Well, this problem I haven’t solved for almost a year (appx. 9 months :D). Link. Recently, I reached the author: vuonghy2442, and focusing to solve it before Christmas 😆
If you come here for a solution, sorry :(, There is no hint for you. Due to the rules of pwnable.vn, I can’t let you read the writeup without knowing the flag. If you need a hint, come search something related to the problem name & thinking about ECM :)
The writeup is located at: $HOST/posts/<md5sum(flag)>