Description

The page stores preferences in a cookie named data. The value is XOR-encrypted with a secret key then base64-encoded. The goal is to forge a cookie that sets showpassword to yes.

Overview

• The server XOR-encrypts a JSON object with a short repeating key, then base64-encodes the result before setting it as a cookie.
• XOR encryption is reversible: if you know both the plaintext and ciphertext, you can recover the key.
• We know the default plaintext ({"showpassword":"no","bgcolor":"#ffffff"}), so we XOR it against the decoded cookie to find the key.
• We then re-encrypt modified JSON (showpassword: "yes") with the recovered key and send the forged cookie.

Background: Known-Plaintext XOR Attack

• XOR has the property that C = P XOR K, so K = P XOR C.
• If the key is shorter than the plaintext, it repeats cyclically; the key length can be found by looking for a repeating pattern in P XOR C.
• Base64 encoding is not encryption; it only obscures the binary data from casual inspection.

Solution

Step 1: Extract the Default Cookie

• Make a GET request to the page and read the data cookie from the response.
• Base64-decode the cookie value to get the raw ciphertext bytes.

Step 2: Recover the XOR Key

• XOR the decoded ciphertext byte-by-byte against the known default plaintext json_encode(["showpassword"=>"no","bgcolor"=>"#ffffff"]).
• The result is a repeating sequence; use a regex to find the repeating unit.
• The key is qw8J.

import base64, json

default = json.dumps({"showpassword": "no", "bgcolor": "#ffffff"}, separators=(',', ':'))
ciphertext = base64.b64decode("MGw7JCQ5OC04PT8jOSpqdmkgJ25gbCorKCEkIzlscm5oKC4qLSgubjY=")

key_stream = "".join(chr(ord(p) ^ c) for p, c in zip(default, ciphertext))
# key_stream repeats "qw8J"
key = "qw8J"

Step 3: Forge the Cookie

• Construct the modified JSON with showpassword set to "yes".
• XOR each byte with the repeating key, then base64-encode the result.

target = json.dumps({"showpassword": "yes", "bgcolor": "#ffffff"}, separators=(',', ':'))
cipher = bytes(ord(c) ^ ord(key[i % len(key)]) for i, c in enumerate(target))
forged = base64.b64encode(cipher).decode()

Step 4: Send the Forged Cookie

• Make a GET request with cookies={"data": forged}.
• The server decrypts, finds showpassword == "yes", and prints the password.

Password

YWqo0pjpcXzSIl5NMAVxg12QxeC1w9QG