Page 403 - (ISC)² CISSP Certified Information Systems Security Professional Official Study Guide

subkeys. The subkeys then act on the input text using a combination of
XOR and modulus operations to produce the encrypted/decrypted

version of the input message. IDEA is capable of operating in the same
five modes used by DES: ECB, CBC, CFB, OFB, and CTR.

All of this material on key length block size and the

number of rounds of encryption may seem dreadfully boring;
however, it’s important material, so be sure to brush up on it while

preparing for the exam.

The IDEA algorithm was patented by its Swiss developers. However,
the patent expired in 2012, and it is now available for unrestricted use.
One popular implementation of IDEA is found in Phil Zimmerman’s
popular Pretty Good Privacy (PGP) secure email package. Chapter 7
covers PGP in further detail.

Blowfish

Bruce Schneier’s Blowfish block cipher is another alternative to DES
and IDEA. Like its predecessors, Blowfish operates on 64-bit blocks of
text. However, it extends IDEA’s key strength even further by allowing
the use of variable-length keys ranging from a relatively insecure 32
bits to an extremely strong 448 bits. Obviously, the longer keys will
result in a corresponding increase in encryption/decryption time.

However, time trials have established Blowfish as a much faster
algorithm than both IDEA and DES. Also, Mr. Schneier released
Blowfish for public use with no license required. Blowfish encryption
is built into a number of commercial software products and operating
systems. A number of Blowfish libraries are also available for software
developers.

Skipjack

The Skipjack algorithm was approved for use by the U.S. government
in Federal Information Processing Standard (FIPS) 185, the Escrowed
Encryption Standard (EES). Like many block ciphers, Skipjack

operates on 64-bit blocks of text. It uses an 80-bit key and supports