CSChaCha20.cs

/*
 * Copyright (c) 2015, 2018 Scott Bennett
 *           (c) 2018-2023 Kaarlo Räihä
 *
 * Permission to use, copy, modify, and distribute this software for any
 * purpose with or without fee is hereby granted, provided that the above
 * copyright notice and this permission notice appear in all copies.
 *
 * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
 * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
 * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
 * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
 * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
 * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
 * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 */

using System;
using System.IO;
using System.Threading.Tasks;
using System.Runtime.Intrinsics;
using System.Runtime.CompilerServices; // For MethodImplOptions.AggressiveInlining

namespace CSChaCha20;

/// <summary>
/// Chosen SIMD mode
/// </summary>
public enum SimdMode
{
	/// <summary>
	/// Autodetect
	/// </summary>
	AutoDetect = 0,

	/// <summary>
	/// 128 bit SIMD
	/// </summary>
	V128,

	/// <summary>
	/// 256 bit SIMD
	/// </summary>
	V256,

	/// <summary>
	/// 512 bit SIMD
	/// </summary>
	V512,

	/// <summary>
	/// No SIMD
	/// </summary>
	None
}

/// <summary>
/// Class for ChaCha20 encryption / decryption
/// </summary>
public sealed class ChaCha20 : IDisposable 
{
	/// <summary>
	/// Only allowed key lenght in bytes
	/// </summary>
	public const int allowedKeyLength = 32;

	/// <summary>
	/// Only allowed nonce lenght in bytes
	/// </summary>
	public const int allowedNonceLength = 12;

	/// <summary>
	/// How many bytes are processed per loop
	/// </summary>
	public const int processBytesAtTime = 64;

	private const int stateLength = 16;

	/// <summary>
	/// The ChaCha20 state (aka "context")
	/// </summary>
	private readonly uint[] state = new uint[stateLength];

	/// <summary>
	/// Determines if the objects in this class have been disposed of. Set to true by the Dispose() method.
	/// </summary>
	private bool isDisposed = false;

	/// <summary>
	/// Set up a new ChaCha20 state. The lengths of the given parameters are checked before encryption happens.
	/// </summary>
	/// <remarks>
	/// See <a href="https://tools.ietf.org/html/rfc7539#page-10">ChaCha20 Spec Section 2.4</a> for a detailed description of the inputs.
	/// </remarks>
	/// <param name="key">
	/// A 32-byte (256-bit) key, treated as a concatenation of eight 32-bit little-endian integers
	/// </param>
	/// <param name="nonce">
	/// A 12-byte (96-bit) nonce, treated as a concatenation of three 32-bit little-endian integers
	/// </param>
	/// <param name="counter">
	/// A 4-byte (32-bit) block counter, treated as a 32-bit little-endian integer
	/// </param>
	public ChaCha20(byte[] key, byte[] nonce, uint counter) 
	{
		this.KeySetup(key);
		this.IvSetup(nonce, counter);
	}

	/// <summary>
	/// Set up a new ChaCha20 state. The lengths of the given parameters are checked before encryption happens.
	/// </summary>
	/// <remarks>
	/// See <a href="https://tools.ietf.org/html/rfc7539#page-10">ChaCha20 Spec Section 2.4</a> for a detailed description of the inputs.
	/// </remarks>
	/// <param name="key">A 32-byte (256-bit) key, treated as a concatenation of eight 32-bit little-endian integers</param>
	/// <param name="nonce">A 12-byte (96-bit) nonce, treated as a concatenation of three 32-bit little-endian integers</param>
	/// <param name="counter">A 4-byte (32-bit) block counter, treated as a 32-bit little-endian unsigned integer</param>
	public ChaCha20(ReadOnlySpan<byte> key, ReadOnlySpan<byte> nonce, uint counter) 
	{
		this.KeySetup(key.ToArray());
		this.IvSetup(nonce.ToArray(), counter);
	}

	/// <summary>
	/// The ChaCha20 state (aka "context"). Read-Only.
	/// </summary>
	public uint[] State 
	{
		get 
		{
			return this.state;
		}
	}


	// These are the same constants defined in the reference implementation.
	// http://cr.yp.to/streamciphers/timings/estreambench/submissions/salsa20/chacha8/ref/chacha.c
	private static readonly byte[] sigma = "expand 32-byte k"u8.ToArray();
	private static readonly byte[] tau   = "expand 16-byte k"u8.ToArray();

