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1. IT346 Information
System Security
Week 4-1: Stream Cipher
& Asymmetric Key
Cryptography
Faculty of Information Technology
Page
1

2. Symmetric Key
Cryptography
Cryptography

Symmetric Key
Plaintext
(Block Cipher)
‣
• DES:
• 3DES:
• AES:
1
1 Data Block = 64 bits
1 Data Block = 64 bits
1 Data Block = 128 bits
(Stream Cipher)
‣
Faculty of Information Technology
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2

3. Stream Ciphers
bit

‣
 Key
‣ Keystream
bit
(Stream)
Keystream
Stream
Cipher
pseudorandom keystream
‣ Pseudorandom
Faculty of Information Technology
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3

4. Stream Ciphers
bit

(


Keystream
XOR)
bit
random
keystream
plaintext
plaintext
Keystream
C = P XOR
plaintext StreamKey Key
encrypt
i
Faculty of Information Technology
i
i
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4

5. Stream Ciphers

Secret Key
Seed
Keystream
Key
Stream Cipher
Pseudorandom
K
Key
K
KeyStream Generator
(Pseudorandom byte
generator)
KeyStream Generator
(Pseudorandom byte
generator)
Plaintext
Byte
Stream
P
+
ENCRYPTION
Faculty of Information Technology
Ciphertext
Byte
Stream
C
+
Plaintext
Byte
Stream
P
DECRYPTION
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5

6. One-Time-Pad
(OTP)
 Stream Cipher
One-Time-Pad
Cipher
break
‣ Keystream
Vernam
(unbreakable cipher)
random number
Secret Key
Pseudorandom number generator
‣ Secret Key
OTP
Keystream
plaintext
OTP
Faculty of Information Technology
OTP
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6

7. Stream Cipher
Properties

Encryption
‣
‣ Keystream
random
Stream Cipher
random
‣ Secret Key
Brute-force Attack
bits

Faculty of Information Technology
Page
7

8. RC4
 RC4
Stream Cipher
Ron Rivest
RSA Security (Security
Company)

Key
(variable key size)
(Byte-oriented
Stream Cipher)
random permutation
 RC4
SSL/TLS
wireless WEP
Faculty of Information Technology
Page
8

9. Attack on
Cryptography
 Cryptanalysis
plaintext
break
encrypt
‣ Ciphertext-only attack
encrypt
Key
key
ciphertext
key
‣ Known-plaintext attack
plaintext
key
key
‣ Chosen-plaintext attack
Faculty of Information Technology
plaintext
ciphertext
ciphertext
Page
9

10. Asymmetric Key
Cryptography
(Public Key
Cryptography)
Faculty of Information Technology
Page
10

11. Asymmetric Key
Cryptography

encrypt
‣
‣
‣
symmetric key cryptography
key
decrypt
key
Key
Key
key
‣
ciphertext
Faculty of Information Technology
Symmetric Key Encryption
Page
11

12. Asymmetric Key
Cryptography

cryptosystem
cryptography
‣ Public Key
asymmetric key
key
key
‣ Private Key
key
Encryption

‣
public key
encrypt
Faculty of Information Technology
public key
Page
12

13. Asymmetric Key
Cryptography
Faculty of Information Technology
Page
13

14. Asymmetric
Encryption
 Public-Key Cryptosystem
‣
encrypt
ciphertext
decrypt
plaintext P = Plaintext
E(P, PKreceiver) = C
‣
public key
private key
encryption
decryption
‣
Plaintext
Faculty of Information Technology
E(C, SKreceiver) = P
C = Cipherte
PK = Public K
SK = Private
public key
private
Public Key key Private Key
Ciphertext
Encryp
Decryp
tion
tion
Plaintext
Page
14

15. Public Key
Cryptography
 Public Key Cryptography
one-way function
‣ One-Way Function
(Multiplication)
(Factorization)
‣
•
x12 = 144
•
x12
144
= 12x12 = 144x1 = 24x6, …
Faculty of Information Technology
Page
15

16. 
Symmetric Key
Cryptography
Asymmetric Key
Cryptography
‣
Security Goals
Confidentiality, Integrity, Authenticity, Av
ailability
Non-Repudiation
‣
Faculty of Information Technology
Page
16

17.  RSA Algorithm
 Diffie-Hellman Algorithm (
Key Exchange)
 Elliptic Curve Cryptography
 Digital Signature Algorithm
Faculty of Information Technology
Page
17

18. RSA
 RSA
Adi Shamir)
Len Adleman) MIT
Faculty of Information Technology
Ron Rivest)
RSA
Page
18

19. RSA

(prime number) p
q
‣



gcd
‣
Faculty of Information Technology
n = pq
m = (p-1)(q-1)
e 1<e<m
e
m
e
e
m
gcd(e, m)
Page
19

20. RSA Encryption

M
M<n
Public Key (e, n)
‣ Ciphertext C = Me mod n
RSA Decryption

ciphertext C
(d, n)
‣ Message
Private Key
M = Cd mod n

‣ p = 5, q = 7, n = 35, m = 24
Faculty of Information Technology
Page
20

21.  n = 35, e = 5
Plaintext
M
Me
Ciphertext = Me mod n
L
12
248832
17
O
15
759375
15
V
22
5153632
22
E
5
3125
10
Faculty of Information Technology
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21

22.  n = 35, d = 29
Cd
Ciphertext
M= Cd mod n
Plaintext
17
48196857210675091509141182522307000
12
L
15
12783403948858939111232757568359400
15
O
22
8.5164331908653770195619449972111e+38
22
V
5
E
10
Faculty of Information Technology
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22