BN_kronecker
r = BN_kronecker(y, q, ctx); /* here 'q' is |p| */
kron = BN_kronecker(x, group->field, ctx);
int BN_kronecker(const BIGNUM *a, const BIGNUM *b, BN_CTX *ctx); /* returns
if (!TEST_int_ge(kronecker = BN_kronecker(a, b, ctx), -1))