CRYPTOHACK

Flipping Cookie

Description

You can get a cookie for my website, but it won't help you read the flag... I think.

Help

This page offers a convenient way for you to interact with the challenge functions. You can also use GET requests to send and receive data directly from the listed routes/endpoints if you wish. For more information see the FAQ.

Your aim is to recover the FLAG value. Once you have have it, submit it on the CryptoHack Symmetric Ciphers page.

Source

from Crypto.Cipher import AES
import os
from Crypto.Util.Padding import pad, unpad
from datetime import datetime, timedelta


KEY = ?
FLAG = ?


@chal.route('/flipping_cookie/check_admin/<cookie>/<iv>/')
def check_admin(cookie, iv):
    cookie = bytes.fromhex(cookie)
    iv = bytes.fromhex(iv)

    try:
        cipher = AES.new(KEY, AES.MODE_CBC, iv)
        decrypted = cipher.decrypt(cookie)
        unpadded = unpad(decrypted, 16)
    except ValueError as e:
        return {"error": str(e)}

    if b"admin=True" in unpadded.split(b";"):
        return {"flag": FLAG}
    else:
        return {"error": "Only admin can read the flag"}


@chal.route('/flipping_cookie/get_cookie/')
def get_cookie():
    expires_at = (datetime.today() + timedelta(days=1)).strftime("%s")
    cookie = f"admin=False;expiry={expires_at}".encode()

    iv = os.urandom(16)
    padded = pad(cookie, 16)
    cipher = AES.new(KEY, AES.MODE_CBC, iv)
    encrypted = cipher.encrypt(padded)
    ciphertext = iv.hex() + encrypted.hex()

    return {"cookie": ciphertext}


Interact

check_admin(cookie,iv)
Hex Input Only
Hex Input Only
get_cookie()
Output

XOR tool

Use this form to XOR two hex strings together.

Output
Hex Encoder/Decoder

This is a convenient encoder designed for ASCII <-> Hex translations. It won't work for decoding hex to byte streams and will just show [unprintable] in that case.

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