I came across a problem very similar recently and wondered if anyone knew how to solve this.
So say for example I have an RAR archive called archiveA.rar and a password to protect it the password is known as keyA. keyA is stored on the server but I can send the server any file and it will compress it into a RAR file and encrypt it with keyA. I can keep sending files to the server to retrieve encrypted versions of different files. So my question is with all this given information and the fact that I can send unlimited files to the server can I solve for keyA, if so please explain how I would do so?