that damage to the 2772 ciphertext will corrupt the affected cipher blocks and the block 2773 following. Additionally, if data is removed from the end of a 2774 CFB-encrypted block, that removal is undetectable. (Note also 2775 that CBC mode has a similar limitation, but data removed from 2776 the front of the block is undetectable.) 2777 2778 The obvious way to protect or authenticate an encrypted block is 2779 to digitally sign it. However, many people do not wish to 2780 habitually sign data, for a large number of reasons beyond the 2781 scope of this document. Suffice it to say that many people 2782 consider properties such as deniability to be as valuable as 2783 integrity. 2784 2785 OpenPGP addresses this desire to have more security than raw 2786 encryption and yet preserve deniability with the MDC system. An 2787 MDC is intentionally not a MAC. Its name was not selected by 2788 accident. It is analogous to a checksum. 2789 2790 2791 2792 Callas, et al. Expires Oct 24, 2007 [Page 49] 2793 INTERNET-DRAFT OpenPGP Message Format Apr 24, 2007 2795 2796 Despite the fact that it is a relatively modest system, it has 2797 proved itself in the real world. It is an effective defense to 2798 several attacks that have surfaced since it has been created. It 2799 has met its modest goals admirably. 2800 2801 Consequently, because it is a modest security system, it has 2802 modest requirements on the hash function(s) it employs. It does 2803 not rely on a hash function being collision-free, it relies on a 2804 hash function being one-way. If a forger, Frank, wishes to send 2805 Alice a (digitally) unsigned message that says, "I've always 2806 secretly loved you, signed Bob" it is far easier for him to 2807 construct a new message than it is to modify anything 2808 intercepted from Bob. (Note also that if Bob wishes to 2809 communicate secretly with Alice, but without authentication nor 2810 identification and with a threat model that includes forgers, he 2811 has a problem that transcends mere cryptography.) 2812 2813 Note also that unlike nearly every other OpenPGP subsystem, 2814 there are no parameters in the MDC system. It hard-defines SHA-1 2815 as its hash function. This is not an accident. It is an 2816 intentional choice to avoid downgrade and cross-grade attacks 2817 while making a simple, fast system. (A downgrade attack would be 2818 an attack that replaced SHA-256 with SHA-1, for example. A 2819 cross-grade attack would replace SHA-1 with another 160-bit 2820 hash, such as RIPE-MD/160, for example.) 2821 2822 However, given the present state of hash function cryptanalysis 2823 and cryptography, it may be desirable to upgrade the MDC system 2824 to a new hash function. See section 10.5 in the IANA 2825 considerations for guidance. 2826 2827 5.14. Modification Detection Code Packet (Tag 19) 2828 2829 The Modification Detection Code packet contains a SHA-1 hash of 2830 plaintext data which is used to detect message modification. It is 2831 only used with a Symmetrically Encrypted Integrity Protected Data 2832 packet. The Modification Detection Code packet MUST be the last 2833 packet in the plaintext data which is encrypted in the Symmetrically 2834 Encrypted Integrity Protected Data packet, and MUST appear in no 2835 other place. 2836 2837 A Modification Detection Code packet MUST have a length of 20 2838 octets. 2839 2840 The body of this packet consists of: 2841 2842 - A 20-octet SHA-1 hash of the preceding plaintext data of the 2843 Symmetrically Encrypted Integrity Protected Data packet, 2844 including prefix data, the tag octet, and length octet of the 2845 Modification Detection Code packet. 2846 2847 2848 2849 Callas, et al. Expires Oct 24, 2007 [Page 50] 2850 INTERNET-DRAFT OpenPGP Message Format Apr 24, 2007 2852 2853 Note that the Modification Detection Code packet MUST always use a 2854 new-format encoding of the packet tag, and a one-octet encoding of 2855 the packet length. The reason for this is that the hashing rules for 2856 modification detection include a one-octet tag and one-octet length 2857 in the data hash. While this is a bit restrictive, it reduces 2858 complexity. 2859 2860 6. Radix-64 Conversions 2861 2862 As stated in the introduction, OpenPGP's underlying native 2863 representation for objects is a stream of arbitrary octets, and some 2864 systems desire these objects to be immune to damage caused by 2865 character set translation, data conversions, etc. 2866 2867 In principle, any printable encoding scheme that met the 2868 requirements of the unsafe channel would suffice, since it would not 2869 change the underlying binary bit streams of the native OpenPGP data 2870 structures. The OpenPGP standard specifies one such printable 2871 encoding scheme to ensure interoperability. 2872 2873 OpenPGP's Radix-64 encoding is composed of two parts: a base64 2874 encoding of the binary data, and a checksum. The base64 encoding is 2875 identical to the MIME base64 content-transfer-encoding [RFC2045]. 2876 2877 The checksum is a 24-bit CRC converted to four characters of 2878 radix-64 encoding by the same MIME base64 transformation, preceded 2879 by an equals sign (=). The CRC is computed by using the generator 2880 0x864CFB and an initialization of 0xB704CE. The accumulation is done 2881 on the data before it is converted to radix-64, rather than on the 2882 converted data. A sample implementation of this algorithm is in the 2883 next section. 2884 2885 The checksum with its leading equal sign MAY appear on the first 2886 line after the Base64 encoded data. 2887 2888 Rationale for CRC-24: The size of 24 bits fits evenly into printable 2889 base64. The nonzero initialization can detect more errors than a 2890 zero initialization. 2891 2892 6.1. An Implementation of the CRC-24 in "C" 2893 2894 #define CRC24_INIT 0xb704ceL 2895 #define CRC24_POLY 0x1864cfbL 2896 2897 typedef long crc24; 2898 crc24 crc_octets(unsigned char *octets, size_t len) 2899 { 2900 crc24 crc = CRC24_INIT; 2901 int i; 2902 2903 2904 2905 2906 Callas, et al. Expires Oct 24, 2007 [Page 51] 2907 INTERNET-DRAFT OpenPGP Message Format Apr 24, 2007 2909 2910 while (len--) { 2911 crc ^= (*octets++) << 16; 2912 for (i = 0; i < 8; i++) { 2913 crc <<= 1; 2914 if (crc & 0x1000000) 2915 crc ^= CRC24_POLY; 2916 } 2917 } 2918 return crc & 0xffffffL; 2919 } 2920 2921 6.2. Forming ASCII Armor 2922 2923 When OpenPGP encodes data into ASCII Armor, it puts specific headers 2924 around the Radix-64 encoded data, so OpenPGP can reconstruct the 2925 data later. An OpenPGP implementation MAY use ASCII armor to protect 2926 raw binary data. OpenPGP informs the user what kind of data is 2927 encoded in the ASCII armor through the use of the headers. 2928 2929 Concatenating the following data creates ASCII Armor: 2930 2931 - An Armor Header Line, appropriate for the type of data 2932 2933 - Armor Headers 2934 2935 - A blank (zero-length, or containing only whitespace) line 2936 2937 - The ASCII-Armored data 2938 2939 - An Armor Checksum 2940 2941 - The Armor Tail, which depends on the Armor Header Line. 2942 2943 An Armor Header Line consists of the appropriate header line text 2944 surrounded by five (5) dashes ('-', 0x2D) on either side of the 2945 header line text. The header line text is chosen based upon the type 2946 of data that is being encoded in Armor, and how it is being encoded. 2947 Header line texts include the following strings: 2948 2949 BEGIN PGP MESSAGE 2950 Used for signed, encrypted, or compressed files. 2951 2952 BEGIN PGP PUBLIC KEY BLOCK 2953 Used for armoring public keys 2954 2955 BEGIN PGP PRIVATE KEY BLOCK 2956 Used for armoring private keys 2957 2958 BEGIN PGP MESSAGE, PART X/Y 2959 Used for multi-part messages, where the armor is split amongst Y 2960 parts, and this is the Xth part out of Y. 2961 2962 2963 Callas, et al. Expires Oct 24, 2007 [Page 52] 2964 INTERNET-DRAFT OpenPGP Message Format Apr 24, 2007 2966 2967 BEGIN PGP MESSAGE, PART X 2968 Used for multi-part messages, where this is the Xth part of an 2969 unspecified number of parts. Requires the MESSAGE-ID Armor 2970 Header to be used. 2971 2972 BEGIN PGP SIGNATURE 2973 Used for detached signatures, OpenPGP/MIME signatures, and 2974 cleartext signatures. Note that PGP 2.x uses BEGIN PGP MESSAGE 2975 for detached signatures. 2976 2977 Note that all these Armor Header Lines are to consist of a complete 2978 line. That is to say, there is always a line ending preceding the 2979 starting five dashes, and following the ending five dashes. The 2980 header lines, therefore, MUST start at the beginning of a line, and 2981 MUST NOT have text other than whitespace following them on the same 2982 line. These line endings are considered a part of the Armor Header 2983 Line for the purposes of determining the content they delimit. This 2984 is particularly important when computing a cleartext signature (see 2985 below). 2986 2987 The Armor Headers are pairs of strings that can give the user or the 2988 receiving OpenPGP implementation some information about how to 2989 decode or use the message. The Armor Headers are a part of the 2990 armor, not a part of the message, and hence are not protected by any 2991 signatures applied to the message. 2992 2993 The format of an Armor Header is that of a key-value pair. A colon 2994 (':' 0x38) and a single space (0x20) separate the key and value. 2995 OpenPGP should consider improperly formatted Armor Headers to be 2996 corruption of the ASCII Armor. Unknown keys should be reported to 2997 the user, but OpenPGP should continue to process the message. 2998 2999 Note that some transport methods are sensitive to line length. While 3000 there is a limit of 76 characters for the Radix-64 data (section 3001 6.3), there is no limit to the length of Armor Headers. Care should 3002 be taken that the Armor Headers are short enough to survive 3003 transport. One way to do this is to repeat an Armor Header key 3004 multiple times with different values for each so that no one line is 3005 overly long. 3006 3007 Currently defined Armor Header Keys are: 3008 3009 - "Version", that states the OpenPGP implementation and version 3010 used to encode the message. 3011 3012 - "Comment", a user-defined comment. OpenPGP defines all text to 3013 be in UTF-8. A comment may be any UTF-8 string. However, the 3014 whole point of armoring is to provide seven-bit-clean data. 3015 Consequently, if a comment has characters that are outside the 3016 US-ASCII range of UTF, they may very well not survive transport. 