lgoldstein commented on a change in pull request #132:
URL: https://github.com/apache/mina-sshd/pull/132#discussion_r429326530



##########
File path: 
sshd-common/src/main/java/org/apache/sshd/common/cipher/BaseGCMCipher.java
##########
@@ -0,0 +1,62 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements. See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership. The ASF licenses this file
+ * to you 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 org.apache.sshd.common.cipher;
+
+import java.security.spec.AlgorithmParameterSpec;
+
+import javax.crypto.Cipher;
+import javax.crypto.spec.GCMParameterSpec;
+
+import org.apache.sshd.common.util.buffer.Buffer;
+import org.apache.sshd.common.util.buffer.ByteArrayBuffer;
+
+public class BaseGCMCipher extends BaseAEADCipher {
+
+    public BaseGCMCipher(
+                         int ivsize, int authSize, int kdfSize, String 
algorithm, int keySize, String transformation,
+                         int blkSize) {
+        super(ivsize, authSize, kdfSize, algorithm, keySize, transformation, 
blkSize);
+    }
+
+    @Override
+    protected AlgorithmParameterSpec initializeAlgorithmParameters(byte[] iv) {
+        Buffer buffer = new ByteArrayBuffer(iv);
+        buffer.rpos(Integer.BYTES);
+        long ic = buffer.getLong();
+        // decrement IV as it will be incremented back to initial value on 
first call to updateWithAAD
+        ic = (ic - 1) & 0x0ffffffffL;
+        buffer.wpos(Integer.BYTES);
+        buffer.putLong(ic);
+        return new GCMParameterSpec(getAuthenticationTagSize() * Byte.SIZE, 
buffer.array());
+    }
+
+    @Override
+    protected AlgorithmParameterSpec getNextAlgorithmParameters() {
+        Cipher cipher = getCipherInstance();
+        Buffer iv = new ByteArrayBuffer(cipher.getIV());
+        iv.rpos(Integer.BYTES);
+        long ic = iv.getLong();
+        ic = (ic + 1) & 0x0ffffffffL;
+        iv.wpos(Integer.BYTES);
+        iv.putLong(ic);
+        return new GCMParameterSpec(getAuthenticationTagSize() * Byte.SIZE, 
iv.array());
+    }

Review comment:
       Pretty expensive in memory - every time it is called (which is for every 
packet) a new `byte[]` (`cipher.getIv()` returns a **clone**), new `Buffer` and 
new `GCMParameterSpec` instance is created. Can we do something about it ?
   
   Moreover, this code is exactly the same as `initializeAlgorithmParameters` 
except it does `+1` instead of `-1` - perhaps we could use an internal 
(protected) `updateAlgorithmParameters(byte[] iv, long delta)` and call it once 
with `+1` and once with `-1`




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