Secure Shell Maintenance S. Kinne
Internet-Draft
Intended status: Standards Track D. Miller
Expires: 22 September 2025 OpenSSH
S. Josefsson, Ed.
21 March 2025
Lightweight Secure Shell (SSH) Signature Format
draft-josefsson-sshsig-format-01
Abstract
This document describes a lightweight SSH Signature format that is
compatible with SSH keys and wire formats.
About This Document
This note is to be removed before publishing as an RFC.
Status information for this document may be found at
https://datatracker.ietf.org/doc/draft-josefsson-sshsig-format/.
Discussion of this document takes place on the SSHM Working Group
mailing list (mailto:ssh@ietf.org), which is archived at
https://mailarchive.ietf.org/arch/browse/ssh/.
Source for this draft and an issue tracker can be found at
https://gitlab.com/jas/ietf-sshsig-format.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on 22 September 2025.
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Copyright Notice
Copyright (c) 2025 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
extracted from this document must include Revised BSD License text as
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provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions Used In This Document . . . . . . . . . . . . . . 2
3. Armored format . . . . . . . . . . . . . . . . . . . . . . . 3
4. Blob format . . . . . . . . . . . . . . . . . . . . . . . . . 3
5. Signed Data, of which the signature goes into the blob
above . . . . . . . . . . . . . . . . . . . . . . . . . . 4
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
7. Security Considerations . . . . . . . . . . . . . . . . . . . 5
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 5
9. Implementation Status . . . . . . . . . . . . . . . . . . . . 5
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
10.1. Normative References . . . . . . . . . . . . . . . . . . 6
10.2. Informative References . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7
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].
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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>
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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
[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.
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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.
This signature format embeds the public key, which is usually already
available for a verifier to make trust decisions. When verifying a
signature, care must be made to not unintentionally use the public
key within the signature for trust decisions.
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.
9. 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 [RFC7942].
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
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exist.
According to [RFC7942], "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
10. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
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[RFC4251] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
Protocol Architecture", RFC 4251, DOI 10.17487/RFC4251,
January 2006, <https://www.rfc-editor.org/info/rfc4251>.
[RFC4253] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
Transport Layer Protocol", RFC 4253, DOI 10.17487/RFC4253,
January 2006, <https://www.rfc-editor.org/info/rfc4253>.
[RFC5656] Stebila, D. and J. Green, "Elliptic Curve Algorithm
Integration in the Secure Shell Transport Layer",
RFC 5656, DOI 10.17487/RFC5656, December 2009,
<https://www.rfc-editor.org/info/rfc5656>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8709] Harris, B. and L. Velvindron, "Ed25519 and Ed448 Public
Key Algorithms for the Secure Shell (SSH) Protocol",
RFC 8709, DOI 10.17487/RFC8709, February 2020,
<https://www.rfc-editor.org/info/rfc8709>.
10.2. Informative References
[RFC7942] Sheffer, Y. and A. Farrel, "Improving Awareness of Running
Code: The Implementation Status Section", BCP 205,
RFC 7942, DOI 10.17487/RFC7942, July 2016,
<https://www.rfc-editor.org/info/rfc7942>.
Authors' Addresses
Sebastian Kinne
Email: skinne@google.com
Damien Miller
OpenSSH
Email: djm@mindrot.org
Simon Josefsson (editor)
Email: simon@josefsson.org
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