Cryptography Reference
In-Depth Information
Historical
Cryptosystems
2
We nowdiscuss a number of simple historical cryptosystems. These cryptosystems
are unsuitable for modern use, but they are simple to understand and help to
illustrate many of the issues discussed in Chapter 1, including the basic model
of a cryptosystem. These historical cryptosystems also provide us with simple
examples that allow some fundamental cryptosystem design principles to be
explored.
At the end of this chapter you should be able to:
• Describe a number of simple historical cryptosystems.
• Relate a number of historical cryptosystems to the basic model of a
cryptosystem.
• Appreciate the direction of some historical advances in cryptosystem design.
• Illustrate properties of these historical cryptosystems that make them
unsuitable for modern use.
• Formulate some fundamental design principles for a modern cryptosystem.
Before we proceed further, it is worth noting that all the cryptosystems that we
will discuss in this chapter have four common features:
Symmetric
. They are all symmetric cryptosystems. Indeed, they all predate the
discovery of public-key cryptography.
Confidentiality
. They are all designed to provide confidentiality only.
Alphabetic
. They are all described as operating on alphabetic characters. This
is in contrast to modern cryptosystems which generally operate on numbers,
most commonly binary numbers. This has some implications: for example, we
normally consider the size of the keyspace of these cryptosystems rather than
discuss key length (see Section 1.6.2).
Unsuitable
. Each, in their own different way, is completely unsuitable for use in
modern cryptographic applications. In most cases this is because they are not
secure enough.
