/// <summary>
/// Generic Z_n FPE encryption, FE1 scheme
/// </summary>
/// <param name="modulus">Use to determine the range of the numbers. Example, if the
/// numbers range from 0 to 999, use "1000" here.</param>
/// <param name="plaintext">The number to encrypt.</param>
/// <param name="key">Secret key</param>
/// <param name="tweak">Non-secret parameter, think of it as an IV</param>
/// <returns>The encrypted number.</returns>
public static BigInteger Encrypt(BigInteger modulus, BigInteger plaintext,
byte[] key,
byte[] tweak)
{
FPE_Encryptor F = new FPE_Encryptor(key, modulus, tweak);
BigInteger a, b;
NumberTheory.factor(modulus, out a, out b);
int r = rounds(a, b);
BigInteger X = plaintext;
for (int i = 0; i != r; ++i)
{
BigInteger L = X / b;
BigInteger R = X.mod(b);
BigInteger W = (L + F.F(i, R)).mod(a);
X = a * R + W;
}
return(X);
}
/// <summary>
/// Generic Z_n FPE decryption, FD1 scheme
/// </summary>
/// <param name="modulus">Use to determine the range of the numbers. Example, if the
/// numbers range from 0 to 999, use "1000" here.</param>
/// <param name="ciphertext">The number to decrypt.</param>
/// <param name="key">Secret key</param>
/// <param name="tweak">Non-secret parameter, think of it as an IV - use the same one used to encrypt</param>
/// <returns>The decrypted number</returns>
public static BigInteger Decrypt(BigInteger modulus, BigInteger ciphertext,
byte[] key,
byte[] tweak)
{
FPE_Encryptor F = new FPE_Encryptor(key, modulus, tweak);
BigInteger a, b;
NumberTheory.factor(modulus, out a, out b);
int r = rounds(a, b);
BigInteger X = ciphertext;
for (int i = 0; i != r; ++i)
{
BigInteger W = X.mod(a);
BigInteger R = X / a;
BigInteger bigInteger = (W - F.F(r - i - 1, R));
BigInteger L = bigInteger.mod(a);
X = b * L + R;
}
return(X);
}