ccna app

ccna app.72.4.4 Choosing an Encryption Key198 Chapter 7 CryptosystemsPerformance is another issue that may affect the choice of key length. The administrator must find a suitable balance between the speed of the algorithm and the strength of the protection, since some algorithms, such as the RSA (Rivest, Shamir, and Adleman) algorithm, run slowly due to the long key length. Every effort should be made to obtain sufficient protection while communication over untrusted networks can flow unimpeded.The attacker's budget money will also influence the choice of key length. When assessing the risk of an encryption algorithm being breached, one needs to estimate the attacker's resources and how long the data must be protected. For example, classical DES can be broken in minutes by a machine worth a million dollars. If the value of the data being protected is much higher than the million dollars needed to obtain a cracked device, classical DES is a bad choice. It would take an attacker a million years or more to crack 168-bit 3DES or 128-bit RC4, making the choice of either of these key lengths easily sufficient.The size of the key required for a particular application continues to grow due to rapid advances in technology and cryptanalysis methods. For example, part of the strength of the RSA algorithm stems from the difficulty of factoring large numbers. If a 1,024-bit number is difficult to factor, a 2048-bit number will be even more difficult to factor. Even using today's fastest computers, it would take many people a lifetime to factor a 1024-bit number (which is a multiple of two 512-bit primes). Of course, this advantage would be lost if an easy way of factoring large numbers were found, but cryptographers do not consider this a possibility. The rule of "the longer the key, the better" still applies, unless there is some possible performance reason.7.3 Confidentiality7.3.1 Encryption7.3.1.1 Cryptographic encryptionCryptographic encryption can provide confidentiality at multiple layers of the OSI model by using a mix of tools and protocols.Proprietary link encryption devices provide confidentiality at the data link layer.