ZyXEL Communications PLA-470 V2 - V3.0.5 Instalační příručka stáhnout pdf (Strana 81)

to send data; and integrity control, which is used to know whether the data sent was not

modified during the transmission.

Cryptography

Making a text or message incomprehensible through the use of an algorithm is not

new. The Egyptians, like the Romans, employed methods used to encode a text or a

message. These techniques, which were relatively simple originally, have changed,

and cryptography has been recognized as a science since World War II.

The basic principle of cryptography is illustrated in Figure 4.1. An encryption

key is used to encode a plain text. The cryptogram is then sent to the recipient. The

recipient uses a deciphering key in order to reconstitute the plain text. At any time

during the transmission, somebody can recover the encrypted text, called a crypto-

gram, and try to decipher it using various methods.

Cryptology

Cryptography only involves encryption design and methods. Trying to decipher encrypted

text is called cryptanalysis. Cryptology designates the study of cryptography and crypta-

nalysis.

In France, for example, there are strict regulations concerning the length of the

keys used for encryption purposes. A key with a maximum length of 40 bits can be

used for any public or private use. For private use, the length of the key may not

exceed 128 bits. For a key length exceeding 128 bits, the key must be transmitted to

the local cyber security authorities. In the USA or in Japan, the regulations are differ-

ent and one should take care to know the restrictions on the length of keys autho-

rized to be used.

There are two cryptography techniques: symmetric-key cryptography and

asymmetric-key cryptography, better known as public-key cryptography.

Symmetric-Key Cryptography

Symmetric-key cryptography is based on the use of a single key used to encrypt and

unscramble data. All persons wishing to transmit data securely must therefore share

the same secret: the key. This process is illustrated in Figure 4.2.

The clear fault in this system resides in how this secret key is shared and trans-

mitted between the sender and the receiver.

62 Security

Figure 4.1 Data encryption

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ZyXEL Communications PLA-470 V2 - V3.0.5 Instalační příručka Strana 81