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/*
 * Copyright 2013-2015 Amazon.com, Inc. or its affiliates. All Rights Reserved.
 *
 * Licensed under the Apache License, Version 2.0 (the "License").
 * You may not use this file except in compliance with the License.
 * A copy of the License is located at
 *
 *  http://aws.amazon.com/apache2.0
 *
 * or in the "license" file accompanying this file. This file 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.amazonaws.services.s3.internal.crypto;

class AesCtr extends ContentCryptoScheme {
    @Override String getKeyGeneratorAlgorithm() { return AES_GCM.getKeyGeneratorAlgorithm(); }
    @Override String getCipherAlgorithm() { return "AES/CTR/NoPadding"; }
    @Override int getKeyLengthInBits() { return AES_GCM.getKeyLengthInBits(); }
    @Override int getBlockSizeInBytes() { return AES_GCM.getBlockSizeInBytes(); }
    @Override int getIVLengthInBytes() { return 16; }
    @Override long getMaxPlaintextSize() {  return MAX_CTR_BYTES;  }

    @Override
    byte[] adjustIV(byte[] iv, long byteOffset) {
        // currently only support iv of length 12 for AES/GCM.
        // Anything else is quite a bit complicated.
        if (iv.length != 12)
            throw new UnsupportedOperationException();
        final int blockSize = getBlockSizeInBytes();
        final long blockOffset = byteOffset / blockSize;
        if (blockOffset * blockSize != byteOffset) {
            throw new IllegalArgumentException(
                    "Expecting byteOffset to be multiple of 16, but got blockOffset="
                            + blockOffset + ", blockSize=" + blockSize
                            + ", byteOffset=" + byteOffset);
        }
        byte[] J0 = computeJ0(iv);
        return incrementBlocks(J0, blockOffset);
    }

    /**
     * See 
     * NIST Special Publication 800-38D. for the definition of J0, the
     * "pre-counter block".
     * 
     * Reference: GCMBlockCipher.java
     */
     private byte[] computeJ0(byte[] nonce) {
         final int blockSize = getBlockSizeInBytes();
         byte[] J0 = new byte[blockSize];
         System.arraycopy(nonce, 0, J0, 0, nonce.length);
         J0[blockSize - 1] = 0x01;
         return incrementBlocks(J0, 1);
     }
}