Hexpresso FIC Quals 2019: Step 1

The first step is a JavaScript challenge.

Here is the source code that is embedded into the page:

const play = () => {
  var game = new Array(
    116, 228, 203, 270, 334, 382, 354, 417, 485, 548, 586, 673, 658, 761, 801,
    797, 788, 850, 879, 894, 959, 1059, 1071, 1140, 1207, 1226, 1258, 1305,
    1376, 1385, 1431, 1515
  );

  const u_u = "CTF.By.HexpressoCTF.By.Hexpresso";
  const flag = document.getElementById("flag").value;

  for (i = 0; i < u_u.length; i++) {
    if (u_u.charCodeAt(i) + flag.charCodeAt(i) + i * 42 != game[i]) {
      alert("NOPE");
      return;
    }
  }

  // Good j0b
  alert("WELL DONE!");

  document.location.replace(
    document.location.protocol +
      "//" +
      document.location.hostname +
      "/" +
      flag
  );
};

/**
 ** Thanks all <3
 ** @HexpressoCTF
 **
 ** The next step is here : https://ctf.hexpresso.fr/{p_p}
 **/

This line is important:

if (u_u.charCodeAt(i) + flag.charCodeAt(i) + i * 42 != game[i]) {

Its a simple equation (pseudocode):

u_u[i] + flag[i] + i * 42 = game[i]

u_u and game are hardcoded and given to us. What we are looking for is flag. So:

flag[i] = game[i] - u_u[i] - i * 42;

Here is my JavaScript solution:

const play = () => {
  const game = new Array(
    116, 228, 203, 270, 334, 382, 354, 417, 485, 548, 586, 673, 658, 761, 801,
    797, 788, 850, 879, 894, 959, 1059, 1071, 1140, 1207, 1226, 1258, 1305, 1376,
    1385, 1431, 1515
  );

  const u_u = "CTF.By.HexpressoCTF.By.Hexpresso";

  let r = "";
  for (const i in u_u) {
    r += String.fromCharCode(game[i] - u_u.charCodeAt(i) - i * 42);
  }

  return r;
}

console.log(play());

It outputs the flag: 1f1bd383026a5db8145258efb869c48f. Click here for the next step!

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