ssh: support rsa-sha2-256/512 on the server side

This lets clients know we support rsa-sha2-256/512 signatures from
ssh-rsa public keys. OpenSSH prefers to break the connection rather than
attempting trial and error, apparently.

We don't enable support for the "ext-info-s" because we're not
interested in any client->server extensions.

This also replaces isAcceptableAlgo which was rejecting the
rsa-sha2-256/512-cert-v01@openssh.com public key algorithms.

Tested with OpenSSH 9.1 on macOS Ventura.

Fixes golang/go#49269
Updates golang/go#49952

Co-authored-by: Nicola Murino <nicola.murino@gmail.com>
Co-authored-by: Kristin Davidson <kdavidson@atlassian.com>
Change-Id: I4955c3b12bb45575e9977ac657bb5805b49d00c3
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/447757
Run-TryBot: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Roland Shoemaker <roland@golang.org>
Reviewed-by: Nicola Murino <nicola.murino@gmail.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>

ssh/server.go

@@ -291,15 +291,6 @@ func (s *connection) serverHandshake(config *ServerConfig) (*Permissions, error)
 	return perms, err
 }
 
-func isAcceptableAlgo(algo string) bool {
-	switch algo {
-	case KeyAlgoRSA, KeyAlgoRSASHA256, KeyAlgoRSASHA512, KeyAlgoDSA, KeyAlgoECDSA256, KeyAlgoECDSA384, KeyAlgoECDSA521, KeyAlgoSKECDSA256, KeyAlgoED25519, KeyAlgoSKED25519,
-		CertAlgoRSAv01, CertAlgoDSAv01, CertAlgoECDSA256v01, CertAlgoECDSA384v01, CertAlgoECDSA521v01, CertAlgoSKECDSA256v01, CertAlgoED25519v01, CertAlgoSKED25519v01:
-		return true
-	}
-	return false
-}
-
 func checkSourceAddress(addr net.Addr, sourceAddrs string) error {
 	if addr == nil {
 		return errors.New("ssh: no address known for client, but source-address match required")
@@ -514,7 +505,7 @@ userAuthLoop:
 				return nil, parseError(msgUserAuthRequest)
 			}
 			algo := string(algoBytes)
-			if !isAcceptableAlgo(algo) {
+			if !contains(supportedPubKeyAuthAlgos, underlyingAlgo(algo)) {
 				authErr = fmt.Errorf("ssh: algorithm %q not accepted", algo)
 				break
 			}
@@ -572,7 +563,7 @@ userAuthLoop:
 				// algorithm name that corresponds to algo with
 				// sig.Format.  This is usually the same, but
 				// for certs, the names differ.
-				if !isAcceptableAlgo(sig.Format) {
+				if !contains(supportedPubKeyAuthAlgos, sig.Format) {
 					authErr = fmt.Errorf("ssh: algorithm %q not accepted", sig.Format)
 					break
 				}