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534 Chapter 9 • Basis of Cryptography

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Literally thousands of different cryptographic algorithms have been
developed over the years. Cryptographic algorithms can be classified as
follows:

■ Encryption Algorithms Used to encrypt data and provide con-
fidentiality
■ Signature Algorithms Used to digitally “sign” data to provide
authentication
■ Hashing Algorithms Used to provide data integrity

Algorithms (ciphers) are also categorized by the way they work at
the technical level (stream ciphers and block ciphers). This categoriza-
tion refers to whether the algorithm is applied to a stream of data,
operating on individual bits, or to an entire block of data. Stream
ciphers are faster, because they work on smaller units of data. The key is
generated as a keystream, which is combined with the plaintext to be
encrypted. RC4 is the most commonly used stream cipher. Another is
ISAAC.
Block ciphers take a block of plaintext and turn it into a block of
ciphertext. (Usually the block is 64 or 128 bits in size.) Common block
ciphers include DES, CAST, Blowfish, IDEA, RC5/RC6, and SAFER. Most
AES candidates are block ciphers.

Instead of relying on the techniques of substitution and transposition that sym-
metric key cryptography uses, asymmetric algorithms rely on the use of large-
integer mathematics problems. Many of these problems are simple to do in one
direction but difficult to do in the opposite direction. For example, it is easy to
multiply two numbers together, but it is more difficult to factor them back into the
original numbers, especially if the integers used contain hundreds of digits.Thus, in
general, the security of asymmetric algorithms is dependent not upon the feasibility
of brute-force attacks, but the feasibility of performing difficult mathematical
inverse operations and advances in mathematical theory that may propose new
“shortcut” techniques.
Asymmetric cryptography is much slower than symmetric cryptography.There
are several reasons for this. First, it relies on exponentiation of both a secret and
public exponent, as well as generation of a modulus. Computationally, exponentia-
tion is a processor-intensive operation. Second, the keys used by asymmetric algo-

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