writeups

IceCTF - 20 - Rotated! - Cryptography

noraj (Alexandre ZANNI)
, , ,
🇬🇧

Information#

Version#

By Version Comment
noraj 1.0 Creation

CTF#

Description#

They went and ROTated the flag by 5 and then ROTated it by 8! The scoundrels! Anyway once they were done this was all that was left VprPGS{jnvg_bar_cyhf_1_vf_3?}

Solution#

  1. A ROT13 Caesar cipher again. 5 + 8 = 13 right?
  2. Let's use a script:
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
#!/usr/bin/python

import sys
import string

def caesar(plaintext, shift):
    alphabet_lower = string.ascii_lowercase
    alphabet_upper = string.ascii_uppercase

    shifted_alphabet_lower = alphabet_lower[shift:] + alphabet_lower[:shift]
    shifted_alphabet_upper = alphabet_upper[shift:] + alphabet_upper[:shift]

    alphabet = alphabet_lower + alphabet_upper
    shifted_alphabet = shifted_alphabet_lower + shifted_alphabet_upper

    if (sys.version_info > (3, 0)):
        # made with python 3.5.2
        table = str.maketrans(alphabet, shifted_alphabet)
    else:
        # made with python 2.7.12
        table = string.maketrans(alphabet, shifted_alphabet)

    return plaintext.translate(table)

rot = 'VprPGS{jnvg_bar_cyhf_1_vf_3?}'
print(caesar(rot, 5 + 8))
  1. Flag: IceCTF{wait_one_plus_1_is_3?}.
Share