public (ElGamalKey, ElGamalKey) GenerateKeys()
{
BigInteger prime, safePrime, generator;
while (true)
{
prime = Algos.GenetateRandomPrime(
BigInteger.Pow(2, dimension / 4),
BigInteger.Pow(2, dimension - 1),
Algos.MillerRabin
);
safePrime = 2 * prime + 1;
if (Algos.MillerRabin(safePrime))
{
break;
}
}
while (true)
{
generator = Algos.GenetateRandomPrime(3, safePrime - 2, Algos.MillerRabin);
if (BigInteger.ModPow(generator, 2, safePrime) != 1 && BigInteger.ModPow(generator, prime, safePrime) != 1)
{
break;
}
}
var closeKey = new ElGamalKey(Algos.GenerateRandom(2, safePrime - 2), generator, safePrime);
var openKey = new ElGamalKey(BigInteger.ModPow(generator, closeKey.Key, safePrime), generator, safePrime);
return(openKey, closeKey);
}
public ElGamalSignature CreateSignature(ElGamalKey closeKey)
{
using var sHA256 = SHA256.Create();;
using var file = File.OpenRead(fileName);
var hash = new BigInteger(sHA256.ComputeHash(file), true);
var k = Algos.GenerateRandomCoprime(closeKey.Prime - 1);
var r = BigInteger.ModPow(closeKey.Generator, k, closeKey.Prime);
var s = ((hash - closeKey.Key * r) * Algos.ReverseByMod(k, closeKey.Prime - 1)) % (closeKey.Prime - 1);
return(new ElGamalSignature(r, s));
}
public bool IsValidSignature(ElGamalSignature signature, ElGamalKey openKey)
{
if (!(0 < signature.r && signature.r < openKey.Prime) || !(0 < signature.s && signature.s < openKey.Prime - 1))
{
return(false);
}
using var sHA256 = SHA256.Create();;
using var file = File.OpenRead(fileName);
var hash = new BigInteger(sHA256.ComputeHash(file), true);
return((BigInteger.ModPow(openKey.Key, signature.r, openKey.Prime)
* BigInteger.ModPow(signature.r, signature.s, openKey.Prime)) % openKey.Prime
== BigInteger.ModPow(openKey.Generator, hash, openKey.Prime));
}