Cryptography

Question 1

Which of the following is NOT a goal of cryptography?

a) Confidentiality

b) Integrity

c) Availability

d) Obscurity

Answer 1

Answer:
d) Obscurity

Explanation:
“Security by obscurity doesn’t work”, implied that it is not a goal of cryptography.
Confidentiality, integrity, and availability are explicitly listed as security goals.

Question 2

What is the process of transforming a message to hide its meaning?

a) Cryptanalysis

b) Encryption

c) Decryption

d) Steganography

Answer 2

Answer:
b) Encryption

Explanation:
Encryption is defined as “a process of scrambling data so that it can only be read by the intended recipient”.

Question 3

In cryptography, what is the original message called?

a) Ciphertext

b) Key

c) Plaintext

d) Hash

Answer 3

Answer:
c) Plaintext

Explanation:
“Plaintext – original message”

Question 4

Which type of cryptography uses the same key for encryption and decryption?

a) Asymmetric key cryptography

b) Symmetric key cryptography

c) Hash functions

d) Public key cryptography

Answer 4

Answer:
b) Symmetric key cryptography

Explanation:
Symmetric key cryptography uses the same key for encryption and decryption.

Question 5

What is a potential vulnerability of the shift cipher?

a) Large key space

b) Frequency analysis

c) Perfect secrecy

d) Computational security

Answer 5

Answer:
b) Frequency analysis

Explanation:
The shift cipher is a type of mono-alphabetic substitution cipher, and “Substitution ciphers are vulnerable to frequency analysis attacks”.

Question 6

What is the key characteristic of a One-Time Pad?

a) Key reuse

b) Short key length

c) Key is at least as long as the plaintext

d) Predictable key

Answer 6

Answer:
c) Key is at least as long as the plaintext

Explanation:
Key is a random string that is at least as long as the plaintext.

Question 7

What is the term for breaking cryptography?

a) Cryptography

b) Cryptanalysis

c) Cryptology

d) Encryption

Answer 7

Answer:
b) Cryptanalysis

Explanation:
Cryptanalysis - Breaking cryptography

Question 8

Which of the following is NOT a property of stream ciphers?

a) Fast encryption

b) Perfect secrecy

c) Highly malleable

d) Security depends on PRNG

Answer 8

Answer:
b) Perfect secrecy

Explanation:
Stream ciphers does not have perfect secrecy and are highly malleable.

Question 9

What is a block cipher?

a) Cipher that encrypts one bit at a time

b) Cipher that encrypts blocks of data

c) Cipher that uses a stream of keys

d) Cipher that uses asymmetric keys

Answer 9

Answer:
b) Cipher that encrypts blocks of data

Explanation:
Block ciphers work by rather than encrypting letter by letter, encrypting block by block.

Question 10

What is the block size of DES?

a) 128 bits

b) 256 bits

c) 64 bits

d) 56 bits

Answer 10

Answer:
c) 64 bits

Question 11

What is a major vulnerability of DES?

a) Large block size

b) Short key size

c) Complex algorithm

d) Slow encryption speed

Answer 11

Answer:
b) Short key size

Explanation:
DES’s main vulnerability is short key size. Key size is 56 bits, considered insecure now - vulnerable to brute-force attacks.

Question 12

What is the main goal of AES?

a) Replace RSA

b) Provide perfect secrecy

c) Replace DES

d) Implement asymmetric key cryptography

Answer 12

Answer:
c) Replace DES

Question 13

Which encryption mode is deterministic?

a) CBC

b) CTR

c) ECB

d) OFB

Answer 13

Answer:
c) ECB

Explanation:
Deterministic: the same data block gets encrypted the same way. This describes ECB mode.

