Cryptography Instructor : Dr. Yanqing Zhang Presented by : Rajapaksage Jayampthi S

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 )

Section I Theory

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.

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 ). plaintext ciphertext K A encryption algorithm decryption algorithm Alice’s encryption key Bob’s decryption key K B

Symmetric Key Cryptography Cryptographic algorithms involve substituting one thing for another, in many possible ways.  Caesar cipher : Substitution with an offset of β for all letters Eg if β = 4 then a -> d b -> 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

Symmetric Key Cryptography : Examples Examples :  ROT 13: 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.

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.

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 ? plaintext ciphertext K A-B encryption algorithm decryption algorithm K A-B plaintext message, m K (m) A-B K (m) A-B m = K ( ) A-B

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

Public Key Encryption ( contd.) plaintext message, m ciphertext encryption algorithm decryption algorithm Bob’s public key plaintext message K (m) B + K B + Bob’s private key K B - m = K ( K (m) ) B + B -

Public Key Encryption ( contd.) plaintext message, m ciphertext encryption algorithm decryption algorithm Alice’s private key plaintext message K (m) A - K A - Alice’s public key K A + m = K ( K (m) ) A - A +

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. K ( K (m) ) = m B B - + K ( K (m) ) A A + - =

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 ).

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.

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.

Section II

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

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

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 world ’ s first network that delivers end - to - end network security via highspeed quantum key distribution, and tested that network against sophisticated eavesdropping attacks.

Section III Future Work

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 )

Referances [1]. http :// en. wikipedia. org / wiki / Quantum _ Crypto graphy http :// en. wikipedia. org / wiki / Quantum _ Crypto graphy [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 _ Computation %3 F [5]. http :// www. quantiki. org / wiki / index. php / Shor % 27 s _ factoring _ algorithm