skinne@google.com

OpenSSH

djm@mindrot.org

simon@josefsson.org

IETF Secure Shell Maintenance SSH Secure Shell Signature secsh This document describes a lightweight SSH Signature format that is compatible with SSH keys and wire formats. Status information for this document may be found at . Discussion of this document takes place on the SSHM Working Group mailing list (), which is archived at . Source for this draft and an issue tracker can be found at .

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 ]]>

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) ]]>

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. 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 and RSA2-512, and implementations MUST NOT let library defaults chose the variant to use but instead instruct the library specifically which algorithm to use. 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

The Secure Shell (SSH) Protocol is a protocol for secure remote login and other secure network services over an insecure network. This document describes the architecture of the SSH protocol, as well as the notation and terminology used in SSH protocol documents. It also discusses the SSH algorithm naming system that allows local extensions. The SSH protocol consists of three major components: The Transport Layer Protocol provides server authentication, confidentiality, and integrity with perfect forward secrecy. The User Authentication Protocol authenticates the client to the server. The Connection Protocol multiplexes the encrypted tunnel into several logical channels. Details of these protocols are described in separate documents. [STANDARDS-TRACK] The Secure Shell (SSH) is a protocol for secure remote login and other secure network services over an insecure network. This document describes the SSH transport layer protocol, which typically runs on top of TCP/IP. The protocol can be used as a basis for a number of secure network services. It provides strong encryption, server authentication, and integrity protection. It may also provide compression. Key exchange method, public key algorithm, symmetric encryption algorithm, message authentication algorithm, and hash algorithm are all negotiated. This document also describes the Diffie-Hellman key exchange method and the minimal set of algorithms that are needed to implement the SSH transport layer protocol. [STANDARDS-TRACK] This document describes algorithms based on Elliptic Curve Cryptography (ECC) for use within the Secure Shell (SSH) transport protocol. In particular, it specifies Elliptic Curve Diffie-Hellman (ECDH) key agreement, Elliptic Curve Menezes-Qu-Vanstone (ECMQV) key agreement, and Elliptic Curve Digital Signature Algorithm (ECDSA) for use in the SSH Transport Layer protocol. [STANDARDS-TRACK] This document describes the use of the Ed25519 and Ed448 digital signature algorithms in the Secure Shell (SSH) protocol. Accordingly, this RFC updates RFC 4253. In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. This document describes a simple process that allows authors of Internet-Drafts to record the status of known implementations by including an Implementation Status section. 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. This process is not mandatory. Authors of Internet-Drafts are encouraged to consider using the process for their documents, and working groups are invited to think about applying the process to all of their protocol specifications. This document obsoletes RFC 6982, advancing it to a Best Current Practice.