	/// <summary>
	/// Set up the ChaCha state with the given key. A 32-byte key is required and enforced.
	/// </summary>
	/// <param name="key">
	/// A 32-byte (256-bit) key, treated as a concatenation of eight 32-bit little-endian integers
	/// </param>
	private void KeySetup(byte[] key) 
	{
		if (key == null) 
		{
			throw new ArgumentNullException("Key is null");
		}

		if (key.Length != allowedKeyLength) 
		{
			throw new ArgumentException($"Key length must be {allowedKeyLength}. Actual: {key.Length}");
		}	

		state[4] = Util.U8To32Little(key, 0);
		state[5] = Util.U8To32Little(key, 4);
		state[6] = Util.U8To32Little(key, 8);
		state[7] = Util.U8To32Little(key, 12);

		byte[] constants = (key.Length == allowedKeyLength) ? sigma : tau;
		int keyIndex = key.Length - 16;

		state[8]  = Util.U8To32Little(key, keyIndex + 0);
		state[9]  = Util.U8To32Little(key, keyIndex + 4);
		state[10] = Util.U8To32Little(key, keyIndex + 8);
		state[11] = Util.U8To32Little(key, keyIndex + 12);

		state[0] = Util.U8To32Little(constants, 0);
		state[1] = Util.U8To32Little(constants, 4);
		state[2] = Util.U8To32Little(constants, 8);
		state[3] = Util.U8To32Little(constants, 12);
	}

	/// <summary>
	/// Set up the ChaCha state with the given nonce (aka Initialization Vector or IV) and block counter. A 12-byte nonce and a 4-byte counter are required.
	/// </summary>
	/// <param name="nonce">
	/// A 12-byte (96-bit) nonce, treated as a concatenation of three 32-bit little-endian integers
	/// </param>
	/// <param name="counter">
	/// A 4-byte (32-bit) block counter, treated as a 32-bit little-endian integer
	/// </param>
	private void IvSetup(byte[] nonce, uint counter) 
	{
		if (nonce == null) 
		{
			// There has already been some state set up. Clear it before exiting.
			Dispose();
			throw new ArgumentNullException("Nonce is null");
		}

		if (nonce.Length != allowedNonceLength) 
		{
			// There has already been some state set up. Clear it before exiting.
			Dispose();
			throw new ArgumentException($"Nonce length must be {allowedNonceLength}. Actual: {nonce.Length}");
		}

		state[12] = counter;
		state[13] = Util.U8To32Little(nonce, 0);
		state[14] = Util.U8To32Little(nonce, 4);
		state[15] = Util.U8To32Little(nonce, 8);
	}

	private static SimdMode DetectSimdMode()
	{
		if (Vector512.IsHardwareAccelerated)
		{
			return SimdMode.V512;
		}
		else if (Vector256.IsHardwareAccelerated)
		{
			return SimdMode.V256;
		}
		else if (Vector128.IsHardwareAccelerated)
		{
			return SimdMode.V128;
		}

		return SimdMode.None;
	}

#region Encryption methods

	/// <summary>
	/// Encrypt arbitrary-length byte array (input), writing the resulting byte array to preallocated output buffer.
	/// </summary>
	/// <remarks>Since this is symmetric operation, it doesn't really matter if you use Encrypt or Decrypt method</remarks>
	/// <param name="output">Output byte array, must have enough bytes</param>
	/// <param name="input">Input byte array</param>
	/// <param name="numBytes">Number of bytes to encrypt</param>
	/// <param name="simdMode">Chosen SIMD mode (default is auto-detect)</param>
	public void EncryptBytes(byte[] output, byte[] input, int numBytes, SimdMode simdMode = SimdMode.AutoDetect)
	{
		if (output == null)
		{
			throw new ArgumentNullException("output", "Output cannot be null");
		}

		if (input == null)
		{
			throw new ArgumentNullException("input", "Input cannot be null");
		}

		if (numBytes < 0 || numBytes > input.Length) 
		{
			throw new ArgumentOutOfRangeException("numBytes", "The number of bytes to read must be between [0..input.Length]");
		}

		if (output.Length < numBytes)
		{
			throw new ArgumentOutOfRangeException("output", $"Output byte array should be able to take at least {numBytes}");
		}

