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, 16 insertions, 4 deletions

diff --git a/PROTOCOL.certkeys b/PROTOCOL.certkeys
index 65f11f538..11363fdc3 100644
--- a/PROTOCOL.certkeys
+++ b/PROTOCOL.certkeys
@@ -25,6 +25,10 @@ raw user keys. The ssh client will support automatic verification of
 acceptance of certified host keys, by adding a similar ability to
 specify CA keys in ~/.ssh/known_hosts.
 
+All certificate types include certification information along with the
+public key that is used to sign challenges. In OpenSSH, ssh-keygen
+performs the CA signing operation.
+
 Certified keys are represented using new key types:
 
     ssh-rsa-cert-v01@openssh.com
@@ -33,9 +37,17 @@ Certified keys are represented using new key types:
     ecdsa-sha2-nistp384-cert-v01@openssh.com
     ecdsa-sha2-nistp521-cert-v01@openssh.com
 
-These include certification information along with the public key
-that is used to sign challenges. ssh-keygen performs the CA signing
-operation.
+Two additional types exist for RSA certificates to force use of
+SHA-2 signatures (SHA-256 and SHA-512 respectively):
+
+    rsa-sha2-256-cert-v01@openssh.com
+    rsa-sha2-512-cert-v01@openssh.com
+
+These RSA/SHA-2 types should not appear in keys at rest or transmitted
+on their wire, but do appear in a SSH_MSG_KEXINIT's host-key algorithms
+field or in the "public key algorithm name" field of a "publickey"
+SSH_USERAUTH_REQUEST to indicate that the signature will use the
+specified algorithm.
 
 Protocol extensions
 -------------------
@@ -291,4 +303,4 @@ permit-user-rc          empty         Flag indicating that execution of
                                       of this script will not be permitted if
                                       this option is not present.
 
-$OpenBSD: PROTOCOL.certkeys,v 1.14 2018/04/10 00:10:49 djm Exp $
+$OpenBSD: PROTOCOL.certkeys,v 1.15 2018/07/03 11:39:54 djm Exp $