upstream: Improve strictness and control over RSA-SHA2 signature

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@

OpenBSD-Commit-ID: c6e9f6d45eed8962ad502d315d7eaef32c419dde

1 files changed, 2 insertions, 1 deletions

diff --git a/ssherr.h b/ssherr.h
index c0b59211e..348da5a20 100644
--- a/ssherr.h
+++ b/ssherr.h
@@ -1,4 +1,4 @@
-/*      $OpenBSD: ssherr.h,v 1.5 2017/09/12 06:32:08 djm Exp $  */
+/*      $OpenBSD: ssherr.h,v 1.6 2018/07/03 11:39:54 djm Exp $  */
 /*
  * Copyright (c) 2011 Damien Miller
  *
@@ -79,6 +79,7 @@
 #define SSH_ERR_PROTOCOL_ERROR                  -55
 #define SSH_ERR_KEY_LENGTH                      -56
 #define SSH_ERR_NUMBER_TOO_LARGE                -57
+#define SSH_ERR_SIGN_ALG_UNSUPPORTED            -58
 
 /* Translate a numeric error code to a human-readable error string */
 const char *ssh_err(int n);