Cryptography Reference
In-Depth Information
Chapter 5
Message Authentication
In the preceding chapters we studied encryption schemes whose goal is to ensure
confidentiality and prevent an adversary from learning information about messages
sent over a communication channel. This chapter is devoted to the study of schemes
that pursue an entirely different goal, namely, to ensure authenticity.
5.1 Confidentiality Versus Authenticity
Since ancient times, cryptography has been closely associated with providing confi-
dentiality and so it is often implicitly or explicitly identifiedwith the use of encryption
and decryption. But, important as confidentiality is, modern cryptography goes far
beyond this objective and
message authentication
and
message integrity
are perhaps
even more important goals. Indeed, when one receives a message from another party
through an open channel, one often may not care whether privacy has been main-
tained but certainly one wants to know for sure who the originator of the message
is and whether or not the message has been altered during transmission. An obvious
example where message integrity is essential while confidentiality is not required is
in the transmission of stock quotes.
The goal of message authentication is to address these concerns by providing
assurance to the receiver that the message is
authentic
, i.e., that it was really sent
by the legitimate sender, and that the message
integrity
was not compromised or, in
other words, that the message was not modified en route by an adversary.
In this chapter we will look at techniques that provide message integrity in the
private-key setting, i.e., we will assume that the honest parties share some secret key.
These techniques will try to guarantee that any modification of the message will be
detected by the honest parties.
Before going into the details of how authentication may be achieved, it is conve-
nient to emphasize that encryption does not provide it and hence the techniques seen
so far are not adequate for this purpose. It is a common source of misunderstanding to
