ZyXEL Communications PLA-470 V2 - V3.0.5 Instalační příručka stáhnout pdf (Strana 82)

Various symmetric-key cryptography algorithms have been developed, in par-

ticular DES (data encryption standard), IDEA (international data encryption algo-

rithm), series RC2 to RC6, and AES (advanced encryption standard).

DES (Data Encryption Standard)

The DES algorithm was jointly developed in the seventies by IBM and the NSA

(National Security Agency). The DES is an encryption algorithm known as “by

blocks.” The length of the key used is fixed (40 or 56 bits). The purpose of the DES

is to carry out a set of permutations and substitutions between the key and the text

to be encrypted so as to encode the information.

The encryption mechanism follows several steps:

1. The text to be encrypted is divided into 64-bit blocks (8 bits are used for par-

ity check).

2. The various blocks are subjected to an initial permutation.

3. Each block is divided into two 32-bit parts, a right part and a left part.

4. Sixteen rounds are performed on half blocks. A round is a set of permuta-

tions and substitutions. On each round, the data and the key are combined.

5. At the end of the sixteen rounds, the two right and left half blocks are

merged and a reverse initial permutation is carried out on the blocks.

Once all the blocks have been encrypted, they are reassembled in order to create

the encrypted text that will be sent over the network. Decryption is carried out in

the encryption reverse order by still using the same key.

Until recently, the DES was the reference for symmetric-key cryptography. It

was used and is still used by many systems. It is used by the information exchange

protocol secured by SSL (secure sockets layer) Internet v1.0, for example, with a

40-bit key.

Overview of Network Security Issues 63

Figure 4.2 Symmetric-key cryptography

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ZyXEL Communications PLA-470 V2 - V3.0.5 Instalační příručka Strana 82