ssh: deprecate and replace SigAlgo constants

RFC 8332, Section 2 sets up two overlapping namespaces: public key
formats and public key algorithms.

* The formats are what we currently have KeyAlgo constants for, and they
  appear in PublicKey.Type.

* The algorithms are the set of both KeyAlgo and SigAlgo constants, and
  they appear in Signature.Format (amongst other places).

This is incoherent, because that means Signature.Format can be both a
KeyAlgo (like KeyAlgoECDSA256) or a SigAlgo (like SigAlgoRSASHA2256).

One solution would be to duplicate all the KeyAlgo constants into the
SigAlgo namespace, but that would be confusing because applications are
currently using KeyAlgos where they'd be supposed to use the new
SigAlgos (while we can't deprecate the KeyAlgos because they are still
necessary for the PublicKey.Type namespace).

Instead, drop the separate namespaces, and use KeyAlgos throughout.
There are simply some KeyAlgos that can't be a PublicKey.Type.

Take the opportunity to fix the stuttering SHA22565/SHA2512 names. It's
totally ok to call those hashes SHA-256 and SHA-512 without the family
infix.

For golang/go#49952

Change-Id: Ia1fce3912a7e60aa70a88f75ed311be331fd19d5
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/392354
Trust: Filippo Valsorda <filippo@golang.org>
Run-TryBot: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Roland Shoemaker <roland@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>

ssh/agent/client_test.go

@@ -182,9 +182,9 @@ func testAgentInterface(t *testing.T, agent ExtendedAgent, key interface{}, cert
 				t.Fatalf("Verify(%s): %v", pubKey.Type(), err)
 			}
 		}
-		sshFlagTest(0, ssh.SigAlgoRSA)
-		sshFlagTest(SignatureFlagRsaSha256, ssh.SigAlgoRSASHA2256)
-		sshFlagTest(SignatureFlagRsaSha512, ssh.SigAlgoRSASHA2512)
+		sshFlagTest(0, ssh.KeyAlgoRSA)
+		sshFlagTest(SignatureFlagRsaSha256, ssh.KeyAlgoRSASHA256)
+		sshFlagTest(SignatureFlagRsaSha512, ssh.KeyAlgoRSASHA512)
 	}
 
 	// If the key has a lifetime, is it removed when it should be?

ssh/agent/keyring.go

@@ -205,9 +205,9 @@ func (r *keyring) SignWithFlags(key ssh.PublicKey, data []byte, flags SignatureF
 					var algorithm string
 					switch flags {
 					case SignatureFlagRsaSha256:
-						algorithm = ssh.SigAlgoRSASHA2256
+						algorithm = ssh.KeyAlgoRSASHA256
 					case SignatureFlagRsaSha512:
-						algorithm = ssh.SigAlgoRSASHA2512
+						algorithm = ssh.KeyAlgoRSASHA512
 					default:
 						return nil, fmt.Errorf("agent: unsupported signature flags: %d", flags)
 					}