HackVent 2023 - [HV23.01] A letter from Santa

Difficulty: Easy

Category: Fun

Author: Coderion

Finally, after 11 months of resting, Santa can finally send out his presents and challenges again. He was writing a letter to his youngest baby elf, who’s just learning his ABC/A-Z’s. Can you help the elf read the message?

Upon opening the page, we can select a letter and enter some text:

If we use something like |||, the output looks like a part of a QR code:

We can write a python script to automate the process:

import requests
import string

URL = "<challenge url>"
data = ""
# Loop over every lowercase letter
for char in string.ascii_lowercase:
    params = {
        "alphabet_select": char,
        "user_input": "|||"
    }
    response = requests.post(URL, data=params)
    # append everything to an HTML file
    data += f"<br>{response.text}"

with open("output.html", "w") as f:
    # save the file
    f.write(data)

When opening output.html, in the browser, we get a QR code:

When reading it, we get the flag: HV23{qr_c0des_fun}