run run run disasm - Hitcon CTF 2020

run.mc

var idx;
var flag;

function onLayout(dc) {
    var v2;

    WatchFace.onUpdate(dc);

    myTimer.stop();

    if (0 <= idx < size) {
        v2 = solve() & 0xff;
        v2 ^= magix[idx]; // ?
        flag += v2.toChar().toString();
    }

    // 06CE
    drawFlag(flag);

    if (idx < size) {
        // 06FE
        myTimer.start(
            method(:timerCallback), 
            (target[idx] + 1) * 100, 
            false
        );
    }
}

function timerCallback() {
    idx += 1;
    UI.requestUpdate();
}

function solve() {
    var v1 = 0;
    var v2 = 1;
    var v3 = 1;
    var v4 = 1;

    while (v1 < target.idx) {
        v1 += 1;

        var temp = ((v2 * 2) + v3 + (v4 * 7)) % 0x7a69;
        v4 = v3;
        v3 = v2;
        v2 = temp;
    }

    return v2;
}

function drawFlag(flag) {
    setColor(Toybox_Graphics.COLOR_BLACK, Toybox_Graphics.COLOR_BLACK);
    clear();
    setColor(Toybox_Graphics.COLOR_WHITE, Toybox_Graphics.COLOR_TRANSPARENT);
    drawText(
        getWidth() / 2,
        getHeight() / 2,
        flag,
        Toybox_Graphics.FONT_LARGE,
        Toybox_Graphics.TEXT_JUSTIFY_CENTER | Toybox_Graphics.TEXT_JUSTIFY_VCENTER
    );
}

magic = [0x62, 0x20, 0x54, 0xfd, 0xd9, 0x12, 0x5c, 0x16, 0x70, 0x8a, 0x93, 0x2e, 0xa8, 0xe5, 0x1f, 0x95, 0x48, 0x5e, 0xbf, 0x7c, 0x15, 0xb0, 0xa, 0x68, 0x9a, 0xd5, 0xeb, 0x19, 0xed, 0x3d, 0x12, 0xf]
target = [0x1, 0x3, 0x9, 0x1b, 0x51, 0xf3, 0x2d9, 0x88b, 0x19a1, 0x4ce3, 0xe6a9, 0x2b3fb, 0x81bf1, 0x1853d3, 0x48fb79, 0xdaf26b, 0x290d741, 0x7b285c3, 0x17179149, 0x4546b3db, 0xcfd41b91, 0x26f7c52b3, 0x74e74f819, 0x15eb5ee84b, 0x41c21cb8e1, 0xc546562aa3, 0x24fd3027fe9, 0x6ef79077fbb, 0x14ce6b167f31, 0x3e6b41437d93, 0xbb41c3ca78b9, 0x231c54b5f6a2b]