AES Counter Mode implementation in C# (should work for AES 128, 192, 256 -> just initialize with proper key length)

AesCounterMode.cs

// The MIT License (MIT)

// Copyright (c) 2020 Hans Wolff

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// of this software and associated documentation files (the "Software"), to deal
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// THE SOFTWARE.

using System;
using System.Collections.Generic;
using System.Security.Cryptography;

public class AesCounterMode : SymmetricAlgorithm
{
    private readonly ulong _nonce;
    private readonly ulong _counter;
    private readonly AesManaged _aes;

    public AesCounterMode(byte[] nonce, ulong counter)
      : this(ConvertNonce(nonce), counter)
    {
    }

    public AesCounterMode(ulong nonce, ulong counter)
    {
        _aes = new AesManaged
        {
            Mode = CipherMode.ECB,
            Padding = PaddingMode.None
        };

        _nonce = nonce;
        _counter = counter;
    }

    private static ulong ConvertNonce(byte[] nonce)
    {
        if (nonce == null) throw new ArgumentNullException(nameof(nonce));
        if (nonce.Length < sizeof(ulong)) throw new ArgumentException($"{nameof(nonce)} must have at least {sizeof(ulong)} bytes");

        return BitConverter.ToUInt64(nonce);
    }

    public override ICryptoTransform CreateEncryptor(byte[] rgbKey, byte[] ignoredParameter)
    {
        return new CounterModeCryptoTransform(_aes, rgbKey, _nonce, _counter);
    }

    public override ICryptoTransform CreateDecryptor(byte[] rgbKey, byte[] ignoredParameter)
    {
        return new CounterModeCryptoTransform(_aes, rgbKey, _nonce, _counter);
    }

    public override void GenerateKey()
    {
        _aes.GenerateKey();
    }

    public override void GenerateIV()
    {
        // IV not needed in Counter Mode
    }
}

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

    private ulong _counter;

    public CounterModeCryptoTransform(SymmetricAlgorithm symmetricAlgorithm, byte[] key, ulong nonce, ulong counter)
    {
        if (key == null) throw new ArgumentNullException(nameof(key));

        _symmetricAlgorithm = symmetricAlgorithm ?? throw new ArgumentNullException(nameof(symmetricAlgorithm));
        _counter = counter;
        _nonceAndCounter = new byte[16];
        BitConverter.TryWriteBytes(_nonceAndCounter, nonce);
        BitConverter.TryWriteBytes(new Span<byte>(_nonceAndCounter, sizeof(ulong), sizeof(ulong)), 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 byte[] _counterModeBlock;
    private void EncryptCounterThenIncrement()
    {
        _counterModeBlock ??= new byte[_symmetricAlgorithm.BlockSize / 8];

        _counterEncryptor.TransformBlock(_nonceAndCounter, 0, _nonceAndCounter.Length, _counterModeBlock, 0);
        IncrementCounter();

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

    private void IncrementCounter()
    {
        _counter++;
        var span = new Span<byte>(_nonceAndCounter, sizeof(ulong), sizeof(ulong));
        BitConverter.TryWriteBytes(span, _counter);
    }

    public int InputBlockSize => _symmetricAlgorithm.BlockSize / 8;
    public int OutputBlockSize => _symmetricAlgorithm.BlockSize / 8;
    public bool CanTransformMultipleBlocks => true;
    public bool CanReuseTransform => false;

    public void Dispose()
    {
        _counterEncryptor.Dispose();
    }
}

//public void ExampleUsage()
//{
//    var key = new byte[16];
//    RandomNumberGenerator.Create().GetBytes(key);
//    var nonce = new byte[8];
//    RandomNumberGenerator.Create().GetBytes(nonce);
//
//    var dataToEncrypt = new byte[12345];
//
//    var counter = 0;
//    using var counterMode = new AesCounterMode(nonce, counter);
//    using var encryptor = counterMode.CreateEncryptor(key, null);
//    using var decryptor = counterMode.CreateDecryptor(key, null);
//
//    var encryptedData = new byte[dataToEncrypt.Length];
//    var bytesWritten = encryptor.TransformBlock(dataToEncrypt, 0, dataToEncrypt.Length, encryptedData, 0);
//
//    var decrypted = new byte[dataToEncrypt.Length];
//    decryptor.TransformBlock(encryptedData, 0, bytesWritten, decrypted, 0);
//
//    //decrypted.Should().BeEquivalentTo(dataToEncrypt);
//}

This is what bugs me. We want an implementation that ensures that other parties decrypting the text... get the text I encrypted. So using the nonce and the counter exactly the same way is crucial, right?

Sorry, but you need to understand other parties' code or their document, you may do some experiment to make it work. My version match my other parties from golang, and it works well.

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

Hi can you please share the working example, Many Thanks