This document provides an overview of symmetric-key cryptography and the DES and AES encryption algorithms. It discusses symmetric-key cryptography, how DES and AES work, including their data encryption processes using secret keys, rounds of substitutions and transformations, and how decryption reverses the encryption process. It also covers the strengths and limitations of DES, the move to AES as the new encryption standard, and the key components of AES encryption rounds like substitution tables, shifting rows, and mixing columns.

1. IT346 Information
System Security
Week 3-1: Symmetric-Key
Cryptography
Faculty of Information Technology
Page
1

2. Cryptography
 Cryptography
graph
crypto
“
 Cryptography
Cryptography
”
3
‣ Symmetric Key Cryptography
Key Cryptography
‣ Asymmetric Key Cryptography
Key Cryptography
Faculty of Information Technology
Secret
Public
Page

3. Cryptography
 Cryptanalysis
Cryptography
 Sensitive Data
Faculty of Information Technology
Page

4. Symmetric-Key
Cryptography
 Plaintext –
 Encryption algorithm –
(substitutions)
(transformations)
plaintext
 Secret Key –
substitutions/transformations
encryption
 Ciphertext –
substitutions/transformations
Key
Key
 Decryption –
Plaintext
Ciphertext
encryption
Encryp
Decryp

tion C encryption  P
tion
E(P,K) 
D(C,K)
key
Faculty of Information Technology
Plaintex
Page

5. 
DES (Data Encryption
Standard)
‣ Key size 56 bits
3-DES (Triple-DES)

‣ Key size 112 bits

IDEA (International Data
Encryption Algorithm)
‣ Key size 128 bits

AES (Advanced Encryption
Standard)
‣ Key size 128, 192, 256 bits
Faculty of Information Technology
Page
5

6. DES Algorithm
 DES
Data Encryption
Standard
(Block cipher)
Lucifer
IBM
Lucifer
DES
US NIST (US
National Institute of Standards and
Technology)
 DES
Faculty of Information Technology
(Block
Page
6

7. DES Overview
64-bit Plaintext
…
64-bit Key
Initial Permutation
PC1
64 bit
Round 0
64 bit
Round 1
…
K1
48 bit
K2
48 bit
56 bit
PC2
PC2
Left Circular Shift
56 bit
64 bit
Round 15
56-bit Key
56-bit Key
Left Circular Shift
56-bit Key
K16
48 bit
PC2
56 bit
Left Circular Shift
64 bit
32-bit Swap
64 bit
Inverse Initial Permutation
…
64-bit Ciphertext
Faculty of Information Technology
Page
7

8. DES Overview

Data Block
1)Data Block
(IP)
2)Data Block
DES Algorithm
bits
Initial Permutation
16
(substitution)
(permutation)
key
3)Data Block
4)Data Block
Permutation

Key Block
Faculty of Information Technology
48 bits
Inverse Initial
DES Algorithm
Page
8

9. Data Block Data Block

16
Round 0
64-bit
L0 (32 bits)
R0 (32 bits)
f
K1
XOR
Round 1
L1=R0
R1=(L0 XOR f(R0,K1))
f
K2
XOR
L2=R3
Faculty of Information Technology
R2=(L1 XOR f(R1,K2))
Page
9

10. Data Block

K

f(Ri,Ki-1)

Faculty of Information Technology
f
(Round 0 – 15)
(Ri)
bits
f(Ri,Ki-1)
(Li)
XOR
XOR
(Ri+1)
(Ri)
(Li+1)
Page
10

11. Data f
Block

(Ri)
f
bits
bits
key

Key
bits
bits
XOR
bits
(substitution)
bits

(Permutation)
Faculty of Information Technology
Page
11

12. Data Block

f
S-box
f
8
map
6 bits
4 bits
S(18 09 12 3d 11 17 38 39) =
5fd25e03
Faculty of Information Technology
Page
12

13. Data Block
Faculty of Information Technology
Page
13

14. Key
Block

subkeys
DES
key
initial permutation (Permutation Choice
1, PC1)
56bits
2
28bit
‣
16
:
‣
•
•
Faculty of Information Technology
28 bit
1
2
key rotation schedule
24 bits
PC2
data block
Page
14

15. DES Decryption
(decryption)

Block Cipher

DES
(encryption)
(decryption)
(encryption)
subkeys (K1 … K16)

