Berserker's Brain Teaser

Posted on Mon 20 March 2023 in ctf

This is a writeup of the Berserker's Brain Teaser challenge which was part of the Crypto category during vikeCTF.

7402bfa4fa5e403778ca745990635cff.png

The attached python script BerserkerBrainTeaserEncrypt.py that was used to encrypt the flag had the following contents:

import random

alphabet = "abcdefghijklmnopqrstuvwxyz"

plaintext = input("text to encrypt:\n")
ciphertext = ""

key = [random.randint(1, len(alphabet)) for _ in range(len("vikeCTF"))]
print("key:", *key)

for i, c in enumerate(plaintext):
    if not c.isalpha():
        ciphertext += c
        continue

    offset = alphabet.find(c.lower())
    rotation = key[i % len(key)]

    result = alphabet[(offset + rotation) % len(alphabet)]
    if c.islower():
        ciphertext += result
    else:
        ciphertext += result.upper()

print("ciphertext:")
print(ciphertext)

Since we know the encrypted text as wenn as the flag prefix it was easy to first rebuild the encryption key:

alphabet = "abcdefghijklmnopqrstuvwxyz"

cipher = "zexqSNE{cVaLuM_xRxBuRs_vE_mTtAe_ToOiN_oEiK}"
cipher_prefix = "zexqSNE"
flag_prefix = "vikeCTF"

key = []
for i, c in enumerate(flag_prefix):
    offset = alphabet.find(c.lower())
    rotation = alphabet.find(cipher_prefix[i].lower()) - offset + 26
    key.append(rotation)

print(f"key: {key}")

With that key we can decrypt the flag:

# Decrypt flag with previously generated key:
flag = ""
for i, c in enumerate(cipher):
    if not c.isalpha():
        flag += c
        continue

    offset = alphabet.find(c.lower())
    rotation = key[i % len(key)]
    result = alphabet[(offset - rotation) % len(alphabet)]

    if c.islower():
        flag += result
    else:
        flag += result.upper()

print(f"flag: {flag}")

This gave: vikeCTF{gIoVaN_bElLaSo_iS_sUpEr_DuPeR_cOoL}

ctf crypto