Crypto - Leonine Misbegotten

Challenge Summary

The challenge gives you a huge file. Looking at chall.py, we see that the flag is encoded 16 times in layers using a random choice of one of four encoding schemes:

• Base16 (b16encode)

• Base32 (b32encode)

• Base64 (b64encode)

• Base85 (b85encode)

After each encoding, the program appends the SHA-1 hash of the data before encoding.

The output.txt file is therefore:

ENCODED_DATA + SHA1
ENCODED_DATA + SHA1
...

And the goal is to recover the original flag.

---

Solve.py

from base64 import b16decode, b32decode, b64decode, b85decode
from hashlib import sha1

SCHEMES = [b16decode, b32decode, b64decode, b85decode]

with open("output", "rb") as f:
    data = f.read()

for round in range(16):
    new_data = None
    checksum = data[-20:]  # SHA-1 is 20 bytes
    candidate = data[:-20]

    for dec in SCHEMES:
        try:
            decoded = dec(candidate)
            if sha1(decoded).digest() == checksum:
                new_data = decoded
                break
        except Exception:
            pass

    if new_data is None:
        raise Exception("Failed at round", round)

    data = new_data

print(data.decode())

Flag: 0xfun{p33l1ng_l4y3rs_l1k3_an_0n10n}