3017 3018 3019 3020 Callas, et al. Expires Oct 24, 2007 [Page 53] 3021 INTERNET-DRAFT OpenPGP Message Format Apr 24, 2007 3023 3024 - "MessageID", a 32-character string of printable characters. The 3025 string must be the same for all parts of a multi-part message 3026 that uses the "PART X" Armor Header. MessageID strings should be 3027 unique enough that the recipient of the mail can associate all 3028 the parts of a message with each other. A good checksum or 3029 cryptographic hash function is sufficient. 3030 3031 The MessageID SHOULD NOT appear unless it is in a multi-part 3032 message. If it appears at all, it MUST be computed from the 3033 finished (encrypted, signed, etc.) message in a deterministic 3034 fashion, rather than contain a purely random value. This is to 3035 allow the legitimate recipient to determine that the MessageID 3036 cannot serve as a covert means of leaking cryptographic key 3037 information. 3038 3039 - "Hash", a comma-separated list of hash algorithms used in this 3040 message. This is used only in cleartext signed messages. 3041 3042 - "Charset", a description of the character set that the plaintext 3043 is in. Please note that OpenPGP defines text to be in UTF-8. An 3044 implementation will get best results by translating into and out 3045 of UTF-8. However, there are many instances where this is easier 3046 said than done. Also, there are communities of users who have no 3047 need for UTF-8 because they are all happy with a character set 3048 like ISO Latin-5 or a Japanese character set. In such instances, 3049 an implementation MAY override the UTF-8 default by using this 3050 header key. An implementation MAY implement this key and any 3051 translations it cares to; an implementation MAY ignore it and 3052 assume all text is UTF-8. 3053 3054 The Armor Tail Line is composed in the same manner as the Armor 3055 Header Line, except the string "BEGIN" is replaced by the string 3056 "END". 3057 3058 6.3. Encoding Binary in Radix-64 3059 3060 The encoding process represents 24-bit groups of input bits as 3061 output strings of 4 encoded characters. Proceeding from left to 3062 right, a 24-bit input group is formed by concatenating three 8-bit 3063 input groups. These 24 bits are then treated as four concatenated 3064 6-bit groups, each of which is translated into a single digit in the 3065 Radix-64 alphabet. When encoding a bit stream with the Radix-64 3066 encoding, the bit stream must be presumed to be ordered with the 3067 most-significant-bit first. That is, the first bit in the stream 3068 will be the high-order bit in the first 8-bit octet, and the eighth 3069 bit will be the low-order bit in the first 8-bit octet, and so on. 3070 3071 +--first octet--+-second octet--+--third octet--+ 3072 |7 6 5 4 3 2 1 0|7 6 5 4 3 2 1 0|7 6 5 4 3 2 1 0| 3073 +-----------+---+-------+-------+---+-----------+ 3074 |5 4 3 2 1 0|5 4 3 2 1 0|5 4 3 2 1 0|5 4 3 2 1 0| 3075 +--1.index--+--2.index--+--3.index--+--4.index--+ 3076 3077 Callas, et al. Expires Oct 24, 2007 [Page 54] 3078 INTERNET-DRAFT OpenPGP Message Format Apr 24, 2007 3080 3081 Each 6-bit group is used as an index into an array of 64 printable 3082 characters from the table below. The character referenced by the 3083 index is placed in the output string. 3084 3085 Value Encoding Value Encoding Value Encoding Value Encoding 3086 0 A 17 R 34 i 51 z 3087 1 B 18 S 35 j 52 0 3088 2 C 19 T 36 k 53 1 3089 3 D 20 U 37 l 54 2 3090 4 E 21 V 38 m 55 3 3091 5 F 22 W 39 n 56 4 3092 6 G 23 X 40 o 57 5 3093 7 H 24 Y 41 p 58 6 3094 8 I 25 Z 42 q 59 7 3095 9 J 26 a 43 r 60 8 3096 10 K 27 b 44 s 61 9 3097 11 L 28 c 45 t 62 + 3098 12 M 29 d 46 u 63 / 3099 13 N 30 e 47 v 3100 14 O 31 f 48 w (pad) = 3101 15 P 32 g 49 x 3102 16 Q 33 h 50 y 3103 3104 The encoded output stream must be represented in lines of no more 3105 than 76 characters each. 3106 3107 Special processing is performed if fewer than 24 bits are available 3108 at the end of the data being encoded. There are three possibilities: 3109 3110 1. The last data group has 24 bits (3 octets). No special 3111 processing is needed. 3112 3113 2. The last data group has 16 bits (2 octets). The first two 6-bit 3114 groups are processed as above. The third (incomplete) data group 3115 has two zero-value bits added to it, and is processed as above. 3116 A pad character (=) is added to the output. 3117 3118 3. The last data group has 8 bits (1 octet). The first 6-bit group 3119 is processed as above. The second (incomplete) data group has 3120 four zero-value bits added to it, and is processed as above. Two 3121 pad characters (=) are added to the output. 3122 3123 6.4. Decoding Radix-64 3124 3125 In Radix-64 data, characters other than those in the table, line 3126 breaks, and other white space probably indicate a transmission 3127 error, about which a warning message or even a message rejection 3128 might be appropriate under some circumstances. Decoding software 3129 must ignore all white space. 3130 3131 3132 3133 3134 Callas, et al. Expires Oct 24, 2007 [Page 55] 3135 INTERNET-DRAFT OpenPGP Message Format Apr 24, 2007 3137 3138 Because it is used only for padding at the end of the data, the 3139 occurrence of any "=" characters may be taken as evidence that the 3140 end of the data has been reached (without truncation in transit). No 3141 such assurance is possible, however, when the number of octets 3142 transmitted was a multiple of three and no "=" characters are 3143 present. 3144 3145 6.5. Examples of Radix-64 3146 3147 Input data: 0x14fb9c03d97e 3148 Hex: 1 4 f b 9 c | 0 3 d 9 7 e 3149 8-bit: 00010100 11111011 10011100 | 00000011 11011001 11111110 3150 6-bit: 000101 001111 101110 011100 | 000000 111101 100111 111110 3151 Decimal: 5 15 46 28 0 61 37 62 3152 Output: F P u c A 9 l + 3153 Input data: 0x14fb9c03d9 3154 Hex: 1 4 f b 9 c | 0 3 d 9 3155 8-bit: 00010100 11111011 10011100 | 00000011 11011001 3156 pad with 00 3157 6-bit: 000101 001111 101110 011100 | 000000 111101 100100 3158 Decimal: 5 15 46 28 0 61 36 3159 pad with = 3160 Output: F P u c A 9 k = 3161 Input data: 0x14fb9c03 3162 Hex: 1 4 f b 9 c | 0 3 3163 8-bit: 00010100 11111011 10011100 | 00000011 3164 pad with 0000 3165 6-bit: 000101 001111 101110 011100 | 000000 110000 3166 Decimal: 5 15 46 28 0 48 3167 pad with = = 3168 Output: F P u c A w = = 3169 3170 6.6. Example of an ASCII Armored Message 3171 3172 -----BEGIN PGP MESSAGE----- 3173 Version: OpenPrivacy 0.99 3174 3175 yDgBO22WxBHv7O8X7O/jygAEzol56iUKiXmV+XmpCtmpqQUKiQrFqclFqUDBovzS 3176 vBSFjNSiVHsuAA== 3177 =njUN 3178 -----END PGP MESSAGE----- 3179 3180 Note that this example has extra indenting; an actual armored 3181 message would have no leading whitespace. 3182 3183 7. Cleartext signature framework 3184 3185 It is desirable to be able to sign a textual octet stream without 3186 ASCII armoring the stream itself, so the signed text is still 3187 readable without special software. In order to bind a signature to 3188 such a cleartext, this framework is used. (Note that this framework 3189 is not intended to be reversible. RFC 3156 defines another way to 3190 3191 Callas, et al. Expires Oct 24, 2007 [Page 56] 3192 INTERNET-DRAFT OpenPGP Message Format Apr 24, 2007 3194 3195 sign cleartext messages for environments that support MIME.) 3196 3197 The cleartext signed message consists of: 3198 3199 - The cleartext header '-----BEGIN PGP SIGNED MESSAGE-----' on a 3200 single line, 3201 3202 - One or more "Hash" Armor Headers, 3203 3204 - Exactly one empty line not included into the message digest, 3205 3206 - The dash-escaped cleartext that is included into the message 3207 digest, 3208 3209 - The ASCII armored signature(s) including the '-----BEGIN PGP 3210 SIGNATURE-----' Armor Header and Armor Tail Lines. 3211 3212 If the "Hash" armor header is given, the specified message digest 3213 algorithm(s) are used for the signature. If there are no such 3214 headers, MD5 is used. If MD5 is the only hash used, then an 3215 implementation MAY omit this header for improved V2.x compatibility. 3216 If more than one message digest is used in the signature, the "Hash" 3217 armor header contains a comma-delimited list of used message 3218 digests. 3219 3220 Current message digest names are described below with the algorithm 3221 IDs. 3222 3223 An implementation SHOULD add a line break after the cleartext, but 3224 MAY omit it if the cleartext ends with a line break. This is for 3225 visual clarity. 3226 3227 7.1. Dash-Escaped Text 3228 3229 The cleartext content of the message must also be dash-escaped. 3230 3231 Dash escaped cleartext is the ordinary cleartext where every line 3232 starting with a dash '-' (0x2D) is prefixed by the sequence dash '-' 3233 (0x2D) and space ' ' (0x20). This prevents the parser from 3234 recognizing armor headers of the cleartext itself. An implementation 3235 MAY dash escape any line, SHOULD dash escape lines commencing "From" 3236 followed by a space, and MUST dash escape any line commencing in a 3237 dash. The message digest is computed using the cleartext itself, not 3238 the dash escaped form. 3239 3240 As with binary signatures on text documents, a cleartext signature 3241 is calculated on the text using canonical <CR><LF> line endings. The 3242 line ending (i.e. the <CR><LF>) before the '-----BEGIN PGP 3243 SIGNATURE-----' line that terminates the signed text is not 3244 considered part of the signed text. 3245 3246 3247 3248 Callas, et al. Expires Oct 24, 2007 [Page 57] 3249 INTERNET-DRAFT OpenPGP Message Format Apr 24, 2007 3251 3252 When reversing dash-escaping, an implementation MUST strip the 3253 string "- " if it occurs at the beginning of a line, and SHOULD warn 3254 on "-" and any character other than a space at the beginning of a 3255 line. 3256 3257 Also, any trailing whitespace -- spaces (0x20) and tabs (0x09) -- at 3258 the end of any line is removed when the cleartext signature is 3259 generated. 3260 3261 8. Regular Expressions 3262 3263 A regular expression is zero or more branches, separated by '|'. It 3264 matches anything that matches one of the branches. 3265 3266 A branch is zero or more pieces, concatenated. It matches a match 3267 for the first, followed by a match for the second, etc. 3268 3269 A piece is an atom possibly followed by '*', '+', or '?'. An atom 3270 followed by '*' matches a sequence of 0 or more matches of the atom. 3271 An atom followed by '+' matches a sequence of 1 or more matches of 3272 the atom. An atom followed by '?' matches a match of the atom, or 3273 the null string. 3274 3275 An atom is a regular expression in parentheses (matching a match for 3276 the regular expression), a range (see below), '.' (matching any 3277 single character), '^' (matching the null string at the beginning of 3278 the input string), '$' (matching the null string at the end of the 3279 input string), a '\' followed by a single character (matching that 3280 character), or a single character with no other significance 3281 (matching that character). 