IT-100 The model of a cryptosystem with the best speed and selectable security. WeaCheslaw Oleinik Dekart, 1, Ghioceilor str., Kishinev MD-2008, Republic of Moldova Tel. +(3732) 245580, fax. +(3732) 242580, E-mail: email@example.com http://www.dekart.com Key words: information security, cryptography, cipher, model.In many if not in all practical applications using symmetric cryptography systems,the main requirement is reaching the greatest possible speed of encoding. Thus thesystem should provide a necessary level of cryptography security. These tworequirements are as a matter of fact contradictory, i.e. rise of speed of the systemresults in its lowering cryptographic security and on the contrary. Therefore atcreation of cryptosystems very important there is a correct choice of a relationbetween speed of encoding and cryptographic security of systems.In the paper it is shown, that cryptosystems on the basis of ciphers such as Vernamcipher  together with customized generators of keys provide possibility of thebalanced choice between cryptographic security of the created cipher and its speed. Itis possible due to that Vernam cipher has (as has shown C. Shannon ) oneprominent feature, consisting that it is theoretically proof cryptography system.As Vernam cipher uses only one operation its speed will be greatest possible andconstant for concrete implementation for encoding. For obtaining a key with longequal to length of the message some ideal generator of a pseudo-random sequence isused. The ideal generator is understood as the generator producing an infinite, notrepeating, random sequence which complexity of a prediction depends only onlength of "priming", i.e. from first time load of the generator.Application of the similar generator of a key sequence for Vernam cipher results tothat cryptography security of the system the whole becomes dependent only from arelong first time load of the generator. The Ideal generators exist, but, unfortunately,they are difficultly sold in practice. However for these purposes it is possible to usethe generators constructed on a basis so-called of hashing functions. One ofproperties such functions are that they provide impossibility of a prediction of anentry sequence at known output values and the algorithm of conversion. The truthmaximum length of a sequence received with the help of such generators will not beinfinite, but it usually enough big, that is acceptable to practical application.References G. S. Vernam, “Cipher printing telegraph systems for secret wire and radio telegraphic communications,” J. Atner. Inst. Elec. Eng., vol. 55, pp. 109-115, 1926. C. E. Shannon, “Communication theory of secrecy systems”, Bell System Technical Journal, 28 (1949), 656-715.WeaCheslaw Oleinik, Dr. of C.S., Ass. Academician of IIA, born: July 4, 1956.Author: over 60 papers.