Lightweight Secure Shell (SSH) Signature Format

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Table of Contents

1. Introduction

Secure Shell (SSH) [RFC4251] 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.¶

2. Conventions Used In This Document

The descriptions of key and signature formats use the notation introduced in [RFC4251].¶

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 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶

3. 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-----

4. Blob format

<CODE BEGINS>
#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

<CODE ENDS>

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], [RFC8709], 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.¶

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

<CODE BEGINS>
#define MAGIC_PREAMBLE "SSHSIG"

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

<CODE ENDS>

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.¶

6. IANA Considerations

None¶

7. 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.¶

8. 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, and Pedro Martelletto.¶

Authors' Addresses