		if (simdMode == SimdMode.AutoDetect)
		{
			simdMode = DetectSimdMode();
		}

		this.WorkBytes(output, input, numBytes, simdMode);
	}

	/// <summary>
	/// Encrypt arbitrary-length byte stream (input), writing the resulting bytes to another stream (output)
	/// </summary>
	/// <param name="output">Output stream</param>
	/// <param name="input">Input stream</param>
	/// <param name="howManyBytesToProcessAtTime">How many bytes to read and write at time, default is 1024</param>
	/// <param name="simdMode">Chosen SIMD mode (default is auto-detect)</param>
	public void EncryptStream(Stream output, Stream input, int howManyBytesToProcessAtTime = 1024, SimdMode simdMode = SimdMode.AutoDetect)
	{
		if (simdMode == SimdMode.AutoDetect)
		{
			simdMode = DetectSimdMode();
		}

		this.WorkStreams(output, input, simdMode, howManyBytesToProcessAtTime);
	}

	/// <summary>
	/// Async encrypt arbitrary-length byte stream (input), writing the resulting bytes to another stream (output)
	/// </summary>
	/// <param name="output">Output stream</param>
	/// <param name="input">Input stream</param>
	/// <param name="howManyBytesToProcessAtTime">How many bytes to read and write at time, default is 1024</param>
	/// <param name="simdMode">Chosen SIMD mode (default is auto-detect)</param>
	public async Task EncryptStreamAsync(Stream output, Stream input, int howManyBytesToProcessAtTime = 1024, SimdMode simdMode = SimdMode.AutoDetect)
	{
		if (simdMode == SimdMode.AutoDetect)
		{
			simdMode = DetectSimdMode();
		}

		await this.WorkStreamsAsync(output, input, simdMode, howManyBytesToProcessAtTime);
	}

	/// <summary>
	/// Encrypt arbitrary-length byte array (input), writing the resulting byte array to preallocated output buffer.
	/// </summary>
	/// <remarks>Since this is symmetric operation, it doesn't really matter if you use Encrypt or Decrypt method</remarks>
	/// <param name="output">Output byte array, must have enough bytes</param>
	/// <param name="input">Input byte array</param>
	/// <param name="simdMode">Chosen SIMD mode (default is auto-detect)</param>
	public void EncryptBytes(byte[] output, byte[] input, SimdMode simdMode = SimdMode.AutoDetect)
	{
		if (output == null)
		{
			throw new ArgumentNullException("output", "Output cannot be null");
		}

		if (input == null)
		{
			throw new ArgumentNullException("input", "Input cannot be null");
		}

		if (simdMode == SimdMode.AutoDetect)
		{
			simdMode = DetectSimdMode();
		}

		this.WorkBytes(output, input, input.Length, simdMode);
	}

	/// <summary>
	/// Encrypt arbitrary-length byte array (input), writing the resulting byte array that is allocated by method.
	/// </summary>
	/// <remarks>Since this is symmetric operation, it doesn't really matter if you use Encrypt or Decrypt method</remarks>
	/// <param name="input">Input byte array</param>
	/// <param name="numBytes">Number of bytes to encrypt</param>
	/// <param name="simdMode">Chosen SIMD mode (default is auto-detect)</param>
	/// <returns>Byte array that contains encrypted bytes</returns>
	public byte[] EncryptBytes(byte[] input, int numBytes, SimdMode simdMode = SimdMode.AutoDetect)
	{
		if (input == null)
		{
			throw new ArgumentNullException("input", "Input cannot be null");
		}

		if (numBytes < 0 || numBytes > input.Length) 
		{
			throw new ArgumentOutOfRangeException("numBytes", "The number of bytes to read must be between [0..input.Length]");
		}

		if (simdMode == SimdMode.AutoDetect)
		{
			simdMode = DetectSimdMode();
		}

		byte[] returnArray = new byte[numBytes];
		this.WorkBytes(returnArray, input, numBytes, simdMode);
		return returnArray;
	}

	/// <summary>
	/// Encrypt arbitrary-length byte array (input), writing the resulting byte array that is allocated by method.
	/// </summary>
	/// <remarks>Since this is symmetric operation, it doesn't really matter if you use Encrypt or Decrypt method</remarks>
	/// <param name="input">Input byte array</param>
	/// <param name="simdMode">Chosen SIMD mode (default is auto-detect)</param>
	/// <returns>Byte array that contains encrypted bytes</returns>
	public byte[] EncryptBytes(byte[] input, SimdMode simdMode = SimdMode.AutoDetect)
	{
		if (input == null)
		{
			throw new ArgumentNullException("input", "Input cannot be null");
		}

		if (simdMode == SimdMode.AutoDetect)
		{
			simdMode = DetectSimdMode();
		}

		byte[] returnArray = new byte[input.Length];
		this.WorkBytes(returnArray, input, input.Length, simdMode);
		return returnArray;
	}

