decipher.txt

var enc = "hmqxc sewet ho sdd tc hbm yyrcbq bb ggq uivaa wwnbax la zvxwu ai ikq zor qnlo fnbka nc jaw kto zb miwa fhlshm hc hbl lyg jir";
var msg = "hel** *avi* ** *** ** *** ****** ** *** ***** ****** ** ***** ** *** *** **** ***** ** *** *** ** **** ****** ** *** *** **z";
var alpha = "abcdefghijklmnopqrstuvwxyz";
console.log(alpha.length);

function ltrToValue(letter) {
  return alpha.indexOf(letter)+1;
};

function valueToLtr(value) {
  return alpha[value-1];
};

function shiftAlpha(value) {
  if (value < alpha.length && value > 0)
    return value;
  if (value > alpha.length)
    return value - alpha.length;
  if (value <= 0)
    return value + alpha.length;
};

function decipher() {
  let tmpmsg = "";
  for (i=0; i < enc.length; i++) {
    if (enc[i] == " ")
      tmpmsg += " ";
    else {
      let key = tmpmsg[i-1];
      if (key == " ")
        key = tmpmsg[i-2];
      tmpmsg += valueToLtr(shiftAlpha(ltrToValue(enc[i]) - ltrToValue(key)));
    }
  };
  console.log("Comparison:\n%s\n%s\n%s", enc, msg, tmpmsg);
};

decipher();