Question 14

Which mode of operation uses an Initialization Vector (IV)?

a) ECB

b) CTR

c) CBC

d) None of the above

Answer 14

Answer:
c) CBC

Explanation:
DES Encryption Modes: CBC

Question 15

Which mode of operation gives a stream cipher from a block cipher?

a) ECB

b) CBC

c) CTR

d) CFB

Answer 15

Answer:
c) CTR

Explanation:
Gives a stream cipher from a block cipher

Question 16

What type of cryptography uses different keys for encryption and decryption?

a) Symmetric key cryptography

b) Asymmetric key cryptography

c) Hash functions

d) Stream ciphers

Answer 16

Answer:
b) Asymmetric key cryptography

Explanation:
Public-key cryptography separates the keys used for encryption and decryption.

Question 17

Which algorithm is commonly used in public-key cryptography?

a) DES

b) AES

c) RSA

d) RC4

Answer 17

Answer:
c) RSA

Explanation:
Integer factorization (e.g., RSA)

Question 18

What is a hash function?

a) A function that encrypts data

b) A function that decrypts data

c) A function that maps a message of arbitrary length to a fixed-size output

d) A function that generates keys

Answer 18

Answer:
c) A function that maps a message of arbitrary length to a fixed-size output

Question 19

What is a key property of a cryptographic hash function?

a) It is reversible

b) It produces variable-length output

c) It is unique to the input data

d) It is used for encryption

Answer 19

Answer:
c) It is unique to the input data

Explanation:
Hash functions create fixed-length codes (hashes) unique to the input data.

Question 20

What does HMAC provide?

a) Encryption

b) Decryption

c) Message authentication

d) Key generation

Answer 20

Answer:
c) Message authentication

Explanation:
HMAC (Hash-based Message Authentication Code): HMAC is a special type of MAC that uses a cryptographic hash function and a secret cryptographic key.
MAC is a tag or piece of information that authenticates a message.

Question 21

What are the three goals of information security (CIA)?

Answer 21

Answer:
Confidentiality (secrecy, privacy), Integrity, and Availability.

Question 22

Explain the difference between a passive and an active attack.

Answer 22

Answer:
A passive attack involves eavesdropping without modifying the data.
An active attack involves modifying the data or the communication channel.

Question 23

What is steganography?

Answer 23

Answer:
Steganography is “covered writing” and “Hides the existence of a message”.

Question 24

Define “ciphertext.”

Answer 24

Answer:
Ciphertext is the transformed message.

Question 25

What is the key space in cryptography?

Answer 25

Answer: The key space is the set of all possible keys that can be used in a cipher.

Question 26

Why is the one-time pad considered perfectly secure?

Answer 26

Answer: The one-time pad is considered perfectly secure because the ciphertext reveals no "information" about Plaintext, and it has perfect secrecy.

Question 27

What is a stream cipher?

Answer 27

Answer: A stream cipher replaces the random key of the same length as the message in a one-time pad with a pseudo-randomly generated key.

Question 28

Explain the concept of a Pseudo-Random Number Generator (PRNG).

Answer 28

Answer: A PRNG expands a short random seed into a long string that "looks" random.

Question 29

List two adversarial models for ciphers.

Answer 29

Answer: Ciphertext-only attack, Known-plaintext attack, Chosen-plaintext attack, Chosen-ciphertext attack.

Question 30

What is the difference between computational security and information-theoretic security?

Answer 30

Answer: Computational security means the cipher can be broken with enough computing power. Information-theoretic security (like the one-time pad) cannot be broken even with unlimited computing power.

Question 31

What is a block cipher?

Answer 31

Answer: A block cipher encrypts data in fixed-size blocks, rather than bit-by-bit or character-by-character.

Question 32

What are the block and key sizes for AES?

Answer 32

Answer: AES has a block size of 128 bits and supports key sizes of 128-, 192-, and 256-bits.

Question 33

Explain the ECB mode of operation.

Answer 33

Answer: ECB (Electronic Codebook) mode encrypts each block of plaintext independently with the same key.

Question 34

What is an Initialization Vector (IV) in cryptography?