	/// <summary>
	/// Encrypt string as UTF8 byte array, returns byte array that is allocated by method.
	/// </summary>
	/// <remarks>Here you can NOT swap encrypt and decrypt methods, because of bytes-string transform</remarks>
	/// <param name="input">Input string</param>
	/// <param name="simdMode">Chosen SIMD mode (default is auto-detect)</param>
	/// <returns>Byte array that contains encrypted bytes</returns>
	public byte[] EncryptString(string input, SimdMode simdMode = SimdMode.AutoDetect)
	{
		if (input == null)
		{
			throw new ArgumentNullException("input", "Input cannot be null");
		}

		if (simdMode == SimdMode.AutoDetect)
		{
			simdMode = DetectSimdMode();
		}

		byte[] utf8Bytes = System.Text.Encoding.UTF8.GetBytes(input);
		byte[] returnArray = new byte[utf8Bytes.Length];

		this.WorkBytes(returnArray, utf8Bytes, utf8Bytes.Length, simdMode);
		return returnArray;
	}

	#endregion // Encryption methods


	#region // Decryption methods

	/// <summary>
	/// Decrypt arbitrary-length byte array (input), writing the resulting byte array to the output buffer.
	/// </summary>
	/// <remarks>Since this is symmetric operation, it doesn't really matter if you use Encrypt or Decrypt method</remarks>
	/// <param name="output">Output byte array</param>
	/// <param name="input">Input byte array</param>
	/// <param name="numBytes">Number of bytes to decrypt</param>
	/// <param name="simdMode">Chosen SIMD mode (default is auto-detect)</param>
	public void DecryptBytes(byte[] output, byte[] input, int numBytes, SimdMode simdMode = SimdMode.AutoDetect)
	{
		if (output == null)
		{
			throw new ArgumentNullException("output", "Output cannot be null");
		}

		if (input == null)
		{
			throw new ArgumentNullException("input", "Input cannot be null");
		}

		if (numBytes < 0 || numBytes > input.Length) 
		{
			throw new ArgumentOutOfRangeException("numBytes", "The number of bytes to read must be between [0..input.Length]");
		}

		if (output.Length < numBytes)
		{
			throw new ArgumentOutOfRangeException("output", $"Output byte array should be able to take at least {numBytes}");
		}

		if (simdMode == SimdMode.AutoDetect)
		{
			simdMode = DetectSimdMode();
		}

		this.WorkBytes(output, input, numBytes, simdMode);
	}

	/// <summary>
	/// Decrypt arbitrary-length byte stream (input), writing the resulting bytes to another stream (output)
	/// </summary>
	/// <param name="output">Output stream</param>
	/// <param name="input">Input stream</param>
	/// <param name="howManyBytesToProcessAtTime">How many bytes to read and write at time, default is 1024</param>
	/// <param name="simdMode">Chosen SIMD mode (default is auto-detect)</param>
	public void DecryptStream(Stream output, Stream input, int howManyBytesToProcessAtTime = 1024, SimdMode simdMode = SimdMode.AutoDetect)
	{
		if (simdMode == SimdMode.AutoDetect)
		{
			simdMode = DetectSimdMode();
		}

		this.WorkStreams(output, input, simdMode, howManyBytesToProcessAtTime);
	}

	/// <summary>
	/// Async decrypt arbitrary-length byte stream (input), writing the resulting bytes to another stream (output)
	/// </summary>
	/// <param name="output">Output stream</param>
	/// <param name="input">Input stream</param>
	/// <param name="howManyBytesToProcessAtTime">How many bytes to read and write at time, default is 1024</param>
	/// <param name="simdMode">Chosen SIMD mode (default is auto-detect)</param>
	public async Task DecryptStreamAsync(Stream output, Stream input, int howManyBytesToProcessAtTime = 1024, SimdMode simdMode = SimdMode.AutoDetect)
	{
		if (simdMode == SimdMode.AutoDetect)
		{
			simdMode = DetectSimdMode();
		}

		await this.WorkStreamsAsync(output, input, simdMode, howManyBytesToProcessAtTime);
	}

