ssh: fix RSA certificate and public key authentication with older clients

After adding support for rsa-sha2-256/512 on the server side some edge
cases started to arise with old clients:

1) public key authentication with gpg-agent < 2.2.6 fails because we
   receive ssh-rsa as signature format and rsa-sha2-256 or rsa-sha2-512
   as algorithm.
   This is a bug in gpg-agent fixed in this commit:

   https://github.com/gpg/gnupg/commit/80b775bdbb852aa4a80292c9357e5b1876110c00

2) certificate authentication fails with OpenSSH 7.2-7.7 because we
   receive ssh-rsa-cert-v01@openssh.com as algorithm and rsa-sha2-256
   or rsa-sha2-512 as signature format.

This patch is based on CL 412854 and has been tested with every version
of OpenSSH from 7.1 to 7.9 and OpenSSH 9.3.

Fixes golang/go#53391

Change-Id: Id71f596f73d84efb5c76d6d5388432cccad3e3b1
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/506835
Auto-Submit: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
Run-TryBot: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Roland Shoemaker <roland@golang.org>

ssh/common.go

@@ -119,6 +119,13 @@ func algorithmsForKeyFormat(keyFormat string) []string {
 	}
 }
 
+// isRSA returns whether algo is a supported RSA algorithm, including certificate
+// algorithms.
+func isRSA(algo string) bool {
+	algos := algorithmsForKeyFormat(KeyAlgoRSA)
+	return contains(algos, underlyingAlgo(algo))
+}
+
 // supportedPubKeyAuthAlgos specifies the supported client public key
 // authentication algorithms. Note that this doesn't include certificate types
 // since those use the underlying algorithm. This list is sent to the client if