Chapter 7 1 Because There Chaining Multiple Blocks Can Encrypted

CHAPTER 7: BLOCK CIPHER OPERATION

TRUE OR FALSE

T F 1. Once the plaintext is converted to ciphertext using the

encryption algorithm the plaintext is then used as input and the

algorithm is applied again.

T F 2. There are no practical cryptanalytic attacks on 3DES.

T F 3. A mode of operation is a technique for enhancing the effect of a

cryptographic algorithm or adapting the algorithm for an

application.

T F 4. The XTS–AES standard describes a method of decryption for data

stored in sector–based devices where the threat model includes

possible access to stored data by the adversary.

T F 5. S–AES is the most widely used multiple encryption scheme.

T F 6. Given the potential vulnerability of DES to a brute–force attack, an

alternative has been found.

T F 7. A number of Internet based applications have adopted two–key

3DES, including PGP and S/MIME.

T F 8. The sender is the only one who needs to know an initialization

vector.

T F 9. A typical application of Output Feedback mode is stream oriented

transmission over noisy channel, such as satellite communication.

T F 10. Cipher Feedback (CFB) is used for the secure transmission of

single values.

T F 11. Format-preserving encryption refers to any encryption technique

that takes a plaintext in a given format and produces a ciphertext

in the same format.

T F 12. It is possible to convert a block cipher into a stream cipher using

cipher feedback, output feedback and counter modes.

T F 13. Cipher Feedback Mode conforms to the typical construction of a

stream cipher.

T F 14. OFB mode requires an initialization vector that must be unique to

each execution of the encryption operation.

T F 15. The XTS–AES mode is based on the concept of a tweakable block

cipher.

MULTIPLE CHOICE

1. In the first instance of multiple encryption plaintext is converted to __________

using the encryption algorithm.

A. block cipher B. ciphertext

C. S-AES mode D. Triple DES

2. Triple DES makes use of __________ stages of the DES algorithm, using a total of

two or three distinct keys.

A. nine B. six

C. twelve D. three

3. Another important mode, XTS–AES, has been standardized by the __________

Security in Storage Working Group.

A. IEEE B. ISO

C. NIST D. ITIL

4. The _________ and _________ block cipher modes of operation are used for

authentication.

A. OFB, CTR B. ECB, CBC

C. CFB, OFB D. CBC, CFB

5. __________ modes of operation have been standardized by NIST for use with

symmetric block ciphers such as DES and AES.

A. Three B. Five

C. Nine D. Seven

6. The output of the encryption function is fed back to the shift register in

Output Feedback mode, whereas in ___________ the ciphertext unit is fed back

to the shift register.

A. Cipher Block Chaining mode B. Electronic Codebook mode

C. Cipher Feedback mode D. Counter mode

7. The simplest form of multiple encryption has __________ encryption stages and

__________ keys.

A. four, two B. two, three

C. two, two D. three, two

8. The __________ algorithm will work against any block encryption cipher and

does not depend on any particular property of DES.

A. cipher block chaining B. meet-in-the-middle attack

C. counter mode attack D. ciphertext stealing

9. The __________ method is ideal for a short amount of data and is the

appropriate mode to use if you want to transmit a DES or AES key securely.

A. cipher feedback mode B. counter mode

C. output feedback mode D. electronic codebook mode

10. _________ mode is similar to Cipher Feedback, except that the input to the

encryption algorithm is the preceding DES output.

A. Cipher Feedback B. Counter

C. Output Feedback D. Cipher Block Chaining

11. “Each block of plaintext is XORed with an encrypted counter. The counter is

incremented for each subsequent block”, is a description of ___________ mode.

A. Cipher Block Chaining B. Counter

C. Cipher Feedback D. Electronic Codebook

12. The __________ mode operates on full blocks of plaintext and ciphertext, as

opposed to an s–bit subset.

A. CBC B. ECB

C. OFB D. CFB

13. A finite set of two or more symbols is called a(n) ___________ .

A. character B. alphabet

C. radix D. character string

14. __________ mode is suitable for parallel operation. Because there is no

chaining, multiple blocks can be encrypted or decrypted simultaneously.

Unlike CTR mode, this mode includes a nonce as well as a counter.

A. OFB B. S-AES

C. 3DES D. XTS-AES

15. Both __________ produce output that is independent of both the plaintext and

the ciphertext. This makes them natural candidates for stream ciphers that

encrypt plaintext by XOR one full block at a time.

A. CBC and ECB B. OFB and CTR

C. ECB and OFB D. CTR and CBC

SHORT ANSWER

1. The__________ is a technique in which an encryption algorithm is used multiple

times.

2. The most significant characteristic of __________ is that if the same b-bit block

of plaintext appears more than once in the message, it always produces the

same ciphertext.

3. A __________ is a technique for enhancing the effect of a cryptographic

algorithm or adapting the algorithm for an application, such as applying a

block cipher to a sequence of data blocks or a data stream.

4. Five modes of operation have been standardized by NIST for use with

symmetric block ciphers such as DES and AES: electronic codebook mode,

cipher block chaining mode, cipher feedback mode, __________, and counter

mode.

5. One of the most widely used multiple-encryption schemes is __________ .

6. “The input to the encryption algorithm is the XOR of the next 64 bits of

plaintext and the preceding 64 bits of ciphertext” is a description of __________

mode.

7. The simplest mode of operation is the ___________ mode, in which plaintext is

handled one block at a time and each block of plaintext is encrypted using the

same key.

8. __________ refers to any encryption technique that takes a plaintext in a given

format and produces a ciphertext in the same format.

9. The __________ block cipher mode of operation is a general purpose block

oriented transmission useful for high speed requirements.

10. “Input is processed s bits at a time. Preceding ciphertext is used as input to

the encryption algorithm to produce pseudorandom output, which is XORed

with plaintext to produce next unit of ciphertext”, is a description of the

_________ mode of operation.

11. The _________ must be a data block that is unique to each execution of the

encryption operation and may be a counter, a timestamp, or a message

number.

12. A __________ cipher can operate in real time and eliminates the need to pad a

message to be an integral number of blocks.

13. Hardware efficiency, software efficiency, preprocessing, random access,

provable security, and simplicity are all advantages of __________ mode.

14. The plaintext of a sector or data unit is organized in to blocks of 128 bits. For

encryption and decryption, each block is treated independently. The only

exception occurs when the last block has less than 128 bits. In that case the

last two blocks are encrypted/decrypted using a ___________ technique instead

of padding.

15. The __________ standard describes a method of encryption for data stored in

sector-based devices where the threat model includes possible access to

stored data by the adversary. Some characteristics of this standard include:

the ciphertext is freely available for an attacker, the data layout is not

changed on the storage medium and in transit, and the same plaintext is

encrypted to different ciphertexts at different locations.

TRUE OR FALSE

MULTIPLE CHOICE

SHORT ANSWER