/// <summary>
/// Finds modular square root r such that r^2≡a (mod p) for a < p and p is the secp256k1 prime (p%4 = 3).
/// Return value indicates success.
/// </summary>
/// <param name="a"></param>
/// <param name="p">Secp256k1 prime</param>
/// <param name="result">Result if square root exists</param>
/// <returns>True if the square root exists; false if otherwise.</returns>
public static bool TryFind(BigInteger a, BigInteger p, out BigInteger result)
{
if (Legendre.Symbol(a, p) != 1)
{
return(false);
}
result = BigInteger.ModPow(a, (p + 1) / 4, p);
return(true);
}
private static BigInteger TonelliShanks(BigInteger a, BigInteger p)
{
if (a >= p)
{
throw new ArithmeticException("The residue, 'a' cannot be greater than the modulus 'p'!");
}
if (Legendre.Symbol(a, p) != 1) // a^(p-1 / 2) % p == p-1
{
throw new ArithmeticException($"Parameter 'a' is not a quadratic residue, mod 'p'");
}
// This will be true for secp256k1 curve prime
if (p % 4 == 3)
{
return(BigInteger.ModPow(a, (p + 1) / 4, p));
}
//Initialize
BigInteger s = p - 1;
BigInteger e = 0;
while (s % 2 == 0)
{
s /= 2;
e += 1;
}
BigInteger n = FindGenerator(p);
BigInteger x = BigInteger.ModPow(a, (s + 1) / 2, p);
BigInteger b = BigInteger.ModPow(a, s, p);
BigInteger g = BigInteger.ModPow(n, s, p);
BigInteger r = e;
BigInteger m = Order(b, p);
if (m == 0)
{
return(x);
}
while (m > 0)
{
x = (x * BigInteger.ModPow(g, TwoExp(r - m - 1), p)) % p;
b = (b * BigInteger.ModPow(g, TwoExp(r - m), p)) % p;
g = BigInteger.ModPow(g, TwoExp(r - m), p);
r = m;
m = Order(b, p);
}
return(x);
}
