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

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

diff --git a/PROTOCOL.u2f b/PROTOCOL.u2f
index 7b1049c3e9293d3440a648fa93fe5eec43d2c19b..4e3896419f1e21012e3cda096e9ed3999815577c 100644 (file)
--- a/PROTOCOL.u2f
+++ b/PROTOCOL.u2f
@@ -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:
 

diff --git a/ssh-ecdsa-sk.c b/ssh-ecdsa-sk.c
index 3559246578818c5e822831e8c3463580dcc5d70a..7bdecd58413f9c93644b3c1975d87e0a100977e6 100644 (file)
--- a/ssh-ecdsa-sk.c
+++ b/ssh-ecdsa-sk.c
@@ -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;
        }

diff --git a/ssh-sk.c b/ssh-sk.c
index df2f040efbbfe9fd0e74e8cd3e8affed0c3b7834..2b25c42ff759b0c653b7a9d3cdbf41d23af4dbbb 100644 (file)
--- a/ssh-sk.c
+++ b/ssh-sk.c
@@ -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;
        }