	/// <summary>
	/// Decrypt arbitrary-length byte array (input), writing the resulting byte array to preallocated output buffer.
	/// </summary>
	/// <remarks>Since this is symmetric operation, it doesn't really matter if you use Encrypt or Decrypt method</remarks>
	/// <param name="output">Output byte array, must have enough bytes</param>
	/// <param name="input">Input byte array</param>
	/// <param name="simdMode">Chosen SIMD mode (default is auto-detect)</param>
	public void DecryptBytes(byte[] output, byte[] input, SimdMode simdMode = SimdMode.AutoDetect)
	{
		if (output == null)
		{
			throw new ArgumentNullException("output", "Output cannot be null");
		}

		if (input == null)
		{
			throw new ArgumentNullException("input", "Input cannot be null");
		}

		if (simdMode == SimdMode.AutoDetect)
		{
			simdMode = DetectSimdMode();
		}

		WorkBytes(output, input, input.Length, simdMode);
	}

	/// <summary>
	/// Decrypt arbitrary-length byte array (input), writing the resulting byte array that is allocated by method.
	/// </summary>
	/// <remarks>Since this is symmetric operation, it doesn't really matter if you use Encrypt or Decrypt method</remarks>
	/// <param name="input">Input byte array</param>
	/// <param name="numBytes">Number of bytes to encrypt</param>
	/// <param name="simdMode">Chosen SIMD mode (default is auto-detect)</param>
	/// <returns>Byte array that contains decrypted bytes</returns>
	public byte[] DecryptBytes(byte[] input, int numBytes, SimdMode simdMode = SimdMode.AutoDetect)
	{
		if (input == null)
		{
			throw new ArgumentNullException("input", "Input cannot be null");
		}

		if (numBytes < 0 || numBytes > input.Length) 
		{
			throw new ArgumentOutOfRangeException("numBytes", "The number of bytes to read must be between [0..input.Length]");
		}

		if (simdMode == SimdMode.AutoDetect)
		{
			simdMode = DetectSimdMode();
		}

		byte[] returnArray = new byte[numBytes];
		WorkBytes(returnArray, input, numBytes, simdMode);
		return returnArray;
	}

	/// <summary>
	/// Decrypt arbitrary-length byte array (input), writing the resulting byte array that is allocated by method.
	/// </summary>
	/// <remarks>Since this is symmetric operation, it doesn't really matter if you use Encrypt or Decrypt method</remarks>
	/// <param name="input">Input byte array</param>
	/// <param name="simdMode">Chosen SIMD mode (default is auto-detect)</param>
	/// <returns>Byte array that contains decrypted bytes</returns>
	public byte[] DecryptBytes(byte[] input, SimdMode simdMode = SimdMode.AutoDetect)
	{
		if (input == null)
		{
			throw new ArgumentNullException("input", "Input cannot be null");
		}

		if (simdMode == SimdMode.AutoDetect)
		{
			simdMode = DetectSimdMode();
		}

		byte[] returnArray = new byte[input.Length];
		WorkBytes(returnArray, input, input.Length, simdMode);
		return returnArray;
	}

	/// <summary>
	/// Decrypt UTF8 byte array to string.
	/// </summary>
	/// <remarks>Here you can NOT swap encrypt and decrypt methods, because of bytes-string transform</remarks>
	/// <param name="input">Byte array</param>
	/// <param name="simdMode">Chosen SIMD mode (default is auto-detect)</param>
	/// <returns>Byte array that contains encrypted bytes</returns>
	public string DecryptUTF8ByteArray(byte[] input, SimdMode simdMode = SimdMode.AutoDetect)
	{
		if (input == null)
		{
			throw new ArgumentNullException("input", "Input cannot be null");
		}

		if (simdMode == SimdMode.AutoDetect)
		{
			simdMode = DetectSimdMode();
		}

		byte[] tempArray = new byte[input.Length];

		WorkBytes(tempArray, input, input.Length, simdMode);
		return System.Text.Encoding.UTF8.GetString(tempArray);
	}

