S.H.E.L.L CTF 2021: Subsi | szymanski.ninja

Subsi

Category: Cryptography

chal

50 points

cipher : HITSS{5X65Z1ZXZ10F_E1LI3J}

file: script.py

script.py

alpha = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ{}_1234567890'
key   = 'QWERTPOIUYASDFGLKJHZXCVMNB{}_1234567890'

text = <flag>

def encrypter(text,key):
    encrypted_msg = ''
    for i in text:
        index = alpha.index(i)
        encrypted_msg += key[index]
    # print(encrypted_msg)
    return encrypted_msg

Solution

It’s a classic substitution cipher, so I’ve made small change in given Python script to parametrize the alpha and used alpha as key and key as alpha.

alpha = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ{}_1234567890'
key   = 'QWERTPOIUYASDFGLKJHZXCVMNB{}_1234567890'

encrypted_flag = 'HITSS{5X65Z1ZXZ10F_E1LI3J}'

def encrypter(text, key, alpha):
    encrypted_msg = ''
    for i in text:
        index = alpha.index(i)
        encrypted_msg += key[index]
    return encrypted_msg

print(encrypter(encrypted_flag, alpha, key))

Output:

SHELL{5U65T1TUT10N_C1PH3R}

Flag

SHELL{5U65T1TUT10N_C1PH3R}