OpenSSL::PKey::DH#compute_key

static VALUE
ossl_dh_compute_key(VALUE self, VALUE pub)
{
    DH *dh;
    const BIGNUM *pub_key, *dh_p;
    VALUE str;
    int len;

    GetDH(self, dh);
    DH_get0_pqg(dh, &dh_p, NULL, NULL);
    if (!dh_p)
        ossl_raise(eDHError, "incomplete DH");
    pub_key = GetBNPtr(pub);
    len = DH_size(dh);
    str = rb_str_new(0, len);
    if ((len = DH_compute_key((unsigned char *)RSTRING_PTR(str), pub_key, dh)) < 0) {
        ossl_raise(eDHError, NULL);
    }
    rb_str_set_len(str, len);

    return str;
}