1 This document describes a lightweight SSH Signature format 2 that is compatible with SSH keys and wire formats. 3 4 At present, only detached and armored signatures are supported. 5 6 1. Armored format 7 8 The Armored SSH signatures consist of a header, a base64 9 encoded blob, and a footer. 10 11 The header is the string "-----BEGIN SSH SIGNATURE-----" 12 followed by a newline. The footer is the string 13 "-----END SSH SIGNATURE-----" immediately after a newline. 14 15 The header MUST be present at the start of every signature. 16 Files containing the signature MUST start with the header. 17 Likewise, the footer MUST be present at the end of every 18 signature. 19 20 The base64 encoded blob SHOULD be broken up by newlines 21 every 76 characters. 22 23 Example: 24 25 -----BEGIN SSH SIGNATURE----- 26 U1NIU0lHAAAAAQAAADMAAAALc3NoLWVkMjU1MTkAAAAgJKxoLBJBivUPNTUJUSslQTt2hD 27 jozKvHarKeN8uYFqgAAAADZm9vAAAAAAAAAFMAAAALc3NoLWVkMjU1MTkAAABAKNC4IEbt 28 Tq0Fb56xhtuE1/lK9H9RZJfON4o6hE9R4ZGFX98gy0+fFJ/1d2/RxnZky0Y7GojwrZkrHT 29 FgCqVWAQ== 30 -----END SSH SIGNATURE----- 31 32 2. Blob format 33 34 #define MAGIC_PREAMBLE "SSHSIG" 35 #define SIG_VERSION 0x01 36 37 byte[6] MAGIC_PREAMBLE 38 uint32 SIG_VERSION 39 string publickey 40 string namespace 41 string reserved 42 string hash_algorithm 43 string signature 44 45 The publickey field MUST contain the serialisation of the 46 public key used to make the signature using the usual SSH 47 encoding rules, i.e RFC4253, RFC5656, 48 draft-ietf-curdle-ssh-ed25519-ed448, etc. 49 50 Verifiers MUST reject signatures with versions greater than those 51 they support. 52 53 The purpose of the namespace value is to specify a unambiguous 54 interpretation domain for the signature, e.g. file signing. 55 This prevents cross-protocol attacks caused by signatures 56 intended for one intended domain being accepted in another. 57 The namespace value MUST NOT be the empty string. 58 59 The reserved value is present to encode future information 60 (e.g. tags) into the signature. Implementations should ignore 61 the reserved field if it is not empty. 62 63 Data to be signed is first hashed with the specified hash_algorithm. 64 This is done to limit the amount of data presented to the signature 65 operation, which may be of concern if the signing key is held in limited 66 or slow hardware or on a remote ssh-agent. The supported hash algorithms 67 are "sha256" and "sha512". 68 69 The signature itself is made using the SSH signature algorithm and 70 encoding rules for the chosen key type. For RSA signatures, the 71 signature algorithm must be "rsa-sha2-512" or "rsa-sha2-256" (i.e. 72 not the legacy RSA-SHA1 "ssh-rsa"). 73 74 This blob is encoded as a string using the RFC4253 encoding 75 rules and base64 encoded to form the middle part of the 76 armored signature. 77 78 79 3. Signed Data, of which the signature goes into the blob above 80 81 #define MAGIC_PREAMBLE "SSHSIG" 82 83 byte[6] MAGIC_PREAMBLE 84 string namespace 85 string reserved 86 string hash_algorithm 87 string H(message) 88 89 The preamble is the six-byte sequence "SSHSIG". It is included to 90 ensure that manual signatures can never be confused with any message 91 signed during SSH user or host authentication. 92 93 The reserved value is present to encode future information 94 (e.g. tags) into the signature. Implementations should ignore 95 the reserved field if it is not empty. 96 97 The data is concatenated and passed to the SSH signing 98 function. 99 100 $OpenBSD: PROTOCOL.sshsig,v 1.4 2020/08/31 00:17:41 djm Exp $ 101
Indexes created Mon Apr 06 00:22:49 UTC 2026