Authentication and identification

Authentication is a term which is used (and often abused) in a very broad sense. By itself it has little meaning other than to convey the idea that some means has been provided to guarantee that entities are who they claim to be, or that information has not been manipulated by unauthorized parties. Authentication is specific to the security objective which one is trying to achieve. Examples of specific objectives include access control, entity authentication, message authentication, data integrity, non-repudiation, and key authentication. These instances of authentication are dealt with at length in Chapters 9 through 13. For the pui-poses of this chapter, it suffices to give a brief introduction to authentication by describing several of the most obvious applications.

Authentication is one of the most important of all information security objectives. Until the mid 1970s it was generally believed that secrecy and authentication were intrinsically connected. With the discovery of hash functions (§1.9) and digital signatures (§1.6), it was realized that secrecy and authentication were truly separate and independent information security objectives. It may at first not seem important to separate the two but there are situations where it is not only useful but essential. For example, if a two-party communication between Alice and Bob is to take place where Alice is in one country and Bob in another, the host countries might not permit secrecy on the channel; one or both countries might want the ability to monitor all communications. Alice and Bob, however, would like to be assured of the identity of each other, and of the integrity and origin of the information they send and receive.

The preceding scenario illustrates several independent aspects of authentication. If Alice and Bob desire assurance of each other’s identity, there are two possibilities to consider.

  • 1. Alice and Bob could be communicating with no appreciable time delay. That is, they are both active in the communication in “real time”.
  • 2. Alice or Bob could be exchanging messages with some delay. That is, messages might be routed through various networks, stored, and forwarded at some later time.

In the first instance Alice and Bob would want to verify identities in real time. This might be accomplished by Alice sending Bob some challenge, to which Bob is the only entity which can respond correctly. Bob could perform a similar action to identify Alice. This type of authentication is commonly referred to as entity authentication or more simply identification.

For the second possibility, it is not convenient to challenge and await response, and moreover the communication path may be only in one direction. Different techniques are now required to authenticate the originator of the message. This form of authentication is called data origin authentication.

 
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