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// Copyright 2016 The Closure Library Authors. All Rights Reserved.
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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//      http://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS-IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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goog.provide('goog.crypt.Ctr');
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goog.require('goog.array');
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goog.require('goog.asserts');
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goog.require('goog.crypt');
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/**
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 * Implementation of Ctr mode for block ciphers.  See
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 * http://en.wikipedia.org/wiki/Block_cipher_modes_of_operation
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 * #Cipher-block_chaining_.28Ctr.29. for an overview, and
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 * http://csrc.nist.gov/publications/nistpubs/800-38a/sp800-38a.pdf
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 * for the spec.
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 *
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 * @param {!goog.crypt.BlockCipher} cipher The block cipher to use.
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 * @constructor
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 * @final
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 * @struct
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 */
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goog.crypt.Ctr = function(cipher) {
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  /**
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   * Block cipher.
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   * @type {!goog.crypt.BlockCipher}
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   * @private
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   */
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  this.cipher_ = cipher;
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};
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/**
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 * Encrypts a message.
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 *
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 * @param {!Array<number>|!Uint8Array} plainText Message to encrypt. An array of
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 *     bytes. The length does not have to be a multiple of the blocksize.
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 * @param {!Array<number>|!Uint8Array} initialVector Initial vector for the Ctr
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 *     mode. An array of bytes with the same length as the block size, that
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 *     should be not reused when using the same key.
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 * @return {!Array<number>} Encrypted message.
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 */
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goog.crypt.Ctr.prototype.encrypt = function(plainText, initialVector) {
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  goog.asserts.assert(
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      initialVector.length == this.cipher_.BLOCK_SIZE,
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      'Initial vector must be size of one block.');
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  // Copy the IV, so it's not modified.
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  var counter = goog.array.clone(initialVector);
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  var keyStreamBlock = [];
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  var encryptedArray = [];
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  var plainTextBlock = [];
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  while (encryptedArray.length < plainText.length) {
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    keyStreamBlock = this.cipher_.encrypt(counter);
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    goog.crypt.Ctr.incrementBigEndianCounter_(counter);
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    plainTextBlock = goog.array.slice(
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        plainText, encryptedArray.length,
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        encryptedArray.length + this.cipher_.BLOCK_SIZE);
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    goog.array.extend(
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        encryptedArray,
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        goog.crypt.xorByteArray(
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            plainTextBlock,
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            goog.array.slice(keyStreamBlock, 0, plainTextBlock.length)));
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  }
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  return encryptedArray;
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};
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/**
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 * Decrypts a message. In CTR, this is the same as encrypting.
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 *
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 * @param {!Array<number>|!Uint8Array} cipherText Message to decrypt. The length
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 *     does not have to be a multiple of the blocksize.
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 * @param {!Array<number>|!Uint8Array} initialVector Initial vector for the Ctr
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 *     mode. An array of bytes with the same length as the block size.
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 * @return {!Array<number>} Decrypted message.
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 */
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goog.crypt.Ctr.prototype.decrypt = goog.crypt.Ctr.prototype.encrypt;
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/**
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 * Increments the big-endian integer represented in counter in-place.
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 *
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 * @param {!Array<number>|!Uint8Array} counter The array of bytes to modify.
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 * @private
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 */
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goog.crypt.Ctr.incrementBigEndianCounter_ = function(counter) {
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  for (var i = counter.length - 1; i >= 0; i--) {
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    var currentByte = counter[i];
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    currentByte = (currentByte + 1) & 0xFF;  // Allow wrapping around.
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    counter[i] = currentByte;
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    if (currentByte != 0) {
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      // This iteration hasn't wrapped around, which means there is
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      // no carry to add to the next byte.
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      return;
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    }  // else, repeat with next byte.
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  }
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};
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