HackTheBox Cyber Santa CTF 2021: Intercept

TL;DR

The encryption is a custom stream cipher using a predictable keystream.
Each byte is XORed with a value derived from its index.
The keystream formula is (55 * i + 19) % 256.
Reversing the XOR recovers the plaintext directly.

Video Walkthrough

Challenge Description

Intercept We managed to covertly spy on some of the elves' communications, as well as obtain partial code for their experimental encryption algorithm. Can you find where they're planning their next meeting?

Solution

from pwn import *

enc = unhex(b'5b2fedd4801914e7eb765119d4fe6223f1d1984638a9816b5419dac07b27eed9d35e09fdef65521ac5877a24eed19b0c0ae9f16d4c02cc86773bfaa8924a2ae9a12a2f1dd7923d39eea78d5909f9f57b2a16ddc87d33ada58f1208d4f737755283da1168a3e6cc075e8ce920774ef88d483fb1bb8a440884af7d69e2c5874b3bb3be695d4fd5a97b27e7d7d0572cf0bf665405dbfe4225e19b824813e4b96a4e178a95776fe1d8800b0bf7f0705719c0c37834a8f7a26f1febbe3d7119dad66427d5f58b4259eabc3f3626ded46621d3b0ca441afce552274bd6da1f2a')

dec = b''

for i, byte in enumerate(enc):
    dec += xor(chr((55 * i + 19) % 256), byte)

success(dec.decode())

Flag: HTB{pl41nt3xt_4sm?wh4t_n3xt_s0urc3_c0d3?}