Computer Networking

1. Computer –Based Symmetric
Key Cryptographic Algorithm

2. Ciphers
• Mechanism that decides the process of
encryption/decryption
• Stream Cipher: Bit-by-bit encryption /
decryption
• Block Cipher: Block-by-block encryption /
decryption

3. Types of Cipher
Algorithm Types
Stream Ciphers Block Ciphers

4. Stream Cipher Example
Pay 100 010111001
100101011
ZTU91_^%D
+
11001001
Plain text
Cipher text
XOR operation with the key
In normal
format
In binary
format

5. Block Cipher Example
FOUR _AND_ FOUR Plain text
Encrypt Encrypt Encrypt
VFa% VFa%
*yT1x Cipher text
(a) The Encryption Process at the sender’s end
VFa% VFa%
*yT1x Cipher text
Decrypt Decrypt Decrypt
FOUR _AND_ FOUR Plain text
(b) The Decryption Process at the receiver’s end

6. Algorithm Modes
• Add randomness to block cipher
• Otherwise, block cipher becomes
predictable
• Four main modes

7. Algorithm Modes
Algorithm Modes
Electronic Code
Book (ECB)
Cipher Block
Chaining (CBC)
Cipher Feedback
(CFB)
Output Feedback
(OFB)
These two modes work
on block ciphers.
These two modes work
on block ciphers acting
as stream ciphers.

8. Encryption in ECB Mode
Encrypt
Plain text block 1
Key
Cipher text block 1
Step 1
Encrypt
Plain text block 2
Key
Cipher text block 2
Step 2
Encrypt
Plain text block n
Key
Cipher text block n
Step n

9. Decryption in ECB Mode
Decrypt
Cipher text block 1
Key
Plain text block 1
Step 1
Decrypt
Cipher text block 2
Key
Plain text block 2
Step 2
Decrypt
Cipher text block n
Key
Plain text block n
Step n

10. Encryption in CBC Mode
Encrypt
Plain text block 1
IV
Cipher text block
1
Step 1
Encrypt
Plain text block 2
Cipher text block
2
Step 2
Encrypt
Plain text block n
Cipher text block
n
Step n
Key
XOR XOR
Key Key
XOR

11. Decryption in CBC Mode
Decrypt
Cipher text block 1
IV
Plain text block 1
Step 1
Decrypt
Cipher text block 2
Plain text block 2
Step 2
Decrypt
Cipher text block n
Plain text block n
Step n
Key
XOR XOR
Key Key
XOR

12. Encryption in CFB Mode
IV
(Shift register)
Encrypt
Key
Take just the
leftmost 8 bits
XOR
Plain text 8 bits
Cipher text
8 bits
IV
(Shift register)
Encrypt
Key
Take just the
leftmost 8 bits
XOR
Plain text 8 bits
IV
(Shift register)
Encrypt
Key
Take just the
leftmost 8 bits
XOR
Plain text 8 bits
Cipher text
8 bits
Cipher text
8 bits

13. Encryption in OFB Mode
IV
(Shift register)
Encrypt
Key
Take just the
leftmost 8 bits
XOR
Plain text j bits
Cipher text j
bits
IV
(Shift register)
Encrypt
Key
Take just the
leftmost 8 bits
XOR
Plain text j bits
IV
(Shift register)
Encrypt
Key
Take just the
leftmost 8 bits
XOR
Plain text j bits
Cipher text j
bits
Cipher text j
bits

14. Symmetric Key Cryptography
• Same key used for encryption and
decryption
• Examples: DES, IDEA, RC5, Blowfish,
AES
• Quite popular and fast

15. Symmetric Key Cryptography
Plain
text
Encrypt
with
symmetric
key
Plain
text
Decrypt
with
symmetric
key
Sender
(A)
Net
wor
k
Receive
r
(B)
Cipher
text
Cipher
text

16. Conceptual View of DES
64-bit
Plain text
56-bit Key
DES
64-bit
Cipher
text
Block 1
64-bit
Plain text
56-bit Key
DES
64-bit
Cipher
text
Block 2
64-bit
Plain text
56-bit Key
DES
64-bit
Cipher
text
Block n


17. Discarding of every 8th bit of the
original key
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32
33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48
49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64

