exploit.js

// Example usage:
// node exploit.js encode < input.txt | node exploit.js decode
//
// To solve:
// cat instructions.txt | node exploit.js decode

const fs = require('fs');

function _encode(input) {
  let a = [];
  for (let i = 0; i < input.length; i++) {
    let t = input.charCodeAt(i);
    for (let j = 0; j < 8; j++) {
      // if t >> j is odd
      if ((t >> j) & 1) {
        a.push(1 + j + (input.length - 1 - i) * 8);
      }
    }
  }

  let b = [];
  while (a.length) {
    const rand = Math.random();
    let t = (rand * a.length) | 0; // round down
    b.push(a[t]);
    a = a.slice(0, t).concat(a.slice(t + 1)); // remove a[t] from a
  }

  let r = '';
  while (b.length) {
    let t = b.pop();
    r = r + '-'.repeat(t) + '.';
  }
  return r;
}

function generateLookup() {
  const lookup = {};
  for (var i = 32; i < 127; ++i) {
    const char = String.fromCharCode(i);

    const a = [];
    for (let j = 0; j < 8; j++) {
      if ((i >> j) & 1) a.push(j);
    }

    lookup[a] = char;
  }

  return lookup;
}

function _decode(input) {
  const lookup = generateLookup();

  // Reverse encoded string into b[].
  const b = input
    .split('.')
    .map(x => x.length)
    .reverse();

  // Resultant b[] is in random order.
  // Sorting it always gives rise to the same array regardless of random() above.
  b.sort((a, b) => a - b);

  // Input length can be determined from the maximal value.
  const len = (b[b.length - 1] / 8) | (0 + 1);

  // Find the binary set bits and character index from each encoded value.
  const byIdx = {};
  b.forEach(x => {
    // Represents the binary bit (0 to 7).
    const j = (x - 1) % 8;

    // Represents the character index (0 to 161).
    const i = ((x - j - 1) / 8 - len) * -1;

    // Append the binary bit value for a specific character index.
    if (!byIdx[i]) byIdx[i] = [];
    byIdx[i].push(j);
  });

  // Fix last character (for some reason)
  byIdx[len].shift();

  // Decode all characters lookup table
  const chars = Object.values(byIdx).map(j => lookup[j]);

  return chars.join('');
}

if (process.argv.length !== 3) {
  console.error('Usage: node exploit.js [encode|decode]');
  return;
}

// Read from stdin
const file = fs.readFileSync(0, 'utf-8').toString();

if (process.argv[2] === 'encode') {
  console.log(_encode(file));
} else if (process.argv[2] === 'decode') {
  console.log(_decode(file));
} else {
  console.error('Usage: node exploit.js [encode|decode]');
}