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AES128 Counter Mode implementation in C#
// 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.
using System;
using System.Collections.Generic;
using System.Security.Cryptography;
public class Aes128CounterMode : SymmetricAlgorithm
{
private readonly byte[] _counter;
private readonly AesManaged _aes;
public Aes128CounterMode(byte[] counter)
{
if (counter == null) throw new ArgumentNullException("counter");
if (counter.Length != 16)
throw new ArgumentException(String.Format("Counter size must be same as block size (actual: {0}, expected: {1})",
counter.Length, 16));
_aes = new AesManaged
{
Mode = CipherMode.ECB,
Padding = PaddingMode.None
};
_counter = counter;
}
public override ICryptoTransform CreateEncryptor(byte[] rgbKey, byte[] ignoredParameter)
{
return new CounterModeCryptoTransform(_aes, rgbKey, _counter);
}
public override ICryptoTransform CreateDecryptor(byte[] rgbKey, byte[] ignoredParameter)
{
return new CounterModeCryptoTransform(_aes, rgbKey, _counter);
}
public override void GenerateKey()
{
_aes.GenerateKey();
}
public override void GenerateIV()
{
// IV not needed in Counter Mode
}
}
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()
{
}
}
@ghost

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ghost commented Oct 5, 2014

Any chance you can give a call-example (from a main of a console app.) of how to encrypt/decrypt a plain text with your code?
And how to use it if there is actually a random IV present?
Thanks

@nick-wellinghoff-sp

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nick-wellinghoff-sp commented Jan 6, 2015

This code does not work AT ALL. You don't even call the base constructor....

@masantiago

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masantiago commented Feb 14, 2015

Thank you very much! It works perfect. I've even compared it to openssl implementation and it yields the same result. Good job!

@anzun

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anzun commented Oct 16, 2015

Hello, could you please give me a hint how to call the Aes128CounterMode?

Aes128CounterMode aes128 = new Aes128CounterMode(Counter);
aes128.GenerateKey(); // this is not generating a key? is it?
aes128.GenerateIV();
byte[] byteKey = aes128.Key; // aes128.Key is Null
byte[] byteIV = aes128.IV;
aes128.CreateEncryptor(byteKey, byteIV);

@anzun

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anzun commented Oct 16, 2015

2nd question: what has to be adjusted in the above code to create an Aes256CounterMode

@mochasoft

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mochasoft commented Nov 5, 2015

try

Aes128CounterMode am;
ICryptoTransform ict;
am = new Aes128CounterMode(iv);
ict = am.CreateEncryptor(key, null);
ict.TransformBlock(input, 0, input.Length, result, 0);

Good job making the class. It works correct when accessing a Mac running as a SSH2 server.

@martinprikryl

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martinprikryl commented Jul 5, 2018

Despite some previous comments, the code works correctly.

For a compact standalone code based on this, see https://stackoverflow.com/q/6374437/850848#51188472

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