Cryptography Reference
In-Depth Information
issues that need to be resolved before we can comfortably claim that public-
key cryptography
does
overcome these problems. We will see in Chapter 11
that public-key cryptography still requires an
indirect
trust relationship between
entities employing it. We will see in Section 5.5 that one of the most compelling
uses of public-key cryptography is actually to support the establishment of
symmetric keys.
HISTORY OF PUBLIC-KEY CRYPTOGRAPHY
There was no public-key cryptography until the final quarter of the last century.
Thus, in terms of cryptographic history, public-key cryptography is a relatively
new idea. The timing of its invention is, however, more than coincidental. This
is because the problematic issues that we have just associated with symmetric
cryptography are relatively simple to overcome in the types of application
environment in which cryptography was used prior to the 1970s. These were
typically large, closed organisations such as governments, the military and big
financial corporations. Symmetric cryptography was perfectly suitable for use in
such organisations, and indeed still is, because:
• trust relationships exist between users of cryptography, since they are typically
part of the same, or allied, organisations;
• symmetric key establishment can be facilitated and managed by an organisa-
tion's internal procedures and policies.
It was only with the spread of more open computer networks that a genuine need
arose for deploying cryptography in environments where the problems with using
symmetric cryptography present a significant challenge.
The invention of public-key cryptography is an interesting tale in itself. The
intrigue lies in the fact that years after the 'public' mid-1970s invention of public-
key cryptography in the US, it emerged that the idea had also been discovered
several years earlier by UK government researchers. What is particularly poignant
is that the UK government researchers had set aside the idea, primarily due to
practical implementation concerns. Addressing some of these concerns is the
subject of Chapter 11.
5.1.2
Properties of public-key cryptosystems
We now work towards a blueprint for a public-key cryptosystem that identifies
the properties that we might want from such a system.
THE BRIEFCASE PROTOCOL
We have just observed that using symmetric encryption to protect a communica-
tion channel requires a trust relationship between the two communicating entities
and prior establishment of a symmetric key. In fact this is not strictly true, as we
will see in the following example, which we term the
briefcase protocol
. We begin
with a physical analogy.