3282 3283 A range is a sequence of characters enclosed in '[]'. It normally 3284 matches any single character from the sequence. If the sequence 3285 begins with '^', it matches any single character not from the rest 3286 of the sequence. If two characters in the sequence are separated by 3287 '-', this is shorthand for the full list of ASCII characters between 3288 them (e.g. '[0-9]' matches any decimal digit). To include a literal 3289 ']' in the sequence, make it the first character (following a 3290 possible '^'). To include a literal '-', make it the first or last 3291 character. 3292 3293 9. Constants 3294 3295 This section describes the constants used in OpenPGP. 3296 3297 Note that these tables are not exhaustive lists; an implementation 3298 MAY implement an algorithm not on these lists, so long as the 3299 algorithm number(s) are chosen from the private or experimental 3300 algorithm range. 3301 3302 3303 3304 3305 Callas, et al. Expires Oct 24, 2007 [Page 58] 3306 INTERNET-DRAFT OpenPGP Message Format Apr 24, 2007 3308 3309 See the section "Notes on Algorithms" below for more discussion of 3310 the algorithms. 3311 3312 9.1. Public Key Algorithms 3313 3314 ID Algorithm 3315 -- --------- 3316 1 - RSA (Encrypt or Sign) [HAC] 3317 2 - RSA Encrypt-Only [HAC] 3318 3 - RSA Sign-Only [HAC] 3319 16 - Elgamal (Encrypt-Only), see [ELGAMAL] [HAC] 3320 17 - DSA (Digital Signature Algorithm) [FIPS186] [HAC] 3321 18 - Reserved for Elliptic Curve 3322 19 - Reserved for ECDSA 3323 20 - Reserved (formerly Elgamal Encrypt or Sign) 3324 21 - Reserved for Diffie-Hellman (X9.42, 3325 as defined for IETF-S/MIME) 3326 100 to 110 - Private/Experimental algorithm. 3327 3328 Implementations MUST implement DSA for signatures, and Elgamal for 3329 encryption. Implementations SHOULD implement RSA keys (1). RSA 3330 Encrypt-Only (2) and RSA Sign-Only are deprecated and SHOULD NOT be 3331 generated, but may be interpreted. See Section 13.5. See Section 3332 13.8 for notes on Elliptic Curve (18), ECDSA (19), Elgamal Encrypt 3333 or Sign (20), and X9.42 (21). Implementations MAY implement any 3334 other algorithm. 3335 3336 9.2. Symmetric Key Algorithms 3337 3338 ID Algorithm 3339 -- --------- 3340 0 - Plaintext or unencrypted data 3341 1 - IDEA [IDEA] 3342 2 - TripleDES (DES-EDE, [SCHNEIER] [HAC] - 3343 168 bit key derived from 192) 3344 3 - CAST5 (128 bit key, as per RFC 2144) 3345 4 - Blowfish (128 bit key, 16 rounds) [BLOWFISH] 3346 5 - Reserved 3347 6 - Reserved 3348 7 - AES with 128-bit key [AES] 3349 8 - AES with 192-bit key 3350 9 - AES with 256-bit key 3351 10 - Twofish with 256-bit key [TWOFISH] 3352 100 to 110 - Private/Experimental algorithm. 3353 3354 Implementations MUST implement TripleDES. Implementations SHOULD 3355 implement AES-128 and CAST5. Implementations that interoperate with 3356 PGP 2.6 or earlier need to support IDEA, as that is the only 3357 symmetric cipher those versions use. Implementations MAY implement 3358 any other algorithm. 3359 3360 3361 3362 Callas, et al. Expires Oct 24, 2007 [Page 59] 3363 INTERNET-DRAFT OpenPGP Message Format Apr 24, 2007 3365 3366 9.3. Compression Algorithms 3367 3368 ID Algorithm 3369 -- --------- 3370 0 - Uncompressed 3371 1 - ZIP [RFC 1951] 3372 2 - ZLIB [RFC 1950] 3373 3 - BZip2 [BZ2] 3374 100 to 110 - Private/Experimental algorithm. 3375 3376 Implementations MUST implement uncompressed data. Implementations 3377 SHOULD implement ZIP. Implementations MAY implement any other 3378 algorithm. 3379 3380 9.4. Hash Algorithms 3381 3382 ID Algorithm Text Name 3383 -- --------- ---- ---- 3384 1 - MD5 [HAC] "MD5" 3385 2 - SHA-1 [FIPS180] "SHA1" 3386 3 - RIPE-MD/160 [HAC] "RIPEMD160" 3387 4 - Reserved 3388 5 - Reserved 3389 6 - Reserved 3390 7 - Reserved 3391 8 - SHA256 [FIPS180] "SHA256" 3392 9 - SHA384 [FIPS180] "SHA384" 3393 10 - SHA512 [FIPS180] "SHA512" 3394 11 - SHA224 [FIPS180] "SHA224" 3395 100 to 110 - Private/Experimental algorithm. 3396 3397 Implementations MUST implement SHA-1. Implementations MAY implement 3398 other algorithms. MD5 is deprecated. 3399 3400 10. IANA Considerations 3401 3402 OpenPGP is highly parameterized and consequently there are a number 3403 of considerations for allocating parameters for extensions. This 3404 section describes how IANA should look at extensions to the protocol 3405 as described in this document. 3406 3407 10.1. New String-to-Key specifier types 3408 3409 OpenPGP S2K specifiers contain a mechanism for new algorithms to 3410 turn a string into a key. This specification creates a registry of 3411 S2K specifier types. The registry includes the S2K type, the name of 3412 the S2K and a reference to the defining specification. The initial 3413 values for this registry can be found in 3.7.1. Adding a new S2K 3414 specifier MUST be done through the IETF CONSENSUS method, as 3415 described in [RFC2434]. 3416 3417 3418 3419 Callas, et al. Expires Oct 24, 2007 [Page 60] 3420 INTERNET-DRAFT OpenPGP Message Format Apr 24, 2007 3422 3423 10.2. New Packets 3424 3425 Major new features of OpenPGP are defined though new packet types. 3426 This specification creates a registry of packet types. The registry 3427 includes the packet type, the name of the packet and a reference to 3428 the defining specification. The initial values for this registry can 3429 be found in 4.3. Adding a new packet type MUST be done through the 3430 IETF CONSENSUS method, as described in [RFC2434]. 3431 3432 10.2.1. User Attribute Types 3433 3434 The User Attribute packet permits an extensible mechanism for other 3435 types of certificate identification. This specification creates a 3436 registry of User Attribute types. The registry includes the User 3437 Attribute type, the name of the User Attribute and a reference to 3438 the defining specification. The initial values for this registry can 3439 be found in 5.12. Adding a new User Attribute type MUST be done 3440 through the IETF CONSENSUS method, as described in [RFC2434]. 3441 3442 10.2.1.1. Image Format Subpacket Types 3443 3444 Within User Attribute packets, there is an extensible mechanism for 3445 other types of image-based user attributes. This specification 3446 creates a registry of Image Attribute subpacket types. The registry 3447 includes the Image Attribute subpacket type, the name of the Image 3448 Attribute subpacket and a reference to the defining specification. 3449 The initial values for this registry can be found in 5.12.1. Adding 3450 a new Image Attribute subpacket type MUST be done through the IETF 3451 CONSENSUS method, as described in [RFC2434]. 3452 3453 10.2.2. New Signature Subpackets 3454 3455 OpenPGP signatures contain a mechanism for signed (or unsigned) data 3456 to be added to them for a variety of purposes in the signature 3457 subpackets as discussed in section 5.2.3.1. This specification 3458 creates a registry of signature subpacket types. The registry 3459 includes the signature subpacket type, the name of the subpacket and 3460 a reference to the defining specification. The initial values for 3461 this registry can be found in 5.2.3.1. Adding a new signature 3462 subpacket MUST be done through the IETF CONSENSUS method, as 3463 described in [RFC2434]. 3464 3465 10.2.2.1. Signature Notation Data Subpackets 3466 3467 OpenPGP signatures further contain a mechanism for extensions in 3468 signatures. These are the Notation Data subpackets, which contain a 3469 key/value pair. Notations contain a user space which is completely 3470 unmanaged and an IETF space. 3471 3472 This specification creates a registry of Signature Notation Data 3473 types. The registry includes the Signature Notation Data type, the 3474 name of the Signature Notation Data, its allowed values, and a 3475 3476 Callas, et al. Expires Oct 24, 2007 [Page 61] 3477 INTERNET-DRAFT OpenPGP Message Format Apr 24, 2007 3479 3480 reference to the defining specification. The initial values for this 3481 registry can be found in 5.2.3.16. Adding a new Signature Notation 3482 Data subpacket MUST be done through the EXPERT REVIEW method, as 3483 described in [RFC2434]. 3484 3485 10.2.2.2. Key Server Preference Extensions 3486 3487 OpenPGP signatures contain a mechanism for preferences to be 3488 specified about key servers. This specification creates a registry 3489 of key server preferences. The registry includes the key server 3490 preference, the name of the preference and a reference to the 3491 defining specification. The initial values for this registry can be 3492 found in 5.2.3.17. Adding a new key server preference MUST be done 3493 through the IETF CONSENSUS method, as described in [RFC2434]. 3494 3495 10.2.2.3. Key Flags Extensions 3496 3497 OpenPGP signatures contain a mechanism for flags to be specified 3498 about key usage. This specification creates a registry of key usage 3499 flags. The registry includes the key flags value, the name of the 3500 flag and a reference to the defining specification. The initial 3501 values for this registry can be found in 5.2.3.21. Adding a new key 3502 usage flag MUST be done through the IETF CONSENSUS method, as 3503 described in [RFC2434]. 3504 3505 10.2.2.4. Reason For Revocation Extensions 3506 3507 OpenPGP signatures contain a mechanism for flags to be specified 3508 about why a key was revoked. This specification creates a registry 3509 of reason-for-revocation flags. The registry includes the 3510 reason-for-revocation flags value, the name of the flag and a 3511 reference to the defining specification. The initial values for this 3512 registry can be found in 5.2.3.23. Adding a new feature flag MUST be 3513 done through the IETF CONSENSUS method, as described in [RFC2434]. 3514 3515 10.2.2.5. Implementation Features 3516 3517 OpenPGP signatures contain a mechanism for flags to be specified 3518 stating which optional features an implementation supports. This 3519 specification creates a registry of feature-implementation flags. 3520 The registry includes the feature-implementation flags value, the 3521 name of the flag and a reference to the defining specification. The 3522 initial values for this registry can be found in 5.2.3.24. Adding a 3523 new feature-implementation flag MUST be done through the IETF 3524 CONSENSUS method, as described in [RFC2434]. 3525 3526 Also see section 10.6 for more information about when feature flags 3527 are needed. 