Computer security principles and practice 3rd by williams stallings and brown ch20

Chapter 20
Symmetric Encryption and
Message Confidentiality


Symmetric Encryption
• Also referred to as:
o
o

Conventional encryption
Secret-key or single-key encryption

• Only alternative before public-key
encryption in 1970’s
o

Still most widely used alternative

• Has five ingredients:
o
o
o
o
o

Plaintext
Encryption algorithm
Secret key
Ciphertext

Decryption algorithm


Cryptography
Classified along three
independent dimensions:
The type of
operations used
for transforming
plaintext to
ciphertext
• Substitution – each
element in the
plaintext is mapped
into another element
• Transposition –
elements in plaintext
are rearranged

The number of
keys used
• Sender and receiver
use same key –
symmetric
• Sender and receiver
each use a different
key - asymmetric

The way in which
the plaintext is

processed
• Block cipher –
processes input one
block of elements at a
time
• Stream cipher –
processes the input
elements
continuously



Computationally Secure
Encryption Schemes
• Encryption is computationally secure if:
o Cost of breaking cipher exceeds value of information
o Time required to break cipher exceeds the useful lifetime of the
information

• Usually very difficult to estimate the
amount of effort required to break
• Can estimate time/cost of a brute-force
attack


Plaintext (2w bits)

Round 1

L0

wbits

wbits

R0
K1

F

L1

R1

Round i
Ki

F

Li

Ri

Round n
Kn

F

Ln

Rn

Ln+1

Rn+1

Ciphertext (2wbits)

Figure20.1 Classical Feistel Network


Block Cipher Structure
• Symmetric block cipher consists of:
o A sequence of rounds
o With substitutions and permutations controlled by key

• Parameters and design features:
Block size

Key size

Number of
rounds

Subkey
generatio
n
algorithm

Ease of

analysis

Fast
software
encryption
/decryptio
n

Round
function




Most widely used
encryption scheme



Adopted in 1977 by
National Bureau of
Standards (Now
NIST)



FIPS PUB 46



Algorithm is
referred to as the
Data Encryption
Algorithm (DEA)



Minor variation of
the Feistel network

Data
Encryption
Standard
(DES)


K1
P

E

K2
A

D

K3
B

E

C

(a) Encryption

K3
C

D

K2
B

E

K1
A

D

(b) Decryption

Figure20.2 TripleDES

P


Plaintext

Add round key

w[0, 3]

Add round key

Substitutebytes

Expand key

Inversesub bytes

Shift rows

Inverseshift rows

Mix columns

Inversemix cols

Add round key

w[4, 7]

Round 10

Key

Add round key
Inversesub bytes

Round 9

Round 1

Plaintext

Inverseshift rows

Shift rows
Mix columns

Round 10

Add round key

Inversemix cols
w[36, 39]

Add round key

Substitutebytes

Inversesub bytes

Shift rows

Inverseshift rows

Add round key

w[40, 43]