	#endregion // Decryption methods

	private void WorkStreams(Stream output, Stream input, SimdMode simdMode, int howManyBytesToProcessAtTime = 1024)
	{
		int readBytes;

		byte[] inputBuffer = new byte[howManyBytesToProcessAtTime];
		byte[] outputBuffer = new byte[howManyBytesToProcessAtTime];

		while ((readBytes = input.Read(inputBuffer, 0, howManyBytesToProcessAtTime)) > 0)
		{
			// Encrypt or decrypt
			WorkBytes(output: outputBuffer, input: inputBuffer, numBytes: readBytes, simdMode);

			// Write buffer
			output.Write(outputBuffer, 0, readBytes);
		}	
	}

	private async Task WorkStreamsAsync(Stream output, Stream input, SimdMode simdMode, int howManyBytesToProcessAtTime = 1024)
	{
		byte[] readBytesBuffer = new byte[howManyBytesToProcessAtTime];
		byte[] writeBytesBuffer = new byte[howManyBytesToProcessAtTime];
		int howManyBytesWereRead = await input.ReadAsync(readBytesBuffer, 0, howManyBytesToProcessAtTime);

		while (howManyBytesWereRead > 0)
		{
			// Encrypt or decrypt
			WorkBytes(output: writeBytesBuffer, input: readBytesBuffer, numBytes: howManyBytesWereRead, simdMode);

			// Write
			await output.WriteAsync(writeBytesBuffer, 0, howManyBytesWereRead);

			// Read more
			howManyBytesWereRead = await input.ReadAsync(readBytesBuffer, 0, howManyBytesToProcessAtTime);
		}		
	}

	/// <summary>
	/// Encrypt or decrypt an arbitrary-length byte array (input), writing the resulting byte array to the output buffer. The number of bytes to read from the input buffer is determined by numBytes.
	/// </summary>
	/// <param name="output">Output byte array</param>
	/// <param name="input">Input byte array</param>
	/// <param name="numBytes">How many bytes to process</param>
	/// <param name="simdMode">Chosen SIMD mode (default is auto-detect)</param>
	private void WorkBytes(byte[] output, byte[] input, int numBytes, SimdMode simdMode) 
	{
		if (isDisposed) 
		{
			throw new ObjectDisposedException("state", "The ChaCha state has been disposed");
		}

		uint[] x = new uint[stateLength];    // Working buffer
		byte[] tmp = new byte[processBytesAtTime];  // Temporary buffer
		int offset = 0;

		int howManyFullLoops = numBytes / processBytesAtTime;
		int tailByteCount = numBytes - (howManyFullLoops * processBytesAtTime);

		for (int loop = 0; loop < howManyFullLoops; loop++) 
		{
			UpdateStateAndGenerateTemporaryBuffer(this.state, x, tmp);

			if (simdMode == SimdMode.V512)
			{
				// 1 x 64 bytes
				Vector512<byte> inputV = Vector512.Create(input, offset);
				Vector512<byte> tmpV = Vector512.Create(tmp, 0);
				Vector512<byte> outputV = inputV ^ tmpV;
				outputV.CopyTo(output, offset);
			}
			else if (simdMode == SimdMode.V256)
			{
				// 2 x 32 bytes
				Vector256<byte> inputV = Vector256.Create(input, offset);
				Vector256<byte> tmpV = Vector256.Create(tmp, 0);
				Vector256<byte> outputV = inputV ^ tmpV;
				outputV.CopyTo(output, offset);

				inputV = Vector256.Create(input, offset + 32);
				tmpV = Vector256.Create(tmp, 32);
				outputV = inputV ^ tmpV;
				outputV.CopyTo(output, offset + 32);
			}
			else if (simdMode == SimdMode.V128)
			{
				// 4 x 16 bytes
				Vector128<byte> inputV = Vector128.Create(input, offset);
				Vector128<byte> tmpV = Vector128.Create(tmp, 0);
				Vector128<byte> outputV = inputV ^ tmpV;
				outputV.CopyTo(output, offset);

				inputV = Vector128.Create(input, offset + 16);
				tmpV = Vector128.Create(tmp, 16);
				outputV = inputV ^ tmpV;
				outputV.CopyTo(output, offset + 16);

				inputV = Vector128.Create(input, offset + 32);
				tmpV = Vector128.Create(tmp, 32);
				outputV = inputV ^ tmpV;
				outputV.CopyTo(output, offset + 32);

