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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
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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
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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
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R2=(L1 XOR f(R1,K2))
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10. Data Block

K

f(Ri,Ki-1)

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f
(Round 0 – 15)
(Ri)
bits
f(Ri,Ki-1)
(Li)
XOR
XOR
(Ri+1)
(Ri)
(Li+1)
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11. Data f
Block

(Ri)
f
bits
bits
key

Key
bits
bits
XOR
bits
(substitution)
bits

(Permutation)
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12. Data Block

f
S-box
f
8
map
6 bits
4 bits
S(18 09 12 3d 11 17 38 39) =
5fd25e03
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13. Data Block
Faculty of Information Technology
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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
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15. DES Decryption
(decryption)

Block Cipher

DES
(encryption)
(decryption)
(encryption)
subkeys (K1 … K16)

Hardware
Faculty of Information Technology
decryption
subkeys
Encryption
K16 … K1
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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
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17. Avalanche Effect
 Avalanche Effect

 DES
Faculty of Information Technology
encryption algorithm
input
key
bit
bits
output
Avalanche Effect
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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
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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
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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
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22. AES (Advance
Encryption Standard)
 AES (Advance Encryption Standard)
AES

DES
break
NIST
data block
DES
NIST
bit


Faculty of Information Technology
Rijndael
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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
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0
4
8
12
out1 out5 out9 out13
Current
out2 out6 out10 out14
out3 out7 out11 out15
output
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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
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25. AES
AES
(Initial Round)

AddRoundKey
‣
(Rounds)

‣
SubBytes, ShiftRows, MixColumns
AddRoundKey
(Final Rounds)

‣

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SubBytes, ShiftRows
AddRoundKey (
MixColumns)
round key (sub-key
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26. AES (Initial Round)
(Rounds)
(Final Round)
Mix Columns
Faculty of Information Technology
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27. AES
AddRoundKey

XOR
round
‣
‣
key
Key
words
= 16 bytes
w
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28. AES
SubBytes

‣
non-linear substitution
lookup table
‣
Faculty of Information Technology
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29. AES
ShiftRows

‣
,3
‣
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30. AES
MixColumns

‣
‣
Faculty of Information Technology
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30