test

    auto add_key_share_p256 = [&](){

        _client_offered_tls13 = true;

        // generate ephemeral priv scalar (32 bytes big-endian)

        // generate valid ephemeral priv scalar: 0 < k < n

        for (;;) {

            fillRandomBytes(_client_priv_ecdhe, 32);

            // check not all zeros

            bool allZero = true;

            for (int i = 0; i < 32; i++) {

                if (_client_priv_ecdhe[i]) { allZero = false; break; }

            }

            if (allZero) continue;

            // check k < n (P-256 curve order)

            u256 k = bytes_be_to_u256(_client_priv_ecdhe);

            if (u256_cmp(k, P256_N) >= 0) continue;

            break; // valid scalar

        }

        // compute pub = priv * G

        netplus::P256Point pub = netplus::scalar_mul_G(_client_priv_ecdhe);