10.4. Cryptographic method of information security

The most effective means of increasing security is cryptographic transformation. In order to increase safety, one of the following actions is carried out:

1) data transmission in computer networks;

2) transfer of data stored in remote memory devices;

3) the transfer of information during the exchange between remote objects.

Protecting information using the cryptographic transformation method consists in reducing it to an implicit form through the transformation of the constituent parts of information (letters, numbers, syllables, words) using special algorithms or hardware and key codes. The key is a variable part of the cryptographic system, which is kept secret and determines which encryption from the possible transformation is performed in this case.

To change (encrypt), use some algorithm or device that implements a given algorithm. Algorithms can be known to a wide range of people. The encryption process is controlled with the help of a periodically changing key code, which ensures the original presentation of information each time when using the same algorithm or device. With a well-known key, it is possible to decrypt text relatively quickly, simply and securely. Without knowledge of the key, this procedure can become practically impossible even with the use of a computer.

The following necessary requirements are imposed on cryptographic conversion methods:

1) it must be sufficiently resistant to attempts to open the source text using encrypted;

2) key exchange should not be difficult to remember;

3) the cost of protective transformations should be made acceptable at a given level of information security;

4) encryption errors should not cause an obvious loss of information;

5) the size of the cipher text should not exceed the size of the source text.

Methods intended for defense transformations are divided into four main groups: permutations, substitutions, additive, and combined methods.

The permutation and replacement (substitution) methods are characterized by short keys, and the reliability of protection is determined by the complexity of the conversion algorithms. For additive methods, on the contrary, simple algorithms and long keys are peculiar. Combined methods are more reliable. They most often combine the virtues of the components used.

The four cryptographic transform methods mentioned refer to symmetric encryption methods. One key is used for both encryption and decryption.

The main methods of cryptographic transformation are permutation and replacement methods. The basis of the permutation method consists in splitting the source text into blocks, and then in writing these blocks and reading the cipher text along different paths of a geometric figure.

Encryption method replacement is that the symbols of the source text (block), written in one alphabet, are replaced by symbols of another alphabet in accordance with the used transformation key.

The combination of these methods led to the formation of the derivative cipher method , which has strong cryptographic capabilities. The algorithm of the method is implemented both in hardware and software, but it is designed for implementation with the help of special-purpose electronic devices, which allows achieving high performance and simplified organization of information processing. Industrial production of cryptographic encryption equipment established in some Western countries can dramatically increase the level of security of commercial information during its storage and electronic exchange in computer systems.