BN_mod_exp
int BN_mod_exp(BIGNUM *r, const BIGNUM *a, const BIGNUM *p,
BN_mod_exp(base, base, i, val, ctx);
BN_mod_exp(common, public, secret, modulus, ctx);
BN_mod_exp(pk, root, sk, modulus, ctx);
BN_mod_exp(pk, root, sk, modulus, ctx);
BN_mod_exp(common, public, secret, modulus, ctx);
BN_mod_exp(common, public, secret, modulus, ctx);