Digital Credential - Anonymous

Anonymous

The main idea behind anonymous digital credentials is that users are given cryptographic tokens which allow them to prove statements about themselves and their relationships with public and private organizations anonymously. This is seen as a more privacy-friendly alternative to keeping and using large centralized and linkable user records. Anonymous digital credentials are thus related to privacy and anonymity.

Paper world analogues of personalized, or non-anonymous credentials are: passports, driving licenses, credit cards, health insurance cards, club membership cards etc. These contain the name of the owner and have some authenticating information such as a signature, PIN or photograph, to stop them being used by anyone other than the rightful owner. Paper world analogues of anonymous credentials are: money, bus and train tickets, and game-arcade tokens. These don't have any personally identifying information and consequently can be transferred between users without the issuers or relying parties being aware of this. Credentials are issued by organizations that ascertain the authenticity of the information which can be provided to verifying entities on demand.

In order to investigate certain privacy specific properties of credentials, we take a more detailed look at two kind of 'credentials', physical money and credit cards. Without doubt both of them provide adequate information for doing payment transactions. However the amount and quality of the information disclosed varies. Money is protected from forgery by its physical properties. Beyond that, only very little information is revealed: Coins feature an engrained value and the year of coining; in addition bank notes contain a unique serial number in order to provide the traceability required by law enforcement.

On the other hand the use of a credit card, whose main purpose is similar to money, allows for the creation of highly detailed records about the card owner. Credit cards are therefore not privacy protecting. The main privacy advantage of money is that its users can remain anonymous. There are however other security and usability properties that make real world cash popular.

Credentials used in a national identification system are also especially privacy relevant. Such an ID, be it a passport, a driver's license, or some other type of card usually contains essential personal information. In certain situations it may be advantageous to reveal only parts of the information contained on the ID, e.g., some lower limit for the person's age or the fact that the person is capable of driving a car.