## XTEA Block Cipher

Usually people recommend to never implement cryptographic algorithms yourself unless you are an expert in the field. This of course makes a lot of sense mainly because there are numerous things that can go wrong, resulting in a vulnerable implementation.

However, for learning purposes I decided to write an implementation of the XTEA block cipher myself nonetheless. XTEA is a 64-bit block feistel cipher with a 128-bit key.

What can be improved in my implementation? Are there any major flaws?

ISymmetricEncryptionProvider.cs

``namespace Crypto {     /// <summary>     /// Provides a base for symmetric encryption algorithms     /// </summary>     public interface ISymmetricEncryptionProvider     {          /// <summary>         /// Symmetric encryption routine         /// </summary>         /// <param name="data">The data that should get encrypted</param>         /// <returns>The encrypted data</returns>         byte[] Encrypt(byte[] data);           /// <summary>         /// Symmetric decryption routine          /// </summary>         /// <param name="data">The data that should get decrypted</param>         /// <returns>The decrypted data</returns>         byte[] Decrypt(byte[] data);      } } ``

XTEA.cs

``using System; using System.IO;  namespace Crypto {     /// <summary>     /// Like TEA, XTEA is a 64-bit block Feistel cipher with a 128-bit key and a suggested     /// 64 rounds. Several differences from TEA are apparent, including a somewhat     /// more complex key-schedule and a rearrangement of the shifts, XORs, and additions.     ///      /// More information can be found here:     /// + https://en.wikipedia.org/wiki/XTEA     /// + http://www.tayloredge.com/reference/Mathematics/TEA-XTEA.pdf     /// </summary>     public class XTEA : ISymmetricEncryptionProvider     {         /// <summary>         /// The 128 bit key used for encryption and decryption         /// </summary>         private readonly uint[] _key;           /// <summary>         /// The number of rounds, default is 32 because each iteration performs two Feistel-cipher rounds.         /// </summary>         private readonly uint _cycles;           /// <summary>         /// XTEA operates with a block size of 8 bytes         /// </summary>         private readonly uint _blockSize = 8;           /// <summary>         /// The delta is derived from the golden ratio where delta = (sqrt(2) - 1) * 2^31         /// A different multiple of delta is used in each round so that no bit of         /// the multiple will not change frequently         /// </summary>         private const uint Delta = 0x9E3779B9;           /// <summary>         /// Instantiate new XTEA object for encryption/decryption         /// </summary>         /// <param name="key">The encryption/decryption key</param>         /// <param name="cycles">Number of cycles performed, default is 32</param>         public XTEA(byte[] key, uint cycles = 32)         {             _key = GenerateKey(key);             _cycles = cycles;         }           /// <summary>         /// Calculates the next multiple of the block size of the input data because         /// XTEA is a 64-bit cipher.         /// </summary>         /// <param name="length">Input data size</param>         /// <returns>Input data extended to the next multiple of the block size.</returns>         private uint NextMultipleOfBlockSize(uint length)         {             return (length + 7) / _blockSize * _blockSize;         }           /// <summary>         /// Encrypts the provided data with XTEA         /// </summary>         /// <param name="data">The data to encrypt</param>         /// <returns>Encrypted data as byte array</returns>         public byte[] Encrypt(byte[] data)         {             var blockBuffer = new uint[2];             var dataBuffer = new byte[NextMultipleOfBlockSize((uint)data.Length + 4)];             var lengthBuffer = BitConverter.GetBytes(data.Length);              Buffer.BlockCopy(lengthBuffer, 0, dataBuffer, 0, lengthBuffer.Length);             Buffer.BlockCopy(data, 0, dataBuffer, lengthBuffer.Length, data.Length);              using (var memoryStream = new MemoryStream(dataBuffer))             using (var binaryWriter = new BinaryWriter(memoryStream))                 for (uint i = 0; i < dataBuffer.Length; i += _blockSize)                 {                     blockBuffer[0] = BitConverter.ToUInt32(dataBuffer, (int) i);                     blockBuffer[1] = BitConverter.ToUInt32(dataBuffer, (int) i + 4);                      Encode(_cycles, blockBuffer, _key);                      binaryWriter.Write(blockBuffer[0]);                     binaryWriter.Write(blockBuffer[1]);                 }              return dataBuffer;         }           /// <summary>         /// Decrypts the provided data with XTEA         /// </summary>         /// <param name="data">The data to decrypt</param>         /// <returns>The decrypted data as byte array</returns>         public byte[] Decrypt(byte[] data)         {             // Encrypted data size must be a multiple of the block size             if (data.Length % _blockSize != 0)                 throw new ArgumentOutOfRangeException(nameof(data));              var