algorithm.js

/*
Copyright 2021, Ludwig V. <https://github.com/ludwig-v>
Original algorithm by Wouter Bokslag & Jason F. <https://github.com/prototux>

This program is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU General Public License at <http://www.gnu.org/licenses/> for
more details.

The above copyright notice and this permission notice shall be included in
all copies or substantial portions of the Software.
*/

function transform(data, sec) {
    if (data > 32767) {
        data = -(32768 - (data % 32768));
    }

    result = ((((data % sec[0]) * sec[2]) & 0xFFFFFFF) - ((((data / sec[0]) & 0xFFFFFFFF) * sec[1]) & 0xFFFFFFF) & 0xFFFFFFFF);

    if (result < 0)
        result += (sec[0] * sec[2]) + sec[1];

    return (result & 0xFFFF);
}

function getKey(seedTXT, appKeyTXT = "0000") {
    seed = [seedTXT.substring(0, 2), seedTXT.substring(2, 4), seedTXT.substring(4, 6), seedTXT.substring(6, 8)];
    appKey = [appKeyTXT.substring(0, 2), appKeyTXT.substring(2, 4)];

    // Hardcoded secrets
    sec_1 = [0xB2, 0x3F, 0xAA];
    sec_2 = [0xB1, 0x02, 0xAB];
    res_msb = transform(parseInt(appKey[0] + appKey[1], 16), sec_1) | transform(parseInt(seed[0] + seed[3], 16), sec_2);
    res_lsb = transform(parseInt(seed[1] + seed[2], 16), sec_1) | transform(res_msb, sec_2);
    res = (res_msb << 16) | res_lsb;
    return res.toString(16).padStart(8, '0').toUpperCase();
}

console.log(getKey("11111111", "D91C"));