Thursday, January 29, 2009

Encryption in C# .NET

This class uses a symmetric key algorithm (Rijndael/AES) to encrypt and decrypt data. As long as encryption and decryption routines use the same parameters to generate the keys, the keys are guaranteed to be the same.

In a real-life application, this may not be the most efficient way of handling encryption, so as soon as you feel comfortable with it you may want to redesign this class.


public class Encryption
{
    /// <summary>
    /// Encrypts specified text using Rijndael symmetric key algorithm
    /// and returns a base64-encoded result.
    /// </summary>
    /// <param name="plainText">
    /// Plaintext value to be encrypted.
    /// </param>
    /// <param name="passPhrase">
    /// Passphrase from which a pseudo-random password will be derived. The
    /// derived password will be used to generate the encryption key.
    /// Passphrase can be any string. In this example we assume that this
    /// passphrase is an ASCII string.        
    /// </param>
    /// <param name="saltValue">
    /// Salt value used along with passphrase to generate password. Salt can
    /// be any string. In this example we assume that salt is an ASCII string.
    /// </param>
    /// <param name="hashAlgorithm">
    /// Hash algorithm used to generate password. Allowed values are: "MD5" and
    /// "SHA1". SHA1 hashes are a bit slower, but more secure than MD5 hashes.
    /// </param>
    /// <param name="passwordIterations">
    /// Number of iterations used to generate password. One or two iterations
    /// should be enough.
    ///</param>
    /// <param name="initVector">
    /// Initialization vector (or IV). This value is required to encrypt the
    /// first block of plaintext data. For RijndaelManaged class IV must be
    /// exactly 16 ASCII characters long.
    ///</param>
    /// <param name="keySize">
    /// Size of encryption key in bits. Allowed values are: 128, 192, and 256.
    /// Longer keys are more secure than shorter keys.
    ///</param>
    /// <returns>
    /// Encrypted value formatted as a base64-encoded string.
    /// </returns>
    public string Encrypt(string plainText, string passPhrase, string saltValue, string hashAlgorithm, int passwordIterations, string initVector, int keySize)
    {
        // Convert strings into byte arrays.
        // Let us assume that strings only contain ASCII codes.
        // If strings include Unicode characters, use Unicode, UTF7, or UTF8
        // encoding.
        byte[] initVectorBytes = Encoding.ASCII.GetBytes(initVector);
        byte[] saltValueBytes = Encoding.ASCII.GetBytes(saltValue);

        // Convert our plaintext into a byte array.
        // Let us assume that plaintext contains UTF8-encoded characters.
        byte[] plainTextBytes = Encoding.UTF8.GetBytes(plainText);

        // First, we must create a password, from which the key will be derived.
        // This password will be generated from the specified passphrase and
        // salt value. The password will be created using the specified hash
        // algorithm. Password creation can be done in several iterations.
        PasswordDeriveBytes password = new PasswordDeriveBytes(passPhrase, saltValueBytes, hashAlgorithm, passwordIterations);

        // Use the password to generate pseudo-random bytes for the encryption
        // key. Specify the size of the key in bytes (instead of bits).
        byte[] keyBytes = password.GetBytes(keySize / 8);

        // Create uninitialized Rijndael encryption object.
        RijndaelManaged symmetricKey = new RijndaelManaged();

        // It is reasonable to set encryption mode to Cipher Block Chaining
        // (CBC). Use default options for other symmetric key parameters.
        symmetricKey.Mode = CipherMode.CBC;

        // Generate encryptor from the existing key bytes and initialization
        // vector. Key size will be defined based on the number of the key
        // bytes.
        ICryptoTransform encryptor = symmetricKey.CreateEncryptor(keyBytes, initVectorBytes);

        // Define memory stream which will be used to hold encrypted data.
        MemoryStream memoryStream = new MemoryStream();

        // Define cryptographic stream (always use Write mode for encryption).
        CryptoStream cryptoStream = new CryptoStream(memoryStream, encryptor, CryptoStreamMode.Write);
        // Start encrypting.
        cryptoStream.Write(plainTextBytes, 0, plainTextBytes.Length);

        // Finish encrypting.
        cryptoStream.FlushFinalBlock();

        // Convert our encrypted data from a memory stream into a byte array.
        byte[] cipherTextBytes = memoryStream.ToArray();

        // Close both streams.
        memoryStream.Close();
        cryptoStream.Close();

        // Convert encrypted data into a base64-encoded string.
        string cipherText = Convert.ToBase64String(cipherTextBytes);

