Date post: | 31-Dec-2015 |
Category: |
Documents |
Upload: | gabriel-quinn |
View: | 215 times |
Download: | 0 times |
Title: Cryptography
Instructor: Dr. Yanqing ZhangPresented by: Jiangling, Yin
Department of Computer Science
Georgia State University
CSC 8320 Advanced Operating Systems
Outline Introduction & Motivation
What is cryptography and why it is necessary?
Modern cryptography1. Private Key Cryptosystem2. Public Key Cryptosystem3. Comparison of Cryptographic Systems
Future work
A Simple Example
Suppose two lovers try to meet at a certain place. And the girl sends the information to the boy:
meet me at ###
A Simple Example
Instead of sending the intelligible message to the boy, the girl plays a trick and change the information.
phhw ph dw fv ghvduwphqw
meet me at ###
A Simple Example
If the boy happens to know Cryptography, and he may do following things…
phhw ph dw fv ghvduwphqw
meet me at CS department
!!!!!
So, What Is Cryptography
To make thing hard to understand if you don’t know the behind principles…
To convert intelligible information into unintelligible.
To hidden information.
9
Application Model of Cryptography
B and A (lovers!) want to communicate “securely”
C (intruder) may intercept, delete, add messages
securesender
securereceiver
channel data, control messages
data data
A B
C
10
Who Might B, A be?
Distributing OS authenticated principals
Web browser/server for electronic transactions (e.g., on-line purchases)
on-line banking client/server
DNS servers
routers exchanging routing table updates
11
The Language of Cryptography
m plaintext messageKA(m) ciphertext, encrypted with key KA
m = KB(KA(m))
plaintext plaintextciphertext
KA
encryptionalgorithm
decryption algorithm
A’s encryptionkey
B’s decryptionkey
KB
12
Mapping Language Into The Example
Encryption (decryption) algorithm : substitute one letter for another
Plaintext: meet me at CS department Ciphertext: phhw ph dw fv ghvduwphqw
Key: the mapping from the set of 26 letters to the set of 26 letters
Private & Public Key Cryptosystems
Symmetric key cryptography: && are identical.The keys must be kept secret. The encryption and decryption functions used can be the
same or different.Public key cryptography:
&& are different (one public, the other private).
plaintext plaintextciphertext
KA
encryptionalgorithm
decryption algorithm
A’s encryptionkey
B’s decryptionkey
KB
AK KB
AK KB
Symmetric Key Cryptography: Examples
Examples:ROT13: Very simple rotation algorithmCaesar cipher: Another (better) rotation algorithmcrypt: Original Unix encryption programDES: Data Encryption Standard [NIST 1993]AES: Advanced Encryption Standard Skipjack: U.S. National Security Agency developed
algorithm (classified)
DES: Data Encryption StandardIn 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: 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?
plaintextciphertext
KA-B
encryptionalgorithm
decryption algorithm
KA-B
plaintextmessage, m
K (m)A-B
K (m)A-Bm = 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 onePrivate key is known by only receiver
17
Public key cryptography
plaintextmessage, m
ciphertextencryptionalgorithm
decryption algorithm
B’s public key
plaintextmessage
K (m)B+
K B+
B’s privatekey
K B-
m = K (K (m))B+
B-
18
Public key encryption algorithms
need K ( ) and K ( ) such thatB B. .
given public key K , it should be impossible to compute private key K B
B
Requirements:
1
2
RSA: Rivest, Shamir, Adelson algorithm
+ -
K (K (m)) = m BB
- +
+
-
19
RSA: Creating public/private key pair
1. Choose two large prime numbers p, q. (e.g., 1024 bits each)
2. Compute n = pq, z = (p-1)(q-1)
3. Choose e (with e<n) that has no common factors with z. (e, z are “relatively prime”).
4. Choose d such that ed-1 is exactly divisible by z. (in other words: ed mod z = 1 ).
5. Public key is (n,e). Private key is (n,d).
K B+ K B
-
20
RSA: Encryption, decryption
0. Given (n,e) and (n,d) as computed above
1. To encrypt message m (<n), compute
c = m mod n
e
2. To decrypt received bit pattern, c, compute
m = c mod n
d
m = (m mod n)
e mod n
dMagichappens!
c
21
RSA example:
Bob chooses p=5, q=7. Then n=35, z=24.e=5 (so e, z relatively prime).d=29 (so ed-1 exactly divisible by z).
bit pattern m me c = m mod ne
00001100 12 24832 17
c m = c mod nd
17 481968572106750915091411825223071697 12
cd
encrypt:
decrypt:
Encrypting 8-bit messages.
22
Why does RSA work?
Must show that cd mod n = m where c = me mod n
Fact: for any x and y: xy mod n = x(y mod z) mod n where n= pq and z = (p-1)(q-1)
Thus, cd mod n = (me mod n)d mod n
= med mod n = m(ed mod z) mod n = m1 mod n = m
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.
1. KEY security is very important.2. Cryptography based on Image or
watermarking 3. Application in wireless environment.
Ongoing / Future Work --- key security
Quantum Cryptography
Apply the phenomena of quantum physics
Relies onThe Heisenberg Uncertainty principle The principle of photon polarization
Mehrdad S. Sharbaf,” Quantum Cryptography: A New Generation of Information Technology Sec urity System”, 2009 IEEE[2]. Mehrdad S. Sharbaf,” Quantum Cryptography: A New Generation of Information Technology Sec urity System”, 2009 IEEE
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.
Cryptography Based on Watermarking
International Journal of Computer Science and Security (IJCSS), Volume (1) : Issue (3), 2011
Sonal Chugh & Mr. Rajesh Malik, Quality Improvement of Grey Scale and Color Images Using Cryptography and Robust Watermarking, International Journal of Computer Science and Security (IJCSS), Volume (1) : Issue (3), 2011
Application in wireless environmentUser authentication is a crucial service in
wireless sensor networks (WSNs) wireless sensor nodes are typically deployed
in an unattended environment, leaving them open to possible hostile network attack.
However, wireless sensor nodes are limited in computing power, data storage and communication capabilities, any user authentication protocol must be designed to operate efficiently in a resource constrained environment.Yeh, H.-L.; Chen, T.-H.; Liu, P.-C.; Kim, T.-H.; Wei, H.-W. A Secured
Authentication Protocol for Wireless Sensor Networks Using Elliptic Curves Cryptography. Sensors 2011, 11, 4767-4779.
Cryptography toolkithttp://nsfsecurity.pr.erau.edu/crypto/generich
ash.htmlhttp://ats.oka.nu/titaniumcore/js/crypto/Ciphe
r.sample.htmlhttp://www.privacycrypt.com/https://www.dlitz.net/software/pycrypto/
TRY…