writeup.md

Case64AR

CRYPTO - EASY

WU author: inexp-mf

Statement

Someone script kiddie just invented a new encryption scheme. It is described as a blend of modern and ancient cryptographic techniques. Can you prove that the encryption scheme is insecure by decoding the ciphertext below? Ciphertext OoDVP4LtFm7lKnHk+JDrJo2jNZDROl/1HH77H5Xv

Author: k3vin

Solution

The caption and other clues suggest that two encodings were combined somehow: base64 and Caesar cipher. The question is, how exactly?

The simplest options:

• Caesar within ASCII, then base64.
  Let's try it out to no avail:

import base64 as b64
enc  = 'OoDVP4LtFm7lKnHk+JDrJo2jNZDROl/1HH77H5Xv'
decoded = b64.b64decode(enc)
for shift in range(128):
    shifted = ''.join(chr((c + shift) % 128) for c in decoded)
    print(shifted)

None of the strings looks like a flag or anything meaningful.

• base64, then Caesar.
  Seems unlike because the ciphertext is still in base64 character range. Unless the translation table is not ASCII but the base64 alphabet instead?
  Let's check this one:

import base64 as b64
table = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/='
untable = {table[i]:i for i in range(len(table))}
enc  = 'OoDVP4LtFm7lKnHk+JDrJo2jNZDROl/1HH77H5Xv'
for shift in range(len(table)):
    shifted = ''.join(table[(untable[c] + shift) % len(table)] for c in enc)
    try:
        print(b64.b64decode(shifted))
    except:
        print(f'Failed on {shift}')

Among others: b'sdctV{OB3cUr-ty_`\xf1nt_s3\x02UR\x08Ti}'

We're quite close. Let's try without = in the alphabet: b'sdctf{OBscUr1ty_a1nt_s3CURITy}

Success!