Add round key

Ciphertext

Ciphertext

(a) Encryption

(b) Decryption

Figure20.3AES Encryption and Decryption

Round 1

Round 9

Substitutebytes


State

S

SubBytes

S

S

S

S

S

S

S

S

S

S

S

S

S

S

S

State

ShiftRows

State

MixColumns

M

M

M

M

State
r0

r1

r2

r3

r4

r5

r6

r7

r8

r9

r10

r11

r12

AddRoundKey

State

Figure20.4 AES Encryption Round

r13

r14

r15


Table 20.2

AES S-Boxes

(a) S-box
y

x

0

1

2

3

4

5

6

7

8

9

A

B

C

D

E

F

0

63

7C

77

7B

F2

6B

6F

C5

30

01

67

2B

FE

D7

AB

76

1
2

CA
B7

82
FD

C9
93

7D
26

FA
36

59
3F

47

F7

F0
CC

AD
34

D4
A5

A2
E5

AF
F1

9C
71

A4
D8

72
31

C0
15

3

04

C7

23

C3

18

96

05

9A

07

12

80

E2

EB

27

B2

75

4
5

09
53

83
D1

2C
00

1A
ED

1B
20

6E
FC

5A
B1

A0
5B

52
6A

3B
CB

D6
BE

B3
39

29
4A

E3
4C

2F
58

84
CF

6

D0

EF

AA

FB

43

4D

33

85

45

F9

02

7F

50

3C

9F

A8

7
8

51
CD

A3
0C

40
13

8F
EC

92
5F

9D
97

38
44

F5
17

BC
C4

B6
A7

DA
7E

21
3D

10
64

FF
5D

F3
19

D2
73

9

60

81

4F

DC

22

2A

90

88

46

EE

B8

14

DE

5E

0B

DB

A
B

E0
E7

32

C8

3A
37

0A
6D

49
8D

06
D5

24
4E

5C
A9

C2
6C

D3
56

AC
F4

62

EA

91
65

95
7A

E4
AE

79
08

C

BA

78

25

2E

1C

A6

B4

C6

E8

DD

74

1F

4B

BD

8B

8A

D
E

70
E1

3E
F8

B5
98

66
11

48
69

03
D9

F6
8E

0E
94

61
9B

35
1E

57
87

B9
E9

86
CE

C1
55

1D
28

9E
DF

F

8C

A1

89

0D

BF

E6

42

68

41

99

2D

0F

B0

54

BB

16


Table
20.2
Table
20.2

AES
S-Boxes
AES
S-Boxes

(b) InverseS-box
y

x

0

0
52

1
09

2
6A

3
D5

4
30

5
36

6
A5

7
38

8
BF

9
40

A
A3

B
9E

C
81

D
F3

E
D7

F
FB

1

7C

E3

39

82

9B

2F

FF

87

34

8E

43

44

C4

DE

E9

CB

2
3

54
08

7B

2E

94
A1

32
66

A6
28

C2
D9

23
24

3D
B2

EE
76

4C
5B

95
A2

0B

49

42
6D

FA
8B

C3
D1

4E
25

4

72

F8

F6

64

86

68

98

16

D4

A4

5C

CC

5D

65

B6

92

5
6

6C
90

70
D8

48
AB

50
00

FD
8C

ED
BC

B9
D3

DA
0A

5E
F7

15
E4

46
58

57
05

A7
B8

8D
B3

9D
45

84
06

7

D0

2C

1E

8F

CA

3F

0F

02

C1

AF

BD

03

01

13

8A

6B

8
9

3A
96

91
AC

11
74

41
22

4F
E7

67
AD

DC
35

EA
85

97
E2

F2
F9

CF
37

CE
E8

F0
1C

B4
75

E6
DF

73
6E

A

47

F1

1A

71

1D

29

C5

89

6F

B7

62

0E

AA

18

BE

1B

B
C

FC
1F

56
DD

3E
A8

4B
33

C6
88

D2
07

79

C7

20
31

9A
B1

DB
12

C0
10

FE
59

78
27

CD
80

5A
EC

F4
5F

D

60

51

7F

A9

19

B5

4A

0D

2D

E5

7A

9F

93

C9

9C

EF

E
F

A0
17

E0
2B

3B
04

4D
7E

AE
BA

2A
77

F5
D6

B0
26

C8
E1

EB
69

BB
14

3C
63

83
55

53
21

99
0C

61
7D


Shift
Rows
Decryption does
reverse

On encryption left rotate
each row of State by
0,1,2,3 bytes respectively


Mix Columns and Add Key
• Mix columns
o Operates on each column individually
o Mapping each byte to a new value that is a function of
all four bytes in the column
o Use of equations over finite fields
o To provide good mixing of bytes in column

• Add round key
o Simply XOR State with bits of expanded key
o Security from complexity of round key expansion and
other stages of AES


Stream Ciphers
Processes input
elements
continuously

Key input to a
pseudorandom
bit generator

• Produces stream of
random like

numbers
• Unpredictable
without knowing
input key
• XOR keystream
output with
plaintext bytes


Table20.3 Speed Comparisons of Symmetric Ciphers on a Pentium 4
Cipher

Key Length

Speed (Mbps)

DES

56

21

3DES

168

10

AES

128

61

RC4

Variable

Source: />
113


S

0

1

2

3

4

253 253 255

keylen

K

T
(a) Initial stateof S and T

T

T[i]
j =j +S[i] +T[i]

S

S[i]

S[j]

i

Swap
(b) Initial permutation of S
j =j +S[i]

S

S[i]
i

S[j]

S[t]

Swap

t =S[i] +S[j]
(c) Stream Generation

Figure20.5 RC4

k


Table 20.4
Block Cipher Modes of Operation


Electronic Codebook
(ECB)
• Simplest mode
• Plaintext is handled b bits at a time and each
block is encrypted using the same key
• “Codebook” because have unique ciphertext
value for each plaintext block
o Not secure for long messages since repeated plaintext is seen
in repeated ciphertext

• To overcome security deficiencies you need a
technique where the same plaintext block, if
repeated, produces different ciphertext blocks


IV

Time=1

P1

Time=2
P2

Time=N
PN
CN–1

Encrypt

K

K

C1

Encrypt

K

C2

Encrypt

CN

(a) Encryption
C1

Decrypt

K

C2

K

CN

Decrypt

K

Decrypt

CN–1

IV

P1

P2
(b) Decryption

Figure20.6 Cipher Block Chaining(CBC) Mode

PN

CM–1

IV
Shift register
b – s bits
s bits

Shift register
b – s bits
s bits

64

K

64

K

Encrypt

P1

64

K

Encrypt

64

Select
s bits

Shift register
b – s bits
s bits

Encrypt

64

Discard
b – s bits

Select
s bits

P2

s
s

64

Discard
b– s bits

Select
s bits

PM

s
s

Discard
b– s bits

s
s

s

C1

C2
(a) Encryption

CM
CM–1

IV
Shift register
b – s bits
s bits

Shift register
b – s bits
s bits

64

K

64

K

Encrypt
64
Select
s bits

Discard
b – s bits

64

K

Encrypt
Select
s bits

64

Discard
b– s bits

Select

s bits

s
s

C1

Encrypt

64

s

s

P1

Shift register
b – s bits
s bits

Discard
b– s bits

s
s

s

C2

P2
(b) Decryption

Figure20.7 s-bit Cipher Feedback (CFB) Mode

PM

CM


Counter

Encrypt

K

Counter +1

Encrypt

K

P1

Counter +N – 1

P2

C1

Encrypt

K

PN

C2

CN

(a) Encryption
Counter

Encrypt

K

C1

Counter +1

Encrypt

K

C2

P1

Counter +N – 1

Encrypt

K

CN

P2
(b) Decryption

Figure20.8 Counter (CTR) Mode

PN


FRN

FRN

Framerelay
network

FRN

FRN
= end-to-end encryption device
= link encryption device
FRN = frame relay node

Figure20.9 Encryption Across a FrameRelay Network