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Annotation of src/usr.bin/ssh/PROTOCOL.sshsig, Revision 1.1

1.1     ! djm         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 RFC4243 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: