Finding additive inverse using OpenSSL BIGNUM

Question

I need to calculate the additive inverse of a large (BIGNUM) prime in C. So, basically, i need to multiply by -1. Is there an easy way to do this that i've missed? I don't see any functions in the documentation. All i've come up with is this, which i think is rather ugly:

//bigprime = my big prime
inverse = BN_new();
one = BN_new();
negOne = BN_new();
BN_one(one);  // one = 1
BN_zero(negOne); // set negOne to zero
BN_sub(negOne, negOne, one) // subtract one from zero
BN_mul(inverse, bigprime, negOne, ctx); //multiply big prime by -1

More so, i'm not familiar enough with the bignum library to know whether "subtracting" one from zero is really going to do what i want it to. TIA!

Answer 1

The simpler solution is to just subtract the prime from zero.

inverse = BN_new();
nil = BN_new();
BN_zero(nil);
BN_sub(inverse,nil,bigprime);

should do it.