ZyXEL Communications PLA-470 V2 - V3.0.5 Guía de instalación descargar pdf (Pagina 89)

Figure 4.8 illustrates a sender who wishes to send a message while making sure

of its authenticity. For this purpose, a message digest is created by the sender by

means of hash function H. The message and its digest are sent to the recipient apply-

ing the same hash function H to the received message in order to compare the new

digest with the received digest. If the digests are the same, this means that the mes-

sage has not been modified.

MD5

On the Internet, we increasingly come across files to be downloaded with their digests,

generally MD5, intended to check the integrity of the received data.

The hash function is often combined with public-key cryptography. The process

is the following:

1. The sender hashes the message.

2. The digest is encrypted with the sender’s private key.

3. The message, the public key of the sender, and the encrypted digest are sent

over the network.

4. The recipient receives the message, which he hashes in turn to extract a new

digest from it.

5. This digest is compared with the digest he has received in the encrypted con-

dition. The digest is decrypted by the recipient using the public key provided

by the sender.

6. If the two digests match, the message is authenticated.

This process is illustrated in Figure 4.9.

70 Security

Figure 4.8 Message hash

1 2 ... 84 85 86 87 88 89 90 91 92 93 94 ... 348 349

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ZyXEL Communications PLA-470 V2 - V3.0.5 Guía de instalación Pagina 89