Title: Cryptography
1Cryptography
- Instructor Dr. Yanqing Zhang
- Presented by Rajapaksage Jayampthi S
2Outline
- 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)
3Section I
4Introduction
- 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 .
5Introduction (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).
6Symmetric 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
7Symmetric 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.
8Symmetric 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.
9Symmetric 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?
10Public 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
11Public Key Encryption (contd.)
12Public Key Encryption (contd.)
13Public 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.
14Public 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).
15Comparison 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.
16Hybrid 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.
17Section II
18Introduction
- 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
19Quantum 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
20Quantum 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.
21Section III
22Future 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)
23Referances
- 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