FINAL EXAM

Question 1

What is the difference between stream and block ciphers?

Answer 1

Stream cipher encrypts one element at a time.

Block Ciphers encrypts encrypt the plaintext multiple elements at time

Question 2

What is confusion ?

Answer 2

obscures the relationship between the key and ciphertext

Question 3

What is diffusion?

Answer 3

obscures the relationship between the plaintext and the ciphertext

Question 4

Why are confusion and diffusion important when designing a block cipher? Give an example

Question 5

How is confusion achieved?

Answer 5

through substitution , replacing elements of plaintext with elements of cipher text

Question 6

Consider a block cipher with a property where if one bit of the key changes,
many bits of the ciphertext change. Is this example of confusion or diffusion? Explain

Answer 6

Confusion, since its obscuring the relationship between the key and cipher text

Question 7

How is diffusion achieved?

Answer 7

through premutation, shuffling the elements up.

Question 8

What makes a block cipher reversible?

Answer 8

Each block of plaintext must give you a distinct block of cipher text.

Example:
00 -> 11
01 -> 10
10 -> 00
11 -> 01

Question 9

How many possible keys can I have in an ideal block cipher which encrypts
64-bit text blocks?

Answer 9

(2^64)!

Question 10

What is the size of the key in a 64-bit ideal block cipher?

Answer 10

64 * (2^64)

Question 11

How does Feistel cipher perform substitution and permutation

Answer 11

Substitution is applied on the left half,

permutation occurs when we swap the two half’s

Question 12

What are the advantages of the Feistel cipher network?

Answer 12

Encryption and decryption are very smilar

Question 13

Consider binary key string:
0011000110110111000000111001111011101111000010100111100101110100

What will be the first five bits after running this key through PC-1 table?

Answer 13

use python, don’t forget to do -1

Question 14

What will be the output of S-Box 8 given input 111011

Answer 14

row: 11 -> 0b11 -> 3
col: 1101 -> 0b1101 -> 13

find the intercept, of row, col and then get the output, and make it into binary.

intercept: 5 -> bin(5) -> 0101

Question 15

How do you do left circular shift?

Ex:
0000100011000110101001011111 1111001100111100111111110000

Answer 15

take first bit add it to the end
0000100011000110101001011111 => 0001000110001101010010111110

1111001100111100111111110000 => 111001100111100111111110001

Combine the two 0001000110001101010010111110 + 111001100111100111111110001

Question 16

What important function do S-Boxes perform in DES encryption?

Answer 16

substitution

Question 17

Do not need to know all of the S-Box design criteria but need to know at least a
few and describe them.

Answer 17

Any change to an input => results in random looking changes in the output

No output bit of any S-box should be linear

Each row should have 0-16

Question 18

What is the important property of block cipher security achieved by swapping two halves
of the text block after every round

Answer 18

Diffusion

Question 19

What is the avalanche effect? Why is it important?

Answer 19

Small changes in plaintext or key => significant change in the cipher text

Making attempts of guessing keys impossible

Question 20

Why not double DES? What is the method of breaking double DES?

Answer 20

Meet-in-the-middle, the attacker creates two tables, The Encryption and Decryption. If one value within the encryption table matches with one in the decryption.

Question 21

Understand why when using 3DES, it’s better to use
EDE instead of EEE?

Answer 21

EDE, encrypt using key1, decrypt using key2, and encrypt again using key 1

Its backwards compatible with Single DES

Question 22

What is the difference between differential and linear cryptoanalysis?

Answer 22

Differential: Trying to derive the secret key by studying how the input plaintext affects the ciphertext

Linear: Attacker tries to find a linear function tries to find a linear function that can approximate the relationship between the plaintext key inputs, and the encryption

Question 23

What key sizes and block sizes does Twofish support?

Answer 23

128/192/256

Question 24

What is the distinguishing feature of Twofish S-Boxes? How do they compare with, say,
DES S-Boxes.

Answer 24

They are dynamically generated based on the key

Question 25

What is the distinguishing feature of Twofish key scheduling algorithm?

Answer 25

It uses half of the key to do scheduling and the rest is used for deriving the keys

Question 26

Compare and contrast AES and DES in terms of security, key size, block size, structure, and the number of rounds.

Answer 26

AES, has not been broken Key Size: 128(10 rounds) , 192(12 Rounds), 256(14 rounds) bit keys Block Size: 128

Question 27

Is AES based on the Feistel cipher network?

Answer 27

Based on Substitution Permutation network

Question 28

Be able to compare and contrast the Fiestal cipher framework to the Substitution Permu- tation framework.

Answer 28

S.P.F, is mainly focused on S-boxes and P-boxes. Fiestal takes input and splits it in half, with the right hand going through the round function

Question 29

Know basic structure of the single AES round.

Answer 29

1. First round key XOR O.G P.T 2. P.T => SubBytes. 3. ShiftRows & MixColumns 4. AddRoundKey makes new key for next round , and XOR matrix

Question 30

What sequence of operations does the first AES round perform?

Answer 30

SubBytes Permutation, Shift Rows, Mix Column, AddroundKey

Question 31

How does the last round of AES differ from all other rounds?

Answer 31

It drops the mixColumn Step

Question 32

In DES, the number of rounds is fixed. Is the same true of AES?

Answer 32

AES rounds are dependent on the size of the key.

Question 33

Are Decryption and Encryption the same for the S.P.F?

Answer 33

No they are not the same

Question 34

What happens to the plaintext block in AES?

Answer 34

it's put into a 4x4 matrix, known as the state

Question 35

What are the main differences between public key and symmetric cryptography?

Answer 35

Symmetric does not provide digital signature.

Question 36

Is public cryptography inherently more secure than symmetric?

Answer 36

No public key is not more secure. Its only advantage is if one party looses their key the other end is still secure

Question 37

Does the invention of public key cryptography render symmetric obsolete?

Answer 37

no, because public has a much greater over head than symmetric

Question 38

What is public key cryptography good for?

Answer 38

Key distribution and digital signatures

Question 39

what are the requirements for secure public key algo?

Answer 39

Attacker isn't able to start with cipher text, and derive private key or p.T

Question 40

What key do you use for confidentiality

Answer 40

You use public key of reciever

Question 41

What key do you use for digital Signature

Answer 41

Sign using the private key of the sender

Question 42

What can public key cryptography be used for?

Answer 42

Encryption / Decryption Digital Signatures Key Exchange