It is clear from section 2 that there are many similarities between the different protocol goals specified in the literature. While many authors have given extensional goals for key establishment only intensional goals seem to have been proposed for entity authentication. If protocol designers are to provide secure entity authentication protocols they might be helped greatly by knowing extensional specifications for what they are trying to achieve.
The aim in this section is to consider what are the reasonable and desirable extensional goals for cryptographic protocols. A hierarchy is proposed which incorporates key establishment on one side and entity authentication on the other, as well as extended goals which include both key establishment and entity authentication. These are certainly not the only possible extensional goals and may not be the most useful, but they do seem to fit together in a logical way and cover a broad range of reasonable protocol goals. The approach taken is to consider the fundamental elements used in practical protocols and to abstract the properties that are obtained by employing those elements.