Stream Crypto AES-256-CTR C# Implementation

aes-ctr.cs

using System.Security.Cryptography;

namespace TgServer.Crypto;

public sealed class AesCtr {
    private readonly byte[] _key;
    private readonly byte[] _counter;
    private readonly byte[] _blockKey;

    private readonly ICryptoTransform _encryptor;
    private long _bytesTransformed = 0;

    public AesCtr(byte[] key, byte[] iv) {
        this._key = key;
        _counter = iv;
        _blockKey = new byte[16];
        var aes = Aes.Create();
        aes.Mode = CipherMode.ECB;
        aes.Padding = PaddingMode.None;
        _encryptor = aes.CreateEncryptor(key, new byte[16]);
    }

    public byte[] Encrypt(byte[] data) {
        var ret = new byte[data.Length];
        for (var i = 0; i < data.Length; ++i) {
            var keyIdx = _bytesTransformed % 16;
            if (keyIdx == 0) {
                _encryptor.TransformBlock(_counter, 0, 16, _blockKey, 0);
                for (var k = 15; k >= 0; k--) {
                    _counter[k]++;
                    if (_counter[k] != 0) {
                        break;
                    }
                }
            }

            ret[i] = (byte) (data[i] ^ _blockKey[keyIdx]);
            _bytesTransformed++;
        }

        return ret;
    }
    public byte[] Decrypt(byte[] data) {
        return Encrypt(data);
    }
}