write-ups-challenges-2024-2025/royal-message-decryptor/SOLUTION.md

Difficulty

50/100 | MEDIUM

They need to figure out that the first part of the message is a substitution alphabet, then decrypt the code, and then finally figure out the flag is embedded in the last message they sent.

Category

Crypto

How To Solve

The first part of every message line was a substitution table, the second part the message. Just substitute the message and the last message will contain the flag.

from pwn import *
import string

p = remote("localhost", 3006)
p.recvline()


while True:
    msg = p.recvline().decode()
    p.recvuntil(b"> ")
    print(msg)

    alphabet = msg.split(" | ")[0]
    message = msg.split(" | ")[1][:-1]
    alphabet_map = dict(zip(alphabet, string.ascii_letters))

    decoded = "".join([alphabet_map[c] if c in alphabet_map else c for c in message])
    print(f"Sending back decoded: '{decoded}'")
    p.sendline(decoded.encode())

    p.recvline()

Flag

IGCTF{Some-wicked-story-unfolded-here-great-work}