2 cissp ® Official Study Guide Eighth Edition

International Data Encryption Algorithm 
The International Data Encryption Algorithm (IDEA) block cipher was developed in 
response to complaints about the insuffi cient key length of the DES algorithm. Like DES, 
IDEA operates on 64-bit blocks of plaintext/ciphertext. However, it begins its operation with 
a 128-bit key. This key is broken up in a series of operations into 52 16-bit 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 fi ve 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 Blowfi sh block cipher is another alternative to DES and IDEA. Like its 
predecessors, Blowfi sh 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 
Blowfi sh as a much faster algorithm than both IDEA and DES. Also, Mr. Schneier released 
Blowfi sh for public use with no license required. Blowfi sh encryption is built into a number 
of commercial software products and operating systems. A number of Blowfi sh libraries are 
also available for software developers.