3 General characteristic of des

DES is a block encryption algorithm. When 64-bit blocks of plaintext go in, 64-bit blocks of ciphertext come out. It is also a symmetric algorithm, meaning the same key is used for encryption and decryption. It uses a 64-bit key, 56 bits make up the true key, and 8 bits are used for parity.

The DES system utillizes combinations from substitutions and transpositions of characters (numbers) and addition on the module 2.

The comfort of the applied algorithm is that operations of encryption and decryptions in DES are convertible. The generalized chart of process of encipherement in the DES algorithm has a next kind (Fig.3)

Key

Figure 3- The generalized chart of encipherement in the algorithm of DES

The structural chart of algorithm of DES, consisting of 16 steps, is presented on a fig. 4.

Figure 4 - Structural chart of algorithm DES

The algorithm of encipherement is carried out as follows.

From the file of input text the next 64-bit block of T is read. Process his encryption consists of initial transposition, sixteen cycles of encipherement and, finally, in final transposition of blocks.

By the matrix of initial transposition of IP, containing 8 columns and 8 lines transposition of characters of initial block is carried out.

Finishing transformation of algorithm - reverse transposition is carried out by the matrix of reverse transposition.

The 64-bit sequence got after the first transposition is broken up on two 32-bit blocks of L₀ and R₀ .

Then the iterative process of encipherement, which consists of 16-ti steps, is executed.

Let Ti is a result of i - iteration:

T_i = L_i R_i

where L_i = t₁,t₂,…,t₃₂;R_i=t₃₃,t₃₄,…,t₆₄... In this case the result of iteration is described the followings formulas:

L_i = R_i-1, i =1,2,…,16;

R_i = L_i-1 f (R_i-1, K_i ), i =1,2,…,16;

One cycle of DES-transformation is resulted on a figure 5.

DES works with the 64-bit blocks of plaintext. After primary transposition a block is broken up on right and left halves long for 32 bits. Then 16 transformations (function of F) in which information team up with the key are then executed. After the sixteenth cycle right and left halves unite and an algorithm is completed final transposition (reverse in relation to primary). On every cycle bits of key moved, and then 48 bits get out from 56 bits of the key. The right half of information is increased to 48 bits by transposition with expansion, unites by means of XOR with 48 bits of the displaced and moved key, passes through 8 S-blocks, forming 32 new bits, and moved again. These 4 operations are executed the function of F.

Then the result of function of F unites with a left half by other XOR. In the total these actions a new right half appears, and old right - becomes a new left half. These actions repeat 16 times, forming 16 cycles of DES.