https://app.hackthebox.com/challenges/453

Description

My friend send me a encrypted message by using encryption bot. This is a important message. Can you decrypt the message for me?

Exploitation

#!/usr/bin/python3

def decode(encoded):
    chars = "RSTUVWXYZ0123456789ABCDEFGHIJKLMNOPQabcdefghijklmnopqrstuvwxyz"
    binary = ''
    for c in encoded:
        idx = chars.index(c)
        binary += format(idx, '06b')
    flag = ''
    for i in range(0, len(binary), 8):
        chunk = binary[i:i+8]
        if len(chunk) == 8:
            flag += chr(int(chunk, 2))
    return flag

encrypted = "9W8TLp4k7t0vJW7n3VvMCpWq9WzT3C8pZ9Wz"
print(f"Encrypted: {encrypted}")
print(f"Flag: {decode(encrypted)}")

Summary

Encryption Bot: reverse the validation logic, model the transform, and recover the accepted input.