Tags: xor crypto graphics
The creators of the SHA2017 CTF "appetizer" built an own crypto algorithm which needed to be cracked to solve the challenge. This is a good example that building your own proprietary crypto algorithms is always not a good idea.
The fun thing about this crypto is, that the ciphertexts are encoded in pictures. An "encryption engine" (encrypting oracle) is given where text can be entered to generate the corresponding picture. Additionally, the ciphertext of the flag is given.
The full writeup is divided in firstly finding out, how the ciphertext is encoded in the pictures and secondly, how the cipher itself works. In a last section, a link to a python-script which can act as a automatic decryptor can be found. This writeup is located on Github: [Full Writeup](https://github.com/Endres/writeups/blob/master/sha2017/sha2017teaser_crypto200.md)