/// <summary>
/// Decrypts authenticated ciphertext using the Rijndael cipher in CBC mode with a password derived
/// HMAC SHA-512 salt.
/// </summary>
/// <param name="etmCiphertext">The EtM ciphertext to decrypt.</param>
/// <param name="password">The password to decrypt the EtM ciphertext with.</param>
/// <param name="keySize">The size of the cipher key used to create the EtM ciphertext.</param>
/// <returns>The plaintext.</returns>
public static new string Decrypt(byte[] etmCiphertext, string password, KeySize keySize)
{
// Generate AE keys
var keyRing = AeKeyRing.Generate(password);
// Extract the ciphertext and MAC from the EtM ciphertext
var mac = new byte[keyRing.MacKey.Length];
var ciphertext = new byte[etmCiphertext.Length - mac.Length];
using (var ms = new MemoryStream(etmCiphertext))
{
// Extract the ciphertext
ms.Read(ciphertext, 0, ciphertext.Length);
// Extract the MAC
ms.Read(mac, 0, mac.Length);
}
// Calculate the MAC from the ciphertext
var newMac = CalculateMac(ciphertext, keyRing.MacKey);
// Authenticate ciphertext
if (!mac.SequenceEqual(newMac))
{
throw new Exception("Authentication failed!");
}
// Decrypt the ciphertext
return(Rijndael.Decrypt(ciphertext, keyRing.CipherKey, keySize));
}
/// <summary>
/// Encrypts plaintext using the Encrypt-then-MAC (EtM) mode via the Rijndael cipher in
/// CBC mode with a password derived HMAC SHA-512 salt.
/// </summary>
/// <param name="plaintext">The plaintext to encrypt.</param>
/// <param name="password">The password to encrypt the plaintext with.</param>
/// <param name="iv">The initialization vector. Must be 128-bits.</param>
/// <param name="keySize">The cipher key size. 256-bit is stronger, but slower.</param>
/// <returns>The EtM ciphertext.</returns>
public static new byte[] Encrypt(byte[] plaintext, string password, byte[] iv, KeySize keySize)
{
// Generate AE keys
var keyRing = AeKeyRing.Generate(password);
// Encrypt the plaintext
var ciphertext = Rijndael.Encrypt(plaintext, keyRing.CipherKey, iv, keySize);
// Calculate the MAC from the ciphertext
var mac = CalculateMac(ciphertext, keyRing.MacKey);
// Append the MAC to the ciphertext
var etmCiphertext = new byte[ciphertext.Length + mac.Length];
Buffer.BlockCopy(ciphertext, 0, etmCiphertext, 0, ciphertext.Length);
Buffer.BlockCopy(mac, 0, etmCiphertext, ciphertext.Length, mac.Length);
// IV + Cipher + MAC
return(etmCiphertext);
}
