syssec 3.3

var key = 8;
var msg = `bpm ncvliumvbit xzwjtmu wn kwuucvqkibqwv qa bpib wn zmxzwlckqvo ib
wvm xwqvb mqbpmz mfikbtg wz ixxzwfquibmtg i umaaiom amtmkbml ib
ivwbpmz xwqvb. nzmycmvbtg bpm umaaioma pidm umivqvo; bpib qa bpmg
zmnmz bw wz izm kwzzmtibml ikkwzlqvo bw awum agabmu eqbp kmzbiqv
xpgaqkit wz kwvkmxbcit mvbqbqma. bpmam amuivbqk iaxmkba wn
kwuucvqkibqwv izm qzzmtmdivb bw bpm mvoqvmmzqvo xzwjtmu. bpm
aqovqnqkivb iaxmkb qa bpib bpm ikbcit umaaiom qa wvm amtmkbml nzwu
i amb wn xwaaqjtm umaaioma. bpm agabmu ucab jm lmaqovml bw wxmzibm
nwz mikp xwaaqjtm amtmkbqwv, vwb rcab bpm wvm epqkp eqtt ikbcittg
jm kpwamv aqvkm bpqa qa cvsvwev ib bpm bqum wn lmaqov.`;

// decodes digit d with key
var getNextDigit = (d) => {
    if(d == " ") return " "; // ignore whitespace and dots
    if (d == ".") return ".";

    let newCode = d.charCodeAt(0) - key;

    if (newCode < 'a'.charCodeAt(0))
        newCode = newCode + 26;

    return String.fromCharCode(newCode);
}

var result = msg.split('').map(d => getNextDigit(d)).join('');

console.log(result);