ssh: support RSA SHA-2 (RFC8332) signatures

This change adds support for RSA SHA-2 based signatures for host keys and certificates. It also switches the default certificate signature algorithm for RSA to use SHA-512. This is implemented by treating ssh.Signer specially when the key type is `ssh-rsa` by also allowing SHA-256 and SHA-512 signatures.

Fixes golang/go#37278

Change-Id: I2ee1ac4ae4c9c1de441a2d6cf1e806357ef18910
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/220037
Trust: Jason A. Donenfeld <Jason@zx2c4.com>
Run-TryBot: Jason A. Donenfeld <Jason@zx2c4.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Jason A. Donenfeld <Jason@zx2c4.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>

ssh/client.go

@@ -115,12 +115,25 @@ func (c *connection) clientHandshake(dialAddress string, config *ClientConfig) e
 
 // verifyHostKeySignature verifies the host key obtained in the key
 // exchange.
-func verifyHostKeySignature(hostKey PublicKey, result *kexResult) error {
+func verifyHostKeySignature(hostKey PublicKey, algo string, result *kexResult) error {
 	sig, rest, ok := parseSignatureBody(result.Signature)
 	if len(rest) > 0 || !ok {
 		return errors.New("ssh: signature parse error")
 	}
 
+	// For keys, underlyingAlgo is exactly algo. For certificates,
+	// we have to look up the underlying key algorithm that SSH
+	// uses to evaluate signatures.
+	underlyingAlgo := algo
+	for sigAlgo, certAlgo := range certAlgoNames {
+		if certAlgo == algo {
+			underlyingAlgo = sigAlgo
+		}
+	}
+	if sig.Format != underlyingAlgo {
+		return fmt.Errorf("ssh: invalid signature algorithm %q, expected %q", sig.Format, underlyingAlgo)
+	}
+
 	return hostKey.Verify(result.H, sig)
 }