2B Firmware Decryption

gistfile1.js

const fs = require("fs");

function decryptWithRC4Key(arr, block) {
  let out_arr = [];
  let previous_val = 0;
  let b = 0;
  for (let i = 0; i < arr.length; ++i) {
    let a = (i + 1) % 256;
    b = ((b + block[a]) % 256);
    [block[a], block[b]] = [block[b], block[a]];
    let c = block[(block[a] + block[b]) % 256] ^ block[b] ^ arr[i];
    out_arr.push(c);
  }

  return out_arr;
}

function createRC4Key() {
  let key = "2012E5-T1mSyst3MFirmware".split("").map(x => x.charCodeAt() );
  let block = [...Array(256).keys()];

  if (key.length > block.length) {
    throw new Error("Key must be shorter than block");
  }

  let new_index = 0;
  for (let i = 0; i < block.length; ++i) {
    new_index += block[i] + key[i % key.length];
    new_index %= block.length;
    [block[i], block[new_index]] = [block[new_index], block[i]];
  }

  return block;
}


function decryptFile() {
  let xorblock = createRC4Key();
  let fw = fs.readFileSync("2B106.2bfx");
  //let fw = fs.readFileSync("bootloader14.2bfx");
  let fwarr = [...fw];
  let buf = new Buffer(decryptWithRC4Key(fwarr, xorblock));
  fs.writeFileSync("fw.hex", buf);
}

decryptFile();