Bouncy Castle AES-GCM

using System;
using System.IO;
using System.Text;
using Org.BouncyCastle.Crypto;
using Org.BouncyCastle.Crypto.Engines;
using Org.BouncyCastle.Crypto.Generators;
using Org.BouncyCastle.Crypto.Modes;
using Org.BouncyCastle.Crypto.Parameters;
using Org.BouncyCastle.Security;
namespace Encryption
{

  public static class AESGCM
  {
    private static readonly SecureRandom Random = new SecureRandom();

    //Preconfigured Encryption Parameters
    public static readonly int NonceBitSize = 128;
    public static readonly int MacBitSize = 128;
    public static readonly int KeyBitSize = 256;

    //Preconfigured Password Key Derivation Parameters
    public static readonly int SaltBitSize = 128;
    public static readonly int Iterations = 10000;
    public static readonly int MinPasswordLength = 12;


    /// <summary>
    /// Helper that generates a random new key on each call.
    /// </summary>
    /// <returns></returns>
    public static byte[] NewKey()
    {
      var key = new byte[KeyBitSize / 8];
      Random.NextBytes(key);
      return key;
    }

    /// <summary>
    /// Simple Encryption And Authentication (AES-GCM) of a UTF8 string.
    /// </summary>
    /// <param name="secretMessage">The secret message.</param>
    /// <param name="key">The key.</param>
    /// <param name="nonSecretPayload">Optional non-secret payload.</param>
    /// <returns>
    /// Encrypted Message
    /// </returns>
    /// <exception cref="System.ArgumentException">Secret Message Required!;secretMessage</exception>
    /// <remarks>
    /// Adds overhead of (Optional-Payload + BlockSize(16) + Message +  HMac-Tag(16)) * 1.33 Base64
    /// </remarks>
    public static string SimpleEncrypt(string secretMessage, byte[] key, byte[] nonSecretPayload = null)
    {
      if (string.IsNullOrEmpty(secretMessage))
        throw new ArgumentException("Secret Message Required!", "secretMessage");

      var plainText = Encoding.UTF8.GetBytes(secretMessage);
      var cipherText = SimpleEncrypt(plainText, key, nonSecretPayload);
      return Convert.ToBase64String(cipherText);
    }


    /// <summary>
    /// Simple Decryption & Authentication (AES-GCM) of a UTF8 Message
    /// </summary>
    /// <param name="encryptedMessage">The encrypted message.</param>
    /// <param name="key">The key.</param>
    /// <param name="nonSecretPayloadLength">Length of the optional non-secret payload.</param>
    /// <returns>Decrypted Message</returns>
    public static string SimpleDecrypt(string encryptedMessage, byte[] key, int nonSecretPayloadLength = 0)
    {
      if (string.IsNullOrEmpty(encryptedMessage))
        throw new ArgumentException("Encrypted Message Required!", "encryptedMessage");

      var cipherText = Convert.FromBase64String(encryptedMessage);
      var plaintext = SimpleDecrypt(cipherText, key, nonSecretPayloadLength);
      return plaintext == null ? null : Encoding.UTF8.GetString(plaintext);
    }

    /// <summary>
    /// Simple Encryption And Authentication (AES-GCM) of a UTF8 String
    /// using key derived from a password (PBKDF2).
    /// </summary>
    /// <param name="secretMessage">The secret message.</param>
    /// <param name="password">The password.</param>
    /// <param name="nonSecretPayload">The non secret payload.</param>
    /// <returns>
    /// Encrypted Message
    /// </returns>
    /// <remarks>
    /// Significantly less secure than using random binary keys.
    /// Adds additional non secret payload for key generation parameters.
    /// </remarks>
    public static string SimpleEncryptWithPassword(string secretMessage, string password,
                             byte[] nonSecretPayload = null)
    {
      if (string.IsNullOrEmpty(secretMessage))
        throw new ArgumentException("Secret Message Required!", "secretMessage");

      var plainText = Encoding.UTF8.GetBytes(secretMessage);
      var cipherText = SimpleEncryptWithPassword(plainText, password, nonSecretPayload);
      return Convert.ToBase64String(cipherText);
    }


    /// <summary>
    /// Simple Decryption and Authentication (AES-GCM) of a UTF8 message
    /// using a key derived from a password (PBKDF2)
    /// </summary>
    /// <param name="encryptedMessage">The encrypted message.</param>
    /// <param name="password">The password.</param>
    /// <param name="nonSecretPayloadLength">Length of the non secret payload.</param>
    /// <returns>
    /// Decrypted Message
    /// </returns>
    /// <exception cref="System.ArgumentException">Encrypted Message Required!;encryptedMessage</exception>
    /// <remarks>
    /// Significantly less secure than using random binary keys.
    /// </remarks>
    public static string SimpleDecryptWithPassword(string encryptedMessage, string password,
                             int nonSecretPayloadLength = 0)
    {
      if (string.IsNullOrWhiteSpace(encryptedMessage))
        throw new ArgumentException("Encrypted Message Required!", "encryptedMessage");

      var cipherText = Convert.FromBase64String(encryptedMessage);
      var plaintext = SimpleDecryptWithPassword(cipherText, password, nonSecretPayloadLength);
      return plaintext == null ? null : Encoding.UTF8.GetString(plaintext);
    }

    public static byte[] SimpleEncrypt(byte[] secretMessage, byte[] key, byte[] nonSecretPayload = null)
    {
      //User Error Checks
      if (key == null || key.Length != KeyBitSize / 8)
        throw new ArgumentException(String.Format("Key needs to be {0} bit!", KeyBitSize), "key");

      if (secretMessage == null || secretMessage.Length == 0)
        throw new ArgumentException("Secret Message Required!", "secretMessage");

