README.md

Level 09

Resolution

As level09, running ls gives us a binary called level09 and a file named token.

  $> cat token 
  f4kmm6p|=pnDBDu{  
  $> ./level09
  You need to provied only one arg.
  $> ./level09 token
  tpmhr
  $> ./level09 aaaaa
  abcde

After some tests the binary seems to add to each byte its index. In order to reverse this we simply need to substract its index from each byte. Bellow is a simple node.js script that does it.

const { Buffer } = require('buffer')
const fs = require('fs')

try {
  const byteArray = []
  const buffer = Buffer.from(fs.readFileSync(process.argv[2], 'hex'), 'hex')

  for (let i = 0; i < buffer.length - 1; i++) {
    byteArray.push(buffer[i] - i)
  }

  console.log(Buffer.from(byteArray, 'hex').toString())
} catch (_) {
  console.log('You must specify a valid file to decipher.')
}

To explain a bit what's happening here, it loads a file content into a buffer, converting each character to its hexadecimal codepoint. After this, reversing the initial encoding is trivial and finaly we convert byteArrayto a string before printing it.

Once again, we need to use scp to retrieve the file and decipher it.

  $> scp -P 4242 level09@192.168.1.73:token .
  $> chmod 777 token
  $> node decipher.js token
  f3iji1ju5yuevaus41q1afiuq

Use this result to log as flag09 and run getflag to finish this level.

  $> su flag09
  Password: f3iji1ju5yuevaus41q1afiuq
  $> getflag
  Check flag.Here is your token : s5cAJpM8ev6XHw998pRWG728z

So, s5cAJpM8ev6XHw998pRWG728z is the flag for this level.