3528 3529 10.2.3. New Packet Versions 3530 3531 The core OpenPGP packets all have version numbers, and can be 3532 3533 Callas, et al. Expires Oct 24, 2007 [Page 62] 3534 INTERNET-DRAFT OpenPGP Message Format Apr 24, 2007 3536 3537 revised by introducing a new version of an existing packet. This 3538 specification creates a registry of packet types. The registry 3539 includes the packet type, the number of the version and a reference 3540 to the defining specification. The initial values for this registry 3541 can be found in 5. Adding a new packet version MUST be done through 3542 the IETF CONSENSUS method, as described in [RFC2434]. 3543 3544 10.3. New Algorithms 3545 3546 Chapter 9 lists the core algorithms that OpenPGP uses. Adding in a 3547 new algorithm is usually simple. For example, adding in a new 3548 symmetric cipher usually would not need anything more than 3549 allocating a constant for that cipher. If that cipher had other than 3550 a 64-bit or 128-bit block size, there might need to be additional 3551 documentation describing how OpenPGP-CFB mode would be adjusted. 3552 Similarly, when DSA was expanded from a maximum of 1024-bit public 3553 keys to 3072-bit public keys, the revision of FIPS 186 contained 3554 enough information itself to allow implementation. Changes to this 3555 document were emphasis more than required. 3556 3557 10.3.1. Public Key Algorithms 3558 3559 OpenPGP specifies a number of public key algorithms. This 3560 specification creates a registry of public key algorithm 3561 identifiers. The registry includes the algorithm name, its key sizes 3562 and parameters, and a reference to the defining specification. The 3563 initial values for this registry can be found in section 9. Adding a 3564 new public key algorithm MUST be done through the IETF CONSENSUS 3565 method, as described in [RFC2434]. 3566 3567 10.3.2. Symmetric Key Algorithms 3568 3569 OpenPGP specifies a number of symmetric key algorithms. This 3570 specification creates a registry of symmetric key algorithm 3571 identifiers. The registry includes the algorithm name, its key sizes 3572 and block size, and a reference to the defining specification. The 3573 initial values for this registry can be found in section 9. Adding a 3574 new symmetric key algorithm MUST be done through the IETF CONSENSUS 3575 method, as described in [RFC2434]. 3576 3577 10.3.3. Hash Algorithms 3578 3579 OpenPGP specifies a number of hash algorithms. This specification 3580 creates a registry of hash algorithm identifiers. The registry 3581 includes the algorithm name, a text representation of that name, its 3582 block size, an OID hash prefix, and a reference to the defining 3583 specification. The initial values for this registry can be found in 3584 section 9 for the algorithm identifiers and text names, and section 3585 5.2.2 for the OIDs and expanded signature prefixes. Adding a new 3586 hash algorithm MUST be done through the IETF CONSENSUS method, as 3587 described in [RFC2434]. 3588 3589 3590 Callas, et al. Expires Oct 24, 2007 [Page 63] 3591 INTERNET-DRAFT OpenPGP Message Format Apr 24, 2007 3593 3594 10.3.4. Compression Algorithms 3595 3596 OpenPGP specifies a number of compression algorithms. This 3597 specification creates a registry of compression algorithm 3598 identifiers. The registry includes the algorithm name, and a 3599 reference to the defining specification. The initial values for this 3600 registry can be found in section 9.3. Adding a new compression key 3601 algorithm MUST be done through the IETF CONSENSUS method, as 3602 described in [RFC2434]. 3603 3604 11. Packet Composition 3605 3606 OpenPGP packets are assembled into sequences in order to create 3607 messages and to transfer keys. Not all possible packet sequences are 3608 meaningful and correct. This section describes the rules for how 3609 packets should be placed into sequences. 3610 3611 11.1. Transferable Public Keys 3612 3613 OpenPGP users may transfer public keys. The essential elements of a 3614 transferable public key are: 3615 3616 - One Public Key packet 3617 3618 - Zero or more revocation signatures 3619 3620 - One or more User ID packets 3621 3622 - After each User ID packet, zero or more signature packets 3623 (certifications) 3624 3625 - Zero or more User Attribute packets 3626 3627 - After each User Attribute packet, zero or more signature packets 3628 (certifications) 3629 3630 - Zero or more Subkey packets 3631 3632 - After each Subkey packet, one signature packet, plus optionally 3633 a revocation. 3634 3635 The Public Key packet occurs first. Each of the following User ID 3636 packets provides the identity of the owner of this public key. If 3637 there are multiple User ID packets, this corresponds to multiple 3638 means of identifying the same unique individual user; for example, a 3639 user may have more than one email address, and construct a User ID 3640 for each one. 3641 3642 Immediately following each User ID packet, there are zero or more 3643 signature packets. Each signature packet is calculated on the 3644 immediately preceding User ID packet and the initial Public Key 3645 packet. The signature serves to certify the corresponding public key 3646 3647 Callas, et al. Expires Oct 24, 2007 [Page 64] 3648 INTERNET-DRAFT OpenPGP Message Format Apr 24, 2007 3650 3651 and User ID. In effect, the signer is testifying to his or her 3652 belief that this public key belongs to the user identified by this 3653 User ID. 3654 3655 Within the same section as the User ID packets, there are zero or 3656 more User Attribute packets. Like the User ID packets, a User 3657 Attribute packet is followed by zero or more signature packets 3658 calculated on the immediately preceding User Attribute packet and 3659 the initial Public Key packet. 3660 3661 User Attribute packets and User ID packets may be freely intermixed 3662 in this section, so long as the signatures that follow them are 3663 maintained on the proper User Attribute or User ID packet. 3664 3665 After the User ID or Attribute packets there may be zero or more 3666 Subkey packets. In general, subkeys are provided in cases where the 3667 top-level public key is a signature-only key. However, any V4 key 3668 may have subkeys, and the subkeys may be encryption-only keys, 3669 signature-only keys, or general-purpose keys. V3 keys MUST NOT have 3670 subkeys. 3671 3672 Each Subkey packet MUST be followed by one Signature packet, which 3673 should be a subkey binding signature issued by the top level key. 3674 For subkeys that can issue signatures, the subkey binding signature 3675 MUST contain an embedded signature subpacket with a primary key 3676 binding signature (0x19) issued by the subkey on the top level key. 3677 3678 Subkey and Key packets may each be followed by a revocation 3679 Signature packet to indicate that the key is revoked. Revocation 3680 signatures are only accepted if they are issued by the key itself, 3681 or by a key that is authorized to issue revocations via a revocation 3682 key subpacket in a self-signature by the top level key. 3683 3684 Transferable public key packet sequences may be concatenated to 3685 allow transferring multiple public keys in one operation. 3686 3687 11.2. Transferable Secret Keys 3688 3689 OpenPGP users may transfer secret keys. The format of a transferable 3690 secret key is the same as a transferable public key except that 3691 secret key and secret subkey packets are used instead of the public 3692 key and public subkey packets. Implementations SHOULD include 3693 self-signatures on any user IDs and subkeys, as this allows for a 3694 complete public key to be automatically extracted from the 3695 transferable secret key. Implementations MAY choose to omit the 3696 self-signatures, especially if a transferable public key accompanies 3697 the transferable secret key. 3698 3699 11.3. OpenPGP Messages 3700 3701 An OpenPGP message is a packet or sequence of packets that 3702 corresponds to the following grammatical rules (comma represents 3703 3704 Callas, et al. Expires Oct 24, 2007 [Page 65] 3705 INTERNET-DRAFT OpenPGP Message Format Apr 24, 2007 3707 3708 sequential composition, and vertical bar separates alternatives): 3709 3710 OpenPGP Message :- Encrypted Message | Signed Message | 3711 Compressed Message | Literal Message. 3712 3713 Compressed Message :- Compressed Data Packet. 3714 3715 Literal Message :- Literal Data Packet. 3716 3717 ESK :- Public Key Encrypted Session Key Packet | 3718 Symmetric-Key Encrypted Session Key Packet. 3719 3720 ESK Sequence :- ESK | ESK Sequence, ESK. 3721 3722 Encrypted Data :- Symmetrically Encrypted Data Packet | 3723 Symmetrically Encrypted Integrity Protected Data Packet 3724 3725 Encrypted Message :- Encrypted Data | ESK Sequence, Encrypted Data. 3726 3727 One-Pass Signed Message :- One-Pass Signature Packet, 3728 OpenPGP Message, Corresponding Signature Packet. 3729 3730 Signed Message :- Signature Packet, OpenPGP Message | 3731 One-Pass Signed Message. 3732 3733 In addition, decrypting a Symmetrically Encrypted Data Packet or a 3734 Symmetrically Encrypted Integrity Protected Data Packet as well as 3735 decompressing a Compressed Data packet must yield a valid OpenPGP 3736 Message. 3737 3738 11.4. Detached Signatures 3739 3740 Some OpenPGP applications use so-called "detached signatures." For 3741 example, a program bundle may contain a file, and with it a second 3742 file that is a detached signature of the first file. These detached 3743 signatures are simply a signature packet stored separately from the 3744 data that they are a signature of. 3745 3746 12. Enhanced Key Formats 3747 3748 12.1. Key Structures 3749 3750 The format of an OpenPGP V3 key is as follows. Entries in square 3751 brackets are optional and ellipses indicate repetition. 3752 3753 RSA Public Key 3754 [Revocation Self Signature] 3755 User ID [Signature ...] 3756 [User ID [Signature ...] ...] 3757 3758 3759 3760 3761 Callas, et al. Expires Oct 24, 2007 [Page 66] 3762 INTERNET-DRAFT OpenPGP Message Format Apr 24, 2007 3764 3765 Each signature certifies the RSA public key and the preceding User 3766 ID. The RSA public key can have many User IDs and each User ID can 3767 have many signatures. V3 keys are deprecated. Implementations MUST 3768 NOT generate new V3 keys, but MAY continue to use existing ones. 3769 3770 The format of an OpenPGP V4 key that uses multiple public keys is 3771 similar except that the other keys are added to the end as "subkeys" 3772 of the primary key. 3773 3774 Primary-Key 3775 [Revocation Self Signature] 3776 [Direct Key Signature...] 3777 User ID [Signature ...] 3778 [User ID [Signature ...] ...] 3779 [User Attribute [Signature ...] ...] 3780 [[Subkey [Binding-Signature-Revocation] 3781 Primary-Key-Binding-Signature] ...] 