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1. IT346 Information
System Security
Week 3-2: Symmetric Key
Cryptography
Faculty of Information Technology
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1

2. Block Cipher
 Block Cipher
block
‣
block
encrypt
block
block
ciphertext
block
encryption
‣
full
encrypt
encrypt
Exrw
Faculty of Information Technology
time
part
encrypt
vjyo
gMsQ
time
encrypt
vjyo
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2

3. Block Cipher

‣ Ciphertext
plaintext
block
‣ Encryption
block
Faculty of Information Technology
ciphertext
plaintext
block cipher
block
block
probabilistic
output
plaintext
input
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3

4. Block Cipher
 Mode
Block Cipher
‣ Electronic Code Book (ECB)
‣ Cipher Block Chaining (CBC)
 Mode
Block Cipher
Stream Cipher
‣ Cipher Feedback (CFB)
‣ Output Feedback (OFB)
Faculty of Information Technology
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4

5. Electronic Code Book
(ECB) mode




block
block
block
block
Faculty of Information Technology
encrypt
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5

6. Electronic Code Book
(ECB) mode
Faculty of Information Technology
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6

7. Cipher Block Chaining
(CBC) mode

ciphertext
Encryption
‣ Chain
Block
‣ Block
Faculty of Information Technology
plaintext
Input
plaintext
block
Initial Vector (IV)
IV
IV
key
XOR
encrypt
ciphertext
key
block
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8. Cipher Block Chaining
(CBC) mode
Decryption

‣ Ciphertext
key
encryption
block
decrypt
XOR
IV
plaintext
block
‣ Decrypt Ciphertext
block
Ciphertext
block
plaintext
block
‣
block
Faculty of Information Technology
XOR
decrypt
plaintext
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8

9. Cipher Block Chaining
(CBC) mode
C0 = IV
Ci = EK(Pi XOR Ci-1)
C0 = IV
Pi = DK(Ci) XOR Ci-1
Faculty of Information Technology
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9

10. Cipher Feedback
(CFB) mode

‣
encrypt
‣
block
Faculty of Information Technology
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10

11. Cipher Feedback
(CFB) mode
 Encryption
‣ IV
encrypt
bits
shift register
cipher IV
bit
‣
cipher IV
XOR
j
plaintext block
ciphertext block
‣
ciphertext block
‣
IV
shift bit
j
ciphertext block
j
‣
Faculty of Information Technology
IV
j
plaintext
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11

12. CFB Mode
Encryption
Decryption
Faculty of Information Technology
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12

13. Output Feedback
(OFB) mode
CFB

‣
IV
‣
Faculty of Information Technology
CFB
ciphertext
encryption
OFB
ciphertext
IV
IV
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13

14. Output Feedback
(OFB) mode
Faculty of Information Technology
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14

15. Symmetric Key Cryptography
Symmetric Key
Cryptography
key
Encryption
Decryption
?
 Alice
Bob
Alice
Bob
Bob
Alice
Symmetric Key

Faculty of Information Technology
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15

16. Diffie-Hellman
 Diffie-Hellman
(Exponential)
(Logarithm)
Secret Key

Primitive
Modulo p)
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g
p
g<p
g
p (Primitive Root
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17. Diffie-Hellman
(Example)
g = 3, p = 5
Primitive

‣
‣
‣
‣
‣
‣
g1 mod 5 = 3 mod 5 = 3
g2 mod 5 = 9 mod 5 = 4
g3 mod 5 = 27 mod 5 = 2
g4 mod 5 = 81 mod 5 = 1
g5 mod 5 = 243 mod 5 = 3
g6 mod 5 = 729 mod 5 = 4

Faculty of Information Technology
3
primitive
5
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18. Diffie-Hellman
Secret Key

Alice


g
a
Bob
p

A = ga
mod p

g, p, A
Bob

B
Faculty of Information Technology
Bob
g, p,
A
B


b

p

g, p, A
B = gb mod
Alice
K=
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19. Diffie-Hellman
 g = 3 and p = 5
Alice

g=3
p=5
g=3, p

a=2
=5, A

A = 32 mod =4
5=4
B

g=3, p=5, A=4
Bob
Faculty of Information Technology
Bob

g=3, p=5, A=4

b=3

B = 33 mod 5
=2
Alice

K = 43
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