Lightweight Secure Shell (SSH) Signature Format

Introduction Secure Shell (SSH) is a secure remote-login protocol. It provides for an extensible variety of public key algorithms for identifying servers and users to one another. The SSH key and signature formats have found uses outside of the interactive online SSH protocol itself. This document specify these formats. At present, only detached and armored signatures are supported. It is suggested that when referring to this signature format that the term "SSHSIG" is used.

Conventions Used In This Document The descriptions of key and signature formats use the notation introduced in . The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here.

Armored format The Armored SSH signatures is ASCII-encoded and consists 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. Newlines here is defined as ASCII LF. Some text transfer mechanism may use other line delimiters, however when Armored SSHSIG signatures are stored in context which are input to cryptographic hashes or otherwise subject to bit-by-bit comparisons, implementations MUST use ASCII LF as the line delimiter to have one canonical representation. 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:

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

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The publickey field MUST contain the serialisation of the public key
used to make the signature using the usual SSH encoding rules, i.e
, , , 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 for this pupose are "sha256" and "sha512". (Signature
algorithms may use other hash algorithms internally.)

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  encoding rules and
base64 encoded to form the middle part of the armored signature.


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)

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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.


IANA Considerations

None


Security Considerations

The security considerations of all referenced specifications are
inherited.

Cryptographic algorithms and parameters are usually broken or weakened
over time.  Implementers and users need to continously re-evaluate
that cryptographic algorithms continue to provide the expected level
of security.

Implementations has to follow best practices to avoid security
concerns, and users needs to continously re-evaulate implementations
for security vulnerabilities.

SSH supports several Public Key Algorithms and new algorithms may be
introduced in the future.  This document does not limit their use in
SSHSIG.  Any SSH Public Key Algorithm is automatically available in
SSHSIG.  Therefor, applications relying on SSHSIG has to decide which
algorithms and parameters are acceptable in their particular use-case.
Things to consider include which algorithms to support (e.g., disallow
'ssh-dss' and 'null' and/or only allow 'ssh-ed25519'), key sizes
(e.g., 'rsa-sha2-256' key sizes below 2048 bits), algorithm properties
(e.g., disallow 'ssh-rsa' due to SHA-1), cryptographic features (e.g.,
malleability of the signature), performance characteristics, code
size, signature size, and so on.

This signature format embeds the public key, which is usually already
available for a verifier to perform the cryptographic verification
with and to make trust decisions.  When verifying a signature
cryptographically, it is RECOMMENDED to use the locally configured
public key rather than the public key provided in the signature.  A
bit-by-bit comparison of the public key could also be done.  The
public key within the signature should be treated as untrusted input,
but it may be used as an identifier to find the locally trusted public
key that can be used to verify the signature.

RSA public keys can be used with both SHA2-256 ('rsa-sha2-256') and
SHA2-512 ('rsa-sha2-512').  Implementations MUST NOT let library
defaults chose the variant to use but instead instruct the library
specifically which algorithm to use.  This is a generic concern, but
historically has only been relevant for RSA.

Some implementation do not explicitly require or validate that the
namespace is not empty.  We RECOMMEND that implementations reject
those signatures as invalid.  Comparing the namespace value before
performing the signature verification MAY be done to provide a better
error condition rather than a generic signature verification failure.


Acknowledgments

The text in this document is from PROTOCOL.sshsig from OpenSSH which
appears to have been contributed to by at least Sebastian Kinne,
Damien Miller, Markus Friedl, HARUYAMA Seigo, Pedro Martelletto, Paul
Tagliamonte, Hidde Beydals, and Castedo Ellerman.


Implementation Status

This section records the status of known implementations of the
protocol defined by this specification at the time of posting of this
Internet-Draft, and is based on a proposal described in .  The
description of implementations in this section is intended to assist
the IETF in its decision processes in progressing drafts to RFCs.
Please note that the listing of any individual implementation here
does not imply endorsement by the IETF.  Furthermore, no effort has
been spent to verify the information presented here that was supplied
by IETF contributors.  This is not intended as, and must not be
construed to be, a catalog of available implementations or their
features.  Readers are advised to note that other implementations may
exist.

According to , "this will allow reviewers and working groups
to assign due consideration to documents that have the benefit of
running code, which may serve as evidence of valuable experimentation
and feedback that have made the implemented protocols more mature.  It
is up to the individual working groups to use this information as they
see fit

The following example projects maintain an implementation of this
protocol:

OpenSSH: C implementation.

Website: https://www.openssh.com/

SSHSIGLIB: Python implementation by Castedo Ellerman.

Website: https://gitlab.com/perm.pub/sshsiglib

SSHSIGN-GO: Go implementation by Benjamin Pannell at SierraSoftworks.

Website: https://github.com/SierraSoftworks/sshsign-go

SSHSIG: Go implementation by Hidde Beydals.

Website: https://github.com/hiddeco/sshsig

GO-SSHSIG: Go implementation by Paul Tagliamonte.

Website: https://github.com/paultag/go-sshsig

REKOR-PKI-SSH: Go implementation by Sigstore/Rekor.

Website: https://github.com/sigstore/rekor/tree/v1.0.1/pkg/pki/ssh