使用数字签名验证真实性时，从java applet将文件上传到php服务器

I am developing an major project for my undergraduate degree. I have an java applet that is encrypting a file using aes algorithm. The file is then stored in local machine. Now I want to store that file in my MySQL database at server. I am using php at backend. I also want to verify that file by using digital signature. I wrote some java code that generates a signature and public key file and some code to verify that signature for the given file. The problem is suppose multiple users are accessing that page and for every instance these files will be generated and stored in their respective local machines. I don't know how can I upload these files to the server and verify individual users. Is there any better way of doing this. I am developing an major project for my undergr