Introduction

How should Peggy prove to Victor that she is who she claims to be?

There is no simple answer to this question, it depends on the situation. For example if Peggy and Victor meet in person, she may show him her passport (hopefully issued by an authority that he trusts). Alternatively she could present him with a fingerprint or other biometric information which he could then check against a central database. In either case it should be possible for Peggy to convince Victor that she really is Peggy. This is the first requirement of any identification scheme: honest parties should be able to prove and verify identities correctly.

A second requirement is that a dishonest third party, say Oscar, should be unable to impersonate Peggy. For example, two crucial properties of any passport are that it is unforgeable and that its issuring authority can be trusted not to issue a fake one. In the case of biometrics Victor needs to know that the central database is correct.

A special and rather important case of this second requirement arises when Victor is himself dishonest. After asking Peggy to prove her identity, Victor should not be able to impersonate her to someone else.