Chapter 6– Cryptography and Symmetric Key

Question 1

1. How many possible keys exist in a 4-bit key space?

16

Answer 1

To determine the number of keys in a key space, raise 2 to the power of the number of bits in the key space. In this example, 24 = 16.

Question 2

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?
   Non repudiation

Answer 2

Non repudiation

Nonrepudiation prevents the sender of a message from later denying that they sent it.

Question 3

3. What is the length of the cryptographic key used in the Data Encryption Standard (DES) cryptosystem?
   56 bits

Answer 3

56 bits

DES uses a 56-bit key. This is considered one of the major weaknesses of this cryptosystem.

Question 4

4. What type of cipher relies on changing the location of characters within a message to achieve confidentiality?
   Transposition cipher

Answer 4

Transposition cipher

Transposition ciphers use a variety of techniques to reorder the characters within a message.

Question 5

5.	Which of the following is not a possible key length for the Advance Encryption Standard Rijndael cipher?
A. 56 bits
B. 128 bits
C. 192 bits
D. 256 bits
56 bits

Answer 5

56 bits
The Rijndael cipher allows users to select a key length of 128, 192, or 256 bits, depending on the specific security requirements of the application

Question 6

6. Which one of the following cannot be achieved by a secret key cryptosystem?
   Nonrepudiation

Answer 6

Nonrepudiation
Nonrepudiation requires the use of a public key cryptosystem to prevent users from falsely denying that they originated a message.

Question 7

7. When correctly implemented, what is the only cryptosystem known to be unbreakable?
   One-time pad

Answer 7

One-time pad

Assuming that it is used properly, the one-time pad is the only known cryptosystem that is not vulnerable to attacks.

Question 8

8. What is the output value of the mathematical function 16 mod 3?
   1

Answer 8

1

Option B is correct because 16 divided by 3 equals 5, with a remainder value of 1.

Question 9

9. In the 1940s, a team of cryptanalysts from the United States successfully broke a Soviet code based on a one-time pad in a project known as VENONA. What rule did the Soviets break that caused this failure?
   Key values must be random.

Answer 9

Key values must be random.
The cryptanalysts from the United States discovered a pattern in the method the Soviets used to generate their one-time pads. After this pattern was discovered, much of the code was eventually broken.

Question 10

10. Which one of the following cipher types operates on large pieces of a message rather than individual characters or bits of a message?
    Block cipher

Answer 10

Block cipher
Block ciphers operate on message “chunks” rather than on individual characters or bits. The other ciphers mentioned are all types of stream ciphers that operate on individual bits or characters of a message.

Question 11

11. What is the minimum number of cryptographic keys required for secure two-way communications in symmetric key cryptography?
    One

Answer 11

One
Symmetric key cryptography uses a shared secret key. All communicating parties utilize the same key for communication in any direction.

Question 12

12. Dave is developing a key escrow system that requires multiple people to retrieve a key but does not depend on every participant being present. What type of technique is he using?
    M of N Control

Answer 12

M of N Control
M of N Control requires that a minimum number of agents (M) out of the total number of agents (N) work together to perform high-security tasks.

Question 13

13. Which one of the following Data Encryption Standard (DES) operating modes can be used for large messages with the assurance that an error early in the encryption/decryption process won't spoil results throughout the communication?
Output Feedback (OFB)

Answer 13

Output Feedback (OFB)
Output Feedback (OFB) mode prevents early errors from interfering with future encryption/decryption. Cipher Block Chaining and Cipher Feedback modes will carry errors throughout the entire encryption/decryption process. Electronic Codebook (ECB) operation is not suitable for large amounts of data.

Question 14

14. Many cryptographic algorithms rely on the difficulty of factoring the product of large prime numbers. What characteristic of this problem are they relying on?
    It is a one-way function.

Answer 14

It is a one-way function.
A one-way function is a mathematical operation that easily produces output values for each possible combination of inputs but makes it impossible to retrieve the input values.

Question 15

15. How many keys are required to fully implement a symmetric algorithm with 10 participants?
    45

Answer 15

45
The number of keys required for a symmetric algorithm is dictated by the formula (n*(n–1))/2, which in this case, where n = 10, is 45.

Question 16

16. What block size is used by the Advanced Encryption Standard?
    128 bits

Answer 16

128 bits
The Advanced Encryption Standard uses a 128-bit block size, despite the fact that the Rijndael algorithm it is based on allows a variable block size.

Question 17

17. What kind of attack makes the Caesar cipher virtually unusable?
    Frequency analysis attack

Answer 17

Frequency analysis attack
The Caesar cipher (and other simple substitution ciphers) are vulnerable to frequency analysis attacks that analyze the rate at which specific letters appear in the ciphertext.

Question 18

18. What type of cryptosystem commonly makes use of a passage from a well-known book for the encryption key?
    Running key cipher

Answer 18

Running key cipher
Running key (or “book”) ciphers often use a passage from a commonly available book as the encryption key.

Question 19

19. Which AES finalist makes use of prewhitening and postwhitening techniques?
    Twofish

Answer 19

Twofish

The Twofish algorithm, developed by Bruce Schneier, uses prewhitening and postwhitening.

Question 20

20. How many encryption keys are required to fully implement an asymmetric algorithm with 10 participants?
    20

Answer 20

20
10 participants x 2 keys = 20
In an asymmetric algorithm, each participant requires two keys: a public key and a private key.