ZyXEL Communications PLA-470 V2 - V3.0.5 Guía de instalación descargar pdf (Pagina 90)

Various hash techniques are used, in particular the following ones:

•

MD2, MD4, and MD5. Message digests 2, 4, and 5 were developed by Ron

Rivest for RSA Security. These are hash functions that all produce digests with

a size of 128 bits. MD2 is the most reliable but is optimized only for 8-bit

machines, whereas the other two are optimized for 32-bit machines. MD4

was abandoned since it is too sensitive to certain attacks. MD5 is an evolution

of MD4. It is considered as reliable, even if it is vulnerable to certain attacks,

and is used in many applications. MD5 has been standardized by IETF under

RFC 1321.

•

SHA and SHA1. SHA (Secure Hash Algorithm) and its evolution were devel-

oped by NSA. These two algorithms produce 160-bit digests for a message

which may reach a size of two million terabytes. The size of its digest makes it

very difficult to crack, but it is slower than MD5 Network attacks.

The networks have been subjected to various types of attacks at all times. These

may be passive attacks, like in the case of listening to a network for the purpose of

recovering information by “cracking” the various passwords and encryption keys.

In other cases, these are active attacks. The attacker attempts to take control of

machines or to damage some machine devices.

The most common attacks are the following:

•

Denial of service (DoS) attack. This attack, which is among the most feared,

consists of flooding a network with messages so that the network devices can

no longer process them, sometimes up to the point of collapse.

Overview of Network Security Issues 71

Figure 4.9 Hash and public key

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ZyXEL Communications PLA-470 V2 - V3.0.5 Guía de instalación Pagina 90