Last weekend I played PBCTF on my own, and instead of binary challenges that I am quite good at, I tried to play some cryptography challenges. This is actually my first non-trivial Crypto challenge that I solved in CTF. Thus I think it is quite worthy to write a writeup to record. Interestingly, the intended approach to solve the challenges is by using slide attack, but I actually didn’t know this technique until I got the flag, which tells the technique I used is actually slide attack that has been a common cryptography technique.