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upstream: adjust on-wire signature encoding for ecdsa-sk keys to

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better match ec25519-sk keys. Discussed with markus@ and Sebastian Kinne

NB. if you are depending on security keys (already?) then make sure you
update both your clients and servers.

OpenBSD-Commit-ID: 53d88d8211f0dd02a7954d3af72017b1a79c0679
This commit is contained in:
djm@openbsd.org 2019-11-19 22:23:19 +00:00 committed by Damien Miller
parent 26369a5f7d
commit a70d92f236
3 changed files with 18 additions and 15 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

13
PROTOCOL.u2f
View File

@ -175,15 +175,18 @@ The signature returned from U2F hardware takes the following format:
For use in the SSH protocol, we wish to avoid server-side parsing of ASN.1
format data in the pre-authentication attack surface. Therefore, the
signature format used on the wire in SSH2_USERAUTH_REQUEST packets will
be reformatted slightly and the ecdsa_signature_blob value has the encoding:
be reformatted to better match the existing signature encoding:
mpint r
mpint s
string "sk-ecdsa-sha2-nistp256@openssh.com"
string ecdsa_signature
byte flags
uint32 counter
Where 'r' and 's' are extracted by the client or token middleware from the
ecdsa_signature field returned from the hardware.
Where the "ecdsa_signature" field follows the RFC5656 ECDSA signature
encoding:
mpint r
mpint s
For Ed25519 keys the signature is encoded as:

10
ssh-ecdsa-sk.c
View File

@ -1,4 +1,4 @@
/* $OpenBSD: ssh-ecdsa-sk.c,v 1.1 2019/10/31 21:15:14 djm Exp $ */
/* $OpenBSD: ssh-ecdsa-sk.c,v 1.2 2019/11/19 22:23:19 djm Exp $ */
/*
* Copyright (c) 2000 Markus Friedl. All rights reserved.
* Copyright (c) 2010 Damien Miller. All rights reserved.
@ -77,7 +77,9 @@ ssh_ecdsa_sk_verify(const struct sshkey *key,
if ((b = sshbuf_from(signature, signaturelen)) == NULL)
return SSH_ERR_ALLOC_FAIL;
if (sshbuf_get_cstring(b, &ktype, NULL) != 0 ||
sshbuf_froms(b, &sigbuf) != 0) {
sshbuf_froms(b, &sigbuf) != 0 ||
sshbuf_get_u8(b, &sig_flags) != 0 ||
sshbuf_get_u32(b, &sig_counter) != 0) {
ret = SSH_ERR_INVALID_FORMAT;
goto out;
}
@ -92,9 +94,7 @@ ssh_ecdsa_sk_verify(const struct sshkey *key,
/* parse signature */
if (sshbuf_get_bignum2(sigbuf, &sig_r) != 0 ||
sshbuf_get_bignum2(sigbuf, &sig_s) != 0 ||
sshbuf_get_u8(sigbuf, &sig_flags) != 0 ||
sshbuf_get_u32(sigbuf, &sig_counter) != 0) {
sshbuf_get_bignum2(sigbuf, &sig_s) != 0) {
ret = SSH_ERR_INVALID_FORMAT;
goto out;
}

10
ssh-sk.c
View File

@ -1,4 +1,4 @@
/* $OpenBSD: ssh-sk.c,v 1.15 2019/11/18 16:08:57 naddy Exp $ */
/* $OpenBSD: ssh-sk.c,v 1.16 2019/11/19 22:23:19 djm Exp $ */
/*
* Copyright (c) 2019 Google LLC
*
@ -411,13 +411,13 @@ sshsk_ecdsa_sig(struct sk_sign_response *resp, struct sshbuf *sig)
if ((r = sshbuf_put_bignum2_bytes(inner_sig,
resp->sig_r, resp->sig_r_len)) != 0 ||
(r = sshbuf_put_bignum2_bytes(inner_sig,
resp->sig_s, resp->sig_s_len)) != 0 ||
(r = sshbuf_put_u8(inner_sig, resp->flags)) != 0 ||
(r = sshbuf_put_u32(inner_sig, resp->counter)) != 0) {
resp->sig_s, resp->sig_s_len)) != 0) {
debug("%s: buffer error: %s", __func__, ssh_err(r));
goto out;
}
if ((r = sshbuf_put_stringb(sig, inner_sig)) != 0) {
if ((r = sshbuf_put_stringb(sig, inner_sig)) != 0 ||
(r = sshbuf_put_u8(sig, resp->flags)) != 0 ||
(r = sshbuf_put_u32(sig, resp->counter)) != 0) {
debug("%s: buffer error: %s", __func__, ssh_err(r));
goto out;
}