Vigenere cipher
https://en.wikipedia.org/wiki/Vigen%C3%A8re_cipher

Again they are giving us an enormous clue with the title and even a link to the wikipedia. It wasn't being too difficult.

If the above wasn't enough they provided us with the charset and the md5 of the reply with which the great master Patatas built the following scripts to resolve the challenge, first a known text attack is performed using the text we know from the flag:

[More](https://blog.kalrong.net/en/2016/12/14/seccon-2016-vigenere-crypto-100/)

Original writeup (https://blog.kalrong.net/en/2016/12/14/seccon-2016-vigenere-crypto-100/).