void
rln_crypt(userkey, cardkey)
        char *userkey;          /* User's string (max 20 chars). */
        u_int8_t *cardkey;      /* 20 bits (3 bytes) */
{
        /*
         * From <http://www.proxim.com/learn/whiteppr/rl2security.shtml> 
         * "RangeLAN2 Security Features":
         *
         *  The Security ID is a unique, 20 character alphanumeric
         *  string defined and configured by the user. It must be
         *  identically configured in every radio intended to
         *  communicate with others in the same network. Once
         *  configured, the Security ID is reduced to 20 bits by a
         *  proprietary algorithm confidential to Proxim. It is
         *  merged with the radio MAC address (a 12 character field
         *  unique to every radio), scrambled and stored using another
         *  proprietary, confidential algorithm.
         */
        int32_t key;
        int8_t ret;
        int i;
        int len;
        int32_t multiplicand = 0x80000181;
        int64_t res;

        /* 
         * This algorithm is `compatible' with Proxim's first
         * `proprietary confidential algorithm': i.e., it appears
         * to be functionally identical.
         */
        len = strlen(s);
        key = 0x030201;
        for (i = 0; i < len; i++) {

                key *= userkey[i];
                res = (int64_t)multiplicand * key;
                key = key - 0xfffffd * 
                    (((key + (int32_t)(res >> 32)) >> 23) - (key >> 31));
        }

        cardkey[0] = (key >> 16) & 0xff;
        cardkey[1] = (key >> 8) & 0xff;
        cardkey[2] = key & 0xff;

        cardkey[0] |= 0x03;     /* Restrict key space by 2 bits. */
}