Ch3. Block Ciphers and the Data Encryption Standard

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Ch3. Block Ciphers and the Data Encryption Standard

Department of Computer ScienceAcademic Year: 2017-2018

Semester: One

Dr. Maytham Mustafa Hammood

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Modern Block Ciphers

now look at modern block ciphers

one of the most widely used types of cryptographic algorithms

provide secrecy /authentication services

focus on DES (Data Encryption Standard)

to illustrate block cipher design principles

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Block vs Stream Ciphers

• block ciphers process messages in blocks, each of which is then en/decrypted

• like a substitution on very big characters

– 64-bits or more

• stream ciphers process messages a bit or byte at a time when en/decrypting

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Block vs Stream Ciphers

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Claude Shannon and Substitution-Permutation Ciphers

Claude Shannon introduced idea of substitution-permutation (S-P) networks in 1949 paper

form basis of modern block ciphers

S-P nets are based on the two primitive cryptographic operations seen before:

substitution (S-box)

permutation (P-box)

provide confusion & diffusion of message & keyClaude Elwood Shannon (April 30, 1916 – February 24, 2001)

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Confusion and Diffusion

• cipher needs to completely obscure statistical properties of original message

• a one-time pad does this

• more practically Shannon suggested combining S & P elements to obtain:

• diffusion – dissipates statistical structure of plaintext over bulk of ciphertext

• confusion – makes relationship between ciphertext and key as complex as possible

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Feistel Cipher Structure

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Feistel Cipher Design Elements

block size key size number of rounds subkey generation algorithmround function fast software en/decryptionease of analysis

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History of DES Algorithm

In 1977, the results of Tuchman’s project of

IBM was adopted as the Data Encryption

Standard by NSA (NIST).

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DES – Initial Permutation Function

• Before first rounds, the plaintext bits are

permuted using an initial permutation. IP

IP Table

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IP (Initial Permutation):

8 16 24 32 40 48 56

8 16 24 32 40 48 56

IP

5842 50

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12

32 bits

32 bits32 bits 28 bits28 bits

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Expansion

32 1 2 3 4 5

4 5 6 7 8 9

8 9 10 11 12 13

12 13 14 15 16 17

16 17 18 19 20 21

20 21 22 23 24 25

24 25 26 27 28 29

28 29 30 31 32 1

E

ExpansionExpansion

13

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1 4 5 8 9 12 13 16 17 20 21 24 25 28 29 32

1 48

Expansion Permutation

32

48

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1 48

X-OR with 48 bit key

1 48

48

48

48

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S-box

1

S-box

2

S-box

3

S-box

4

S-box

5

S-box

6

S-box

7

S-box

8

1 4 5 8 9 12 13 16 17 20 21 24 25 28 29 32

1 48

S-Box Substitution

48

32

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DES: S-Boxes (1-4)

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DES: S-Boxes (5-8)

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Permutation

16 7 20 21 29 12 28 17

1 15 23 26 5 18 31 10

2 8 24 14 32 27 3 9

9 13 30 6 22 11 4 25

P – Box Table

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1 4 5 8 9 12 13 16 17 20 21 24 25 28 29 32

P-Box Permutation

32

32

1 4 5 8 9 12 13 16 17 20 21 24 25 28 29 32

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Inverse Permutation

• Hence, at the end of the 16 rounds the inverse permutation is

applied.

IP-1 Table

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IP-1 (Final Permutation):

8 16 24 32 40 48 56

8 16 24 32 40 48 56

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• PC-157 49 41 33 25 17 9

1 58 50 42 34 26 18

10 2 59 51 43 35 27

19 11 3 60 52 44 36

63 55 47 39 31 23 15

7 62 54 46 38 30 22

14 6 61 53 45 37 29

21 13 5 28 20 12 4

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• PC-2

14 17 11 24 1 5

3 28 15 6 21 10

23 19 12 4 26 8

16 7 27 20 13 2

41 52 31 37 47 55

30 40 51 45 33 48

44 49 39 56 34 53

46 42 50 36 29 32

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• Iteration corresponds to left shifts:

1 2 3 4 5 6 7 8

1 1 2 2 2 2 2 2

9 10 11 12 13 14 15 16

1 2 2 2 2 2 2 1

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Initial Key Permutation

8 16 24 32 40 48 56

8 16 24 32 40 48 56

64

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Key Split & Shift & Compress8 16 24 32 40 48 56

Shift left by Ni Shift left by Ni

8 16 24 32 40 48 56

Ni = {1,1,2,2,2,2,2,2,1,2,2,2,2,2,2,1}

8 16 24 32 40 48

Shift accumulates every round

K48

K56

Permuted choice 2 TableCipher

DES Example - Key• K=581FBC94D3A452EA• P=3570E2F1BA4682C7

0101 1000 0001 1111 1011 1100 1001 0100

1101 0011 1010 0100 0101 0010 1110 1010• PC-1

C= 57 49 41 33 25 17 9 1 58 50 42 34 26 18

10 2 59 51 43 35 27 19 11 3 60 52 44 36

D = 63 55 47 39 31 23 15 7 62 54 46 38 30 22

14 6 61 53 45 37 29 21 13 5 28 20 12 4

K=

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DES Example - Key

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DES Example - Data• K=581FBC94D3A452EA

• P=3570E2F1BA4682C7

0011 0101 0111 0000 1110 0010 1111 0001

1011 1010 0100 0110 1000 0010 1100 0111P=

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DES Example - DataE(R(i-1)) K(i)

S100(1001)=s1

0(9)=10= 1010

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DES Example - Data

16 7 20 21 29 12 28 17

1 15 23 26 5 18 31 10

2 8 24 14 32 27 3 9

9 13 30 6 22 11 4 25

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R(i) = L(i-1) P(S( E(R(i-1)) K(i) ))

DES Example - Data

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DES Example - Data

L(i) = R(i-1)

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DES Example - Data

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DES Example - Data

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DES Example - Data - Done !

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