Chapter 4 1 One The Most Intense Areas Research The

CHAPTER 4: BLOCK CIPHERS AND THE DATA ENCRYPTION STANDARD

TRUE OR FALSE

T F 1. The vast majority of network based symmetric cryptographic

applications make use of stream ciphers.

T F 2. The Feistel cipher structure, based on Shannon’s proposal of 1945,

dates back over a quarter of a century and is the structure used by

many significant symmetric block ciphers currently in use.

T F 3. DES uses a 56-bit block and a 64-bit key.

T F 4. If the bit-stream generator is a key-controlled algorithm the two

users only need to share the generating key and then each can

produce the keystream.

T F 5. A problem with the ideal block cipher using a small block size is

that it is vulnerable to a statistical analysis of the plaintext.

T F 6. Confusion seeks to make the statistical relationship between the

plaintext and ciphertext as complex as possible in order to thwart

attempts to deduce the key.

T F 7. All other things being equal, smaller block sizes mean greater

security.

T F 8. Greater complexity in the subkey generation algorithm should lead

to greater difficulty of cryptanalysis.

T F 9. Fast software encryption/decryption and ease of analysis are two

considerations in the design of a Feistel cipher.

T F 10. A prime concern with DES has been its vulnerability to brute-force

attack because of its relatively short key length.

T F 11. One criteria for an S-box is: “If two inputs to an S-box differ in

exactly one bit, the outputs must also differ in exactly one bit. “

T F 12. The heart of a Feistel block cipher is the function F, which relies

on the use of S-boxes.

T F 13. The strict avalanche criterion and the bit independence criterion

appear to weaken the effectiveness of the confusion function.  

T F 14. An advantage of key-dependent S-boxes is that because they are

not fixed, it is impossible to analyze the S-boxes ahead of time to

look for weaknesses.

T F 15. The key schedule algorithm is more popular and has received

more attention than S-box design.

MULTIPLE CHOICE

1. DES exhibits the classic __________ block cipher structure, which consists of a

number of identical rounds of processing.

A) Feistel B) SAC

C) Shannon D) Rendell

2. A sequence of plaintext elements is replaced by a __________ of that sequence

which means that no elements are added, deleted or replaced in the

sequence, but rather the order in which the elements appear in the sequence

is changed.

A) permutation B) diffusion

C) stream D) substitution

3. A __________ cipher is one that encrypts a digital data stream one bit or one

byte at a time.

A) product B) block

C) key D) stream

4. The vast majority of network-based symmetric cryptographic applications

make use of ________ ciphers.

A) linear B) block

C) permutation D) stream

5. A __________ cipher is one in which a block of plaintext is treated as a whole

and used to produce a ciphertext block of equal length.

A) bit B) product

C) stream D) block

6. __________ is when each plaintext element or group of elements is uniquely

replaced by a corresponding ciphertext element or group of elements.

A) Substitution B) Diffusion

C) Streaming D) Permutation

7. Key sizes of __________ or less are now considered to be inadequate.

A) 128 bits B) 32 bits

C) 16 bits D) 64 bits

8.  Feistel proposed that we can approximate the ideal block cipher by

utilizing the concept of a __________ cipher, which is the execution of two or

more simple ciphers in sequence in such a way that the final result or

product is cryptographically stronger than any of the component ciphers.

A) linear B) permutation

C) differential D) product

9. The criteria used in the design of the __________ focused on the design of the

S-boxes and on the P function that takes the output of the S-boxes.

A) Avalanche Attack B) Data Encryption Standard

C) Product Cipher D) Substitution Key

10. The greater the number of rounds, the __________ it is to perform cryptanalysis.

A) easier B) less difficult

C) equally difficult D) harder

11. The function F provides the element of __________ in a Feistel cipher.

A) clarification B) alignment

C) confusion D) stability

12. One of the most intense areas of research in the field of symmetric block

ciphers is __________ design.

A) S-box B) F-box

C) E-box D) D-box

13. Mister and Adams proposed that all linear combinations of S-box columns

should be _________ which are a special class of Boolean functions that are

highly nonlinear according to certain mathematical criteria.

A) horizontal functions B) angular functions

C) bent functions D) vertical functions

14. The Nyberg approach that is more or less a manual approach with only

simple mathematics to support it is __________ .

A) human-made B) random

C) math-made D) random with testing

15. Allowing for the maximum number of possible encryption mappings from the

plaintext block is referred to by Feistel as the __________ .

A) ideal substitution cipher B) round function

C) ideal block cipher D) diffusion cipher

SHORT ANSWER

1. A _________ is an encryption/decryption scheme in which a block of plaintext is

treated as a whole and used to produce a ciphertext block of equal length.  

2. __________ seeks to make the relationship between the statistics of the

ciphertext and the value of the encryption key as complex as possible so that

even if the attacker can get some handle on the statistics of the ciphertext,

the way in which the key was used to produce that ciphertext is so complex it

is difficult to deduce the key.

3. Many block ciphers have a __________ structure which consists of a number of

identical rounds of processing and in each round a substitution is performed

on one half of the data being processed, followed by a permutation that

interchanges the two halves.

4. Feistel’s is a practical application of a proposal by Claude Shannon to develop

a product cipher that alternates confusion and ________ functions.

5. The __________ criterion is defined as: “An S-box satisfies GA of order y if, for a

1-bit input change, at least y output bits change.”

6. In ___________ the statistical structure of the plaintext is dissipated into long-

range statistics of the ciphertext. This is achieved by having each plaintext

digit affect the value of many ciphertext digits.

7. The most widely used encryption scheme is based on the __________ adopted in

1977 by the National Bureau of Standards as Federal Information Processing

Standard 46.

8. A change in one bit of the plaintext or one bit of the key should produce a

change in many bits of the ciphertext. This is referred to as the __________

effect.

9. Two areas of concern regarding the level of security provided by DES are the

nature of the algorithm and the __________ .

10. A _________ attack exploits the fact that an encryption or decryption algorithm

often takes slightly different amounts of time on different inputs.

11. The __________ criterion states that output bits j and k should change

independently when any single input bit i is inverted for all i, j and k.

12. The ________ cipher structure, which dates back over a quarter century and

which, in turn, is based on Shannon’s proposal of 1945, is the structure used

by many significant symmetric block ciphers currently in use.

13. The cryptographic strength of a Feistel cipher derives from three aspects of

the design: the function F, the key schedule algorithm, and __________ .

14. The __________ criterion states that any output bit j of an S-box should change

with probability 1/2 when any single input bit i is inverted for all i,j.

15. Two alternatives to DES are AES and _________ DES.

TRUE OR FALSE

MULTIPLE CHOICE

SHORT ANSWER