ZyXEL Communications PLA-470 V2 - V3.0.5 Guía de instalación descargar pdf (Pagina 84)

RC5 and RC6

RC5, another proprietary algorithm of RSA Security, is an encryption algorithm in

blocks with a variable block size between 32 and 128 bits, a variable round number

between 0 and 255, and a dynamic key length between 0 and 2,040 bits.

RC6 is an improved version of RC5 so therefore uses its characteristics. The

only difference relates to the addition of new mathematical operations at the

rounds.

Blowfish

Like DES, blowfish is an encryption algorithm in 64-bit blocks. Its key, based on

DES, has a variable size between 40 and 448 bits. This algorithm is particularly fast

and reliable.

Twofish

Like blowfish, twofish is an encryption algorithm in 128-bit blocks on 16 rounds

with a variable key length. It is also both reliable and fast.

AES (Advanced Encryption Standard)

The AES is the result of a call for tender launched in 2000 by the NIST (National

Institute of Standards and Technology) to replace the DES, which was seen as unre-

liable. Several algorithms were proposed, such as RC6 and Twofish, but Rijndael

was chosen because it is simple and fast. Its name is now AES.

AES is an algorithm in 128-bit blocks, or 16 bytes, for K encryption key of 128,

192, or 256 bits. Depending on the key size, the number of rounds is 10, 12, and 14,

respectively.

For each round, AES defines four simple operations:

•

SubBytes, nonlinear substitution (S) mechanism that is different for each

encrypted data block.

•

ShiftRows, permutation (P) mechanism that shifts the block elements.

•

MixColumns, transformation (M) mechanism that carries out a multiplica-

tion between block elements not in a conventional way but in a GF(2

) Galois

body.

•

AddRoundkey, key derivation algorithm. It defines in each round a new

encryption key, Ki, where i corresponds to the ith round from encryption key

The data is divided into 128-bit blocks before encryption. The first encryption

stage consists of adding the data block with the encryption key by means of an

exclusive “or.” Then, each block is subjected to ten rounds in a row, each made up

of a substitution (S), a permutation (P) and a transformation (M). At the end of each

round, a new encryption key is derived from the initial key, and the result of opera-

tion M is added to this key, Ki, by means of an exclusive ”or," all of which is sent to

Overview of Network Security Issues 65

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ZyXEL Communications PLA-470 V2 - V3.0.5 Guía de instalación Pagina 84