DES_AMIES

DESAmies is a Cryptography challenge

First of all we have a network connection that asks for input and gives us what looks like encrypted message.

It’s just that nothing.

Looking at the challenge name I assume that It’s DES encryption and we have to decrypt the message somehow.

Honestly I wasn’t able to solve it without the hint which says “Strong key”. Hmmmm if there is a strong key then there is a weak key.

After some googling I discovered that DES implementation has some ‘weak keys’ that if you encrypt with them twice you will get the original message

“Original” —enc—> “Encrypted” —enc—> “Original”

Now that’s awesome we just have to try every key and wish it will work. After trying the key this one worked and we got this message :)

Key: 0xFFFFFFFFFFFFFFFF

from pwn import *
from des import DesKey

conn = remote('167.71.93.117', 9000)
conn.send('aaaaaaaa')
x = conn.recvline()
s = x[41:-1]
keys = [
	b'\x00\x00\x00\x00\x00\x00\x00\x00',
	b'\x00\x00\x00\x00\xFF\xFF\xFF\xFF',
	b'\xFF\xFF\xFF\xFF\x00\x00\x00\x00',
	b'\xFF\xFF\xFF\xFF\xFF\xFF\xFF\xFF'
]

for k in keys:
	key = DesKey(k)
	y = key.encrypt(s)
	print(y)
	print()