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, 7 insertions, 4 deletions

diff --git a/sshd_config.5 b/sshd_config.5
index 60c5f4bd3..cc019ec7d 100644
--- a/sshd_config.5
+++ b/sshd_config.5
@@ -33,7 +33,7 @@
 .\" (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: sshd_config.5,v 1.278 2018/07/03 10:59:35 djm Exp $
+.\" $OpenBSD: sshd_config.5,v 1.279 2018/07/03 11:39:54 djm Exp $
 .Dd $Mdocdate: July 3 2018 $
 .Dt SSHD_CONFIG 5
 .Os
@@ -674,9 +674,10 @@ 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
@@ -751,9 +752,10 @@ 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
@@ -1399,9 +1401,10 @@ 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