P2L7: Public Key Cryptography

Question 1

Symmetric encryption is also referred to as secret-key or single-key encryption.

Answer 1

True

Question 2

The ciphertext-only attack is the easiest to defend against.

Answer 2

True

Question 3

A brute-force approach involves trying every possible key until an intelligible translation of the ciphertext into plaintext is obtained.

Answer 3

True

Question 4

AES uses a Feistel structure.

Answer 4

False

Question 5

Each block of 64 plaintext bits is encoded independently using the same key? is a description of the CBC mode of operation.

Answer 5

False

Question 6

Timing attacks are only applicable to RSA.

Answer 6

False

Question 7

Using PKCS (public-key cryptography standard), when RSA encrypts the same message twice, different ciphertexts will be produced.

Answer 7

True

Question 8

The Diffie-Hellman algorithm depends for its effectiveness on the difficulty of computing discrete logarithms.

Answer 8

True

Question 9

A key exchange protocol is vulnerable to a man-in-the-middle attack if it does not authenticate the participants.

Answer 9

True

Question 10

Just like RSA can be used for signature as well as encryption, Digital Signature Standard can also be used for encryption.

Answer 10

False

Question 11

In general, public key based encryption is much slower than symmetric key based encryption.

Answer 11

True

Question 12

________ is the original message or data that is fed into the encryption process as input.A. Plaintext B. Encryption algorithm C. Decryption algorithm D. Ciphertext

Answer 12

A

Question 13

Which of the following would allow an attack that to know the (plaintext of) current message must be the same as one previously transmitted because their ciphtertexts are the same?A. CBC B. ECB C. CFB D. OFB

Answer 13

B

Question 14

________ is a term that refers to the means of delivering a key to two parties that wish to exchange data without allowing others to see the key. A. Session key B. Subkey C. Key distribution technique D. Ciphertext key

Answer 14

C

Question 15

Which of the following feature can only be provided by public-key cryptography?A. Confidentiality protection B. Integrity protection C. Non-repudiation D. None of the above

Answer 15

C

Question 16

Cryptographic systems are generically classified by _______.A. the type of operations used for transforming plaintext to ciphertext B. the number of keys used C. the way in which the plaintext is processed D. all of the above

Answer 16

D

Question 17

________ attacks have several approaches, all equivalent in effort to factoring the product of two primes.A. Mathematical B. Brute-force C. Chosen ciphertext D. Timing

Answer 17

A

Question 18

________ are analogous to a burglar guessing a safe combination by observing how long it takes to turn the dial from number to number.A. Digital standards B. Mathematical attacks C. Ciphers D. Timing attacks

Answer 18

D

Question 19

_________ was the first published public-key algorithm.A. NIST B. Diffie-Hellman C. RC4 D. RSA

Answer 19

B

Question 20

The principal attraction of ________ compared to RSA is that it appears to offer equal security for a far smaller bit size, thereby reducing processing overhead.A. ECC B. MD5 C. Diffie-Hellman D. none of the above

Answer 20

A

Question 21

To decrypt using DES, same algorithm is used, but with per-round keys used in the reversed order.

Answer 21

True

Question 22

With Triple DES the effective key length can be 56, 112, and 168

Answer 22

True

Question 23

Each round of DES contains both substitution and permutation operations.

Answer 23

True

Question 24

The logics behind the S-boxes are well-known and verified

Answer 24

False

Question 25

To decrypt using AES, just run the same algorithm in the same order of operations.

Answer 25

False

Question 26

Each operation or stage in AES is reversible.

Answer 26

True

Question 27

AES can support key length of 128, 192, 256

Answer 27

True

Question 28

AES is much more efficient than Triple DES.

Answer 28

True

Question 29

CBC is more secure than ECB.

Answer 29

True

Question 30

We can have both confidentiality and integrity protection with CBC by using just one key.

Answer 30

False

Question 31

RSA is a block cipher in which the plaintext and ciphertext are integers between 0 and n - 1 for some n.

Answer 31

True

Question 32

If someone invents a very efficient method to factor large integers, then RSA becomes insecure.

Answer 32

True

Question 33

What type of math are public key algorithms based on?

Answer 33

Modular arithmetic

Question 34

What is the additive inverse of the below?

8 mod 10

Answer 34

12
because…
8 + 12 mod 10 = 0

Question 35

What is the multiplicative inverse of the below?

3 mod 17

Answer 35

6
because…
3*6 mod 17 = 1

Question 36

Does RSA support public or single key encryption?

Answer 36

Public

Question 37

Does RSA support digital signatures?

Answer 37

Yes

Question 38

What key-length is supported for RSA?

Answer 38

Any, but most use 128

Question 39

Given that p = 3 and q = 11

What is step 1 in the RSA algorithm?

Answer 39

Compute n and totient(n).
n = pq
n = 311 = 33

totient(n) = (p-1)*(q-1)
totient(n) = (3-1)*(11-1)
totient(n) = 2*10 = 20

Question 40

Given that p = 3 and q = 11 and the computed n, therefore, is 33 while the totient(n) is 20. And given that e = 7
What is step 2 in the RSA algorithm?

Answer 40

Calculate d.
We need to find the answer to the equation…
e * d mod totient(n) = 1
7 * d mod 20 = 1
3 = d
So public key is (7, 33) and private key is (3, 33)

Question 41

Why is RSA secure and what would have to happen to make it not secure from a mathematical perspective?

Answer 41

It’s secure because it relies on factoring large numbers, which is computationally slow. If an algorithm is discovered that can efficiently compute the factors of large numbers then RSA encryption would no longer be secure

Question 42

What are the special case plaintext that produce the same cipher texts that are regardless of the keys used?

Answer 42

1, -1 and 0

Question 43

Given that prime q = 23 and primitive root a = 5, and Alice chose 6 as her secret key while Bob chose 15 as his secret key… what number does Alice show Bob and Bob show Alice?

Answer 43

• Alice To Bob –> 5^6 mod 23 = 8

* Bob To Alice –> 5^15 mod 23 = 19

Question 44

What are the key points of the Diffie Hellman Key Exchange algorithm?

Answer 44

• The shared/secret key itself is never transmitted
• Uses discrete logarithms which are hard to crack ASSUMES q is a large prime (at least 300 digits long)
• ɑ^x mod q where alpha is the primitive root, x is the chosen secret key and q is the prime

Question 45

What are the shortcoming of the Diffie Hellman algorithm?

Answer 45

• Expensive exponential operation, so DOS attacks are possible
• Only works for key exchange
• No authentication or digital signatures

Question 46

What is the Bucket Brigade Attack and how is it overcome?

Answer 46

• type of Man-In the-Middle
• Intercepts message Alice sends out and redirects it back to herself, leading Alice to think she received a message from Bob. It does the same thing on Bob’s side.
• This can be overcome just by publishing the public key rather than sending it and risking redirected messages

Question 47

What are two other public key algorithms besides Diffie Hellman and RSA?

Answer 47

DSS and ECC