      //Non-secret Payload Optional
      nonSecretPayload = nonSecretPayload ?? new byte[] { };

      //Using random nonce large enough not to repeat
      var nonce = new byte[NonceBitSize / 8];
      Random.NextBytes(nonce, 0, nonce.Length);

      var cipher = new GcmBlockCipher(new AesFastEngine());
      var parameters = new AeadParameters(new KeyParameter(key), MacBitSize, nonce, nonSecretPayload);
      cipher.Init(true, parameters);

      //Generate Cipher Text With Auth Tag
      var cipherText = new byte[cipher.GetOutputSize(secretMessage.Length)];
      var len = cipher.ProcessBytes(secretMessage, 0, secretMessage.Length, cipherText, 0);
      cipher.DoFinal(cipherText, len);

      //Assemble Message
      using (var combinedStream = new MemoryStream())
      {
        using (var binaryWriter = new BinaryWriter(combinedStream))
        {
          //Prepend Authenticated Payload
          binaryWriter.Write(nonSecretPayload);
          //Prepend Nonce
          binaryWriter.Write(nonce);
          //Write Cipher Text
          binaryWriter.Write(cipherText);
        }
        return combinedStream.ToArray();
      }
    }

    public static byte[] SimpleDecrypt(byte[] encryptedMessage, byte[] key, int nonSecretPayloadLength = 0)
    {
      //User Error Checks
      if (key == null || key.Length != KeyBitSize / 8)
        throw new ArgumentException(String.Format("Key needs to be {0} bit!", KeyBitSize), "key");

      if (encryptedMessage == null || encryptedMessage.Length == 0)
        throw new ArgumentException("Encrypted Message Required!", "encryptedMessage");

      using (var cipherStream = new MemoryStream(encryptedMessage))
      using (var cipherReader = new BinaryReader(cipherStream))
      {
        //Grab Payload
        var nonSecretPayload = cipherReader.ReadBytes(nonSecretPayloadLength);

        //Grab Nonce
        var nonce = cipherReader.ReadBytes(NonceBitSize / 8);

        var cipher = new GcmBlockCipher(new AesFastEngine());
        var parameters = new AeadParameters(new KeyParameter(key), MacBitSize, nonce, nonSecretPayload);
        cipher.Init(false, parameters);

        //Decrypt Cipher Text
        var cipherText = cipherReader.ReadBytes(encryptedMessage.Length - nonSecretPayloadLength - nonce.Length);
        var plainText = new byte[cipher.GetOutputSize(cipherText.Length)];

        try
        {
          var len = cipher.ProcessBytes(cipherText, 0, cipherText.Length, plainText, 0);
          cipher.DoFinal(plainText, len);

        }
        catch (InvalidCipherTextException)
        {
          //Return null if it doesn't authenticate
          return null;
        }

        return plainText;
      }

    }

    public static byte[] SimpleEncryptWithPassword(byte[] secretMessage, string password, byte[] nonSecretPayload = null)
    {
      nonSecretPayload = nonSecretPayload ?? new byte[] {};

      //User Error Checks
      if (string.IsNullOrWhiteSpace(password) || password.Length < MinPasswordLength)
        throw new ArgumentException(String.Format("Must have a password of at least {0} characters!", MinPasswordLength), "password");

      if (secretMessage == null || secretMessage.Length == 0)
        throw new ArgumentException("Secret Message Required!", "secretMessage");

      var generator = new Pkcs5S2ParametersGenerator();

      //Use Random Salt to minimize pre-generated weak password attacks.
      var salt = new byte[SaltBitSize / 8];
      Random.NextBytes(salt);

      generator.Init(
        PbeParametersGenerator.Pkcs5PasswordToBytes(password.ToCharArray()),
        salt,
        Iterations);

      //Generate Key
      var key = (KeyParameter)generator.GenerateDerivedMacParameters(KeyBitSize);

      //Create Full Non Secret Payload
      var payload = new byte[salt.Length + nonSecretPayload.Length];
      Array.Copy(nonSecretPayload, payload, nonSecretPayload.Length);
      Array.Copy(salt,0, payload,nonSecretPayload.Length, salt.Length);

      return SimpleEncrypt(secretMessage, key.GetKey(), payload);
    }

    public static byte[] SimpleDecryptWithPassword(byte[] encryptedMessage, string password, int nonSecretPayloadLength = 0)
    {
      //User Error Checks
      if (string.IsNullOrWhiteSpace(password) || password.Length < MinPasswordLength)
        throw new ArgumentException(String.Format("Must have a password of at least {0} characters!", MinPasswordLength), "password");

      if (encryptedMessage == null || encryptedMessage.Length == 0)
        throw new ArgumentException("Encrypted Message Required!", "encryptedMessage");

      var generator = new Pkcs5S2ParametersGenerator();

      //Grab Salt from Payload
      var salt = new byte[SaltBitSize / 8];
      Array.Copy(encryptedMessage, nonSecretPayloadLength, salt, 0, salt.Length);

      generator.Init(
        PbeParametersGenerator.Pkcs5PasswordToBytes(password.ToCharArray()),
        salt,
        Iterations);

      //Generate Key
      var key = (KeyParameter)generator.GenerateDerivedMacParameters(KeyBitSize);

      return SimpleDecrypt(encryptedMessage, key.GetKey(), salt.Length + nonSecretPayloadLength);
    }
  }
}