Title: Cryptography

1
Department of Computer Science Georgia State
University

• Title Cryptography
• Instructor Dr. Yanqing Zhang
• Presented by Jiangling, Yin

CSC 8320 Advanced Operating Systems
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Outline

• Introduction Motivation
• What is cryptography and why it is necessary?
• Modern cryptography
• Private Key Cryptosystem
• Public Key Cryptosystem
• Comparison of Cryptographic Systems
• Future work

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A Simple Example

• Suppose two lovers try to meet at a certain
  place. And the girl sends the information to the
  boy
• meet me at

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A Simple Example

• Instead of sending the intelligible message to
  the boy, the girl plays a trick and change the
  information.

meet me at

phhw ph dw fv ghvduwphqw
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A Simple Example

• The boy receives the girls message and
  thinking.

phhw ph dw fv ghvduwphqw
???

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A Simple Example

• If the boy happens to know Cryptography, and he
  may do following things

phhw ph dw fv ghvduwphqw
!!!!!

meet me at CS department
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A Simple Example

• Finally.

Meet at CS department
VWXSLW
????
What is VWXSLW ?

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So, What Is Cryptography

• To make thing hard to understand if you dont
  know the behind principles
• To convert intelligible information into
  unintelligible.
• To hidden information.

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Application Model of Cryptography

• B and A (lovers!) want to communicate securely
• C (intruder) may intercept, delete, add messages

A
B
data, control messages
channel
secure sender
secure receiver
data
data
C
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Who Might B, A be?

• Distributing OS authenticated principals
• Web browser/server for electronic transactions
  (e.g., on-line purchases)
• on-line banking client/server
• DNS servers
• routers exchanging routing table updates

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The Language of Cryptography

• m plaintext message
• KA(m) ciphertext, encrypted with key KA
• m KB(KA(m))

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Mapping Language Into The Example

• Encryption (decryption) algorithm substitute
  one letter for another
• Plaintext meet me at CS department
• Ciphertext phhw ph dw fv ghvduwphqw
• Key the mapping from the set of 26 letters to
  the set of 26 letters

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Private Public Key Cryptosystems

• Symmetric key cryptography
• are identical.
• The keys must be kept secret.
• The encryption and decryption functions used can
  be the same or different.
• Public key cryptography
• are different (one public, the
  other private).

K
K
A
B
K
K
A
B
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Symmetric Key Cryptography Examples

• Examples
• ROT13 Very simple rotation algorithm
• Caesar cipher Another (better) rotation
  algorithm
• crypt Original Unix encryption program
• DES Data Encryption Standard NIST 1993
• AES Advanced Encryption Standard
• Skipjack U.S. National Security Agency developed
  algorithm (classified)
• DES Data Encryption Standard
• In 1997 DES was cracked in only 140 days by a
  team
• In 1999 DES was cracked in little over 22 hours
  by a network of volunteers and special purpose
  computer.

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Symmetric Key Cryptography Key Issues

• How do sender and receiver agree on key value?
• How is the agreed upon key distributed to both
  sender and receiver in a secure fashion?

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Public Key Encryption

• Diffie-Hellman 1976 the first public key
  approach proposed.
• Sender and receiver do not share secret key
• Public key is available to every one
• Private key is known by only receiver

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Public key cryptography

Bs public key
K
B
-
Bs private key
K
B
encryption algorithm
decryption algorithm
plaintext message
plaintext message, m
ciphertext
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Public key encryption algorithms
Requirements
.
.

-

• need K ( ) and K ( ) such that

B
B

given public key K , it should be impossible to
compute private key K
B
-
B
RSA Rivest, Shamir, Adelson algorithm
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RSA Creating public/private key pair
1. Choose two large prime numbers p, q.
(e.g., 1024 bits each)
2. Compute n pq, z (p-1)(q-1)
3. Choose e (with eltn) that has no common
factors with z. (e, z are relatively prime).
4. Choose d such that ed-1 is exactly divisible
by z. (in other words ed mod z 1 ).
5. Public key is (n,e). Private key is (n,d).
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RSA Encryption, decryption
0. Given (n,e) and (n,d) as computed above
2. To decrypt received bit pattern, c, compute
Magic happens!
c
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RSA example
Bob chooses p5, q7. Then n35, z24.
e5 (so e, z relatively prime). d29 (so ed-1
exactly divisible by z).
Encrypting 8-bit messages.
e
m
m
bit pattern
encrypt
00001100
12
24832
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c
decrypt
17
12
481968572106750915091411825223071697
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Why does RSA work?

• Must show that cd mod n m where c me mod n
• Fact for any x and y xy mod n x(y mod z) mod
  n
• where n pq and z (p-1)(q-1)
• Thus, cd mod n (me mod n)d mod n
• med mod n
• m(ed mod z) mod n
• m1 mod n
• m

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Comparison of Cryptographic Systems

• With suitable keys and algorithms, both methods
  can be secure enough for most purposes.
• To use symmetric cryptography, both parties must
  know the secret key, which can be quite
  inconvenient.
• To use public key cryptography, one only needs to
  find the public key to communicate with someone
  else, which can be a lot more convenient.
• Encrypting and decrypting a lot of information
  with public key cryptography can be painfully
  slow in comparison to symmetric cryptography.

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Ongoing / Future Work --- key security

• KEY security is very important.
• Cryptography based on Image or watermarking
• Application in wireless environment.

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Quantum Cryptography

• Apply the phenomena of quantum physics
• Relies on
• The Heisenberg Uncertainty principle
• The principle of photon polarization

Mehrdad S. Sharbaf, Quantum Cryptography A New
Generation of Information Technology Sec urity
System, 2009 IEEE2. Mehrdad S. Sharbaf,
Quantum Cryptography A New Generation of
Information Technology Sec urity System, 2009
IEEE
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Quantum Cryptography (contd.)

• Why Quantum Cryptography is secure?
• when measuring the polarization of a photon, the
  choice of what direction to measure affects all
  subsequences measurements.
• photons can be easily polarized (by photon
  polarization principle)
• intruder can not copy unknown qubits (no-cloning
  theorem).
• presence of the intruder can be determined
• Harvard, and Boston University built the DARPA
  quantum network, the worlds first network that
  delivers end-to-end network security via
  highspeed quantum key distribution, and tested
  that network against sophisticated eavesdropping
  attacks.

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Cryptography Based on Watermarking

• International Journal of Computer Science and
  Security (IJCSS), Volume (1) Issue (3), 2011

Sonal Chugh Mr. Rajesh Malik, Quality
Improvement of Grey Scale and Color Images Using
Cryptography and Robust Watermarking,
International Journal of Computer Science and
Security (IJCSS), Volume (1) Issue (3), 2011
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Application in wireless environment

• User authentication is a crucial service in
  wireless sensor networks (WSNs)
• wireless sensor nodes are typically deployed
  in an unattended environment, leaving them open
  to possible hostile network attack.
• However, wireless sensor nodes are limited in
  computing power, data storage and communication
  capabilities, any user authentication protocol
  must be designed to operate efficiently in a
  resource constrained environment.

Yeh, H.-L. Chen, T.-H. Liu, P.-C. Kim, T.-H.
Wei, H.-W. A Secured Authentication Protocol for
Wireless Sensor Networks Using Elliptic Curves
Cryptography. Sensors 2011, 11, 4767-4779.
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Cryptography toolkit

• http//nsfsecurity.pr.erau.edu/crypto/generichash.
  html
• http//ats.oka.nu/titaniumcore/js/crypto/Cipher.sa
  mple.html
• http//www.privacycrypt.com/
• https//www.dlitz.net/software/pycrypto/

TRY