1 =pod 2 3 =head1 NAME 4 5 des_modes - the variants of DES and other crypto algorithms of OpenSSL 6 7 =head1 DESCRIPTION 8 9 Several crypto algorithms for OpenSSL can be used in a number of modes. Those 10 are used for using block ciphers in a way similar to stream ciphers, among 11 other things. 12 13 =head1 OVERVIEW 14 15 =head2 Electronic Codebook Mode (ECB) 16 17 Normally, this is found as the function I<algorithm>_ecb_encrypt(). 18 19 =over 2 20 21 =item * 22 23 64 bits are enciphered at a time. 24 25 =item * 26 27 The order of the blocks can be rearranged without detection. 28 29 =item * 30 31 The same plaintext block always produces the same ciphertext block 32 (for the same key) making it vulnerable to a 'dictionary attack'. 33 34 =item * 35 36 An error will only affect one ciphertext block. 37 38 =back 39 40 =head2 Cipher Block Chaining Mode (CBC) 41 42 Normally, this is found as the function I<algorithm>_cbc_encrypt(). 43 Be aware that des_cbc_encrypt() is not really DES CBC (it does 44 not update the IV); use des_ncbc_encrypt() instead. 45 46 =over 2 47 48 =item * 49 50 a multiple of 64 bits are enciphered at a time. 51 52 =item * 53 54 The CBC mode produces the same ciphertext whenever the same 55 plaintext is encrypted using the same key and starting variable. 56 57 =item * 58 59 The chaining operation makes the ciphertext blocks dependent on the 60 current and all preceding plaintext blocks and therefore blocks can not 61 be rearranged. 62 63 =item * 64 65 The use of different starting variables prevents the same plaintext 66 enciphering to the same ciphertext. 67 68 =item * 69 70 An error will affect the current and the following ciphertext blocks. 71 72 =back 73 74 =head2 Cipher Feedback Mode (CFB) 75 76 Normally, this is found as the function I<algorithm>_cfb_encrypt(). 77 78 =over 2 79 80 =item * 81 82 a number of bits (j) <= 64 are enciphered at a time. 83 84 =item * 85 86 The CFB mode produces the same ciphertext whenever the same 87 plaintext is encrypted using the same key and starting variable. 88 89 =item * 90 91 The chaining operation makes the ciphertext variables dependent on the 92 current and all preceding variables and therefore j-bit variables are 93 chained together and can not be rearranged. 94 95 =item * 96 97 The use of different starting variables prevents the same plaintext 98 enciphering to the same ciphertext. 99 100 =item * 101 102 The strength of the CFB mode depends on the size of k (maximal if 103 j == k). In my implementation this is always the case. 104 105 =item * 106 107 Selection of a small value for j will require more cycles through 108 the encipherment algorithm per unit of plaintext and thus cause 109 greater processing overheads. 110 111 =item * 112 113 Only multiples of j bits can be enciphered. 114 115 =item * 116 117 An error will affect the current and the following ciphertext variables. 118 119 =back 120 121 =head2 Output Feedback Mode (OFB) 122 123 Normally, this is found as the function I<algorithm>_ofb_encrypt(). 124 125 =over 2 126 127 =item * 128 129 a number of bits (j) <= 64 are enciphered at a time. 130 131 =item * 132 133 The OFB mode produces the same ciphertext whenever the same 134 plaintext enciphered using the same key and starting variable. More 135 over, in the OFB mode the same key stream is produced when the same 136 key and start variable are used. Consequently, for security reasons 137 a specific start variable should be used only once for a given key. 138 139 =item * 140 141 The absence of chaining makes the OFB more vulnerable to specific attacks. 142 143 =item * 144 145 The use of different start variables values prevents the same 146 plaintext enciphering to the same ciphertext, by producing different 147 key streams. 148 149 =item * 150 151 Selection of a small value for j will require more cycles through 152 the encipherment algorithm per unit of plaintext and thus cause 153 greater processing overheads. 154 155 =item * 156 157 Only multiples of j bits can be enciphered. 158 159 =item * 160 161 OFB mode of operation does not extend ciphertext errors in the 162 resultant plaintext output. Every bit error in the ciphertext causes 163 only one bit to be in error in the deciphered plaintext. 164 165 =item * 166 167 OFB mode is not self-synchronizing. If the two operation of 168 encipherment and decipherment get out of synchronism, the system needs 169 to be re-initialized. 170 171 =item * 172 173 Each re-initialization should use a value of the start variable 174 different from the start variable values used before with the same 175 key. The reason for this is that an identical bit stream would be 176 produced each time from the same parameters. This would be 177 susceptible to a 'known plaintext' attack. 178 179 =back 180 181 =head2 Triple ECB Mode 182 183 Normally, this is found as the function I<algorithm>_ecb3_encrypt(). 184 185 =over 2 186 187 =item * 188 189 Encrypt with key1, decrypt with key2 and encrypt with key3 again. 190 191 =item * 192 193 As for ECB encryption but increases the key length to 168 bits. 194 There are theoretic attacks that can be used that make the effective 195 key length 112 bits, but this attack also requires 2^56 blocks of 196 memory, not very likely, even for the NSA. 197 198 =item * 199 200 If both keys are the same it is equivalent to encrypting once with 201 just one key. 202 203 =item * 204 205 If the first and last key are the same, the key length is 112 bits. 206 There are attacks that could reduce the effective key strength 207 to only slightly more than 56 bits, but these require a lot of memory. 208 209 =item * 210 211 If all 3 keys are the same, this is effectively the same as normal 212 ecb mode. 213 214 =back 215 216 =head2 Triple CBC Mode 217 218 Normally, this is found as the function I<algorithm>_ede3_cbc_encrypt(). 219 220 =over 2 221 222 =item * 223 224 Encrypt with key1, decrypt with key2 and then encrypt with key3. 225 226 =item * 227 228 As for CBC encryption but increases the key length to 168 bits with 229 the same restrictions as for triple ecb mode. 230 231 =back 232 233 =head1 NOTES 234 235 This text was been written in large parts by Eric Young in his original 236 documentation for SSLeay, the predecessor of OpenSSL. In turn, he attributed 237 it to: 238 239 AS 2805.5.2 240 Australian Standard 241 Electronic funds transfer - Requirements for interfaces, 242 Part 5.2: Modes of operation for an n-bit block cipher algorithm 243 Appendix A 244 245 =head1 SEE ALSO 246 247 L<BF_encrypt(3)>, L<DES_crypt(3)> 248 249 =head1 COPYRIGHT 250 251 Copyright 2000-2017 The OpenSSL Project Authors. All Rights Reserved. 252 253 Licensed under the Apache License 2.0 (the "License"). You may not use 254 this file except in compliance with the License. You can obtain a copy 255 in the file LICENSE in the source distribution or at 256 L<https://www.openssl.org/source/license.html>. 257 258 =cut 259
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