/// <summary>
/// Create instance of SimpleElGamal class and count private key.
/// Base and modulo should be large enough to correctly encrypt and decrypt given messages.
/// </summary>
/// <param name="exponentAlice">Exponent of Alice</param>
/// <param name="exponentBob">Exponent of Bob</param>
/// <param name="base">Shared base value. Must be larger than modulo.</param>
/// <param name="modulo">Shared modulo. Must be less than base.</param>
public SimpleElGamal(ushort exponentAlice, ushort exponentBob, ushort @base, ushort modulo)
{
if (@base >= modulo)
{
throw new ArgumentException("Base shouldn't be larger than modulo");
}
Modulo = modulo;
var halfKeyAlice = NumericUtilities.SolveModulo(@base, exponentAlice, modulo);
var halfKeyBob = NumericUtilities.SolveModulo(@base, exponentBob, modulo);
// Discrete logarithm problem.
// In real situation Alice wouldn't know about Bob's exponent and vice versa.
// They change only "half parts of their own keys".
var aliceKey = NumericUtilities.SolveModulo(halfKeyBob, exponentAlice, modulo);
var bobKey = NumericUtilities.SolveModulo(halfKeyAlice, exponentBob, modulo);
if (aliceKey != bobKey)
{
throw new Exception("Alice's and Bob's keys are not equal!");
}
Key = aliceKey;
}
/// <summary> /// Decrypt value. /// </summary> /// <param name="value">Value to decrypt</param> /// <returns>Decrypted value</returns> public uint Decrypt(uint value) => NumericUtilities.SolveModulo(value, PrivateKey, Modulo);
/// <summary> /// Encrypt value. /// </summary> /// <param name="value">Value to encrypt</param> /// <returns>Encrypted value</returns> public uint Encrypt(uint value) => NumericUtilities.SolveModulo(value, PublicKey, Modulo);