void get_mac(byte * buffer, int nbits, byte * out)
{
    int nbytes = nbits >> 3;
    int rembits = nbits & 7;
    int mask = rembits ? ((1 < rembits) - 1) : 0;
    int i;
    for (i = 0; i < nbytes; i++)
        out[i] = buffer[i];
    if (rembits)
        out[i] = buffer[i] & mask;
}