An intro to reversing challenges, pwntools, tips and tricks in Python, and basic ciphers such as Caesar's, XOR, and RSA.
- Python 3
- incore.py: Reverse challenge
Python is a really simple and powerful tool for CTFs. We went over:
- Intuition on how to recognize what the challenge is actually doing
- Caesar's cipher - calculating offsets etc.
- binary strings
- netcat
- pwntools
A detailed description of the challenge can be found here.
The challenge basically returned a string each time you sent a message, with 1 or 0 characters of the flag in it. This would take hours to solve manually, decrypting the Caesar's cipher everytime. Instead, we wrote a python script to do it for us.
flag = [None]*100
def response(s):
offset = ord(s[0]) - ord('s')
position = 0
res = ''
for i in s:
if not i.isalpha():
if i.isnumeric():
position = position*10 + int(i)
res += i
continue
res += chr((ord(i) - ord('a') - offset) % 26 + ord('a'))
if len(res) == 55:
return res
flag[position] = res[-2]
return res
from pwn import remote
r = remote('jh2i.com', 50034)
while True:
received = r.recv()
x = received.decode().strip()
print(x)
y = response(x)
r.send(y)
print(y)
print()
f = ''.join([i for i in flag if i != None])
print(f)
print()
# Rotten- Caesar's Cipher
- XOR Encryption
- RSA Algorithm