CRYPTOGRAPHY

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CRYPTOGRAPHY

• Gayathri V.R. Kunapuli

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OUTLINE

• History of Cryptography
• Need for cryptography
• Private Key Cryptosystems
• Public Key Cryptosystems
• Comparison between Public and Private Key
  Cryptosystems
• PEM
• Future Work

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History of Cryptography2

• Ceaser Ciphers
• Transposition Cipher
• Substitution Cipher
• Vigenere Cipher
• Enigma Machine

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Need for Cryptography1

• Authentication of the Communicating Principals
• Authenticated message carries a Digital Signature

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Private Key Cryptosystems1,2

• Also called Symmetric Cryptography
• Encryption algorithm E turns plain text message M
  into a cipher text C
• CE(M)
• Decrypt C by using decryption algorithm D which
  is an inverse function of E
• MD(C)

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Private Key Cryptosystems cont1,2

• Algorithm decomposed into Function(public) and
  Key(secret)
• Encrypted using the key Ke and decrypted using
  the key Kd
• MDKd(EKe(M))
• A function and a variable number of keys
  constitute a class of algorithms indexed by the
  keys.

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Cont

• The encryption function is
• -One-to-one injective mapping
• -One way function
• The secrecy rests on the keys rather than on
  algorithms.
• The key should be of sufficient length in bits.

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DES(Jan,1977)1,2

• Encryption consists of 3 stages of Transposition
  and 16 stages of Substitution of bits.
• Easy to implement on VLSI
• The 56-bit length key was found insufficient and
  easy to break
• Repetitions in cipher text give clues to
  eavesdroppers
• Spurious data can be injected

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Contd

• Private Key systems require n(n-1)/2 keys for
  n principals in a system
• The conversation key must be agreed upon
  beforehand
• Management of the keys is a function of the Key
  Distribution Server(KDS)

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Public Key Cryptographic Systems(Need)1

• Also called as the Asymmetric Cryptography
• To avoid the need to transmit secret keys and
• To reduce the key requirement to 2n, the public
  key systems are used

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Public Key Cryptosystems Cont

• Introduced by Diffie and Hellman
• Each principal keeps a set of encryption keys (Ke
  Kd)
• Encryption algorithm E is public and so is the
  key Ke
• Decryption algorithm D and decryption key Kd is
  kept private
• Data sent to a principal is encrypted using that
  corresponding Ke
• E and D can be made public if Ke and Kd are
  chosen such that it is impossible to infer Kd
  from Ke.

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RSA(Aug,1977)1,2,5

• The algorithms E and D are inverses
• Plain text messages are limited to a size is
  limited to k
• Integer k is chosen such that 2k lt N
• N p q where p q are LARGE prime numbers
• Kp (public encyrption key) and Ks (private
  decryption key) are derived from p q

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Contd

• The robustness of RSA algorithm relies on the
  computational complexity in factoring a large
  number upon which the keys are based.
• The authenticity of the sender can also be
  verified.

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Comparison between the cryptosystems1

• Private Key DES is computationally efficient
• Public Key RSA is computationally expensive
• Possible best use is RSA for short/important data
  and DES for long or less critical

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PEM1,5

• Provides mechanism for the mail users to specify
  the cryptographic algorithm and parameters to be
  used for mail messages.
• Essential data fields in PEM are
• DEK
• IK
• MIC

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Extended Works4

• To prevent the Denial-of-Decryption
• To reduce the time taken for the authentication
  of the digital signatures
• Self Generated Certificate Public Key Cryptography

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References

• 1. Chow, Randy Johnson, Theodore Distributed
  Operating Systems Algorithms, 1998
• 2. Aiden A.Bruen,Mario A.Forcinito
  Cryptography, Information theory and
  Error-correction,2005
• 3.www.wikipedia.org/history of cryptography
• 4. Self generated certificate public key
  cryptography and certificateless
  signature/Encryption scheme in the standard model
• ASIACCS07, March 20-22, 2007, Singapore.
• 5.http//www.cybercrimes.net/Cryptography/Articles
  /Hebert.html (April 2007)