Protection of information in the channels and the commutation knots of the communication systems this form of protection is directed toward averting of the possibility of the unsanctioned access to the confidential and valuable information circulating along the communication channels of different it is specific. In the general case it is necessary to read all forms of the threats, which appear in the channels and the communication knots of the communication systems. The most effective means of protection of information in the controlled communication channels is the application of cryptography and special connected protocols.

Cryptographic protection is the totality of methods and means, intended for the coding it is text, i.e., for converting the form of initial (opened) it is text communications in such a way that their sense becomes incomprehensible for any person, who does not manage the secret of inverse transformation. The straight process of converting the plain text for the purpose of the concealment of its sense is called encoding, and its result encoded text.

The algorithms of coding utilized in practice ensure so large a quantity of possible keys, that decoding of encoded text via their complete sorting proves to be economically unfavorable or simply impossible. Contemporary cryptographic systems ensure the high durability of coding, even if algorithm of the coding of data is not secret. In this case the durability of encoded text completely is ensured due to the maintenance of the regime of the secrecy of the cryptographic key, used in this report of coding. A central question of cryptography are the evaluation of the durability of the algorithms of coding used, the determining confidence in the fact that the assumed opponent, who does not have access to the utilized cryptographic key, will not be able to decipher and to understand the sense of the intercepted encoded information. Conducting the studies, which make it possible to obtain this estimation, is the very labor-consuming and expensive matter, feasible only professional cryptographer. Therefore in practice one should employ only certification cryptographic means, which passed thorough examinations and certification of the authoritative specialized organizations. As an example assuredly of the steadfast algorithm of coding it is possible to name widely known algorithm according to GOST 28147-89, realized in a number of the serially produced program and firmware means of protection. Generation and propagation of cryptographic keys is one of the most complex problems of circuit protection. At present most promising are seemed the solutions, connected with hybrid encryption systems, which use traditional methods of encoding secret key for the protection of secrecy and integrity, with the simultaneous use of methods of coding with the open keys for the realization of the functions of the distribution of keys. The important requirement of safety of connection is the presence in the effective procedures of authorization, with the aid of which remote users can identify and check the authenticity of each other.

This problem also can effectively be solved with the aid of the cryptographic methods. In particular, possession by the correct key of coding can be considered as the proof of the fact that the subscriber has the right to enter the exchange of communications. For authorization in the networks, built on the base of the communication channels, it is possible to use the simpler, non-crypt methods, connected with the application of modems, which ensure the function of reverse call. In this case in the process of authorization, besides the traditional checking of secret password, automatically is initiated reverse telephone call (with the previous forced break of connection) to the subscriber, who claims to the access to information.