These two numbers can be run through a program that will create a number sometimes referred to as a cryptographic hash, that can be used to immediately tell when the content has been tampered with. This system has two levels of protection. The first is simply a computation, which is useful for verifying to recipients that what they ordered is what they received. It is useful for publishers to provide them assurance that their customers are also getting what they intended and not something that has been in some way compromised. However, a higher level of authenticity check can also be provided if the product of computing the DOI and the size of the file can be stored in a protected repository so that a later user who wants to be sure that the version he has is the same as the original version can compare the stored content with the new computational output to determine if the versions are the same.
Systems are already in place to provide authenticity checks using digital signatures.
Certificates of authenticity Independent of the DOI system, there are third party agents providing certification to protect both buyer and seller - the certificates prove that the customer is who he says he is and the creator is who she says she is. This is useful to protect commerce and insure smoothness in transactions. Anyone who has used a credit card has had a moment of concern about someone making unauthorized use of a credit card number when ordering over the telephone - the Internet increases the concerns, but with certificates and verification, the problem of credit card fraud might be diminished rather than multiplied. The DOI system can be linked to certificates so that only the authorized customer can receive the digital content. Privilege controls Many of the encryption systems being described for copyright protection involve wrappers or envelopes or interfaces that the customer or user must interact with before gaining access to the content.