I'm not very knowledgeable on this subject, but what if we use two different hash methods? If we use both MD5 and SHA-1, on a file, how hard would it be to make modifications that would be undetectable to both? From what I understand, it would be very difficult because a modification that's undectable to one hash method could be detected by the other hash method. Don't know if it would work, but I think redunant security is a good policy.