int rsa_multip_calc_product(RSA *rsa)
{
    RSA_PRIME_INFO *pinfo;
    BIGNUM *p1 = NULL, *p2 = NULL;
    BN_CTX *ctx = NULL;
    int i, rv = 0, ex_primes;

    if ((ex_primes = sk_RSA_PRIME_INFO_num(rsa->prime_infos)) <= 0) {
        /* invalid */
        goto err;
    }

    if ((ctx = BN_CTX_new()) == NULL)
        goto err;

    /* calculate pinfo->pp = p * q for first 'extra' prime */
    p1 = rsa->p;
    p2 = rsa->q;

    for (i = 0; i < ex_primes; i++) {
        pinfo = sk_RSA_PRIME_INFO_value(rsa->prime_infos, i);
        if (pinfo->pp == NULL) {
            pinfo->pp = BN_secure_new();
            if (pinfo->pp == NULL)
                goto err;
        }
        if (!BN_mul(pinfo->pp, p1, p2, ctx))
            goto err;
        /* save previous one */
        p1 = pinfo->pp;
        p2 = pinfo->r;
    }

    rv = 1;
 err:
    BN_CTX_free(ctx);
    return rv;
}