Counter Mode for any symmentric block cipher in C#

CounterModeCryptoTransform.cs

// The MIT License (MIT)
 
// Copyright (c) 2014 Hans Wolff
 
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
 
// The above copyright notice and this permission notice shall be included in
// all copies or substantial portions of the Software.
 
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
// THE SOFTWARE.

public class CounterModeCryptoTransform : ICryptoTransform
{
    private readonly byte[] _counter;
    private readonly ICryptoTransform _counterEncryptor;
    private readonly Queue<byte> _xorMask = new Queue<byte>();
    private readonly SymmetricAlgorithm _symmetricAlgorithm;

    public CounterModeCryptoTransform(SymmetricAlgorithm symmetricAlgorithm, byte[] key, byte[] counter)
    {
        if (symmetricAlgorithm == null) throw new ArgumentNullException("symmetricAlgorithm");
        if (key == null) throw new ArgumentNullException("key");
        if (counter == null) throw new ArgumentNullException("counter");
        if (counter.Length != symmetricAlgorithm.BlockSize / 8)
            throw new ArgumentException(String.Format("Counter size must be same as block size (actual: {0}, expected: {1})",
                counter.Length, symmetricAlgorithm.BlockSize / 8));

        _symmetricAlgorithm = symmetricAlgorithm;
        _counter = counter;

        var zeroIv = new byte[_symmetricAlgorithm.BlockSize / 8];
        _counterEncryptor = symmetricAlgorithm.CreateEncryptor(key, zeroIv);
    }

    public byte[] TransformFinalBlock(byte[] inputBuffer, int inputOffset, int inputCount)
    {
        var output = new byte[inputCount];
        TransformBlock(inputBuffer, inputOffset, inputCount, output, 0);
        return output;
    }

    public int TransformBlock(byte[] inputBuffer, int inputOffset, int inputCount, byte[] outputBuffer, int outputOffset)
    {
        for (var i = 0; i < inputCount; i++)
        {
            if (NeedMoreXorMaskBytes()) EncryptCounterThenIncrement();

            var mask = _xorMask.Dequeue();
            outputBuffer[outputOffset + i] = (byte)(inputBuffer[inputOffset + i] ^ mask);
        }

        return inputCount;
    }

    private bool NeedMoreXorMaskBytes()
    {
        return _xorMask.Count == 0;
    }

    private void EncryptCounterThenIncrement()
    {
        var counterModeBlock = new byte[_symmetricAlgorithm.BlockSize / 8];

        _counterEncryptor.TransformBlock(_counter, 0, _counter.Length, counterModeBlock, 0);
        IncrementCounter();

        foreach (var b in counterModeBlock)
        {
            _xorMask.Enqueue(b);
        }
    }

    private void IncrementCounter()
    {
        for (var i = _counter.Length - 1; i >= 0; i--)
        {
            if (++_counter[i] != 0)
                break;
        }
    }

    public int InputBlockSize { get { return _symmetricAlgorithm.BlockSize / 8; } }
    public int OutputBlockSize { get { return _symmetricAlgorithm.BlockSize / 8; } }
    public bool CanTransformMultipleBlocks { get { return true; } }
    public bool CanReuseTransform { get { return false; } }

    public void Dispose()
    {
    }
}