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