3782 3783 A subkey always has a single signature after it that is issued using 3784 the primary key to tie the two keys together. This binding signature 3785 may be in either V3 or V4 format, but SHOULD be V4. Subkeys that can 3786 issue signatures MUST have a V4 binding signature due to the 3787 REQUIRED embedded primary key binding signature. 3788 3789 In the above diagram, if the binding signature of a subkey has been 3790 revoked, the revoked key may be removed, leaving only one key. 3791 3792 In a V4 key, the primary key MUST be a key capable of certification. 3793 The subkeys may be keys of any other type. There may be other 3794 constructions of V4 keys, too. For example, there may be a 3795 single-key RSA key in V4 format, a DSA primary key with an RSA 3796 encryption key, or RSA primary key with an Elgamal subkey, etc. 3797 3798 It is also possible to have a signature-only subkey. This permits a 3799 primary key that collects certifications (key signatures) but is 3800 used only used for certifying subkeys that are used for encryption 3801 and signatures. 3802 3803 12.2. Key IDs and Fingerprints 3804 3805 For a V3 key, the eight-octet key ID consists of the low 64 bits of 3806 the public modulus of the RSA key. 3807 3808 The fingerprint of a V3 key is formed by hashing the body (but not 3809 the two-octet length) of the MPIs that form the key material (public 3810 modulus n, followed by exponent e) with MD5. Note that both V3 keys 3811 and MD5 are deprecated. 3812 3813 A V4 fingerprint is the 160-bit SHA-1 hash of the octet 0x99, 3814 followed by the two-octet packet length, followed by the entire 3815 Public Key packet starting with the version field. The key ID is the 3816 low order 64 bits of the fingerprint. Here are the fields of the 3817 3818 Callas, et al. Expires Oct 24, 2007 [Page 67] 3819 INTERNET-DRAFT OpenPGP Message Format Apr 24, 2007 3821 3822 hash material, with the example of a DSA key: 3823 3824 a.1) 0x99 (1 octet) 3825 3826 a.2) high order length octet of (b)-(f) (1 octet) 3827 3828 a.3) low order length octet of (b)-(f) (1 octet) 3829 3830 b) version number = 4 (1 octet); 3831 3832 c) time stamp of key creation (4 octets); 3833 3834 d) algorithm (1 octet): 17 = DSA (example); 3835 3836 e) Algorithm specific fields. 3837 3838 Algorithm Specific Fields for DSA keys (example): 3839 3840 e.1) MPI of DSA prime p; 3841 3842 e.2) MPI of DSA group order q (q is a prime divisor of p-1); 3843 3844 e.3) MPI of DSA group generator g; 3845 3846 e.4) MPI of DSA public key value y (= g**x mod p where x is secret). 3847 3848 Note that it is possible for there to be collisions of key IDs -- 3849 two different keys with the same key ID. Note that there is a much 3850 smaller, but still non-zero probability that two different keys have 3851 the same fingerprint. 3852 3853 Also note that if V3 and V4 format keys share the same RSA key 3854 material, they will have different key IDs as well as different 3855 fingerprints. 3856 3857 Finally, the key ID and fingerprint of a subkey are calculated in 3858 the same way as for a primary key, including the 0x99 as the first 3859 octet (even though this is not a valid packet ID for a public 3860 subkey). 3861 3862 13. Notes on Algorithms 3863 3864 13.1. PKCS#1 Encoding In OpenPGP 3865 3866 This standard makes use of the PKCS#1 functions EME-PKCS1-v1_5 and 3867 EMSA-PKCS1-v1_5. However, the calling conventions of these functions 3868 has changed in the past. To avoid potential confusion and 3869 interoperability problems, we are including local copies in this 3870 document, adapted from those in PKCS#1 v2.1 [RFC3447]. RFC-3447 3871 should be treated as the ultimate authority on PKCS#1 for OpenPGP. 3872 Nonetheless, we believe that there is value in having a 3873 self-contained document that avoids problems in the future with 3874 3875 Callas, et al. Expires Oct 24, 2007 [Page 68] 3876 INTERNET-DRAFT OpenPGP Message Format Apr 24, 2007 3878 3879 needed changes in the conventions. 3880 3881 13.1.1. EME-PKCS1-v1_5-ENCODE 3882 3883 Input: 3884 3885 k = the length in octets of the key modulus 3886 3887 M = message to be encoded, an octet string of length mLen, where 3888 mLen <= k - 11 3889 3890 Output: 3891 3892 EM = encoded message, an octet string of length k 3893 3894 Error: "message too long" 3895 3896 1. Length checking: If mLen > k - 11, output "message too long" and 3897 stop. 3898 3899 2. Generate an octet string PS of length k - mLen - 3 consisting of 3900 pseudo-randomly generated nonzero octets. The length of PS will 3901 be at least eight octets. 3902 3903 3. Concatenate PS, the message M, and other padding to form an 3904 encoded message EM of length k octets as 3905 3906 EM = 0x00 || 0x02 || PS || 0x00 || M. 3907 3908 4. Output EM. 3909 3910 13.1.2. EME-PKCS1-v1_5-DECODE 3911 3912 Input: 3913 3914 EM = encoded message, an octet string 3915 3916 Output: 3917 3918 M = message, an octet string 3919 3920 Error: "decryption error" 3921 3922 To decode an EME-PKCS1_v1_5 message, separate the encoded message EM 3923 into an octet string PS consisting of nonzero octets and a message M 3924 as 3925 3926 EM = 0x00 || 0x02 || PS || 0x00 || M. 3927 3928 If the first octet of EM does not have hexadecimal value 0x00, if 3929 the second octet of EM does not have hexadecimal value 0x02, if 3930 there is no octet with hexadecimal value 0x00 to separate PS from M, 3931 3932 Callas, et al. Expires Oct 24, 2007 [Page 69] 3933 INTERNET-DRAFT OpenPGP Message Format Apr 24, 2007 3935 3936 or if the length of PS is less than 8 octets, output "decryption 3937 error" and stop. See also the security note in section 13 regarding 3938 differences in reporting between a decryption error and a padding 3939 error. 3940 3941 13.1.3. EMSA-PKCS1-v1_5 3942 3943 This encoding method is deterministic and only has an encoding 3944 operation. 3945 3946 Option: 3947 3948 Hash hash function (hLen denotes the length in octets of the hash 3949 function output) 3950 3951 Input: 3952 3953 M = message to be encoded 3954 3955 mL = intended length in octets of the encoded message, at least tLen 3956 + 11, where tLen is the octet length of the DER encoding T of a 3957 certain value computed during the encoding operation 3958 3959 Output: 3960 3961 EM = encoded message, an octet string of length emLen 3962 3963 Errors: "message too long"; "intended encoded message length too 3964 short" 3965 3966 Steps: 3967 3968 1. Apply the hash function to the message M to produce a hash value 3969 H: 3970 3971 H = Hash(M). 3972 3973 If the hash function outputs "message too long," output "message 3974 too long" and stop. 3975 3976 2. Using the list in section 5.2.2, produce an ASN.1 DER value for 3977 the hash function used. Let T be the full hash prefix from 3978 section 5.2.2, and let tLen be the length in octets of T. 3979 3980 3. If emLen < tLen + 11, output "intended encoded message length 3981 too short" and stop. 3982 3983 4. Generate an octet string PS consisting of emLen - tLen - 3 3984 octets with hexadecimal value 0xff. The length of PS will be at 3985 least 8 octets. 3986 3987 3988 3989 Callas, et al. Expires Oct 24, 2007 [Page 70] 3990 INTERNET-DRAFT OpenPGP Message Format Apr 24, 2007 3992 3993 5. Concatenate PS, the hash prefix T, and other padding to form the 3994 encoded message EM as 3995 3996 EM = 0x00 || 0x01 || PS || 0x00 || T. 3997 3998 6. Output EM. 3999 4000 13.2. Symmetric Algorithm Preferences 4001 4002 The symmetric algorithm preference is an ordered list of algorithms 4003 that the keyholder accepts. Since it is found on a self-signature, 4004 it is possible that a keyholder may have multiple, different 4005 preferences. For example, Alice may have TripleDES only specified 4006 for "alice (a] work.com" but CAST5, Blowfish, and TripleDES specified 4007 for "alice (a] home.org". Note that it is also possible for preferences 4008 to be in a subkey's binding signature. 4009 4010 Since TripleDES is the MUST-implement algorithm, if it is not 4011 explicitly in the list, it is tacitly at the end. However, it is 4012 good form to place it there explicitly. Note also that if an 4013 implementation does not implement the preference, then it is 4014 implicitly a TripleDES-only implementation. 4015 4016 An implementation MUST NOT use a symmetric algorithm that is not in 4017 the recipient's preference list. When encrypting to more than one 4018 recipient, the implementation finds a suitable algorithm by taking 4019 the intersection of the preferences of the recipients. Note that the 4020 MUST-implement algorithm, TripleDES, ensures that the intersection 4021 is not null. The implementation may use any mechanism to pick an 4022 algorithm in the intersection. 4023 4024 If an implementation can decrypt a message that a keyholder doesn't 4025 have in their preferences, the implementation SHOULD decrypt the 4026 message anyway, but MUST warn the keyholder that the protocol has 4027 been violated. For example, suppose that Alice, above, has software 4028 that implements all algorithms in this specification. Nonetheless, 4029 she prefers subsets for work or home. If she is sent a message 4030 encrypted with IDEA, which is not in her preferences, the software 4031 warns her that someone sent her an IDEA-encrypted message, but it 4032 would ideally decrypt it anyway. 4033 4034 13.3. Other Algorithm Preferences 4035 4036 Other algorithm preferences work similarly to the symmetric 4037 algorithm preference, in that they specify which algorithms the 4038 keyholder accepts. There are two interesting cases that other 4039 comments need to be made about, though, the compression preferences 4040 and the hash preferences. 4041 4042 13.3.1. Compression Preferences 4043 4044 Compression has been an integral part of PGP since its first days. 4045 4046 Callas, et al. Expires Oct 24, 2007 [Page 71] 4047 INTERNET-DRAFT OpenPGP Message Format Apr 24, 2007 4049 4050 OpenPGP and all previous versions of PGP have offered compression. 4051 In this specification, the default is for messages to be compressed, 4052 although an implementation is not required to do so. Consequently, 4053 the compression preference gives a way for a keyholder to request 4054 that messages not be compressed, presumably because they are using a 4055 minimal implementation that does not include compression. 4056 Additionally, this gives a keyholder a way to state that it can 4057 support alternate algorithms. 4058 4059 Like the algorithm preferences, an implementation MUST NOT use an 4060 algorithm that is not in the preference vector. If the preferences 4061 are not present, then they are assumed to be [ZIP(1), 4062 UNCOMPRESSED(0)]. 