Rating:

# Secret Sheep Society - 120 points - 95 solves
## Attack againgst AES-CBC mode.

We get a cookie `{"admin": false, "handle": "bam"}` AES-CBC encrypted with a known IV. By carefully modifying the IV, we can change the string `false` in the first block to `true`.

The string `false` appears at indices 10 to 15 of the first block. Therefore, with C the being the first block, setting
```
IV[10:15] = C[10:15] ^ 'false' ^ 'true '
```
will make the the admin parameter to `true` in the decrypted cookie.

Here is the python implementation:
```python
import requests
import re
from Crypto.Util.strxor import strxor
from base64 import *

url = 'https://secretsheepsociety.2019.chall.actf.co/'
cookie = "ZuNJ2xfy2eIegJsWEYmVTEwU63bTiSUAieVy8eVd+/qlqzDixgBcrqU6pHRIAsrY0C4/Uz1XTpreWMugeC8Fxw=="
ct = b64decode(cookie)
payload = ct[:10]+strxor(strxor(ct[10:15],b'false'),b'true ')+ct[15:]
r = requests.get(url,cookies={'token':b64encode(payload).decode()})
print(re.findall('actf{.*}',r.text))
```
which outputs the flag `actf{shep_the_conqueror_slumbers}`.