openssh/PROTOCOL.sshsig

This document describes a lightweight SSH Signature format
that is compatible with SSH keys and wire formats.

At present, only detached and armored signatures are supported.

1. Armored format

The Armored SSH signatures consist of a header, a base64
encoded blob, and a footer.

The header is the string "-----BEGIN SSH SIGNATURE-----"
followed by a newline. The footer is the string
"-----END SSH SIGNATURE-----" immediately after a newline.

The header MUST be present at the start of every signature.
Files containing the signature MUST start with the header.
Likewise, the footer MUST be present at the end of every
signature.

The base64 encoded blob SHOULD be broken up by newlines
every 76 characters.

Example:

-----BEGIN SSH SIGNATURE-----
U1NIU0lHAAAAAQAAADMAAAALc3NoLWVkMjU1MTkAAAAgJKxoLBJBivUPNTUJUSslQTt2hD
jozKvHarKeN8uYFqgAAAADZm9vAAAAAAAAAFMAAAALc3NoLWVkMjU1MTkAAABAKNC4IEbt
Tq0Fb56xhtuE1/lK9H9RZJfON4o6hE9R4ZGFX98gy0+fFJ/1d2/RxnZky0Y7GojwrZkrHT
FgCqVWAQ==
-----END SSH SIGNATURE-----

2. Blob format

#define MAGIC_PREAMBLE "SSHSIG"
#define SIG_VERSION    0x01

        byte[6]   MAGIC_PREAMBLE
        uint32    SIG_VERSION
        string    publickey
        string    namespace
        string    reserved
        string    hash_algorithm
        string    signature

The publickey field MUST contain the serialisation of the
public key used to make the signature using the usual SSH
encoding rules, i.e RFC4253, RFC5656,
draft-ietf-curdle-ssh-ed25519-ed448, etc.

Verifiers MUST reject signatures with versions greater than those
they support.

The purpose of the namespace value is to specify a unambiguous
interpretation domain for the signature, e.g. file signing.
This prevents cross-protocol attacks caused by signatures
intended for one intended domain being accepted in another.
The namespace value MUST NOT be the empty string.

The reserved value is present to encode future information
(e.g. tags) into the signature. Implementations should ignore
the reserved field if it is not empty.

Data to be signed is first hashed with the specified hash_algorithm.
This is done to limit the amount of data presented to the signature
operation, which may be of concern if the signing key is held in limited
or slow hardware or on a remote ssh-agent. The supported hash algorithms
are "sha256" and "sha512".

The signature itself is made using the SSH signature algorithm and
encoding rules for the chosen key type. For RSA signatures, the
signature algorithm must be "rsa-sha2-512" or "rsa-sha2-256" (i.e.
not the legacy RSA-SHA1 "ssh-rsa").

This blob is encoded as a string using the RFC4253 encoding
rules and base64 encoded to form the middle part of the
armored signature.


3. Signed Data, of which the signature goes into the blob above

#define MAGIC_PREAMBLE "SSHSIG"

        byte[6]   MAGIC_PREAMBLE
        string    namespace
        string    reserved
        string    hash_algorithm
        string    H(message)

The preamble is the six-byte sequence "SSHSIG". It is included to
ensure that manual signatures can never be confused with any message
signed during SSH user or host authentication.

The reserved value is present to encode future information
(e.g. tags) into the signature. Implementations should ignore
the reserved field if it is not empty.

The data is concatenated and passed to the SSH signing
function.