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

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