18. Key discarding process
Original 64-bit key
Key discarding process
Resulting 56 bit key

19. Broad Level Steps in DES
Initial Permutation (IP)
LPT RPT
16 rounds 16 rounds
Key Key
Final Permutation (FP)
Step 1
Step 2
Step 3
Step 4
Step 5
Step 6
Plain text (64 bits)
Cipher text (64 bits)

20. Idea of Initial Permutation(IP)
Bit position in the plain
text block
To be overwritten with the contents of
this bit position
1
2
3
…
64
58
50
42
…
7

21. Initial Permutation(IP) table
58 50 42 34 26 18 10 2 60 52 44 36 28 20 12 4
62 54 46 38 30 22 14 6 64 56 48 40 32 24 16 8
57 49 41 33 25 17 9 1 59 51 43 35 27 19 11 3
61 53 45 37 29 21 13 5 63 55 47 39 31 23 15 7

22. Details of One Round in DES
Key Transformation
Expansion Permutation
S-Box Substitution
P-Box Permutation
XOR and Swap

23. Key transformation
Round 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Number of key
bits shifted
1 1 2 2 2 2 2 2 1 2 2 2 2 2 2 1

24. Compression permutation
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

25. Expansion permutation
•Division of 32-bit RPT into eight 4 bit blocks

26. RPT expansion permutation
process

27. RPT expansion permutation table
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

28. S-box Subtitution
Key Transformation (Compress
key from 56 bits to 48 bits)
Expansion permutation (Expand
RPT from 32 bits to 48 bits)
48 bit key 48 bit RPT
XOR
S- box Substitution

29. S-Box Substitution
48 bit Input block
6 bit sub block 6 bit sub block 6 bit sub block
S-box 1 S-box 2 S-box 8
4 bit Output 4 bit Output 4 bit Output
32 bit Output block

30. S-box 1
14 4 13 1 2 15 11 8 3 10 6 12 5 9 0 7
0 15 7 4 14 2 13 1 10 6 12 11 9 5 3 8
4 1 14 8 13 6 2 11 15 12 9 7 3 10 5 0
15 12 8 2 4 9 1 7 5 11 3 14 10 0 6 13

31. Selecting an entity in a s-box
based on the 6-bit input
4 bit column number
b1 b6
b2 b3 b4 b5
2 bit row number

32. P-box 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 19 13 30 6 22 11 4 25

33. XOR and Swap
Original 64 bit Plain text block
32 bit LPT block 32 bit RPT block
1. Key transformation
2. Expansion Permutation
3. S-box Substitution
4. P-box Permutation
XOR
32 bit LPT block 32 bit RPT block
Next Round

34. Final Permutation
40 8 48 16 56 24 64 32 39 7 47 15 55 23 63 31
38 6 46 14 54 22 62 30 37 5 45 13 53 21 61 29
36 4 44 12 52 20 60 28 35 3 43 11 51 19 59 27
34 2 42 10 50 18 58 26 33 1 41 9 49 17 57 25

35. Modified Versions of DES
• Double DES: Perform DES twice with two
different keys
• Triple DES with Three Different Keys
• Triple DES with Two Different Keys

36. Double DES Encryption
Original
Plain Text
Encrypt
K1
Cipher
Text
Encrypt
K2
Cipher
Text

37. Double DES Decryption
Original
Plain Text
Decrypt
K2
Decrypt
K1
Cipher
Text
Cipher
Text

38. Double DES Mathematically
Expressed
P Encrypt
K1
Temporary
result (T)
Encrypt
K2
C
EK1(P) EK2(EK1(P))
T =
EK1(P)
C =
EK2(EK1(P))

39. Triple DES
Original
Plain Text
Encrypt
K1
Cipher
Text 1
Encrypt
K2
Cipher
Text 2
Encrypt
K3
Final Cipher
Text

40. Triple DES with Two Keys
Original
Plain Text
Encrypt
K1
Cipher
Text 1
Decrypt
K2
Cipher
Text 2
Encrypt
K1
Final Cipher
Text

41. Broad Level Steps in IDEA
( International Data Encryption Algorithm)
Input Plain Text (64 bits)
P1 (16 bits) P2 (16 bits) P3 (16 bits) P4 (16 bits)
Round 1
K1
.
. K6
Round 2
K7
.
. K12

