/// <summary>
/// Decrypts a cipher with an authentication tag and additional data using AES-GCM.
/// </summary>
/// <param name="cipher">The cipher to be decrypted.</param>
/// <param name="nonce">The 12 byte nonce.</param>
/// <param name="key">The 32 byte key.</param>
/// <param name="additionalData">The additional data; may be null, otherwise between 0 and 16 bytes.</param>
/// <returns>The decrypted cipher.</returns>
/// <exception cref="KeyOutOfRangeException"></exception>
/// <exception cref="NonceOutOfRangeException"></exception>
/// <exception cref="AdditionalDataOutOfRangeException"></exception>
/// <exception cref="CryptographicException"></exception>
public static byte[] DecryptDetached(byte[] cipher, byte[] nonce, byte[] key, byte[] mac, byte[] additionalData = null)
{
//additionalData can be null
if (additionalData == null)
{
additionalData = new byte[0x00];
}
//validate the length of the key
if (key == null || key.Length != KEYBYTES)
{
throw new KeyOutOfRangeException("key", (key == null) ? 0 : key.Length,
string.Format("key must be {0} bytes in length.", KEYBYTES));
}
//validate the length of the nonce
if (nonce == null || nonce.Length != NPUBBYTES)
{
throw new NonceOutOfRangeException("nonce", (nonce == null) ? 0 : nonce.Length,
string.Format("nonce must be {0} bytes in length.", NPUBBYTES));
}
//validate the length of the additionalData
/*if (additionalData.Length > ABYTES || additionalData.Length < 0)
* throw new AdditionalDataOutOfRangeException(
* string.Format("additionalData must be between {0} and {1} bytes in length.", 0, ABYTES));*/
var message = new byte[cipher.Length - additionalData.Length];
var bin = Marshal.AllocHGlobal(message.Length);
var ret = SodiumLibrary.crypto_aead_aes256gcm_decrypt_detached(bin, null, cipher, cipher.Length, mac,
additionalData, additionalData.Length, nonce, key);
Marshal.Copy(bin, message, 0, cipher.Length);
Marshal.FreeHGlobal(bin);
if (ret != 0)
{
throw new CryptographicException("Error decrypting message.");
}
if (message.Length == cipher.Length)
{
return(message);
}
//remove the trailing nulls from the array
var tmp = new byte[cipher.Length];
Array.Copy(message, 0, tmp, 0, (int)cipher.Length);
return(tmp);
}
