Cryptography Reference
In-Depth Information
Default algorithms
(Zone E). This consists of a handful of publicly known
encryption algorithms that are widely recognised and deployed. These are
regarded as safe choices and likely to be supported by many cryptographic
applications.
Note that a publicly known encryption algorithm may well move between these
zones over time. The only modern encryption algorithms that we will make
specific references to in this topic are (or used to be) either default or respected
algorithms. It would normally be unwise to deploy a publicly known encryption
algorithm that belongs to any other zone.
We now discuss the much misunderstood concept of 'breaking' a cryptosystem.
We will focus on:
• Cryptosystems providing confidentiality based on encryption algorithms. We
note that the general principles apply to other cryptosystems supporting other
cryptographic primitives.
• 'Breaks' that are directly related to the underlying cryptographic primitives.
There are many ways in which a cryptosystem could be 'broken' which have
nothing to do with the underlying cryptographic primitives. We discuss these
further in Section 3.2.
1.6.1
Some useful preliminaries
An important objective of this topic is to explain cryptography without the need
for skills in mathematics. While we fully intend to honour this objective, there
are some very basic pieces of notation and terminology that we will need. At the
risk of insulting the intelligence of some readers, this is a good place to make
them clear. Other (optional) mathematical ideas are relegated to the Mathematics
Appendix.
BINARY NUMBERS
It is important to realise that although we will discuss some historical encryption
algorithms in Chapter 2 which operate on letters of the alphabet, all the 'real'
cryptographic algorithms that we will discuss in this topic are designed to run on
computers, and thus process information (including plaintexts, ciphertexts and
cryptographic keys) as
binary
data consisting of zeros and ones. Individual zeros
and ones are conventionally referred to as
bits
and groups of eight bits are referred
to as
bytes
.
For much of our discussion we can probably 'get away' with just considering
binary data as sequences of zeros and ones. However, it is important to realise
