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528 Chapter 9 • Basis of Cryptography

asymmetric, depending on the number of keys that are used. These two basic types
of encryption technology are discussed in the following sections.

Symmetric Encryption Algorithms

The most widely used type of encryption is symmetric encryption, which is aptly
named because it uses one key for both the encryption and decryption processes.
Symmetric encryption is also commonly referred to as secret-key encryption and
shared-secret encryption, but all terms refer to the same class of algorithms. For pur-
poses of the Security+ exam, the term shared key is used.
The reason why symmetric encryption systems are abundant is speed and sim-
plicity.The strength of symmetric algorithms lies primarily in the size of the keys
used in the algorithm, as well as the number of cycles each algorithm employs.The
cardinal rule is “fewer is faster.”
By definition, all symmetric algorithms are theoretically vulnerable to brute-force
attacks (covered in Chapter 2), which are exhaustive searches of all possible keys.
Brute-force attacks involve methodically guessing what the key to a message may
be. Given that all symmetric algorithms have a fixed key length, there are a large
number of possible keys that can unlock a message. Brute-force attacks methodi-
cally attempt to check each key until the key that decrypts the message is found.
However, brute-force attacks are often impractical, because the amount of time
necessary to search the keys is greater than the useful life expectancy of the hidden
information. No algorithm is truly unbreakable, but a strong algorithm takes so
long to crack that it is impractical to try. Because brute-force attacks originate from
computers, and because computers are continually improving in efficiency, an algo-
rithm that may be resistant to a brute-force attack performed by a computer today,
will not necessarily be resistant to attacks by computers 5 to 10 years in the future.

TEST DAY TIP
The numbers of symmetric algorithms used generally outweigh the
number of asymmetric algorithms. If you run into a question concerning
an algorithm you are unfamiliar with, chances are it is a symmetric algo-
rithm.

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