Points calculated using this elliptic curve point multiplication do not lie on the curve and this class brings Arithmetic exception

Question

I get stack on my error of point multiplication using standard projective coordinates. I don\'t know what i missed but the multiplied points do not lie on the curve and some t

Answer 1

The most important errors are in NISTCurves.P192: p and the order are in base-10, not in base-16. Also, when you construct the EllipticCurve-object, you provide the parameters in the wrong order. Your method requires (a, b, p), but you call it with (p, a, b) (so my guess about p not being prime was correct).

Another problem is in your verify-method, when you unwrap r and s. Since they are in unsigned format, you should use new BigInteger(1, rArr) instead of the normal constructor.

With those changes your code works for me (I can validate the signatures - I have not verified the correctness of the implementation).

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(Old answer below:)

Since you have not given us the code that matches the stacktrace, this will merely be a guess:

During elliptic curve addition (with a curve over a prime field), you should only be calling BigInteger.modInverse() with the prime p (the order of the prime field) as the modulus.

The most probable way for this to fail sporadically with "BigInteger not invertible" is if p is not actually a prime.

Where are you getting p from? Try inserting

if(!ec.getP().isProbablePrime(100)) throw new RuntimeException("P is not a prime");

somewhere.