Cryptohack - Flipping Cookie writeup

writeup.md

From cbc decryption: P0 = iv xor block_decryption(C0)

P0 --> "admin=False" P' --> "admin=True" (what we want)

"admin=True" = iv' xor block_decryption(C0)

iv' = "admin=True" xor block_decryption(C0)

block_decryption(C0) = "admin=False" xor iv

iv' = iv xor "admin=False" xor "admin=True"

Code:

from Crypto.Util.number import *
import requests
import json
from pwn import xor

def get_ciphertext():
    url = "http://aes.cryptohack.org/flipping_cookie/get_cookie/"
    r = requests.get(url)
    ct = (json.loads(r.text))['cookie']
    return ct

def check_cookie(cookie, iv):
    url = "http://aes.cryptohack.org/flipping_cookie/check_admin/"+cookie+"/"+iv
    r = requests.get(url)
    try:
        flag = (json.loads(r.text))['flag']
    except:
        flag = (json.loads(r.text))['error']
    return flag

c = get_ciphertext()
iv = bytes.fromhex((c[:32]))
ct = c[32:]

iv1 = xor(iv,b'admin=False',b'admin=True;').hex()
flag = check_cookie(ct, iv1)
print(flag)

Flag = crypto{4u7h3n71c4710n_15_3553n714l}

note: in cbc bit flipping attack the number of bytes of P0 and P' must be equal