Rating:
20 points - 717 solves
Every CTF starts off with a Caesar cipher, but we're more classy.
Source code was provided in the challenge, which looked like
from secret import flag, shift
def encrypt(d, s):
e = ''
for c in d:
e += chr((ord(c)+s) % 0xff)
return e
assert encrypt(flag, shift) == ':<M?TLH8<A:KFBG@V'
We are given a cipher-text and a knowledge about the logic which was used to encrypt the ciphertext. It is evident that the encrypt algorithm is XOR (Exclusive-OR).
The modulus 0xff ensures what chr data point lies until 255. Now, our goal is to figure out the correct shift, as we know that chr would lie within 255. I can simply brute-force the values.
I quickly code a solver for it
def encrypt(d, s):
e = ''
for c in d:
e += chr((ord(c)+s) % 0xff)
return e
for i in xrange(0, 256):
ciphertext=':<M?TLH8<A:KFBG@V'
flag = encrypt(ciphertext, i)
if flag[:5] == 'actf{':
print (i, flag)
Hence, we retrieve
(39, 'actf{so_charming}')
