/*
 * Copyright (C) 2016 The Android Open Source Project
 *
 * 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.google.android.exoplayer2.upstream.crypto;

import com.google.android.exoplayer2.util.Assertions;
import java.nio.ByteBuffer;
import java.security.InvalidAlgorithmParameterException;
import java.security.InvalidKeyException;
import java.security.NoSuchAlgorithmException;
import javax.crypto.Cipher;
import javax.crypto.NoSuchPaddingException;
import javax.crypto.ShortBufferException;
import javax.crypto.spec.IvParameterSpec;
import javax.crypto.spec.SecretKeySpec;

/**
 * A flushing variant of a AES/CTR/NoPadding {@link Cipher}.
 *
 * Unlike a regular {@link Cipher}, the update methods of this class are guaranteed to process all
 * of the bytes input (and hence output the same number of bytes).
 */
public final class AesFlushingCipher {

  private final Cipher cipher;
  private final int blockSize;
  private final byte[] zerosBlock;
  private final byte[] flushedBlock;

  private int pendingXorBytes;

  public AesFlushingCipher(int mode, byte[] secretKey, long nonce, long offset) {
    try {
      cipher = Cipher.getInstance("AES/CTR/NoPadding");
      blockSize = cipher.getBlockSize();
      zerosBlock = new byte[blockSize];
      flushedBlock = new byte[blockSize];
      long counter = offset / blockSize;
      int startPadding = (int) (offset % blockSize);
      cipher.init(mode, new SecretKeySpec(secretKey, cipher.getAlgorithm().split("/")[0]),
          new IvParameterSpec(getInitializationVector(nonce, counter)));
      if (startPadding != 0) {
        updateInPlace(new byte[startPadding], 0, startPadding);
      }
    } catch (NoSuchAlgorithmException | NoSuchPaddingException | InvalidKeyException
        | InvalidAlgorithmParameterException e) {
      // Should never happen.
      throw new RuntimeException(e);
    }
  }

  public void updateInPlace(byte[] data, int offset, int length) {
    update(data, offset, length, data, offset);
  }

  public void update(byte[] in, int inOffset, int length, byte[] out, int outOffset) {
    // If we previously flushed the cipher by inputting zeros up to a block boundary, then we need
    // to manually transform the data that actually ended the block. See the comment below for more
    // details.
    while (pendingXorBytes > 0) {
      out[outOffset] = (byte) (in[inOffset] ^ flushedBlock[blockSize - pendingXorBytes]);
      outOffset++;
      inOffset++;
      pendingXorBytes--;
      length--;
      if (length == 0) {
        return;
      }
    }

    // Do the bulk of the update.
    int written = nonFlushingUpdate(in, inOffset, length, out, outOffset);
    if (length == written) {
      return;
    }

    // We need to finish the block to flush out the remaining bytes. We do so by inputting zeros,
    // so that the corresponding bytes output by the cipher are those that would have been XORed
    // against the real end-of-block data to transform it. We store these bytes so that we can
    // perform the transformation manually in the case of a subsequent call to this method with
    // the real data.
    int bytesToFlush = length - written;
    Assertions.checkState(bytesToFlush < blockSize);
    outOffset += written;
    pendingXorBytes = blockSize - bytesToFlush;
    written = nonFlushingUpdate(zerosBlock, 0, pendingXorBytes, flushedBlock, 0);
    Assertions.checkState(written == blockSize);
    // The first part of xorBytes contains the flushed data, which we copy out. The remainder
    // contains the bytes that will be needed for manual transformation in a subsequent call.
    for (int i = 0; i < bytesToFlush; i++) {
      out[outOffset++] = flushedBlock[i];
    }
  }

  private int nonFlushingUpdate(byte[] in, int inOffset, int length, byte[] out, int outOffset) {
    try {
      return cipher.update(in, inOffset, length, out, outOffset);
    } catch (ShortBufferException e) {
      // Should never happen.
      throw new RuntimeException(e);
    }
  }

  private byte[] getInitializationVector(long nonce, long counter) {
    return ByteBuffer.allocate(16).putLong(nonce).putLong(counter).array();
  }

}
