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100 lines
3.3 KiB
Plaintext
100 lines
3.3 KiB
Plaintext
This document describes a lightweight SSH Signature format
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that is compatible with SSH keys and wire formats.
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At present, only detached and armored signatures are supported.
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1. Armored format
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The Armored SSH signatures consist of a header, a base64
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encoded blob, and a footer.
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The header is the string "-----BEGIN SSH SIGNATURE-----"
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followed by a newline. The footer is the string
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"-----END SSH SIGNATURE-----" immediately after a newline.
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The header MUST be present at the start of every signature.
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Files containing the signature MUST start with the header.
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Likewise, the footer MUST be present at the end of every
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signature.
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The base64 encoded blob SHOULD be broken up by newlines
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every 76 characters.
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Example:
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-----BEGIN SSH SIGNATURE-----
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U1NIU0lHAAAAAQAAADMAAAALc3NoLWVkMjU1MTkAAAAgJKxoLBJBivUPNTUJUSslQTt2hD
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jozKvHarKeN8uYFqgAAAADZm9vAAAAAAAAAFMAAAALc3NoLWVkMjU1MTkAAABAKNC4IEbt
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Tq0Fb56xhtuE1/lK9H9RZJfON4o6hE9R4ZGFX98gy0+fFJ/1d2/RxnZky0Y7GojwrZkrHT
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FgCqVWAQ==
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-----END SSH SIGNATURE-----
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2. Blob format
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#define MAGIC_PREAMBLE "SSHSIG"
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#define SIG_VERSION 0x01
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byte[6] MAGIC_PREAMBLE
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uint32 SIG_VERSION
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string publickey
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string namespace
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string reserved
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string hash_algorithm
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string signature
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The publickey field MUST contain the serialisation of the
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public key used to make the signature using the usual SSH
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encoding rules, i.e RFC4253, RFC5656,
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draft-ietf-curdle-ssh-ed25519-ed448, etc.
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Verifiers MUST reject signatures with versions greater than those
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they support.
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The purpose of the namespace value is to specify a unambiguous
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interpretation domain for the signature, e.g. file signing.
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This prevents cross-protocol attacks caused by signatures
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intended for one intended domain being accepted in another.
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The namespace value MUST NOT be the empty string.
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The reserved value is present to encode future information
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(e.g. tags) into the signature. Implementations should ignore
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the reserved field if it is not empty.
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Data to be signed is first hashed with the specified hash_algorithm.
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This is done to limit the amount of data presented to the signature
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operation, which may be of concern if the signing key is held in limited
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or slow hardware or on a remote ssh-agent. The supported hash algorithms
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are "sha256" and "sha512".
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The signature itself is made using the SSH signature algorithm and
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encoding rules for the chosen key type. For RSA signatures, the
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signature algorithm must be "rsa-sha2-512" or "rsa-sha2-256" (i.e.
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not the legacy RSA-SHA1 "ssh-rsa").
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This blob is encoded as a string using the RFC4243 encoding
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rules and base64 encoded to form the middle part of the
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armored signature.
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3. Signed Data, of which the signature goes into the blob above
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#define MAGIC_PREAMBLE "SSHSIG"
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byte[6] MAGIC_PREAMBLE
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string namespace
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string reserved
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string hash_algorithm
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string H(message)
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The preamble is the six-byte sequence "SSHSIG". It is included to
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ensure that manual signatures can never be confused with any message
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signed during SSH user or host authentication.
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The reserved value is present to encode future information
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(e.g. tags) into the signature. Implementations should ignore
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the reserved field if it is not empty.
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The data is concatenated and passed to the SSH signing
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function.
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