Cryptographic Protocols - Codes: The Guide to Secrecy from Ancient to Modern Times - page 191

Chapter 5
Cryptographic Protocols
The shadow cloaked from head to foot,
Who keeps the keys of all the creeds.
Alfred Lord Tennyson (1809-1892) , English poet
— from In Memoriam A.H.H. (1850), canto 23
5.1 Introduction
The term cryptographic protocol was brieflyintroduced to us in footnote
2.24 on page 101. This chapter is devoted to studying several particular such
protocols including: keyestablishment; keyagreement; identification; commit-
ment; secret sharing; electronic voting; protocol layer analysis with SSL as an
Internet protocol providing authentication and secrecyfor session-based com-
munications; and we conclude with digital cash schemes and e-commerce.
Before presenting the wealth of information listed above, we need an
overview. In anyinteraction among people, there is a certain level of risk,
trust, and expected behavior implicit in the interchange. This maybe inspired
to be conducted properlyfor anyreason ranging from fear of prosecution un-
der the law, to the desire to act in an ethical manner due to societal influences.
However, in a cryptographic protocol, trust has to be kept to the lowest possible
levels. In anysuch protocol, if there is an absence of a mechanism for verif-
ing, say, authenticity, one must assume that other participants are dishonest,
if for no other reason than self-preservation (or in the case of e-commerce, the
preservation of a positive balance in one's bank account). One must take this
approach as a default, unless there is a clearlyspecified secure interface to deal
with authentication of the entities with whom one is communicating.
In Section 5.7, we will discuss the various layers that make up the hierarchy
of communications within a given protocol. These layers allow each level to
speak to the next level up. Moreover, anyalteration on one protocol layer does
not affect other layers. This setup greatly eases the burden of work in creating
and maintaining communications networks. We leave the actual details of these