[🥈 Silver 4] 암호문 배열을 걷어내니 로직은 열 줄도 안 됐다 — DreamHack Secure Mail 풀이

file script.js secure-mail.html; wc -c script.js

<input id="pass" type="password" maxlength="6" ... placeholder="Input your birthday eg.) 850810">
<button type="submit" onclick="_0x9a220(pass.value);">Confirm</button>

// deobfuscate.js
global.window = global;
global.document = { write(s) {} };
global.alert = (s) => {};
const fs = require('fs');
const src = fs.readFileSync(__dirname + '/script.js', 'utf8');
eval(src);
 
const fnSrc = _0x9a220.toString();
console.log('_0x9a220 total source length:', fnSrc.length, 'bytes');
 
const arrMatch = fnSrc.match(/file=\[[^\]]*\]/);
console.log('embedded ciphertext array literal length:', arrMatch[0].length, 'bytes');
 
const cleaned = fnSrc.slice(0, arrMatch.index)
  + '/* ...223616-byte ciphertext array literal omitted... */'
  + fnSrc.slice(arrMatch.index + arrMatch[0].length);
 
console.log('actual check logic once the array is stripped:', cleaned.length, 'bytes');
fs.writeFileSync(__dirname + '/logic_only.js', cleaned);
console.log('---');
console.log(cleaned);

node deobfuscate.js

function _0x9a220(password) {
  var key = Array.from(MD5(password, null, raw=true)).map(c => c.charCodeAt()),
      cipher = new ModeOfOperation.cbc(key, key); // key와 iv가 같은 값
 
  var decrypted = cipher.decrypt(file); // file = 223,616바이트 암호문
  var plaintext = '';
  for (var i = 0; i < decrypted.length; i++) plaintext += String.fromCharCode(decrypted[i]);
 
  if (MD5(plaintext, null, raw=true) != FIXED_HASH) {
    return alert('Wrong'), false;
  }
  return document.write('<img src="' + plaintext + '">'), true;
}

// verify_md5.js
const crypto = require('crypto');
const tests = ['', 'abc', '000101', 'hello world'];
for (const t of tests) {
  const nodeMd5 = crypto.createHash('md5').update(t, 'utf8').digest('hex');
  const custom = _0x3eebe5(t);
  console.log(JSON.stringify(t), 'node_md5=', nodeMd5, 'custom=', custom, 'match=', nodeMd5 === custom);
}

node verify_md5.js

// fast_brute.js
const fs = require('fs');
const crypto = require('crypto');
 
const ciphertext = fs.readFileSync(__dirname + '/ciphertext.bin');   // file[] 223,616바이트
const targetHash = fs.readFileSync(__dirname + '/target_hash.bin');  // 고정 MD5(16바이트)
 
const pad2 = (n) => String(n).padStart(2, '0');
let tried = 0;
let found = null;
const t0 = Date.now();
 
outer:
for (let yy = 0; yy <= 99; yy++) {
  for (let mm = 1; mm <= 12; mm++) {
    for (let dd = 1; dd <= 31; dd++) {
      const cand = pad2(yy) + pad2(mm) + pad2(dd);
      tried++;
      const key = crypto.createHash('md5').update(cand, 'utf8').digest();
      const decipher = crypto.createDecipheriv('aes-128-cbc', key, key);
      decipher.setAutoPadding(false);
      const plainBytes = Buffer.concat([decipher.update(ciphertext), decipher.final()]);
      const plainStr = plainBytes.toString('latin1');
      const reEncoded = Buffer.from(plainStr, 'utf8');
      const h = crypto.createHash('md5').update(reEncoded).digest();
      if (h.equals(targetHash)) {
        found = { cand, plainStr };
        break outer;
      }
    }
  }
}
 
console.log('tried:', tried, 'elapsed ms:', Date.now() - t0);
if (found) {
  console.log('FOUND PASSWORD:', found.cand);
  fs.writeFileSync(__dirname + '/plaintext_output.txt', found.plainStr, 'latin1');
  console.log('plaintext head:', found.plainStr.slice(0, 100));
}

node fast_brute.js

// extract_flag.js
const fs = require('fs');
const data = fs.readFileSync(__dirname + '/plaintext_output.txt', 'latin1');
const m = data.match(/^data:image\/png;base64,(.*)$/s);
const buf = Buffer.from(m[1], 'base64');
fs.writeFileSync(__dirname + '/flag.png', buf);
console.log('decrypted data URI ->', buf.length, 'byte PNG saved to flag.png');

node extract_flag.js