Implement AES-GCM cipher support (#630)

* Implement AES-GCM cipher support

Fixes #217.

A port of AES-GCM cipher support from Apache MINA-SSHD, based on https://github.com/apache/mina-sshd/pull/132.

Included tests for decoding SSH packets sent from Apache MINA-SSHD and OpenSSH (Version 7.9p1 as used by Debian 10).

Manual tests also done on OpenSSH server 7.9p1 running Debian 10 with its available ciphers, including 3des-cbc, aes128-cbc, aes192-cbc, aes256-cbc, aes128-ctr, aes192-ctr, aes256-ctr, aes128-gcm@openssh.com and aes256-gcm@openssh.com.

* Changes per PR feedback

- Fixed variable/statement whitespaces and add back missing braces per coding standard requirement
- Moved Buffer.putLong() and Buffer.getLong() into GcmCipher.CounterGCMParameterSpec since it's the only user
- Moved BaseCipher.authSize into GcmCipher since it is the only cipher that would return a non-zero. BaseCipher will keep return 0 instead
- Made BaseCipher.cipher protected instead of making it publicly accessible
- Combined the three decoding modes in Decoder.decode() into one single method, to reduce code duplication
- Added integration test for the ciphers, along with the newly implemented AES-GCM ciphers

src/itest/groovy/com/hierynomus/sshj/transport/cipher/CipherSpec.groovy

@@ -0,0 +1,54 @@
+/*
+ * Copyright (C)2009 - SSHJ Contributors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.hierynomus.sshj.transport.cipher
+
+import com.hierynomus.sshj.IntegrationBaseSpec
+import net.schmizz.sshj.DefaultConfig
+import spock.lang.Unroll
+
+class CipherSpec extends IntegrationBaseSpec {
+
+    @Unroll
+    def "should correctly connect with #cipher Cipher"() {
+        given:
+        def cfg = new DefaultConfig()
+        cfg.setCipherFactories(cipherFactory)
+        def client = getConnectedClient(cfg)
+
+        when:
+        client.authPublickey(USERNAME, KEYFILE)
+
+        then:
+        client.authenticated
+
+        cleanup:
+        client.disconnect()
+
+        where:
+        cipherFactory << [BlockCiphers.TripleDESCBC(),
+                          BlockCiphers.BlowfishCBC(),
+                          BlockCiphers.AES128CBC(),
+                          BlockCiphers.AES128CTR(),
+                          BlockCiphers.AES192CBC(),
+                          BlockCiphers.AES192CTR(),
+                          BlockCiphers.AES256CBC(),
+                          BlockCiphers.AES256CTR(),
+                          GcmCiphers.AES128GCM(),
+                          GcmCiphers.AES256GCM()]
+        cipher = cipherFactory.name
+    }
+
+}