Crytography

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Crytography Steganography

• Introduction to concept of cryptography
  steganography

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Standard Attack
Denial of service
Preventing legitimate use
Pretending to be someone else
Intruding the flow of a communication
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Terms

• Plain text
• Cipher text
• Key
• Cipher
• Encryption
• Decryption

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Kerckhoffs' principle

• the enemy knows the system by Claude Shannon
• The system must be practically, if not
  mathematically, indecipherable
• It must not be required to be secret, and it must
  be able to fall into the hands of the enemy
  without inconvenience
• Its key must be communicable and retainable
  without the help of written notes, and changeable
  or modifiable at the will of the correspondents
• It must be applicable to telegraphic
  correspondence
• It must be portable, and its usage and function
  must not require the concourse of several people
  
• Finally, it is necessary, given the circumstances
  that command its application, that the system be
  easy to use, requiring neither mental strain nor
  the knowledge of a long series of rules to
  observe

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Simple Cryptography

• Khoor, Zruog!!
• Elppa
• Ctaropyghpry
• 3
• Yvjrhag km ias eiwemciiyeawe nmger
• crypto

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Modern Cryptography

• Symmetric
• Block
• DES, AES, Blowfish, etc
• Stream
• A5/1, A5/2, RC4, etc
• Asymmectric
• Public key Cipher
• RSA

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Symmetric cipher

• Encryption and Decryption is done by same
  key(symmetric key)
• Fast!!
• Most case, easy to implement by hardware

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DES Digital Encryption Standard
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Example

• P Professor Hwang is very nice professor!!
• K plus
• C c4839979081f7f0f37156f1fbadedef125d53f1d607282
  5fa0febf3bb9d9dc21bd0489ee8cb02a5c
• http//cissoft.dju.ac.kr/amho/sub/training/des.htm
  l

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Vernam cipher

• Plain Text xored with same length of key
• Psudo-random number is used as encryption key
• Random seed has meaning of real key

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Vernam Cipher
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Problems?

• Key delivery
• How can we pass key to the receiver?
• Number of key
• For one receiver, sender must have different key

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Asymmetric cipher

• Encryption and Decryption use different key
• A want to send msg B, A encrypt msg B with Bs
  public key
• B decrypt cipher text Bs private key, and get a
  message

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RSA Cipher

• prepare p,q
• compute n pq
• Compute Phi(n)(Phi(pq)(p-1)(q-1))
• Find e,d s.t ed mod Phi(n) 1
• Publish e,n as public key
• encrpyt message m, c me (mod n)
• decrpyt message m cd (mod n)

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Advantage

• Secure
• Factorization of large number is very difficult
• Easy
• No key delivery problem
• Key number is O(n)

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Disadvantage

• Slow
• Finding large prime p,q is very difficult
• Using expensive operation

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Solution?

• Use a asymmetric cipher on the exchange of key
• Use a symmetric cipher on the phase of real
  communication

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Steganography?

• Sometimes, action of send a messages can be a
  hint to enemy
• Cipher text is very hard to break, but can be
  breaked
• Steganography is method of hidding message in
  plain text

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Example

• P I love you!
• p 991, q 919, e 881, d 46421
• block size 5
• c 41007 67986 69008 97985 01000
• http//cissoft.dju.ac.kr/amho/sub/training/rsa.htm
  l

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Example(1)

• News Eight Weather Tonight increasing snow.
  Unexpected precipitation smothers eastern towns.
  Be extremely cautious and use snowtires
  especially heading east. The highway is not
  knowingly slippery. Highway evacuation is
  suspected. Police report emergency situations in
  downtown ending near Tuesday.
• Newt is upset because he thinks he is President.

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Example(2)
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Example(3)
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Example(3)
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Example(3)
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Example(4)
From Wikipidia, Steganography
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Question?
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Thank you!!