Rating:
# picoCTF keygenme-py Write Up
## Details:
Points: 30
Jeopardy style CTF
Category: Reverse Engineering
## Write up:
Looking at the top of the python file we see:
```python
username_trial = "PRITCHARD"
bUsername_trial = b"PRITCHARD"
key_part_static1_trial = "picoCTF{1n_7h3_|<3y_of_"
key_part_dynamic1_trial = "xxxxxxxx"
key_part_static2_trial = "}"
```
Looking through the file a bit more we trace the path from accepting the key till a function that seems to try to decrypt the key:
```python
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
```
From here I wrote a little script to decrypt the missing characters from the flag:
```python
# import hashlib
import hashlib
# username
username_trial = "PRITCHARD"
# known flag
flag = "picoCTF{1n_7h3_|<3y_of_"
# decrypt flag
flag += hashlib.sha256(username_trial.encode()).hexdigest()[4]
flag += hashlib.sha256(username_trial.encode()).hexdigest()[5]
flag += hashlib.sha256(username_trial.encode()).hexdigest()[3]
flag += hashlib.sha256(username_trial.encode()).hexdigest()[6]
flag += hashlib.sha256(username_trial.encode()).hexdigest()[2]
flag += hashlib.sha256(username_trial.encode()).hexdigest()[7]
flag += hashlib.sha256(username_trial.encode()).hexdigest()[1]
flag += hashlib.sha256(username_trial.encode()).hexdigest()[8]
flag += '}'
# print flag
print(flag)
```
Once run we get:
```
picoCTF{1n_7h3_|<3y_of_54ef6292}
```