Hardware
Faculty of Information Technology
decryption
subkeys
Encryption
K16 … K1
Page
15

16. DES Decryption
 DES decryption
Encryption
‣
‣
‣
‣
‣
DES
Initial Permutation
decryption
Inverse Initial Permutation
encryption
Round 0
K16
Round
encryption
….
Round 15
K1
Round
encryption
Inverse Initial Permutation
decryption
Faculty of Information Technology
Page
16

17. Avalanche Effect
 Avalanche Effect

 DES
Faculty of Information Technology
encryption algorithm
input
key
bit
bits
output
Avalanche Effect
Page
17

18. Strength of DES –
Key Size
 Key
56-bit
256 = 7.2 x 1016

brute force search
brute
DES Algorithm

force search
1997
Internet
‣
1998
‣
‣
key
Distribution.net
EFF
hardware
1999 Distribution.net
Faculty of Information Technology
EFF
Page
18

19. DES

bits
‣
Key
DES
DES
Bit

crack
bits
crack
DES
bit
Bit
DES
DES
Triple-DES ( DES)
Faculty of Information Technology
Page
19

20. Triple-DES (3DES) with
Two-Keys
 Triple-DES
DES
DES
DES

key

‣

Triple-
Faculty of Information Technology
C = EK1(DK2(EK1(P)))
Encryption
Encrypt-Decrypt-Encrypt
K1 = K2
DES
Triple-DES
Page
20

21. Triple-DES (3DES) with
Two-Keys

Key
Encryption
3DES
64-bit Plaintext
DES Encryption
Key 1
DES Decryption
Key 2
DES Encryption
Key 1
64-bit Ciphertext
Faculty of Information Technology
Page
21

22. AES (Advance
Encryption Standard)
 AES (Advance Encryption Standard)
AES

DES
break
NIST
data block
DES
NIST
bit


Faculty of Information Technology
Rijndael
Page
22

23. AES
 AES
Bytes
 Data Block
bits = 16 bytes
‣
Data Block 16 bytes
x
4
‣
bytes
Current State
out
in in in in
s s s
s s s s bytesout out out
‣ Current State s
0
4
8
12
0,0
0,1
0,2
0,3
0,0
0,1
0,2
0,3
in1 in5 in9 in13
s10 s1,1 s1,2 s1,3
s10 s1,1 s1,2 s1,3
in2 in6 in10 in14
s2,0 s2,1 s2,2 s2,3
s2,0 s2,1 s2,2 s2,3
in3 in7 in11 in15
s3,0 s3,1 s3,2 s3,3
s3,0 s3,1 s3,2 s3,3
State
ciphertext
input
State Array
encryption
Faculty of Information Technology
0
4
8
12
out1 out5 out9 out13
Current
out2 out6 out10 out14
out3 out7 out11 out15
output
Page
23

24. AES
bits
= 16 bytes
Key Block 16 bytes
‣
x4
‣ Key
k4
k8 k12
k1
k5
k9 k13
k2
 Key Block
k0
k6 k10 k14
k3
k7 k11 k15
w0 w1 w2 w3
(Key
Expansion)
f
+ + + +
w4 w5 w6 w7
word
44 words
k k k k
(1kword = 32 bits = 4
k
k k
w w
bytes)k
k k k
Key Expansion
Key Expansion
0
4
8
12
1
5
9
13
0
2
k3
6
10
1
…
w43
14
k7 k11 k15
Faculty of Information Technology
Page
24

25. AES
AES
(Initial Round)

AddRoundKey
‣
(Rounds)

‣
SubBytes, ShiftRows, MixColumns
AddRoundKey
(Final Rounds)

‣

Faculty of Information Technology
SubBytes, ShiftRows
AddRoundKey (
MixColumns)
round key (sub-key
Page
25

26. AES (Initial Round)
(Rounds)
(Final Round)
Mix Columns
Faculty of Information Technology
Page
26

27. AES
AddRoundKey

XOR
round
‣
‣
key
Key
words
= 16 bytes
w
Faculty of Information Technology
Page
27

28. AES
SubBytes

‣
non-linear substitution
lookup table
‣
Faculty of Information Technology
Page
28

29. AES
ShiftRows

‣
,3
‣
Faculty of Information Technology
Page
29

30. AES
MixColumns

‣
‣
Faculty of Information Technology
Page
30