ZyXEL Communications PLA-470 V2 - V3.0.5 Installationsanleitung PDF herunterladen (Seite 83)

However, the DES hasn’t been used since 1998 as its reliability was considered

to be poor. Its encryption algorithm has been altered and improved.

3-DES

3-DES, or triple-DES, uses three DES one after the other. Therefore, the data is

encrypted then deciphered then encrypted with two or three different keys. The size

of the 3-DES key may be 118 bits in size. Because of this, it cannot be used in France.

3-DES is considered as being reasonably secure.

IDEA (International Data Encryption Algorithm)

The IDEA (international data encryption algorithm) is an algorithm with a 128-bit

key length. The text to be encrypted is divided into four sub-blocks. Eight rounds are

performed on each of these sub-blocks. Each round is a combination of exclusive

“or,” addition modulo 2

and multiplication modulo 2

. On each round, the data

and the key are combined. This technique makes the IDEA particularly secure.

The IDEA is implemented in PGP (Pretty Good Privacy), which is the world’s

most widely used software.

RC2

The RC2 algorithm was developed by Ron Rivest, who gave it the name Ron’s Code

2. It is based on an algorithm in 64-bit blocks. It is twice or even three times faster

than DES with a maximum key length of 2,048 bits.

The algorithm is the property of RSA Security and is used in SSL v2.0.

RC4

RC4 (Ron’s Code 4) no longer uses blocks but encrypts by stream. Its specific char-

acteristic resides in the fact that it uses pseudorandom permutations for data encryp-

tion and deciphering.

Two mechanisms are defined by RC4:

•

KSA (Key Scheduling Algorithm). This algorithm generates a status table

using the encryption key by means of simple permutations.

•

PRGA (Pseudorandom Generator Algorithm). The status table generated by

KSA is placed in a pseudorandom number generator (PRNG) which creates

the key stream by means of complex permutations.

Unlike the other algorithms, the data is not divided into blocks for their encryp-

tion or decryption. In RC4, the encryption corresponds to the addition of data to the

key stream using an exclusive “or,” whereas the decryption corresponds to the addi-

tion of encrypted data to the same key stream still using an exclusive “or.”

RC4 is faster than RC2. Like RC2, it is the property of RSA Security. RC4 is

used in SSL v2.0 and SSL v3.0 to secure connections and in the WEP protocol of

IEEE standard series 802.11.

64 Security

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ZyXEL Communications PLA-470 V2 - V3.0.5 Installationsanleitung Seite 83