				inputV = Vector128.Create(input, offset + 48);
				tmpV = Vector128.Create(tmp, 48);
				outputV = inputV ^ tmpV;
				outputV.CopyTo(output, offset + 48);
			}
			else
			{
				for (int i = 0; i < processBytesAtTime; i+=4 ) 
				{
					// Small unroll
					int start = i + offset;
					output[start] = (byte) (input[start] ^ tmp[i]);
					output[start + 1] = (byte) (input[start + 1] ^ tmp[i + 1]);
					output[start + 2] = (byte) (input[start + 2] ^ tmp[i + 2]);
					output[start + 3] = (byte) (input[start + 3] ^ tmp[i + 3]);
				}
			}

			offset += processBytesAtTime;
		}

		// In case there are some bytes left
		if (tailByteCount > 0)
		{
			UpdateStateAndGenerateTemporaryBuffer(this.state, x, tmp);

			for (int i = 0; i < tailByteCount; i++) 
			{
				output[i + offset] = (byte) (input[i + offset] ^ tmp[i]);
			}
		}
	}

	[MethodImpl(MethodImplOptions.AggressiveInlining)]
	private static void UpdateStateAndGenerateTemporaryBuffer(uint[] stateToModify, uint[] workingBuffer, byte[] temporaryBuffer)
	{
		// Copy state to working buffer
		Buffer.BlockCopy(stateToModify, 0, workingBuffer, 0, stateLength * sizeof(uint));

		for (int i = 0; i < 10; i++) 
		{
			QuarterRound(workingBuffer, 0, 4,  8, 12);
			QuarterRound(workingBuffer, 1, 5,  9, 13);
			QuarterRound(workingBuffer, 2, 6, 10, 14);
			QuarterRound(workingBuffer, 3, 7, 11, 15);

			QuarterRound(workingBuffer, 0, 5, 10, 15);
			QuarterRound(workingBuffer, 1, 6, 11, 12);
			QuarterRound(workingBuffer, 2, 7,  8, 13);
			QuarterRound(workingBuffer, 3, 4,  9, 14);
		}

		for (int i = 0; i < stateLength; i++) 
		{
			Util.ToBytes(temporaryBuffer, Util.Add(workingBuffer[i], stateToModify[i]), 4 * i);
		}

		stateToModify[12] = Util.AddOne(stateToModify[12]);
		if (stateToModify[12] <= 0) 
		{
			/* Stopping at 2^70 bytes per nonce is the user's responsibility */
			stateToModify[13] = Util.AddOne(stateToModify[13]);
		}
	}

	/// <summary>
	/// The ChaCha Quarter Round operation. It operates on four 32-bit unsigned integers within the given buffer at indices a, b, c, and d.
	/// </summary>
	/// <remarks>
	/// The ChaCha state does not have four integer numbers: it has 16. So the quarter-round operation works on only four of them -- hence the name. Each quarter round operates on four predetermined numbers in the ChaCha state.
	/// See <a href="https://tools.ietf.org/html/rfc7539#page-4">ChaCha20 Spec Sections 2.1 - 2.2</a>.
	/// </remarks>
	/// <param name="x">A ChaCha state (vector). Must contain 16 elements.</param>
	/// <param name="a">Index of the first number</param>
	/// <param name="b">Index of the second number</param>
	/// <param name="c">Index of the third number</param>
	/// <param name="d">Index of the fourth number</param>
	[MethodImpl(MethodImplOptions.AggressiveInlining)]
	private static void QuarterRound(uint[] x, uint a, uint b, uint c, uint d) 
	{
		x[a] = Util.Add(x[a], x[b]); 
		x[d] = Util.Rotate(Util.XOr(x[d], x[a]), 16);

		x[c] = Util.Add(x[c], x[d]); 
		x[b] = Util.Rotate(Util.XOr(x[b], x[c]), 12);

		x[a] = Util.Add(x[a], x[b]); 
		x[d] = Util.Rotate(Util.XOr(x[d], x[a]),  8);

		x[c] = Util.Add(x[c], x[d]); 
		x[b] = Util.Rotate(Util.XOr(x[b], x[c]),  7);
	}

	#region Destructor and Disposer

	/// <summary>
	/// Clear and dispose of the internal state. The finalizer is only called if Dispose() was never called on this cipher.
	/// </summary>
	~ChaCha20() 
	{
		Dispose(false);
	}

	/// <summary>
	/// Clear and dispose of the internal state. Also request the GC not to call the finalizer, because all cleanup has been taken care of.
	/// </summary>
	public void Dispose() 
	{
		Dispose(true);
		/*
			* The Garbage Collector does not need to invoke the finalizer because Dispose(bool) has already done all the cleanup needed.
			*/
		GC.SuppressFinalize(this);
	}

