void silc_mp_mod_2exp(SilcMPInt *dst, SilcMPInt *mp1, SilcMPInt *mp2);

    Computes the remainder of `mp1' with 2 ** `exp' and saves the
    result to `dst'. This is equivalent to dst = mp1 mod (2 ^ exp).
    The same result can also be get with silc_mp_div_2exp_qr as that
    function returns the remainder as well.