keygenme-py is a Reverse Engineering puzle worth 30 points.
The puzzle does not come with a description, but provides source code for a small game written in Python named keygenme-trial.py.
This game is far from complete and is only 243 lines of Python. Reading the source, it is clear that the check_key function is the key to obtaining the flag. This code is taking positional values from the SHA256 hex digest of the username this software was licensed to.
def check_key(key, username_trial):
global key_full_template_trial
if len(key) != len(key_full_template_trial):
return False
else:
# Check static base key part --v
i = 0
for c in key_part_static1_trial:
if key[i] != c:
return False
i += 1
# TODO : test performance on toolbox container
# Check dynamic part --v
if key[i] != hashlib.sha256(username_trial).hexdigest()[4]:
return False
else:
i += 1
if key[i] != hashlib.sha256(username_trial).hexdigest()[5]:
return False
else:
i += 1
if key[i] != hashlib.sha256(username_trial).hexdigest()[3]:
return False
else:
i += 1
if key[i] != hashlib.sha256(username_trial).hexdigest()[6]:
return False
else:
i += 1
if key[i] != hashlib.sha256(username_trial).hexdigest()[2]:
return False
else:
i += 1
if key[i] != hashlib.sha256(username_trial).hexdigest()[7]:
return False
else:
i += 1
if key[i] != hashlib.sha256(username_trial).hexdigest()[1]:
return False
else:
i += 1
if key[i] != hashlib.sha256(username_trial).hexdigest()[8]:
return False
return True
At the top of the source code, it has the expected template for the key, which is in picoCTF{} flag format. Writing a key generator for this software was straightforward:
#!/usr/bin/env python3
import hashlib
username = b"YOUR_USERNAME_HERE"
key_prefix = "picoCTF{1n_7h3_|<3y_of_"
user_hash = hashlib.sha256(username).hexdigest()
key_prefix += user_hash[4]
key_prefix += user_hash[5]
key_prefix += user_hash[3]
key_prefix += user_hash[6]
key_prefix += user_hash[2]
key_prefix += user_hash[7]
key_prefix += user_hash[1]
key_prefix += user_hash[8]
key_prefix += "}"
print(key_prefix)
Running this key generator yielded the correct flag.
DMFR SECURITY 
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