/// <summary>
/// D(y) = y^a mod n
/// </summary>
/// <param name="ciphertext"></param>
/// <returns></returns>
public override string Decryption(string ciphertext, int start = 0) =>
new String(ciphertext.ToCharArray().Select(y => Convert.ToChar(
start + BigInteger.ModPow(y - start, PrivateKey.ElementAt(2), PublicKey.ElementAt(0))
)).ToArray());
/// <summary>
/// E(x) = x^b mod n
/// </summary>
/// <param name="plaintext"></param>
/// <returns></returns>
public override string Encryption(string plaintext, int start = 0) =>
new String(plaintext.ToCharArray().Select(x => Convert.ToChar(
BigInteger.ModPow(x - start, PublicKey.ElementAt(1), PublicKey.ElementAt(0))
)).ToArray());
