2- Stream and block ciphers

Question 1

Stream cipher

Answer 1

encrypts one bit at a time

Question 2

Block cipher

Answer 2

Encrypts block of ptext

Question 3

Key management for stream ciphers

Answer 3

short symmetric keys - easier than 1-time pad)

Question 4

Stream ciphers properties

Answer 4

1- no error propagation
2- speed: real time
3- on the fly encryption
4- need for synchronisation (A,B) must have the same keystream

Question 5

Stream ciphers use

Answer 5

internal n/works - no compatibility problems

Question 6

Block ciphers advantages

Answer 6

1) versatility: other uses
2) compatibility: widely implemented
3) adaptability: different modes

Question 7

Block ciphers disadvantages

Answer 7

1) error propagation
2) need for padding

Question 8

DES in words

Answer 8

data encryption standard

Question 9

DES (Feistel) cipher process

Answer 9

1- divide ptext into L and R
2- function f(R,K)
3- X = L xor f(R,K)
4- R1=X, L1=R
Repeat m times
5- concat Rm, Lm

Question 10

3x DES

Answer 10

1- DES Enc.
2- DES Dec.
3- DES Enc.
-ctext

Question 11

AES in words

Answer 11

advanced encryption system

Question 12

AES process

Answer 12

0- input 16 bytes
1- byte substitution (from S-box)
2- shift rows
3- mix columns
4- add round key (xor) from key schedule

Question 13

AES applications

Answer 13

soft- and hardware (RFID)

Question 14

Block cipher modes

Answer 14

1- ECB - electronic code book
2- CBC - cipher block chaining
3- CFB - cipher feedback
4- CTR - counter mode

Question 15

ECB process

Answer 15

encrypts 1 block at a time using the same key

Question 16

CBC process

Answer 16

1- IV xor P0
2- Enc. (1) with key => C1
3- C1xor P1
//=> Ci = Ek(Pi xor C(i-1))

Question 17

CFB process

Answer 17

1- Ek(IV)
2- P0 xor (1)
3- Ek(C0)…
//=> Ci = Pi xor Ek (C[i-1])

Question 18

CTR process

Answer 18

0- initial counter is IV
1- encrypt counter with a key
2- P xor (1)

Question 19

List of modes properties

Answer 19

1) positional dependency
2) error propagation
3) synchronisation
4) efficiency, parallelisation
5) padding