go.crypto/ssh: never negotiate unsupported ciphers

Fixes golang/go#4285.

Adding a new cipher that is supported by the remote end, but not supported by our client causes that cipher to be considered a valid candidate. This fails later in setupKeys when there is no cipherModes configuration.

In summary, unsupported ciphers cannot be willed into existence by adding them to the client config. This change enforces this.

R=golang-dev, agl
CC=golang-dev
https://golang.org/cl/6780047

ssh/client_auth_test.go

@@ -276,3 +276,23 @@ func TestClientHMAC(t *testing.T) {
 		c.Close()
 	}
 }
+
+// issue 4285.
+func TestClientUnsupportedCipher(t *testing.T) {
+	kc := new(keychain)
+	kc.keys = append(kc.keys, rsakey)
+	config := &ClientConfig{
+		User: "testuser",
+		Auth: []ClientAuth{
+			ClientAuthKeyring(kc),
+		},
+		Crypto: CryptoConfig{
+			Ciphers: []string{"aes128-cbc"}, // not currently supported
+		},
+	}
+	c, err := Dial("tcp", newMockAuthServer(t), config)
+	if err == nil {
+		t.Errorf("expected no ciphers in common")
+		c.Close()
+	}
+}