	/// <summary>
	/// This method should only be invoked from Dispose() or the finalizer. This handles the actual cleanup of the resources.
	/// </summary>
	/// <param name="disposing">
	/// Should be true if called by Dispose(); false if called by the finalizer
	/// </param>
	private void Dispose(bool disposing) 
	{
		if (!isDisposed) 
		{
			if (disposing) 
			{
				/* Cleanup managed objects by calling their Dispose() methods */
			}

			/* Cleanup any unmanaged objects here */
			Array.Clear(state, 0, stateLength);
		}

		isDisposed = true;
	}

	#endregion // Destructor and Disposer
}

/// <summary>
/// Utilities that are used during compression
/// </summary>
public static class Util 
{
	/// <summary>
	/// n-bit left rotation operation (towards the high bits) for 32-bit integers.
	/// </summary>
	/// <param name="v"></param>
	/// <param name="c"></param>
	/// <returns>The result of (v LEFTSHIFT c)</returns>
	[MethodImpl(MethodImplOptions.AggressiveInlining)]
	public static uint Rotate(uint v, int c) 
	{
		unchecked 
		{
			return (v << c) | (v >> (32 - c));
		}
	}

	/// <summary>
	/// Unchecked integer exclusive or (XOR) operation.
	/// </summary>
	/// <param name="v"></param>
	/// <param name="w"></param>
	/// <returns>The result of (v XOR w)</returns>
	[MethodImpl(MethodImplOptions.AggressiveInlining)]
	public static uint XOr(uint v, uint w) 
	{
		return unchecked(v ^ w);
	}

	/// <summary>
	/// Unchecked integer addition. The ChaCha spec defines certain operations to use 32-bit unsigned integer addition modulo 2^32.
	/// </summary>
	/// <remarks>
	/// See <a href="https://tools.ietf.org/html/rfc7539#page-4">ChaCha20 Spec Section 2.1</a>.
	/// </remarks>
	/// <param name="v"></param>
	/// <param name="w"></param>
	/// <returns>The result of (v + w) modulo 2^32</returns>
	[MethodImpl(MethodImplOptions.AggressiveInlining)]
	public static uint Add(uint v, uint w) 
	{
		return unchecked(v + w);
	}

	/// <summary>
	/// Add 1 to the input parameter using unchecked integer addition. The ChaCha spec defines certain operations to use 32-bit unsigned integer addition modulo 2^32.
	/// </summary>
	/// <remarks>
	/// See <a href="https://tools.ietf.org/html/rfc7539#page-4">ChaCha20 Spec Section 2.1</a>.
	/// </remarks>
	/// <param name="v"></param>
	/// <returns>The result of (v + 1) modulo 2^32</returns>
	[MethodImpl(MethodImplOptions.AggressiveInlining)]
	public static uint AddOne(uint v) 
	{
		return unchecked(v + 1);
	}

	/// <summary>
	/// Convert four bytes of the input buffer into an unsigned 32-bit integer, beginning at the inputOffset.
	/// </summary>
	/// <param name="p"></param>
	/// <param name="inputOffset"></param>
	/// <returns>An unsigned 32-bit integer</returns>
	[MethodImpl(MethodImplOptions.AggressiveInlining)]
	public static uint U8To32Little(byte[] p, int inputOffset) 
	{
		unchecked 
		{
			return ((uint) p[inputOffset]
				| ((uint) p[inputOffset + 1] << 8)
				| ((uint) p[inputOffset + 2] << 16)
				| ((uint) p[inputOffset + 3] << 24));
		}
	}

	/// <summary>
	/// Serialize the input integer into the output buffer. The input integer will be split into 4 bytes and put into four sequential places in the output buffer, starting at the outputOffset.
	/// </summary>
	/// <param name="output"></param>
	/// <param name="input"></param>
	/// <param name="outputOffset"></param>
	[MethodImpl(MethodImplOptions.AggressiveInlining)]
	public static void ToBytes(byte[] output, uint input, int outputOffset) 
	{
		unchecked 
		{
			output[outputOffset]     = (byte) input;
			output[outputOffset + 1] = (byte) (input >> 8);
			output[outputOffset + 2] = (byte) (input >> 16);
			output[outputOffset + 3] = (byte) (input >> 24);
		}
	}
}