blockBuffer = new uint[2];             var buffer = new byte[data.Length];              Buffer.BlockCopy(data, 0, buffer, 0, data.Length);              using (var memoryStream = new MemoryStream(buffer))             using (var binaryWriter = new BinaryWriter(memoryStream))             {                 for (uint i = 0; i < buffer.Length; i += _blockSize)                 {                     blockBuffer[0] = BitConverter.ToUInt32(buffer, (int) i);                     blockBuffer[1] = BitConverter.ToUInt32(buffer, (int) i + 4);                      Decode(_cycles, blockBuffer, _key);                      binaryWriter.Write(blockBuffer[0]);                     binaryWriter.Write(blockBuffer[1]);                 }             }              // Verify if length of output data is valid             var length = BitConverter.ToUInt32(buffer, 0);             VerifyDataLength(length, buffer.Length, 4);              // Trim first 4 bytes of output data                         return TrimOutputData(length, buffer, 4);         }           /// <summary>         /// Removes the first n bytes from the buffer         /// </summary>         /// <param name="length">Length of the output buffer</param>         /// <param name="buffer">The output buffer</param>         /// <param name="trimSize">Number of bytes to trim from the start of the buffer</param>         /// <returns>Trimmed output buffer array</returns>         private byte[] TrimOutputData(uint length, byte[] buffer, int trimSize)         {             var result = new byte[length];             Buffer.BlockCopy(buffer, trimSize, result, 0, (int) length);             return result;         }           /// <summary>         /// Compares the length of the output data from a specified offset to the length of the buffer         /// </summary>         /// <param name="dataLength">Length of the output data</param>         /// <param name="bufferLength">Length of the buffer</param>         /// <param name="offset">The offset</param>         /// <exception cref="ArgumentOutOfRangeException">Thrown if buffer data is corrupted</exception>         private void VerifyDataLength(uint dataLength, int bufferLength, uint offset)         {             if (dataLength > bufferLength - offset)                 throw new ArgumentOutOfRangeException(nameof(bufferLength));         }           /// <summary>         /// Transforms the key to uint[]         /// </summary>         /// <returns>Transformed key</returns>         private uint[] GenerateKey(byte[] key)         {             if (key.Length != 16)                 throw new ArgumentOutOfRangeException(nameof(key));              return new[]             {                 BitConverter.ToUInt32(key, 0), BitConverter.ToUInt32(key, 4),                 BitConverter.ToUInt32(key, 8), BitConverter.ToUInt32(key, 12)             };         }           /// <summary>         /// TEA inplace encoding routine of the provided data array.         /// </summary>         /// <param name="rounds">The number of encryption rounds, default is 32.</param>         /// <param name="v">The data array containing two values.</param>         /// <param name="k">The key array containing 4 values.</param>         private void Encode(uint rounds, uint[] v, uint[] k)         {             uint sum = 0;             uint v0 = v[0], v1 = v[1];             for (int i = 0; i < rounds; i++)             {                 v0 += (((v1 << 4) ^ (v1 >> 5)) + v1) ^ (sum + k[sum & 3]);                 sum += Delta;                 v1 += (((v0 << 4) ^ (v0 >> 5)) + v0) ^ (sum + k[(sum >> 11) & 3]);             }              v[0] = v0;             v[1] = v1;         }           /// <summary>         /// TEA inplace decoding routine of the provided data array.         /// </summary>         /// <param name="rounds">The number of encryption rounds, default is 32.</param>         /// <param name="v">The data array containing two values.</param>         /// <param name="k">The key array containing 4 values.</param>         private void Decode(uint rounds, uint[] v, uint[] k)         {             uint sum = Delta * rounds;             uint v0 = v[0], v1 = v[1];             for (int i = 0; i < rounds; i++)             {                 v1 -= (((v0 << 4) ^ (v0 >> 5)) + v0) ^ (sum + k[(sum >> 11) & 3]);                 sum -= Delta;                 v0 -= (((v1 << 4) ^ (v1 >> 5)) + v1) ^ (sum + k[sum & 3]);             }              v[0] = v0;             v[1] = v1;         }      } } ``

Here is an example usage of the class:

``public static void Main(string[] args) {     byte[] key = {0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07,                    0x08, 0x09, 0x0A, 0x0B, 0x0C, 0x0D, 0x0E, 0x0F};      byte[] data = Encoding.UTF8.GetBytes("This is a message which will be encrypted with XTEA!");      var xtea = new XTEA(key);     var enc = xtea.Encrypt(data);     var dec = xtea.Decrypt(enc);      Console.WriteLine(Encoding.UTF8.GetString(dec));     Console.ReadKey(); } ``

References

Credit where credit belongs. During the writing of this class used the following websites/books as a guideline:

The Tiny Encryption Algorithm (TEA)

C# implementation of XTEA

C# CarestiaXTEA

C++ XTEA Implementation