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, 8 insertions, 5 deletions

diff --git a/ssh_config.5 b/ssh_config.5
index e5eadcaaf..eff9c5e61 100644
--- a/ssh_config.5
+++ b/ssh_config.5
@@ -33,8 +33,8 @@
 .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 .\"
-.\" $OpenBSD: ssh_config.5,v 1.277 2018/06/09 06:36:31 jmc Exp $
-.Dd $Mdocdate: June 9 2018 $
+.\" $OpenBSD: ssh_config.5,v 1.278 2018/07/03 11:39:54 djm Exp $
+.Dd $Mdocdate: July 3 2018 $
 .Dt SSH_CONFIG 5
 .Os
 .Sh NAME
@@ -772,9 +772,10 @@ ecdsa-sha2-nistp256-cert-v01@openssh.com,
 ecdsa-sha2-nistp384-cert-v01@openssh.com,
 ecdsa-sha2-nistp521-cert-v01@openssh.com,
 ssh-ed25519-cert-v01@openssh.com,
+rsa-sha2-512-cert-v01@openssh.com,rsa-sha2-256-cert-v01@openssh.com,
 ssh-rsa-cert-v01@openssh.com,
 ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
-ssh-ed25519,ssh-rsa
+ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa
 .Ed
 .Pp
 The
@@ -799,9 +800,10 @@ ecdsa-sha2-nistp256-cert-v01@openssh.com,
 ecdsa-sha2-nistp384-cert-v01@openssh.com,
 ecdsa-sha2-nistp521-cert-v01@openssh.com,
 ssh-ed25519-cert-v01@openssh.com,
+rsa-sha2-512-cert-v01@openssh.com,rsa-sha2-256-cert-v01@openssh.com,
 ssh-rsa-cert-v01@openssh.com,
 ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
-ssh-ed25519,ssh-rsa
+ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa
 .Ed
 .Pp
 If hostkeys are known for the destination host then this default is modified
@@ -1255,9 +1257,10 @@ ecdsa-sha2-nistp256-cert-v01@openssh.com,
 ecdsa-sha2-nistp384-cert-v01@openssh.com,
 ecdsa-sha2-nistp521-cert-v01@openssh.com,
 ssh-ed25519-cert-v01@openssh.com,
+rsa-sha2-512-cert-v01@openssh.com,rsa-sha2-256-cert-v01@openssh.com,
 ssh-rsa-cert-v01@openssh.com,
 ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
-ssh-ed25519,ssh-rsa
+ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa
 .Ed
 .Pp
 The list of available key types may also be obtained using