2. The subject of cryptography

3

The decryption process is the inverse procedure, i.e., the trans-

formation of the cryptogram into the plaintext according to some

specific rules, which are also a part of the cipher.

Cryptography is an applied science, and it is based on the most

recent developments of other sciences, first of all mathematics. On the

other hand, all applications and daily needs of cryptography depend

on the development of technology, namely on the communication and

information transmission hardware.

2. The subject of cryptography

Now, what is the subject of cryptography? In order to answer this

question let us return to the ST problem and make both the situation

and the notions more precise.

Note first of all that the problem arises only for information which

must be kept private. Such information is also called secret, or con-

fidential The most typical and frequently occurring forms of confi-

dential information are

- state secrets;

- military secrets;

- business and commercial secrets;

- judicial secrets;

- medical secrets, and so on.

Below, speaking about secret information, we mean the following

properties of it:

- there is a group of authorized users of this information;

- there are unauthorized users intending to get the information

in order to profit from it, and to harm the authorized users.

For simplicity, we start with a single threat, that of information

divulgence. There are also other threats, such as information substi-

tution or imitation; we shall discuss them later.

Now we are able to represent the ST problem in the schematic

form shown in Figure 1. Here A and B are authorized users of the

information who want to communicate through a public line, and O