        // Return encrypted string.
        return cipherText;
    }

    /// <summary>
    /// Decrypts specified ciphertext using Rijndael symmetric key algorithm.
    /// </summary>
    /// <param name="cipherText">
    /// Base64-formatted ciphertext value.
    ///</param>
    /// <param name="passPhrase">
    /// Passphrase from which a pseudo-random password will be derived. The
    /// derived password will be used to generate the encryption key.
    /// Passphrase can be any string. In this example we assume that this
    /// passphrase is an ASCII string.
    ///</param>
    /// <param name="saltValue">
    /// Salt value used along with passphrase to generate password. Salt can
    /// be any string. In this example we assume that salt is an ASCII string.
    ///</param>
    /// <param name="hashAlgorithm">
    /// Hash algorithm used to generate password. Allowed values are: "MD5" and
    /// "SHA1". SHA1 hashes are a bit slower, but more secure than MD5 hashes.
    ///</param>
    /// <param name="passwordIterations">
    /// Number of iterations used to generate password. One or two iterations
    /// should be enough.
    ///</param>
    /// <param name="initVector">
    /// Initialization vector (or IV). This value is required to encrypt the
    /// first block of plaintext data. For RijndaelManaged class IV must be
    /// exactly 16 ASCII characters long.
    ///</param>
    /// <param name="keySize">
    /// Size of encryption key in bits. Allowed values are: 128, 192, and 256.
    /// Longer keys are more secure than shorter keys.
    ///</param>
    /// <returns>
    /// Decrypted string value.
    /// </returns>
    /// <remarks>
    /// Most of the logic in this function is similar to the Encrypt
    /// logic. In order for decryption to work, all parameters of this function
    /// - except cipherText value - must match the corresponding parameters of
    /// the Encrypt function which was called to generate the
    /// ciphertext.
    /// </remarks>
    public string Decrypt(string cipherText, string passPhrase, string saltValue, string hashAlgorithm, int passwordIterations, string initVector, int keySize)
    {
        // Convert strings defining encryption key characteristics into byte
        // arrays. Let us assume that strings only contain ASCII codes.
        // If strings include Unicode characters, use Unicode, UTF7, or UTF8
        // encoding.
        byte[] initVectorBytes = Encoding.ASCII.GetBytes(initVector);
        byte[] saltValueBytes = Encoding.ASCII.GetBytes(saltValue);

        // Convert our ciphertext into a byte array.
        byte[] cipherTextBytes = Convert.FromBase64String(cipherText);

        // First, we must create a password, from which the key will be
        // derived. This password will be generated from the specified
        // passphrase and salt value. The password will be created using
        // the specified hash algorithm. Password creation can be done in
        // several iterations.
        PasswordDeriveBytes password = new PasswordDeriveBytes(passPhrase, saltValueBytes, hashAlgorithm, passwordIterations);

        // Use the password to generate pseudo-random bytes for the encryption
        // key. Specify the size of the key in bytes (instead of bits).
        byte[] keyBytes = password.GetBytes(keySize / 8);

        // Create uninitialized Rijndael encryption object.
        RijndaelManaged symmetricKey = new RijndaelManaged();

        // It is reasonable to set encryption mode to Cipher Block Chaining
        // (CBC). Use default options for other symmetric key parameters.
        symmetricKey.Mode = CipherMode.CBC;

        // Generate decryptor from the existing key bytes and initialization
        // vector. Key size will be defined based on the number of the key
        // bytes.
        ICryptoTransform decryptor = symmetricKey.CreateDecryptor(keyBytes, initVectorBytes);

        // Define memory stream which will be used to hold encrypted data.
        MemoryStream memoryStream = new MemoryStream(cipherTextBytes);

        // Define cryptographic stream (always use Read mode for encryption).
        CryptoStream cryptoStream = new CryptoStream(memoryStream, decryptor, CryptoStreamMode.Read);

        // Since at this point we don't know what the size of decrypted data
        // will be, allocate the buffer long enough to hold ciphertext;
        // plaintext is never longer than ciphertext.
        byte[] plainTextBytes = new byte[cipherTextBytes.Length];

        // Start decrypting.
        int decryptedByteCount = cryptoStream.Read(plainTextBytes, 0, plainTextBytes.Length);

        // Close both streams.
        memoryStream.Close();
        cryptoStream.Close();

        // Convert decrypted data into a string.
        // Let us assume that the original plaintext string was UTF8-encoded.
        string plainText = Encoding.UTF8.GetString(plainTextBytes, 0, decryptedByteCount);

        // Return decrypted string.
        return plainText;
    }
}

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