Issue with publicPointFromPrivate() function

[AmritKumar]

PROBLEM: The following function in ECKeyPair.cs allows one to obtain the public key from an input private key.

public static ECPoint publicPointFromPrivate(BigInteger privKey)
        {
            /*
             * TODO: FixedPointCombMultiplier currently doesn't support scalars longer than the group
             * order, but that could change in future versions.
             */
            if (privKey.BitLength > CURVE.N.BitLength)
            {
                privKey = privKey.Mod(CURVE.N);
            }
            return new FixedPointCombMultiplier().Multiply(CURVE.G, privKey);
}

If the input privKey has a bit length that is larger than the bit length of the group order N, then privKey is reduced modulo N. There are couple of issues here: 1) Any input privKey that does not fall between 1 and N-1 (both inclusive) should be outright rejected. 2) Comparing the bit length of N and privKey is not correct. One should rather compare their values directly.

SOLUTION: Replace the if condition by instead checking whether the input privKey is valid or not. A valid privKey is simply a scalar value that is between 1 and N-1. If privKey is invalid, then the function should simply throw an error instead of reducing privKey modulo N.

@neeboo @yanbin007

[yanbin007]

Thanks，I have fixed it.