http2: require either ECDSA or RSA ciphersuite

The HTTP/2 RFC does indeed mandate TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256
but in practice, people are also using TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256
becuase they are only using an ECDSA certificate. This is the case in acme/autocert.

It doesn't make sense to enforce only RSA in cipher suites if it will
never be used because they are using a ECDSA certificate.

Change-Id: I86dac192a3eb9b74e4268310a3b550b3bd88a37f
Reviewed-on: https://go-review.googlesource.com/30721
Reviewed-by: Tom Bergan <tombergan@google.com>
Run-TryBot: Tom Bergan <tombergan@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

http2/server.go

@@ -220,12 +220,15 @@ func ConfigureServer(s *http.Server, conf *Server) error {
 	} else if s.TLSConfig.CipherSuites != nil {
 		// If they already provided a CipherSuite list, return
 		// an error if it has a bad order or is missing
-		// ECDHE_RSA_WITH_AES_128_GCM_SHA256.
-		const requiredCipher = tls.TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256
+		// ECDHE_RSA_WITH_AES_128_GCM_SHA256 or ECDHE_ECDSA_WITH_AES_128_GCM_SHA256.
 		haveRequired := false
 		sawBad := false
 		for i, cs := range s.TLSConfig.CipherSuites {
-			if cs == requiredCipher {
+			switch cs {
+			case tls.TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256,
+				// Alternative MTI cipher to not discourage ECDSA-only servers.
+				// See http://golang.org/cl/30721 for further information.
+				tls.TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256:
 				haveRequired = true
 			}
 			if isBadCipher(cs) {
@@ -235,7 +238,7 @@ func ConfigureServer(s *http.Server, conf *Server) error {
 			}
 		}
 		if !haveRequired {
-			return fmt.Errorf("http2: TLSConfig.CipherSuites is missing HTTP/2-required TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256")
+			return fmt.Errorf("http2: TLSConfig.CipherSuites is missing an HTTP/2-required AES_128_GCM_SHA256 cipher.")
 		}
 	}
 

http2/server_test.go

@@ -3189,12 +3189,18 @@ func TestConfigureServer(t *testing.T) {
 				CipherSuites: []uint16{tls.TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256},
 			},
 		},
+		{
+			name: "just the alternative required cipher suite",
+			tlsConfig: &tls.Config{
+				CipherSuites: []uint16{tls.TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256},
+			},
+		},
 		{
 			name: "missing required cipher suite",
 			tlsConfig: &tls.Config{
 				CipherSuites: []uint16{tls.TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384},
 			},
-			wantErr: "is missing HTTP/2-required TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256",
+			wantErr: "is missing an HTTP/2-required AES_128_GCM_SHA256 cipher.",
 		},
 		{
 			name: "required after bad",