## Original Source Code

import base64
from cryptography.fernet import Fernet



payload = b'gAAAAABkzWGO_8MlYpNM0n0o718LL-w9m3rzXvCMRFghMRl6CSZwRD5DJOvN_jc8TFHmHmfiI8HWSu49MyoYKvb5mOGm_Jn4kkhC5fuRiGgmwEpxjh0z72dpi6TaPO2TorksAd2bNLemfTaYPf9qiTn_z9mvCQYV9cFKK9m1SqCSr4qDwHXgkQpm7IJAmtEJqyVUfteFLszyxv5-KXJin5BWf9aDPIskp4AztjsBH1_q9e5FIwIq48H7AaHmR8bdvjcW_ZrvhAIOInm1oM-8DjamKvhh7u3-lA=='

key_str = 'correctstaplecorrectstaplecorrec'
key_base64 = base64.b64encode(key_str.encode())
f = Fernet(key_base64)
plain = f.decrypt(payload)
exec(plain.decode())

## Modified

import base64
from cryptography.fernet import Fernet



payload = b'gAAAAABkzWGO_8MlYpNM0n0o718LL-w9m3rzXvCMRFghMRl6CSZwRD5DJOvN_jc8TFHmHmfiI8HWSu49MyoYKvb5mOGm_Jn4kkhC5fuRiGgmwEpxjh0z72dpi6TaPO2TorksAd2bNLemfTaYPf9qiTn_z9mvCQYV9cFKK9m1SqCSr4qDwHXgkQpm7IJAmtEJqyVUfteFLszyxv5-KXJin5BWf9aDPIskp4AztjsBH1_q9e5FIwIq48H7AaHmR8bdvjcW_ZrvhAIOInm1oM-8DjamKvhh7u3-lA=='

key_str = 'correctstaplecorrectstaplecorrec'
key_base64 = base64.b64encode(key_str.encode()) #encode:change key_str to byte str, base64.b64encode: then b64 encode it
f = Fernet(key_base64) # assign the "value" of key to variable `f`
plain = f.decrypt(payload)
print(plain.decode()) #decode: change `plain` from byte str to normal one
# exec(plain.decode()) #exec: run the text as a script

## Output

┌──(kali㉿kali)-[~/code]
└─$ /bin/python /home/kali/code/unpackme.flag.py

pw = input('What\'s the password? ')

if pw == 'batteryhorse':
  print('picoCTF{175_chr157m45_5274ff21}')
else:
  print('That password is incorrect.')

## REFs
- https://www.geeksforgeeks.org/fernet-symmetric-encryption-using-cryptography-module-in-python/
- https://www.geeksforgeeks.org/advanced-encryption-standard-aes/