Answer 34

Answer: An IV is a random value used in some encryption modes (like CBC) to ensure that the same plaintext encrypted multiple times produces different ciphertexts.

Question 35

How does public-key cryptography differ from symmetric-key cryptography?

Answer 35

Answer: Public-key cryptography uses separate keys for encryption and decryption (public and private keys). Symmetric-key cryptography uses the same key for both.

Question 36

Give an example of a public-key cryptography algorithm.

Answer 36

Answer: RSA, Diffie-Hellman, DSA, Elliptic curves

Question 37

What is the purpose of a hash function in cryptography?

Answer 37

Answer: A hash function is used to generate a fixed-size output (hash or message digest) from an arbitrary-length input, used for integrity checks and authentication.

Question 38

What is a collision in the context of hash functions?

Answer 38

Answer: A collision occurs when two different inputs produce the same hash output.

Question 39

What is a Message Authentication Code (MAC)?

Answer 39

Answer: A MAC is a tag or piece of information that authenticates a message.

Question 40

Why is "Encrypt-then-Authenticate" (EtA) considered a secure approach?

Answer 40

Answer: Because it authenticates the ciphertext before decryption, preventing attacks that manipulate the ciphertext.

Question 41

Discuss the importance of cryptography in securing communication over insecure mediums. Provide examples of insecure mediums and the security goals that cryptography helps to achieve.

Answer 41

Answer: 1. Cryptography is fundamental to securing communication over insecure mediums by protecting data from unauthorized access. 2. Insecure mediums are characterized by the adversary's ability to eavesdrop or having full control over communications. The document provides examples of insecure communication over insecure medium. 3. Cryptography primarily achieves confidentiality (ensuring only the intended recipient can see the communication) and integrity (ensuring the communication is generated by the alleged sender). 4. Cryptographic systems often combine symmetric-key and public-key cryptosystems to achieve secure communication.

Question 42

Explain the differences between symmetric and asymmetric key cryptography. Discuss the advantages and disadvantages of each.

Answer 42

Answer: 1. Symmetric key cryptography uses the same key for both encryption and decryption, while asymmetric key cryptography uses different keys (public and private). 2. Symmetric key cryptography is generally faster, making it suitable for encrypting large amounts of data. However, it requires a secure method to exchange the shared key. 3. Asymmetric key cryptography simplifies key distribution, as public keys can be freely shared. But it is generally slower than symmetric key cryptography and is thus typically used for key exchange and digital signatures.

Question 43

Describe the different modes of operation for block ciphers and their security implications.

Answer 43

Answer: 1. Block cipher modes of operation define how to apply a cipher's single-block operation to larger messages. 2. ECB mode encrypts each block independently, which is simple but reveals patterns in the plaintext, making it unsuitable for most applications. 3. CBC mode introduces randomness using an Initialization Vector (IV), providing semantic security and making it resistant to pattern analysis. 4. CTR mode turns a block cipher into a stream cipher by encrypting a counter, offering both semantic security and random access for decryption. 5. The choice of mode significantly impacts security; improper mode selection can lead to vulnerabilities.

Question 44

Discuss the role of hash functions in cryptography. Explain their properties and common applications.

Answer 44

Answer: 1. Hash functions play a crucial role in cryptography by providing a way to ensure data integrity and authenticity. 2. They map arbitrary-length messages to fixed-size outputs (hashes or message digests). 3. Key properties of cryptographic hash functions include being one-way (computationally infeasible to reverse) and collision-resistant (computationally infeasible to find two different inputs that produce the same output). 4. Common applications include software integrity checks, timestamping, message authentication, one-time passwords, and digital signatures.

Question 45

Explain the concept of frequency analysis and how it can be used to break substitution ciphers.

