Improve strictness and control over RSA-SHA2 signature types:

In ssh, when an agent fails to return a RSA-SHA2 signature when
requested and falls back to RSA-SHA1 instead, retry the signature to
ensure that the public key algorithm sent in the SSH_MSG_USERAUTH
matches the one in the signature itself.

In sshd, strictly enforce that the public key algorithm sent in the
SSH_MSG_USERAUTH message matches what appears in the signature.

Make the sshd_config PubkeyAcceptedKeyTypes and
HostbasedAcceptedKeyTypes options control accepted signature algorithms
(previously they selected supported key types). This allows these
options to ban RSA-SHA1 in favour of RSA-SHA2.

Add new signature algorithms "rsa-sha2-256-cert-v01@openssh.com" and
"rsa-sha2-512-cert-v01@openssh.com" to force use of RSA-SHA2 signatures
with certificate keys.

feedback and ok markus@

1 files changed, 7 insertions, 6 deletions

diff --git a/usr.bin/ssh/auth2-pubkey.c b/usr.bin/ssh/auth2-pubkey.c
index 7422ba8b9b8..490cef11407 100644
--- a/usr.bin/ssh/auth2-pubkey.c
+++ b/usr.bin/ssh/auth2-pubkey.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: auth2-pubkey.c,v 1.79 2018/06/06 18:29:18 markus Exp $ */
+/* $OpenBSD: auth2-pubkey.c,v 1.80 2018/07/03 11:39:54 djm Exp $ */
 /*
  * Copyright (c) 2000 Markus Friedl.  All rights reserved.
  *
@@ -106,7 +106,7 @@ userauth_pubkey(struct ssh *ssh)
 	pktype = sshkey_type_from_name(pkalg);
 	if (pktype == KEY_UNSPEC) {
 		/* this is perfectly legal */
-		logit("%s: unsupported public key algorithm: %s",
+		verbose("%s: unsupported public key algorithm: %s",
 		    __func__, pkalg);
 		goto done;
 	}
@@ -133,8 +133,7 @@ userauth_pubkey(struct ssh *ssh)
 		logit("refusing previously-used %s key", sshkey_type(key));
 		goto done;
 	}
-	if (match_pattern_list(sshkey_ssh_name(key),
-	    options.pubkey_key_types, 0) != 1) {
+	if (match_pattern_list(pkalg, options.pubkey_key_types, 0) != 1) {
 		logit("%s: key type %s not in PubkeyAcceptedKeyTypes",
 		    __func__, sshkey_ssh_name(key));
 		goto done;
@@ -185,8 +184,10 @@ userauth_pubkey(struct ssh *ssh)
 		/* test for correct signature */
 		authenticated = 0;
 		if (PRIVSEP(user_key_allowed(ssh, pw, key, 1, &authopts)) &&
-		    PRIVSEP(sshkey_verify(key, sig, slen, sshbuf_ptr(b),
-		    sshbuf_len(b), NULL, ssh->compat)) == 0) {
+		    PRIVSEP(sshkey_verify(key, sig, slen,
+		    sshbuf_ptr(b), sshbuf_len(b),
+		    (ssh->compat & SSH_BUG_SIGTYPE) == 0 ? pkalg : NULL,
+		    ssh->compat)) == 0) {
 			authenticated = 1;
 		}
 		sshbuf_free(b);