ssh: add support for aes256-gcm@openssh.com

Change-Id: I91caf3bda3dfd00c050f5ebf23c2a35a04c5762b
GitHub-Last-Rev: 6e71340e7960b5b6f71f7b96eeeaf8dfb268e306
GitHub-Pull-Request: golang/crypto#127
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/223518
Auto-Submit: Filippo Valsorda <filippo@golang.org>
Run-TryBot: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Roland Shoemaker <roland@golang.org>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Han-Wen Nienhuys <hanwen@google.com>

ssh/cipher_test.go

@@ -141,7 +141,7 @@ func TestCVE202143565(t *testing.T) {
 		constructPacket func(packetCipher) io.Reader
 	}{
 		{
-			cipher: gcmCipherID,
+			cipher: gcm128CipherID,
 			constructPacket: func(client packetCipher) io.Reader {
 				internalCipher := client.(*gcmCipher)
 				b := &bytes.Buffer{}