From 2a9c9f7272c1e8665155118fe6536bebdafb6166 Mon Sep 17 00:00:00 2001 From: "djm@openbsd.org" Date: Tue, 3 Sep 2019 08:34:19 +0000 Subject: upstream: sshsig: lightweight signature and verification ability for OpenSSH This adds a simple manual signature scheme to OpenSSH. Signatures can be made and verified using ssh-keygen -Y sign|verify Signatures embed the key used to make them. At verification time, this is matched via principal name against an authorized_keys-like list of allowed signers. Mostly by Sebastian Kinne w/ some tweaks by me ok markus@ OpenBSD-Commit-ID: 2ab568e7114c933346616392579d72be65a4b8fb --- PROTOCOL.sshsig | 99 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 99 insertions(+) create mode 100644 PROTOCOL.sshsig (limited to 'PROTOCOL.sshsig') diff --git a/PROTOCOL.sshsig b/PROTOCOL.sshsig new file mode 100644 index 00000000..806c35da --- /dev/null +++ b/PROTOCOL.sshsig @@ -0,0 +1,99 @@ +This document describes a lightweight SSH Signature format +that is compatible with SSH keys and wire formats. + +At present, only detached and armored signatures are supported. + +1. 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----- + +2. 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 RFC4253, RFC5656, +draft-ietf-curdle-ssh-ed25519-ed448, 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 RFC4243 encoding +rules and base64 encoded to form the middle part of the +armored signature. + + +3. 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. + -- cgit v1.2.1