Answer 45

Answer: 1. Frequency analysis exploits the fact that in any language, certain letters and combinations of letters appear more frequently than others. 2. Substitution ciphers preserve these frequency characteristics. 3. By analyzing the frequency of letters in the ciphertext and comparing them to the known frequency distribution of the plaintext language, attackers can deduce the key and decrypt the message.

Question 46

Describe the one-time pad and discuss why it is considered to provide perfect secrecy. What are the limitations of the one-time pad in practical use?

Answer 46

Answer: 1. The one-time pad is a symmetric key cipher where the key is a random string at least as long as the plaintext and is used only once. 2. It provides perfect secrecy because each bit of the ciphertext is equally likely to correspond to either a 0 or a 1 in the plaintext, given that the key is truly random. 3. Limitations include the difficulty of generating and securely distributing a truly random key as long as the message, and the requirement that the key never be reused.

Question 47

Explain the operation of a stream cipher and how it differs from a block cipher. Discuss the importance of a strong Pseudo-Random Number Generator (PRNG) in the security of a stream cipher.

Answer 47

Answer: 1. A stream cipher generates a keystream using a PRNG and combines it with the plaintext bitwise (usually using XOR). 2. Unlike block ciphers that encrypt data in fixed-size blocks, stream ciphers encrypt data bit by bit or byte by byte. 3. The security of a stream cipher heavily relies on the unpredictability of the PRNG. If the keystream is predictable, the cipher can be broken.

Question 48

Discuss the different types of attacks that can be performed on cryptographic systems. Explain the adversarial models: ciphertext-only attack, known-plaintext attack, chosen-plaintext attack, and chosen-ciphertext attack.

Answer 48

Answer: 1. Attacks on cryptographic systems aim to recover the key or the plaintext. 2. Ciphertext-only attack: The attacker only has access to the ciphertext. 3. Known-plaintext attack: The attacker has access to both the plaintext and its corresponding ciphertext. 4. Chosen-plaintext attack: The attacker can choose plaintext and obtain the corresponding ciphertext. 5. Chosen-ciphertext attack: The attacker can choose ciphertext and obtain the corresponding plaintext.

Question 49

Explain the difference between computational security and information-theoretic security. How do these concepts apply to modern cryptography?

Answer 49

Answer: 1. Information-theoretic security (perfect secrecy) means that a cipher cannot be broken even with unlimited computational resources (e.g., one-time pad). 2. Computational security means that a cipher can be broken in theory but requires an infeasible amount of computation. 3. Modern cryptography primarily relies on computational security, based on the assumption that certain mathematical problems are hard to solve.

Question 50

Describe the Data Encryption Standard (DES) algorithm. What are its key features and what are the reasons it is considered insecure today?

Answer 50

Answer: 1. DES is a symmetric-key block cipher with a 64-bit block size and a 56-bit key. 2. It uses a 16-round Feistel structure. 3. DES is considered insecure today primarily due to its short key size, which makes it vulnerable to brute-force attacks.

Question 51

Explain the Advanced Encryption Standard (AES). What were the goals in its design, and what are its advantages over DES?

Answer 51

Answer: 1. AES is a symmetric-key block cipher designed to replace DES. 2. Goals included security, efficiency, ease of implementation, and flexibility. 3. AES supports larger key sizes (128, 192, and 256 bits) and is more resistant to known attacks than DES.

Question 52

Discuss the Electronic Codebook (ECB) mode of operation for block ciphers. What are its weaknesses, and why is it generally not recommended for use?

Answer 52

Answer: 1. ECB mode encrypts each block of plaintext independently with the same key. 2. If the same plaintext block appears multiple times, it produces the same ciphertext block, revealing patterns. 3. ECB mode is deterministic and does not provide semantic security, making it vulnerable to attacks.

Question 53

Explain the Cipher Block Chaining (CBC) mode of operation for block ciphers. How does it improve security compared to ECB mode?

