# ASIS CTF Finals 2020 - Coffeehouse writeup

## Coffehouse

We are given a small python code, here’s the relevant part

The flag is then splitted into blocks of 2 bytes and encrypted using the encrypt function. We can recognize the encrypt as the Tiny Encryption Algorithm, a very simple ARX block cipher.

We can also see that the key generation is done using 32 bit of randoms, so without trying to break the algorithm itself we wrote a bruteforce in C++ using the C code of TEA available on wikipedia. The only things to notice are that the round constant (d in the code) is not the standard one and that all the computations are done on 16-bit integers instead of the standard 32-bit integers used in TEA. The following non-optimized C++ code retrieves the flag in 2 minutes on a 32 core machine, with almost zero memory usage.