Eulers license - DCTF 22

I took part to the DCTF 2022 with the team Ulisse of the University of Bologna.

The Bookstore.java challenge stated that:

Someone who doesn't care about bandwidth usage decided to package both the server and client binaries in a single file... The server of course is meant to run on linux, and the client on Windows.

We get a PowerShell file eulers_license.ps1 that contains:

• a binary_linux var containing the server code encoded in base64.
• a binary_win var containing the client code also encoded in base64.

The linux binary

The linux binary is very easy to reverse. In fact by decoding it we get a python server which has a huge SQLi vuln:

lice = request.args.get("license_key")query = "SELECT * FROM license_keys WHERE license_key = '" + lice + "';"

we can proceed with a basic SQLi like ' OR 1=1 -- and get the first part of the flag (which is the second one really):

_python_is_easy_to_reverse}

The windows binary

The windows exe is a little bit harder to reverse. By looking at it with ghidra we understand that it must be:

• a 10 digits number
• a prime number
• it has something to do with Euler

By a combination of chance and testing we come across the number 2147483647 which is a prime number discovered by Euler.

Providing this input to the client gives us the output:

Enter eulers license key: 2147483647dctf{2147483647Failed to contact euler.dragonsec.si for license confirmation...

dctf{2147483647_python_is_easy_to_reverse}