4063 4064 Additionally, an implementation MUST implement this preference to 4065 the degree of recognizing when to send an uncompressed message. A 4066 robust implementation would satisfy this requirement by looking at 4067 the recipient's preference and acting accordingly. A minimal 4068 implementation can satisfy this requirement by never generating a 4069 compressed message, since all implementations can handle messages 4070 that have not been compressed. 4071 4072 13.3.2. Hash Algorithm Preferences 4073 4074 Typically, the choice of a hash algorithm is something the signer 4075 does, rather than the verifier, because a signer rarely knows who is 4076 going to be verifying the signature. This preference, though, allows 4077 a protocol based upon digital signatures ease in negotiation. 4078 4079 Thus, if Alice is authenticating herself to Bob with a signature, it 4080 makes sense for her to use a hash algorithm that Bob's software 4081 uses. This preference allows Bob to state in his key which 4082 algorithms Alice may use. 4083 4084 Since SHA1 is the MUST-implement hash algorithm, if it is not 4085 explicitly in the list, it is tacitly at the end. However, it is 4086 good form to place it there explicitly. 4087 4088 13.4. Plaintext 4089 4090 Algorithm 0, "plaintext," may only be used to denote secret keys 4091 that are stored in the clear. Implementations MUST NOT use plaintext 4092 in Symmetrically Encrypted Data Packets; they must use Literal Data 4093 Packets to encode unencrypted or literal data. 4094 4095 13.5. RSA 4096 4097 There are algorithm types for RSA Sign-Only, and RSA Encrypt-Only 4098 keys. These types are deprecated. The "key flags" subpacket in a 4099 signature is a much better way to express the same idea, and 4100 generalizes it to all algorithms. An implementation SHOULD NOT 4101 create such a key, but MAY interpret it. 4102 4103 Callas, et al. Expires Oct 24, 2007 [Page 72] 4104 INTERNET-DRAFT OpenPGP Message Format Apr 24, 2007 4106 4107 An implementation SHOULD NOT implement RSA keys of size less than 4108 1024 bits. 4109 4110 13.6. DSA 4111 4112 An implementation SHOULD NOT implement DSA keys of size less than 4113 1024 bits. It MUST NOT implement a DSA key with a q size of less 4114 than 160 bits. DSA keys MUST also be a multiple of 64 bits, and the 4115 q size MUST be a multiple of 8 bits. The Digital Signature Standard 4116 (DSS) [FIPS186] specifies that DSA be used in one of the following 4117 ways: 4118 4119 * 1024-bit key, 160-bit q, SHA-1, SHA-224, SHA-256, SHA-384 or 4120 SHA-512 hash 4121 4122 * 2048-bit key, 224-bit q, SHA-224, SHA-256, SHA-384 or SHA-512 4123 hash 4124 4125 * 2048-bit key, 256-bit q, SHA-256, SHA-384 or SHA-512 hash 4126 4127 * 3072-bit key, 256-bit q, SHA-256, SHA-384 or SHA-512 hash 4128 4129 The above key and q size pairs were chosen to best balance the 4130 strength of the key with the strength of the hash. Implementations 4131 SHOULD use one of the above key and q size pairs when generating DSA 4132 keys. If DSS compliance is desired, one of the specified SHA hashes 4133 must be used as well. [FIPS186] is the ultimate authority on DSS, 4134 and should be consulted for all questions of DSS compliance. 4135 4136 Note that earlier versions of this standard only allowed a 160-bit q 4137 with no truncation allowed, so earlier implementations may not be 4138 able to handle signatures with a different q size or a truncated 4139 hash. 4140 4141 13.7. Elgamal 4142 4143 An implementation SHOULD NOT implement Elgamal keys of size less 4144 than 1024 bits. 4145 4146 13.8. Reserved Algorithm Numbers 4147 4148 A number of algorithm IDs have been reserved for algorithms that 4149 would be useful to use in an OpenPGP implementation, yet there are 4150 issues that prevent an implementer from actually implementing the 4151 algorithm. These are marked in the Public Algorithms section as 4152 "(reserved for)". 4153 4154 The reserved public key algorithms, Elliptic Curve (18), ECDSA (19), 4155 and X9.42 (21) do not have the necessary parameters, parameter 4156 order, or semantics defined. 4157 4158 4159 4160 Callas, et al. Expires Oct 24, 2007 [Page 73] 4161 INTERNET-DRAFT OpenPGP Message Format Apr 24, 2007 4163 4164 Previous versions of OpenPGP permitted Elgamal [ELGAMAL] signatures 4165 with a public key identifier of 20. These are no longer permitted. 4166 An implementation MUST NOT generate such keys. An implementation 4167 MUST NOT generate Elgamal signatures. See [BLEICHENBACHER]. 4168 4169 13.9. OpenPGP CFB mode 4170 4171 OpenPGP does symmetric encryption using a variant of Cipher Feedback 4172 Mode (CFB mode). This section describes the procedure it uses in 4173 detail. This mode is what is used for Symmetrically Encrypted Data 4174 Packets; the mechanism used for encrypting secret key material is 4175 similar, but described in those sections above. 4176 4177 In the description below, the value BS is the block size in octets 4178 of the cipher. Most ciphers have a block size of 8 octets. The AES 4179 and Twofish have a block size of 16 octets. Also note that the 4180 description below assumes that the IV and CFB arrays start with an 4181 index of 1 (unlike the C language, which assumes arrays start with a 4182 zero index). 4183 4184 OpenPGP CFB mode uses an initialization vector (IV) of all zeros, 4185 and prefixes the plaintext with BS+2 octets of random data, such 4186 that octets BS+1 and BS+2 match octets BS-1 and BS. It does a CFB 4187 resynchronization after encrypting those BS+2 octets. 4188 4189 Thus, for an algorithm that has a block size of 8 octets (64 bits), 4190 the IV is 10 octets long and octets 7 and 8 of the IV are the same 4191 as octets 9 and 10. For an algorithm with a block size of 16 octets 4192 (128 bits), the IV is 18 octets long, and octets 17 and 18 replicate 4193 octets 15 and 16. Those extra two octets are an easy check for a 4194 correct key. 4195 4196 Step by step, here is the procedure: 4197 4198 1. The feedback register (FR) is set to the IV, which is all zeros. 4199 4200 2. FR is encrypted to produce FRE (FR Encrypted). This is the 4201 encryption of an all-zero value. 4202 4203 3. FRE is xored with the first BS octets of random data prefixed to 4204 the plaintext to produce C[1] through C[BS], the first BS octets 4205 of ciphertext. 4206 4207 4. FR is loaded with C[1] through C[BS]. 4208 4209 5. FR is encrypted to produce FRE, the encryption of the first BS 4210 octets of ciphertext. 4211 4212 6. The left two octets of FRE get xored with the next two octets of 4213 data that were prefixed to the plaintext. This produces C[BS+1] 4214 and C[BS+2], the next two octets of ciphertext. 4215 4216 4217 Callas, et al. Expires Oct 24, 2007 [Page 74] 4218 INTERNET-DRAFT OpenPGP Message Format Apr 24, 2007 4220 4221 7. (The resynchronization step) FR is loaded with C[3] through 4222 C[BS+2]. 4223 4224 8. FR is encrypted to produce FRE. 4225 4226 9. FRE is xored with the first BS octets of the given plaintext, 4227 now that we have finished encrypting the BS+2 octets of prefixed 4228 data. This produces C[BS+3] through C[BS+(BS+2)], the next BS 4229 octets of ciphertext. 4230 4231 10. FR is loaded with C[BS+3] to C[BS + (BS+2)] (which is C11-C18 4232 for an 8-octet block). 4233 4234 11. FR is encrypted to produce FRE. 4235 4236 12. FRE is xored with the next BS octets of plaintext, to produce 4237 the next BS octets of ciphertext. These are loaded into FR and 4238 the process is repeated until the plaintext is used up. 4239 4240 13.10. Private or Experimental Parameters 4241 4242 S2K specifiers, Signature subpacket types, user attribute types, 4243 image format types, and algorithms described in Section 9 all 4244 reserve the range 100 to 110 for private and experimental use. 4245 Packet types reserve the range 60 to 63 for private and experimental 4246 use. These are intentionally managed with the PRIVATE USE method, as 4247 described in [RFC2434]. 4248 4249 However, implementations need to be careful with these and promote 4250 them to full IANA-managed parameters when they grow beyond the 4251 original, limited system. 4252 4253 13.11. Extension of the MDC System 4254 4255 As described in the non-normative explanation in section 5.13, the 4256 MDC system is uniquely unparameterized in OpenPGP, and that this was 4257 an intentional decision to avoid cross-grade attacks. If the MDC 4258 system is extended to a stronger hash function, there must be care 4259 given to avoiding downgrade and cross-grade attacks. 4260 4261 One simple way to do this is to create new packets for a new MDC. 4262 For example, instead of the MDC system using packets 18 and 19, a 4263 new MDC could use 20 and 21. This has obvious drawbacks (it uses two 4264 packet numbers for each new hash function in a space that is limited 4265 to a maximum of 60). 4266 4267 Another simple way to extend the MDC system is to create new 4268 versions of packet 18, and reflect this in packet 19. For example, 4269 suppose that V2 of packet 18 implicitly used SHA-256. This would 4270 require packet 19 to have a length of 32 octets. The change in the 4271 version in packet 18 and the size of packet 19 prevent a downgrade 4272 attack. 4273 4274 Callas, et al. Expires Oct 24, 2007 [Page 75] 4275 INTERNET-DRAFT OpenPGP Message Format Apr 24, 2007 4277 4278 There are two drawbacks to this latter approach. The first is that 4279 using the version number of a packet to carry algorithm information 4280 is not tidy from a protocol-design standpoint. it is possible that 4281 there might be several versions of the MDC system in common use, but 4282 this untidiness would reflect untidiness in cryptographic consensus 4283 about hash function security. The second is that different versions 4284 of packet 19 would have to have unique sizes. If there were two 4285 versions each with 256-bit hashes, they could not both have 32-octet 4286 packet 19s without admitting the chance of a cross-grade attack. 4287 4288 Yet another, complex approach to extend the MDC system would be a 4289 hybrid of the two above -- create a new pair of MDC packets that are 4290 fully parameterized, and yet protected from downgrade and 4291 cross-grade. 4292 4293 Any change to the MDC system MUST be done through the IETF CONSENSUS 4294 method, as described in [RFC2434]. 4295 4296 13.12. Meta-Considerations for Expansion 4297 4298 If OpenPGP is extended in a way that is not backwards-compatible, 4299 meaning that old implementations will not gracefully handle their 4300 absence of a new feature, the extension proposal can be declared in 4301 the key holder's self-signature as part of the Features signature 4302 subpacket. 