/// <summary>
/// Used by the token verifier. It recreates the initial point from the initiator, signs it, and verifies that they are equal.
/// </summary>
/// <param name="k">The private key for the token scheme</param>
/// <param name="curve">Curve parameters</param>
/// <param name="t">Seed for the initial point chosen by the initiator</param>
/// <param name="W">Token received from the initiator</param>
/// <returns>True if the token is valid, otherwise false</returns>
public async Task <bool> VerifyTokenAsync(
BigInteger k,
ECCurve curve,
byte[] t,
ECPoint W)
{
// Check if token t is received earlier
if (await _seedStore.ExistsAsync(t))
{
return(false);
}
// Check that W is a valid point on the currect curve
if (ECPointVerifier.PointIsValid(W, curve) == false)
{
throw new AnonymousTokensException("W is not a valid point on the curve");
}
await _seedStore.SaveAsync(t);
ECPoint?T = ECCurveHash.HashToWeierstrassCurve(curve, t);
if (T == null)
{
return(false);
}
ECPoint?V = T.Multiply(k);
return(V.Equals(W));
}
/// <summary>
/// Used by the initiator. Generates an initial point to be submitted to the token service for signing.
/// </summary>
/// <param name="curve">Curve parameters</param>
/// <returns>The seed t for a random point, the initial mask r of the point, and the masked point P</returns>
public (byte[] t, BigInteger r, ECPoint P) Initiate(ECCurve curve)
{
BigInteger r = ECCurveRandomNumberGenerator.GenerateRandomNumber(curve, _random);
// Sample random bytes t such that x = hash(t) is a valid
// x-coordinate on the curve. Then T = HashToWeierstrassCurve(t).
byte[]? t = new byte[32];
ECPoint?T;
for (; ;)
{
_random.NextBytes(t);
T = ECCurveHash.HashToWeierstrassCurve(curve, t);
if (T == null)
{
continue;
}
break;
}
if (T == null)
{
throw new AnonymousTokensException("Point T is null after unsuccessfull hashing");
}
// Compute P = r*T
ECPoint P = T.Multiply(r);
return(t, r, P);
}
public void HashToWeierstrassCurve_tIsNotWithinRangeOfP_ReturnsNull(string randomBigInteger)
{
// Arrange
var x9 = ECNamedCurveTable.GetByName("prime239v1");
var curve = x9.Curve;
byte[] t = new BigInteger(randomBigInteger).ToByteArray();
// Act
var actual = ECCurveHash.HashToWeierstrassCurve(curve, t);
// Assert
Assert.Null(actual);
}