Round 8
K43
.
. K48
Output Transformation
K49
.
. K52
C1 (16 bits) C2 (16 bits) C3 (16 bits) C4 (16 bits)
Output Cipher Text (64 bits)

42. Details of one round in IDEA
Step 2: Add *P2 and K2
Step 3: Add * P3 and K3
Step 1: Multiply * P1 and K1
Step 4: Multiply *P4 and K4
Step 5: XOR the result of step 1 and step 3
Step 6: XOR the result of step 2 and step 4
Step 7: Multiply* the result of step 5 with K5

43. Details of one round in IDEA
Step 9: Multiply * the results of Step 8 with K6
Step 10: Add* the results of step 7 and step 9
Step 8: Add * the results of Step 6 and step 7
Step11: XOR the results of step 1 and step 9
Step 12: XOR the result of step 3 and step 9
Step 13: XOR the result of step 2 and step 10
Step 14: XOR the result of step 4 and step 10

44. Modulo operation
•Addition with modulo 216 i.e. addition
modulo 65536
•Multiplication with modulo 216 + 1 i.e.
multiplication modulo 65537.

45. Detail of sub key generation and use
• Bit position 1-96 of the key are used. 97-128 remain unused.
•Bit 97-128 are first used. Circular left shift of 25 bits occurs.
As per new key bit position 26-89 are used. Bit position 90-
128 and 1-25 remain unused.

46. Details of the output transformation
Step 2: Add * R2 and K2
Step 3: Add*R3 and K3
step 1: Multiply *R1 and K1
Step 4: Multiply * R4 and K4

47. Encryption using RC5
First, divide the original plain text into two blocks of equal size.
Call them as A and B.
Add A and S[0] to produce C.
Add B and S[1] to produce D.
1. XOR C and D to produce E. 4. XOR D and F to produce G.
2. Circular-left shift E by D bits.
3. Add E and S[2i] to produce F.
5. Circular-left shift G by F bits.
6. Add G and S[2i + 1] to produce H.
Increment i by 1.
Check:
Is i > r?
Stop
Yes
No
Note: First perform all the left-hand side steps,
and then come to the right hand side steps, as
indicated by the step numbers.
Call F as C
(i.e. C = F)
Call H as D
(i.e. D = H)

48. RC5 Encryption
A = A + S[0]
B = B + S[1]
For i = 1 to r
A = ((A XOR B) <<< B) + S[2i]
B = ((B XOR A) <<< A) + S[2i + 1]
Next i

49. RC5 Decryption
For i = r to 1 step –1 (i.e. decrement i each time by 1)
B = ((B – S[2i + 1]) >>> A) XOR A
A = ((A – S[2i]) >>> B) XOR B
Next i
B = B – S[1]
A = A – S[0]

50. Blowfish
Plain text (64 bits)
32 bits 32 bits
XOR
P1 (32 bits)
F XOR
XOR
P2 (32 bits)
F XOR
13 more rounds
XOR F XOR
P16 (32 bits)
XOR
P18 (32 bits) XOR
P17 (32 bits)
32 bits 32 bits
Cipher text (64 bits)

51. Blowfish Function
32-bit
XL block
S-box 1
8 bits 32 bits
S-box 2
8 bits 32 bits
XOR
S-box 3
8 bits 32 bits
XOR
S-box 4
8 bits 32 bits
XOR
32-bit
output

52. The Description of Rijndael
• Do the following one –time initialization
process
–Expand the 16 byte key to get the
actual Key block to be used
–Do one time initialization of the 16
byte plain text block( called as State)
–XOR the State with the Key block

53. The Description of Rijndael
• For each round do the following
–Apply S-box to each of the plain text bytes
–Rotate row k of the plain text block by k
bytes
–Perform a mix columns operation
–XOR the state with the key block

54. Rijndael (AES)
Step 1: Byte Substitution
Step 2: Shift Rows
Step 3: Mix Columns
Step 4: Round Key Addition
Repeat these four
steps 10, 12 or 14
times.

55. AES Key Generation
16-byte key
Expanded into 11 arrays, each of size 4 x 4

56. AES Key Expansion – 1
16-byte key
To be expanded into 11 arrays, each of size 4 x 4
Copied, as is

57. AES S-Box