IrrORversible

Scenario

So reversible it’s practically irreversible

nc irrorversible.ctf.intigriti.io 1330

By CryptoCat

Solution

The challenge is a simple XOR encryption. The server encrypts the input text with a sentence that contains the flag and sends the ciphertext back to the client. The client can then extract the sentence by XORing the ciphertext with the previously sent plaintext.

from pwn import *

def xor(ciphertext_hex, known_plaintext):
    ciphertext = bytes.fromhex(ciphertext_hex)
    plaintext = known_plaintext.decode()

    key = ''
    for i in range(len(plaintext)):
        key += chr(ciphertext[i] ^ ord(plaintext[i]))
    
    return key

context.log_level = 'debug'

r = remote('irrorversible.ctf.intigriti.io', 1330)

r.recvuntil(b'Please enter the text you would like to encrypt:')

plaintext = cyclic(233)
r.sendline(plaintext)

r.recvuntil(b'Your encrypted ciphertext (hex format):')
r.recvline()

ciphertext_hex = r.recvline(231).strip().decode()
ciphertext_hex = ciphertext_hex.replace('\x1b[0m', '')
ciphertext_hex = ciphertext_hex.replace('\x1b[32m', '')

sentence = xor(ciphertext_hex, plaintext)
log.info(f'Sentence: {sentence}')

IrrORversible Flag: INTIGRITI{b451c_x0r_wh47?}