Secure Shell Maintenance D. Miller Internet-Draft OpenSSH Intended status: Standards Track S. Josefsson, Ed. Expires: 12 August 2025 8 February 2025 Lightweight Secure Shell (SSH) Signature Format draft-josefsson-sshsig-format-00 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 working documents as Internet-Drafts. The list of current Internet- Drafts is at https://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on 12 August 2025. Miller & Josefsson Expires 12 August 2025 [Page 1] Internet-Draft SSHSIG February 2025 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 described in Section 4.e of the Trust Legal Provisions and are 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. Normative References . . . . . . . . . . . . . . . . . . . . 5 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 6 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]. Miller & Josefsson Expires 12 August 2025 [Page 2] Internet-Draft SSHSIG February 2025 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 #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 Miller & Josefsson Expires 12 August 2025 [Page 3] Internet-Draft SSHSIG February 2025 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 #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. Miller & Josefsson Expires 12 August 2025 [Page 4] Internet-Draft SSHSIG February 2025 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. 9. 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, . [RFC4251] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH) Protocol Architecture", RFC 4251, DOI 10.17487/RFC4251, January 2006, . [RFC4253] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH) Transport Layer Protocol", RFC 4253, DOI 10.17487/RFC4253, January 2006, . Miller & Josefsson Expires 12 August 2025 [Page 5] Internet-Draft SSHSIG February 2025 [RFC5656] Stebila, D. and J. Green, "Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer", RFC 5656, DOI 10.17487/RFC5656, December 2009, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . [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, . Authors' Addresses Damien Miller OpenSSH Email: djm@mindrot.org Simon Josefsson (editor) Email: simon@josefsson.org Miller & Josefsson Expires 12 August 2025 [Page 6]