2- Stream and block ciphers Flashcards
Stream cipher
encrypts one bit at a time
Block cipher
Encrypts block of ptext
Key management for stream ciphers
short symmetric keys - easier than 1-time pad)
Stream ciphers properties
1- no error propagation
2- speed: real time
3- on the fly encryption
4- need for synchronisation (A,B) must have the same keystream
Stream ciphers use
internal n/works - no compatibility problems
Block ciphers advantages
1) versatility: other uses
2) compatibility: widely implemented
3) adaptability: different modes
Block ciphers disadvantages
1) error propagation
2) need for padding
DES in words
data encryption standard
DES (Feistel) cipher process
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
3x DES
1- DES Enc.
2- DES Dec.
3- DES Enc.
-ctext
AES in words
advanced encryption system
AES process
0- input 16 bytes
1- byte substitution (from S-box)
2- shift rows
3- mix columns
4- add round key (xor) from key schedule
AES applications
soft- and hardware (RFID)
Block cipher modes
1- ECB - electronic code book
2- CBC - cipher block chaining
3- CFB - cipher feedback
4- CTR - counter mode
ECB process
encrypts 1 block at a time using the same key
CBC process
1- IV xor P0
2- Enc. (1) with key => C1
3- C1xor P1
//=> Ci = Ek(Pi xor C(i-1))
CFB process
1- Ek(IV)
2- P0 xor (1)
3- Ek(C0)…
//=> Ci = Pi xor Ek (C[i-1])
CTR process
0- initial counter is IV
1- encrypt counter with a key
2- P xor (1)
List of modes properties
1) positional dependency
2) error propagation
3) synchronisation
4) efficiency, parallelisation
5) padding
