Lecture 6

Theme: Standard of encipherement ГОСТ 28147-89

Content:

1 Modes of DES algorithm operation

2 General description of ГОСТ 28147-89 algorithm

3 Encipherement in the mode of simple replacement (ГОСТ 28147-89)

1 Modes of des algorithm operation

The DES algorithm can be utillized both for an encipherement and for authentification of information. In an order to utillize the DES algorithm for the decision of various cryptographic tasks, four modes of its application were developed.

1.1 Mode of " Electronic Code Book "

As the considered algorithm belongs to the number a block, the general for them lack is inherent it. Identical blocks, belongings the the same message T, will be in ciphers identically. Such mode of encipherement is named the mode of electronic code book (Electronic Code Book ECB). It means that it is possible in theory, not knowing the key, to create a codebook, where the pair of blocks of replacements will be written, proper to opened Ti text and closed Т_i text. Other business, that for all of the candidate keys long in 64 character, it to do it is practically impossible.

In most in-use in practice messages contained standard fragments of texts feel like a regular reiteration. Moreover, the separate fragments of text in standard formalized messages occupy one and also place, for example, headings and completions of texts.

1.2 The mode " Cipher Block Chaining "

This mode (fig. 1) is foreseen by breaking up of transferrable message on blocks

М = М₁М₂М₃М₄…

The first block is folded on the module two with the initial vector S, which regularly changes on receiving and transmitter sides. After it, the got sum is encoded with the use of algorithm of DES. Thus, in a channel passed 64th bit message С₁.

Transm.

Rec.

Figure1- Mode " Cipher Block Chaining "

The sum С₁ got on a receiving side is decoded, where upon from it the initial vector S is subtracted.

On the next stage of encipherement, at the transmission of next block, in place of initial vector S to the encrypted message the massege С₁.passed on the previous stage is added. This procedure repeats oneself at the transmission of every next 64-bit block.

Thus, the last block of ciphertext will be the function of the secret key, initial vector and every bit of plaintext, regardless of his length. This block is named the code of authentification of message (CAM). CAM can be easily got on a receiving side by the reiteration of procedures, executable on a transmitter side.

1.3 Mode " Cipher Feed Back " cfb

This mode (fig. 2) assumes, that block length differed from 64 beaten. We will suppose that length of blocks which the ciphered sequence is broken up on, less of 64 bits.