4303 4304 We cannot state definitively what extensions will not be 4305 upwards-compatible, but typically new algorithms are 4306 upwards-compatible, but new packets are not. 4307 4308 If an extension proposal does not update the Features system, it 4309 SHOULD include an explanation of why this is unnecessary. If the 4310 proposal contains neither an extension to the Features system nor an 4311 explanation of why such an extension is unnecessary, the proposal 4312 SHOULD be rejected. 4313 4314 14. Security Considerations 4315 4316 * As with any technology involving cryptography, you should check 4317 the current literature to determine if any algorithms used here 4318 have been found to be vulnerable to attack. 4319 4320 * This specification uses Public Key Cryptography technologies. It 4321 is assumed that the private key portion of a public-private key 4322 pair is controlled and secured by the proper party or parties. 4323 4324 * Certain operations in this specification involve the use of 4325 random numbers. An appropriate entropy source should be used to 4326 generate these numbers. See RFC 4086. 4327 4328 4329 4330 4331 Callas, et al. Expires Oct 24, 2007 [Page 76] 4332 INTERNET-DRAFT OpenPGP Message Format Apr 24, 2007 4334 4335 * The MD5 hash algorithm has been found to have weaknesses, with 4336 collisions found in a number of cases. MD5 is deprecated for use 4337 in OpenPGP. Implementations MUST NOT generate new signatures 4338 using MD5 as a hash function. They MAY continue to consider old 4339 signatures that used MD5 as valid. 4340 4341 * SHA-224 and SHA-384 require the same work as SHA-256 and SHA-512 4342 respectively. In general, there are few reasons to use them 4343 outside of DSS compatibility. You need a situation where one 4344 needs more security than smaller hashes, but does not want to 4345 have the full 256-bit or 512-bit data length. 4346 4347 * Many security protocol designers think that it is a bad idea to 4348 use a single key for both privacy (encryption) and integrity 4349 (signatures). In fact, this was one of the motivating forces 4350 behind the V4 key format with separate signature and encryption 4351 keys. If you as an implementer promote dual-use keys, you should 4352 at least be aware of this controversy. 4353 4354 * The DSA algorithm will work with any hash, but is sensitive to 4355 the quality of the hash algorithm. Verifiers should be aware 4356 that even if the signer used a strong hash, an attacker could 4357 have modified the signature to use a weak one. Only signatures 4358 using acceptably strong hash algorithms should be accepted as 4359 valid. 4360 4361 * As OpenPGP combines many different asymmetric, symmetric, and 4362 hash algorithms, each with different measures of strength, care 4363 should be taken that the weakest element of an OpenPGP message 4364 is still sufficiently strong for the purpose at hand. While 4365 consensus about the the strength of a given algorithm may 4366 evolve, NIST Special Publication 800-57 [SP800-57] recommends 4367 the following list of equivalent strengths: 4368 4369 Asymmetric | Hash | Symmetric 4370 key size | size | key size 4371 ------------+--------+----------- 4372 1024 160 80 4373 2048 224 112 4374 3072 256 128 4375 7680 384 192 4376 15360 512 256 4377 4378 4379 * There is a somewhat-related potential security problem in 4380 signatures. If an attacker can find a message that hashes to the 4381 same hash with a different algorithm, a bogus signature 4382 structure can be constructed that evaluates correctly. 4383 4384 For example, suppose Alice DSA signs message M using hash 4385 algorithm H. Suppose that Mallet finds a message M' that has the 4386 same hash value as M with H'. Mallet can then construct a 4387 4388 Callas, et al. Expires Oct 24, 2007 [Page 77] 4389 INTERNET-DRAFT OpenPGP Message Format Apr 24, 2007 4391 4392 signature block that verifies as Alice's signature of M' with 4393 H'. However, this would also constitute a weakness in either H 4394 or H' or both. Should this ever occur, a revision will have to 4395 be made to this document to revise the allowed hash algorithms. 4396 4397 * If you are building an authentication system, the recipient may 4398 specify a preferred signing algorithm. However, the signer would 4399 be foolish to use a weak algorithm simply because the recipient 4400 requests it. 4401 4402 * Some of the encryption algorithms mentioned in this document 4403 have been analyzed less than others. For example, although CAST5 4404 is presently considered strong, it has been analyzed less than 4405 TripleDES. Other algorithms may have other controversies 4406 surrounding them. 4407 4408 * In late summer 2002, Jallad, Katz, and Schneier published an 4409 interesting attack on the OpenPGP protocol and some of its 4410 implementations [JKS02]. In this attack, the attacker modifies a 4411 message and sends it to a user who then returns the erroneously 4412 decrypted message to the attacker. The attacker is thus using 4413 the user as a random oracle, and can often decrypt the message. 4414 4415 Compressing data can ameliorate this attack. The incorrectly 4416 decrypted data nearly always decompresses in ways that defeats 4417 the attack. However, this is not a rigorous fix, and leaves open 4418 some small vulnerabilities. For example, if an implementation 4419 does not compress a message before encryption (perhaps because 4420 it knows it was already compressed), then that message is 4421 vulnerable. Because of this happenstance -- that modification 4422 attacks can be thwarted by decompression errors, an 4423 implementation SHOULD treat a decompression error as a security 4424 problem, not merely a data problem. 4425 4426 This attack can be defeated by the use of Modification 4427 Detection, provided that the implementation does not let the 4428 user naively return the data to the attacker. An implementation 4429 MUST treat an MDC failure as a security problem, not merely a 4430 data problem. 4431 4432 In either case, the implementation MAY allow the user access to 4433 the erroneous data, but MUST warn the user as to potential 4434 security problems should that data be returned to the sender. 4435 4436 While this attack is somewhat obscure, requiring a special set 4437 of circumstances to create it, it is nonetheless quite serious 4438 as it permits someone to trick a user to decrypt a message. 4439 Consequently, it is important that: 4440 4441 1. Implementers treat MDC errors and decompression failures as 4442 security problems. 4443 4444 4445 Callas, et al. Expires Oct 24, 2007 [Page 78] 4446 INTERNET-DRAFT OpenPGP Message Format Apr 24, 2007 4448 4449 2. Implementers implement Modification Detection with all due 4450 speed and encourage its spread. 4451 4452 3. Users migrate to implementations that support Modification 4453 Detection with all due speed. 4454 4455 * PKCS#1 has been found to be vulnerable to attacks in which a 4456 system that reports errors in padding differently from errors in 4457 decryption becomes a random oracle that can leak the private key 4458 in mere millions of queries. Implementations must be aware of 4459 this attack and prevent it from happening. The simplest solution 4460 is report a single error code for all variants of decryption 4461 errors so as not to leak information to an attacker. 4462 4463 * Some technologies mentioned here may be subject to government 4464 control in some countries. 4465 4466 * In winter 2005, Serge Mister and Robert Zuccherato from Entrust 4467 released a paper describing a way that the "quick check" in 4468 OpenPGP CFB mode can be used with a random oracle to decrypt two 4469 octets of every cipher block [MZ05]. They recommend as 4470 prevention not using the quick check at all. 4471 4472 Many implementers have taken this advice to heart for any data 4473 that is symmetrically encrypted and for which the session key is 4474 public-key encrypted. In this case, the quick check is not 4475 needed as the public key encryption of the session key should 4476 guarantee that it is the right session key. In other cases, the 4477 implementation should use the quick check with care. 4478 4479 On the one hand, there is a danger to using it if there is a 4480 random oracle that can leak information to an attacker. In 4481 plainer language, there is a danger to using the quick check if 4482 timing information about the check can be exposed to an 4483 attacker, particularly via an automated service that allows 4484 rapidly repeated queries. 4485 4486 On the other hand, it is inconvenient to the user to be informed 4487 that they typed in the wrong passphrase only after a petabyte of 4488 data is decrypted. There are many cases in cryptographic 4489 engineering where the implementer must use care and wisdom, and 4490 this is one. 4491 4492 15. Implementation Nits 4493 4494 This section is a collection of comments to help an implementer, 4495 particularly with an eye to backward compatibility. Previous 4496 implementations of PGP are not OpenPGP-compliant. Often the 4497 differences are small, but small differences are frequently more 4498 vexing than large differences. Thus, this is a non-comprehensive 4499 list of potential problems and gotchas for a developer who is trying 4500 to be backward-compatible. 4501 4502 Callas, et al. Expires Oct 24, 2007 [Page 79] 4503 INTERNET-DRAFT OpenPGP Message Format Apr 24, 2007 4505 4506 * The IDEA algorithm is patented, and yet it is required for PGP 4507 2.x interoperability. It is also the de-facto preferred 4508 algorithm for a V3 key with a V3 self-signature (or no 4509 self-signature). 4510 4511 * When exporting a private key, PGP 2.x generates the header 4512 "BEGIN PGP SECRET KEY BLOCK" instead of "BEGIN PGP PRIVATE KEY 4513 BLOCK". All previous versions ignore the implied data type, and 4514 look directly at the packet data type. 4515 4516 * PGP 2.0 through 2.5 generated V2 Public Key Packets. These are 4517 identical to the deprecated V3 keys except for the version 4518 number. An implementation MUST NOT generate them and may accept 4519 or reject them as it sees fit. Some older PGP versions generated 4520 V2 PKESK packets (Tag 1) as well. An implementation may accept 4521 or reject V2 PKESK packets as it sees fit, and MUST NOT generate 4522 them. 4523 4524 * PGP 2.6.x will not accept key-material packets with versions 4525 greater than 3. 4526 4527 * There are many ways possible for two keys to have the same key 4528 material, but different fingerprints (and thus key IDs). Perhaps 4529 the most interesting is an RSA key that has been "upgraded" to 4530 V4 format, but since a V4 fingerprint is constructed by hashing 4531 the key creation time along with other things, two V4 keys 4532 created at different times, yet with the same key material will 4533 have different fingerprints. 