Answer 53

Answer: 1. CBC mode encrypts each plaintext block by XORing it with the previous ciphertext block (or an Initialization Vector (IV) for the first block). 2. This introduces dependency between blocks, so identical plaintext blocks produce different ciphertext blocks. 3. CBC mode provides semantic security and is more resistant to attacks than ECB.

Question 54

Describe the Counter (CTR) mode of operation for block ciphers. What are its advantages and disadvantages?

Answer 54

Answer: 1. CTR mode encrypts a counter value to generate a keystream, which is then XORed with the plaintext. 2. Advantages include semantic security, parallelization, and random access to blocks. 3. A disadvantage is that it's crucial to never reuse the same counter/IV for the same key, as this can compromise security (similar to stream cipher key reuse).

Question 55

Explain the basic principles of public-key cryptography. How does it address the key distribution problem of symmetric-key cryptography?

Answer 55

Answer: 1. Public-key cryptography uses a pair of keys: a public key for encryption and a private key for decryption. 2. The public key can be freely distributed, and only the corresponding private key can decrypt messages encrypted with that public key. 3. This eliminates the need for a secure channel to exchange keys, as in symmetric-key cryptography.

Question 56

Describe the RSA algorithm. Explain the role of public and private keys in encryption and decryption.

Answer 56

Answer: 1. RSA involves generating a public key (N, e) and a private key (N, d) based on the product of two large prime numbers. 2. To encrypt, the plaintext is raised to the power of the public exponent 'e' modulo N. 3. To decrypt, the ciphertext is raised to the power of the private exponent 'd' modulo N.

Question 57

Discuss the use of hash functions in cryptography. What security properties should a cryptographic hash function possess?

Answer 57

Answer: 1. Hash functions are used to generate a fixed-size "fingerprint" or "message digest" of a message. 2. Security properties include: 2.1 Preimage resistance (one-wayness): It should be computationally infeasible to find the original message given its hash. 2.2 Second preimage resistance: It should be computationally infeasible to find a different message with the same hash as a given message. 2.3 Collision resistance: It should be computationally infeasible to find two different messages that produce the same hash.

Question 58

Explain the purpose of a Message Authentication Code (MAC). How does it differ from a hash function? Describe HMAC.

Answer 58

Answer: 1. A MAC is used to authenticate a message, providing assurance of both data integrity and data origin. 2. Unlike a hash function, a MAC uses a secret key. 3. HMAC (Hash-based Message Authentication Code) is a specific type of MAC that uses a cryptographic hash function and a secret key to provide strong authentication.

Question 59

Discuss the importance of authenticated encryption. Explain the "Encrypt-then-Authenticate" approach and its advantages.

Answer 59

Answer: 1. Authenticated encryption combines encryption and authentication to provide both confidentiality and integrity. 2. Encrypt-then-Authenticate (EtA) first encrypts the plaintext and then generates a MAC of the ciphertext. 3. EtA is considered secure because it prevents attacks where an adversary might manipulate the ciphertext.

Question 60

Compare and contrast the different cryptographic techniques discussed in the lecture (symmetric-key cryptography, asymmetric-key cryptography, and hash functions). For each technique, discuss its primary purpose, general characteristics, advantages, and disadvantages.

Answer 60

Answer: 1. Symmetric-key cryptography: 1.1 Purpose: Confidentiality. 1.2 Characteristics: Uses the same key for encryption and decryption. 1.3 Advantages: Fast, efficient. 1.4 Disadvantages: Requires secure key exchange. 2. Asymmetric-key cryptography: 2.1 Purpose: Key exchange, digital signatures, confidentiality. 2.2 Characteristics: Uses separate keys for encryption and decryption. 2.3 Advantages: Simplifies key distribution. 2.4 Disadvantages: Slower than symmetric-key. 3. Hash functions: 3.1 Purpose: Integrity, authentication. 3.2 Characteristics: One-way function, fixed-size output. 3.3 Advantages: Efficient for integrity checks. 3.4 Disadvantages: Not for confidentiality by themselves.