MetaRed 2022 3rd STAGE - Quemada Broadcast

Quemada Broadcast is a web challenge that was part of the MetaRed CTF 2022 (3rd STAGE).

With the challenge, we get the following info.

We have intercepted a broadcast coming from the "La Quemada" in Zacatecas, Mexico. It appears to be grouped in 7 bits bunches, but we can't decode it. Can you help us to understand what do they trying to tell us? La Quemada represents the most important monumental settlement in northern central Mexico for its architecture. 

In the place, there is a large hall of columns with its square, a field for the large ball game in the traditional form of “I”, and a pyramidal basement called Votive Pyramid. 

Note> 1. run the docker container, instructions  below, 2. access app at: http://127.0.0.1:5000


docker run --rm -it --name quemada -p 5000:5000 hackadvisermx/metaredctf-quemada-broadcast

First approach

We start by launching the docker container with the supplied command. On opening the URL in our Browser we notice a ~ character. On refreshing, a number appears. Doing multiple refreshes we notice that we see 1 or 0 appearing. With the info that the broadcast is grouped in 7 bit parts, we decided to write a script to automatically get all messages from the server separated after receiving 7 numbers. We assume that the ~ character is the divider of the message.

import requests 

i = 0
bits = ''
while True:
    i += 1
    r = requests.get('http://127.0.0.1:5000/seq/e995c28a-9095-49a2-837d-20b2bb50f7f2')
    if r.text != '~':
        bits += r.text
        if i % 7 == 0:
            print(bits)
            bits = ''
    else:
        break

Using the following script, we get this message out of our broadcast.

Solution

Putting this into CyberChef and decoding it from binary using byte length 7 we get the following message.

ZmxhZ01YezdCaXRTdGFuZGFyZEhlbHBVc1BsZWFzZX0=

Applying CyberChef’s magic or our good eyes we see that this is base64 and get the flag by decoding.

Flag

The flag is “flagMX{7BitStandardHelpUsPlease}”.