4534 4535 * If an implementation is using zlib to interoperate with PGP 2.x, 4536 then the "windowBits" parameter should be set to -13. 4537 4538 * The 0x19 back signatures were not required for signing subkeys 4539 until relatively recently. Consquently, there may be keys in the 4540 wild that do not have these back signatures. Implementing 4541 software may handle these keys as it sees fit. 4542 4543 16. Authors' Addresses 4544 4545 The working group can be contacted via the current chair: 4546 4547 Derek Atkins 4548 IHTFP Consulting, Inc. 4549 6 Farragut Ave 4550 Somerville, MA 02144 USA 4551 Email: derek (a] ihtfp.com 4552 Tel: +1 617 623 3745 4553 4554 The principal authors of this draft are: 4555 4556 4557 4558 4559 Callas, et al. Expires Oct 24, 2007 [Page 80] 4560 INTERNET-DRAFT OpenPGP Message Format Apr 24, 2007 4562 4563 Jon Callas 4564 Email: jon (a] callas.org 4565 4566 Lutz Donnerhacke 4567 IKS GmbH 4568 Wildenbruchstr. 15 4569 07745 Jena, Germany 4570 4571 EMail: lutz (a] iks-jena.de 4572 4573 Hal Finney 4574 Email: hal (a] finney.org 4575 4576 David Shaw 4577 Email: dshaw (a] jabberwocky.com 4578 4579 Rodney Thayer 4580 Email: rodney (a] canola-jones.com 4581 4582 This memo also draws on much previous work from a number of other 4583 authors who include: Derek Atkins, Charles Breed, Dave Del Torto, 4584 Marc Dyksterhouse, Gail Haspert, Gene Hoffman, Paul Hoffman, Ben 4585 Laurie, Raph Levien, Colin Plumb, Will Price, David Shaw, William 4586 Stallings, Mark Weaver, and Philip R. Zimmermann. 4587 4588 17. References (Normative) 4589 4590 4591 [AES] NIST, FIPS PUB 197, "Advanced Encryption Standard 4592 (AES)," November 2001. 4593 4594 http://csrc.nist.gov/publications/fips/fips197/ 4595 fips-197.{ps,pdf} 4596 4597 [BLOWFISH] Schneier, B. "Description of a New Variable-Length 4598 Key, 64-Bit Block Cipher (Blowfish)" Fast Software 4599 Encryption, Cambridge Security Workshop Proceedings 4600 (December 1993), Springer-Verlag, 1994, pp191-204 4601 <http://www.counterpane.com/bfsverlag.html> 4602 4603 [BZ2] J. Seward, jseward (a] acm.org, "The Bzip2 and libbzip2 4604 home page" <http://www.bzip.org/> 4605 4606 [ELGAMAL] T. Elgamal, "A Public-Key Cryptosystem and a 4607 Signature Scheme Based on Discrete Logarithms," 4608 IEEE Transactions on Information Theory, v. IT-31, 4609 n. 4, 1985, pp. 469-472. 4610 4611 [FIPS180] Secure Hash Signature Standard (SHS) (FIPS PUB 4612 180-2). 4613 <http://csrc.nist.gov/publications/fips/ 4614 fips180-2/fips180-2withchangenotice.pdf> 4615 4616 Callas, et al. Expires Oct 24, 2007 [Page 81] 4617 INTERNET-DRAFT OpenPGP Message Format Apr 24, 2007 4619 4620 [FIPS186] Digital Signature Standard (DSS) (FIPS PUB 186-2). 4621 <http://csrc.nist.gov/publications/fips/fips186-2/ 4622 fips186-2-change1.pdf> 4623 FIPS 186-3 describes keys greater than 1024 bits. 4624 The latest draft is at: 4625 <http://csrc.nist.gov/publications/drafts/ 4626 fips_186-3/Draft-FIPS-186-3%20_March2006.pdf> 4627 4628 [HAC] Alfred Menezes, Paul van Oorschot, and Scott 4629 Vanstone, "Handbook of Applied Cryptography," CRC 4630 Press, 1996. 4631 <http://www.cacr.math.uwaterloo.ca/hac/> 4632 4633 [IDEA] Lai, X, "On the design and security of block 4634 ciphers", ETH Series in Information Processing, 4635 J.L. Massey (editor), Vol. 1, Hartung-Gorre Verlag 4636 Knostanz, Technische Hochschule (Zurich), 1992 4637 4638 [ISO10646] ISO/IEC 10646-1:1993. International Standard -- 4639 Information technology -- Universal Multiple-Octet 4640 Coded Character Set (UCS) -- Part 1: Architecture 4641 and Basic Multilingual Plane. 4642 4643 [JFIF] JPEG File Interchange Format (Version 1.02). 4644 Eric Hamilton, C-Cube Microsystems, Milpitas, CA, 4645 September 1, 1992. 4646 4647 [RFC1991] Atkins, D., Stallings, W. and P. Zimmermann, "PGP 4648 Message Exchange Formats", RFC 1991, August 1996. 4649 4650 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 4651 Requirement Level", BCP 14, RFC 2119, March 1997. 4652 [RFC2045] Borenstein, N. and N. Freed, "Multipurpose Internet 4653 Mail Extensions (MIME) Part One: Format of Internet 4654 Message Bodies.", RFC 2045, November 1996. 4655 4656 [RFC2144] Adams, C., "The CAST-128 Encryption Algorithm", RFC 4657 2144, May 1997. 4658 4659 [RFC2434] Narten, T. and H. Alvestrand, "Guidelines for 4660 Writing an IANA Considerations Section in RFCs", 4661 BCP 26, RFC 2434, October 1998. 4662 [RFC2822] Resnick, P., "Internet Message Format", RFC 2822. 4663 4664 [RFC3156] M. Elkins, D. Del Torto, R. Levien, T. Roessler, 4665 "MIME Security with OpenPGP", RFC 3156, 4666 August 2001. 4667 4668 [RFC3447] B. Kaliski and J. Staddon, "PKCS #1: RSA 4669 Cryptography Specifications Version 2.1", 4670 RFC 3447, February 2003. 4671 4672 4673 Callas, et al. Expires Oct 24, 2007 [Page 82] 4674 INTERNET-DRAFT OpenPGP Message Format Apr 24, 2007 4676 4677 [RFC3629] Yergeau., F., "UTF-8, a transformation format of 4678 Unicode and ISO 10646", RFC 3629, November 2003. 4679 4680 [RFC4086] Eastlake, D., Crocker, S. and J. Schiller, 4681 "Randomness Recommendations for Security", RFC 4682 4086, June 2005. 4683 4684 [SCHNEIER] Schneier, B., "Applied Cryptography Second Edition: 4685 protocols, algorithms, and source code in C", 1996. 4686 4687 [TWOFISH] B. Schneier, J. Kelsey, D. Whiting, D. Wagner, C. 4688 Hall, and N. Ferguson, "The Twofish Encryption 4689 Algorithm", John Wiley & Sons, 1999. 4690 4691 4692 18. References (Informative) 4693 4694 4695 [BLEICHENBACHER] Bleichenbacher, Daniel, "Generating Elgamal 4696 signatures without knowing the secret key," 4697 Eurocrypt 96. Note that the version in the 4698 proceedings has an error. A revised version is 4699 available at the time of writing from 4700 <ftp://ftp.inf.ethz.ch/pub/publications/papers/ti 4701 /isc/ElGamal.ps> 4702 4703 [JKS02] Kahil Jallad, Jonathan Katz, Bruce Schneier 4704 "Implementation of Chosen-Ciphertext Attacks 4705 against PGP and GnuPG" 4706 http://www.counterpane.com/pgp-attack.html 4707 4708 [MAURER] Ueli Maurer, "Modelling a Public-Key 4709 Infrastructure", Proc. 1996 European Symposium on 4710 Research in Computer Security (ESORICS' 96), 4711 Lecture Notes in Computer Science, Springer-Verlag, 4712 vol. 1146, pp. 325-350, Sep 1996. 4713 4714 [MZ05] Serge Mister, Robert Zuccherato, "An Attack on 4715 CFB Mode Encryption As Used By OpenPGP," IACR 4716 ePrint Archive: Report 2005/033, 8 Feb 2005 4717 http://eprint.iacr.org/2005/033 4718 4719 [RFC1423] Balenson, D., "Privacy Enhancement for Internet 4720 Electronic Mail: Part III: Algorithms, Modes, and 4721 Identifiers", RFC 1423, October 1993. 4722 4723 [RFC1951] Deutsch, P., "DEFLATE Compressed Data Format 4724 Specification version 1.3.", RFC 1951, May 1996. 4725 4726 [RFC2440] Callas, J., Donnerhacke, L., Finney, H., and 4727 Thayer, R. "OpenPGP Message Format", RFC 2440, 4728 November, 1998. 4729 4730 Callas, et al. Expires Oct 24, 2007 [Page 83] 4731 INTERNET-DRAFT OpenPGP Message Format Apr 24, 2007 4733 4734 [SP800-57] NIST Special Publication 800-57, Recommendation on 4735 Key Management 4736 <http://csrc.nist.gov/publications/nistpubs/ 4737 800-57/SP800-57-Part1.pdf> 4738 <http://csrc.nist.gov/publications/nistpubs/ 4739 800-57/SP800-57-Part2.pdf> 4740 4741 4742 19. Full Copyright Statement 4743 4744 Copyright (C) 2007 by The IETF Trust. 4745 4746 This document is subject to the rights, licenses and restrictions 4747 contained in BCP 78, and except as set forth therein, the authors 4748 retain all their rights. 4749 4750 This document and the information contained herein are provided on 4751 an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE 4752 REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE 4753 IETF TRUST AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL 4754 WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY 4755 WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE 4756 ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS 4757 FOR A PARTICULAR PURPOSE. 4758 4759 This document and translations of it may be copied and furnished to 4760 others, and derivative works that comment on or otherwise explain it 4761 or assist in its implementation may be prepared, copied, published 4762 and distributed, in whole or in part, without restriction of any 4763 kind, provided that the above copyright notice and this paragraph 4764 are included on all such copies and derivative works. However, this 4765 document itself may not be modified in any way, such as by removing 4766 the copyright notice or references to the Internet Society or other 4767 Internet organizations, except as needed for the purpose of 4768 developing Internet standards in which case the procedures for 4769 copyrights defined in the Internet Standards process must be 4770 followed, or as required to translate it into languages other than 4771 English. 4772 4773 The limited permissions granted above are perpetual and will not be 4774 revoked by the Internet Society or its successors or assigns. 4775 4776 20. Intellectual Property 4777 4778 The IETF takes no position regarding the validity or scope of any 4779 Intellectual Property Rights or other rights that might be claimed 4780 to pertain to the implementation or use of the technology described 4781 in this document or the extent to which any license under such 4782 rights might or might not be available; nor does it represent that 4783 it has made any independent effort to identify any such rights. 4784 Information on the procedures with respect to rights in RFC 4785 documents can be found in BCP 78 and BCP 79. 4786 4787 Callas, et al. Expires Oct 24, 2007 [Page 84] 4788 INTERNET-DRAFT OpenPGP Message Format Apr 24, 2007 4790 4791 Copies of IPR disclosures made to the IETF Secretariat and any 4792 assurances of licenses to be made available, or the result of an 4793 attempt made to obtain a general license or permission for the use 4794 of such proprietary rights by implementers or users of this 4795 specification can be obtained from the IETF on-line IPR repository 4796 at http://www.ietf.org/ipr. 4797 4798 The IETF invites any interested party to bring to its attention any 4799 copyrights, patents or patent applications, or other proprietary 4800 rights that may cover technology that may be required to implement 4801 this standard. Please address the information to the IETF at 4802 ietf-ipr (a] ietf.org. 4803 4804 4805 4806 4807 4808 4809 4810 4811 4812 4813 4814 4815 4816 4817 4818 4819 4820 4821 4822 4823 4824 4825 4826 4827 4828 4829 4830 4831 4832 4833 4834 4835 4836 4837 4838 4839 4840 4841 4842 4843 4844 Callas, et al. Expires Oct 24, 2007 [Page 85] 4845 4846 4847
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