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Cryptography

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Cryptography Instructor: Dr. Yanqing Zhang Presented by: Rajapaksage Jayampthi S Outline Section I (Theory) Introduction Symmetric Key Cryptography Examples Key ... – PowerPoint PPT presentation

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Title: Cryptography


1
Cryptography
  • Instructor Dr. Yanqing Zhang
  • Presented by Rajapaksage Jayampthi S

2
Outline
  • Section I (Theory)
  • Introduction
  • Symmetric Key Cryptography
  • Examples
  • Key Issues
  • Public Key Encryption
  • Algorithms
  • Comparison of Cryptographic systems
  • Hybrid Secret-Public Key Cryptography
  • Section II (Recent Work)
  • Quantum Cryptography A New Generation of
    Information Technology Security System Mehrdad
    S. Sharbaf, 2009
  • Section III (Future Work)

3
Section I
  • Theory

4
Introduction
  • Intruders can get the encrypted data, but can not
    do anything with it.
  • Encryption techniques are published, standardized
    and available to every one.
  • Must be some bit of secret information (key)
    that prevent an intruder from decrypting .

5
Introduction (contd.)
  • Symmetric key cryptography
  • encryption and decryption keys are identical.
  • the key must be kept secret.
  • The encryption and decryption functions used can
    be the same or different.
  • Public key cryptography
  • different keys for encryption and decryption (one
    public, the other private).

6
Symmetric Key Cryptography
  • Cryptographic algorithms involve substituting one
    thing for another, in many possible ways.
  • Caesar cipher
  • Substitution with an offset of ß for all letters
  • Eg if ß 4 then a-gt d b-gt e
  • there are only 25 possible keys available.
  • Easy to break.
  • monoalphabetic cipher substitute one letter for
    another now there are 26! possibilities.
  • polyalphabetic cipher

plaintext abcdefghijklmnopqrstuvwxyz
ciphertext mnbvcxzasdfghjklpoiuytrewq
Plaintext bob. i love you. alice
ciphertext nkn. s gktc wky. mgsbc
7
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.

8
Symmetric Key Cryptography (contd.)
  • How to break simple encryption scheme
  • Brute force attempt all possibilities
  • Simple with the Caesar cipher, but gets quite
    difficult with monoalphabetic or polyalphabetic
    ciphers.
  • Ciphertext-only attack use statistics and other
    information to decrypt intercepted ciphertext
  • Known-plaintext attack if some of the plaintext
    is known, one could uncover some of the
    plaintext-ciphertext mappings, making decryption
    easier.
  • Chosen-plaintext attack the intruder can choose
    the plaintext message and receive the ciphertext
    form.
  • Can break the encryption scheme.

9
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?

10
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

11
Public Key Encryption (contd.)
12
Public Key Encryption (contd.)
13
Public Key Encryption (contd.)
  • Result is the same
  • if one key can decrypt a message, it must have
    been encrypted by the other.
  • It must be extremely difficult, if not
    impossible, to deduce the private key when given
    a public key.

14
Public Key Encryption Algorithms
  • Diffie-Hellman the first public key approach
    proposed.
  • RSA the best known public key system, developed
    by Rivest, Shamir, and Adleman (hence RSA).
  • DSA Digital Signature Algorithm, developed by
    the U.S. National Security Agency (NSA).

15
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.

16
Hybrid Secret-Public Key Cryptography
  • combine the strengths of symmetric and public key
    cryptography, and avoid their weaknesses.
  • When two parties want to communicate securely,
    public key cryptography is used to exchange a
    random symmetric session key.
  • Since the session key is encrypted, we can ensure
    secrecy and mutual authentication.
  • Since secret key cryptography is used, this can
    be done relatively efficiently.
  • When done, both parties destroy the session key.
    If communication is required in the future, this
    process is repeated from the beginning to obtain
    a completely new session key.

17
Section II
18
Introduction
  • Apply the phenomena of quantum physics
  • Relies on
  • The Heisenberg Uncertainty principle
  • The principle of photon polarization
  • classical cryptography
  • communicating parties need to share the keys
  • protocols based on mathematical algorithms
    introduce security holes
  • rarely on refresh their cryptography keys
  • unproven computational assumptions
  • Not efficient
  • Can break

19
Quantum Cryptography
  • What are qubits?
  • both in state 0 and state 1 can exists
  • In classical register composed of three bits can
    store in a given moment of time only one out of
    eight different numbers
  • register composed of three qubits can store in a
    given moment of time all eight numbers in a
    quantum superposition

20
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.

21
Section III
  • Future Work

22
Future Direction of Quantum Cryptography
  • Distance limitation
  • quantum key distribution distances are limited to
    tens of kilometers because of optical
    amplification destroys the qubit state.
  • Develop optical devices capable of generating,
    detecting and guiding single photons.
  • Lack of a security certification process or
    standard for the equipment.
  • Reassurance QKD is theoretically sound. (By
    experiments)

23
Referances
  • 1.http//en.wikipedia.org/wiki/Quantum_Cryptogra
    phy
  • 2. Mehrdad S. Sharbaf, Quantum Cryptography A
    New Generation of Information Technology Sec
    urity System, 2009 IEEE
  • 3. Computer Networking A Top-Down Approach
    Featuring the Internet James F. Kurose and Keith
    W. Ross
  • 4.http//www.quantiki.org/wiki/index.php/What_is
    _Quantum_Computation3F
  • 5.http//www.quantiki.org/wiki/index.php/Shor27
    s_factoring_algorithm
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