Client-server solution for signing and encrypting files

Question

I'm looking for a solution to perform file encrypting and signing. This would be done on the client side, and signature check and decryption on the server side.

Some details:

• This should work for any kind of file (the signature should be "attached" to the file, not inside it like with PDFs)
• At least the server side should be Java based (the client could be a Java applet)

EDIT: The client part is really the greater issue here. While the server side can be implemented with existing libraries, the client should be an existing application (even if commercial).

Answer 1

The Cryptix project is an open source java PGP library.

Answer 2

You would need to use PGP or something similar on the client side then. There are standards for signing and verifying signatures on messages and/or files, and there are certainly commercial and free products for this. I would recommend using PGP on the clients, and you can look up either a PGP library for Java to use on the server, or you can check out how PGP wraps up the data to emulate that (after all, signing data is a standard and well documented procedure, albeit cumbersome).

The Bouncy Castle library for Java has some OpenPGP support, you can check it out to see how compatible it is with a PGP implementation for Windows or whatever the clients use. Cryptix also has some OpenPGP support but that library was meant for Java 1.3 and hasn't been updated since 2005. I recommend you use BC instead.