Prehistory of Cryptography - A Classical Introduction to Cryptography: Applications for Communications Security - page 15

1.2.5 Methodology

Communication Channels

Communication channels have different kinds of attributes: cost, speed, availability,
reliability, security. Here, reliability refers to resistance against random noise. We do
not consider it since it is addressed by coding theory. So we implicitly consider that all
communication channels perform a transmission in a reliable way: the sent information
is always equal to the received one unless there is a malicious attack.

As we have seen, security may relate to the ability to provide confidentiality, in-
tegrity, or authentication. If we use basic telegraph through radio signal, speed is high,
cost is low, but security is void. Availability is also high since ether is (in principle) al-
ways usable. If we now use the diplomatic case to transmit information (for instance, we
give some information to an ambassador who is physically sent to the information des-
tination), we have a low speed, a high cost, but a high security. Availability also depends
on the airplane and the schedule of the ambassador. If we now use Enigma-encrypted
radio signals, the speed is high, the cost is relatively low (the development of the Enigma
machine is quickly amortized in wartime), and the security should have been high.

Note that we talk about communication channels in a broad sense: we are not only
interested in moving some information from one place of a three-dimensional space to
another. We are also interested in the fourth dimension (time): we also want to archive
some information which can be used later. So an archive system can also be considered
as a communication channel.

Reduction

Computer science is a matter of reduction: instead of solving a problem from scratch,
we try to cut it into several subproblems, and reduce one problem to another one.
Security is the same. As we will see later, it is impossible to consider that we can have
a single communication channel which is cheap, fast, available, and secure. On the
other hand, we can use expensive, slow, hardly available, but secure channels in order
to transform a cheap, fast, available, and insecure channel into a cheap, fast, available,
and secure one. In the next sections and chapters we will see how to do it by improving
the security attributes.

1.3 The Shannon Theory of Secrecy

1.3.1 Secrecy of Communication

The purpose of encryption is to ensure communication secrecy. We assume that we
want to communicate, which means to transmit information through a channel. The
channel is not assumed to be secure.