COMPUTER SYSTEM FOR PROTECTING A FILE AND A METHOD FOR PROTECTING A FILE

A computer system is provided having a protection mechanism for protecting the contents of a file. The protection mechanism has at least one Viewer program, at least one challenge associated with the Viewer program and the file, and at least one response with private keying material that it can access. The challenge has no access to the private keying material. The response can prove that it has access to the private keying material by interacting with the challenge using an asymmetric cryptographic scheme. The challenge can instruct the Viewer program to avoid using some or all of the file's content unless the proof is successful.