2 cissp ® Official Study Guide Eighth Edition

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Be able to explain the basic operational modes of the Data Encryption Standard (DES) and 
Triple DES (3DES). 
The Data Encryption Standard operates in five modes: Electronic Code 
Book (ECB) mode, Cipher Block Chaining (CBC) mode, Cipher Feedback (CFB) mode, 
Output Feedback (OFB) mode, and Counter (CTR) mode. ECB mode is considered the least 
secure and is used only for short messages. 3DES uses three iterations of DES with two or 
three different keys to increase the effective key strength to 112 or 168 bits, respectively.
Know the Advanced Encryption Standard (AES). 
The Advanced Encryption Standard (AES) 
uses the Rijndael algorithm and is the U.S. government standard for the secure exchange of 
sensitive but unclassified data. AES uses key lengths of 128, 192, and 256 bits and a fixed 
block size of 128 bits to achieve a much higher level of security than that provided by the 
older DES algorithm.
Written Lab
1.
What is the major hurdle preventing the widespread adoption of one-time pad crypto-
systems to ensure data confidentiality?
2.
Encrypt the message “I will pass the CISSP exam and become certified next month” 
using columnar transposition with the keyword SECURE.
3.
Decrypt the message “F R Q J U D W X O D W L R Q V B R X J R W L W” using the 
Caesar ROT3 substitution cipher.

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232
Chapter 6 
■
Cryptography and Symmetric Key Algorithms
Review Questions
1.
How many possible keys exist in a 4-bit key space?
A.
4
B.
8
C.
16
D.
128
2.
John recently received an email message from Bill. What cryptographic goal would need to 
be met to convince John that Bill was actually the sender of the message?