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Unformatted text preview: Lecture 4 Page 1 CS 136, Winter 2010 Introduction to Cryptography CS 136 Computer Security Peter Reiher January 14, 2010 Lecture 4 Page 2 CS 136, Winter 2010 Outline • What is data encryption? • Cryptanalysis • Basic encryption methods – Substitution ciphers – Permutation ciphers Lecture 4 Page 3 CS 136, Winter 2010 Introduction to Encryption • Much of computer security is about keeping secrets • One method is to make the secret hard for others to read • While (usually) making it simple for authorized parties to read Lecture 4 Page 4 CS 136, Winter 2010 Encryption • Encryption is the process of hiding information in plain sight • Transform the secret data into something else • Even if the attacker can see the transformed data, he can’t understand the underlying secret Lecture 4 Page 5 CS 136, Winter 2010 Encryption and Data Transformations • Encryption is all about transforming the data • One bit or byte pattern is transformed to another bit or byte pattern • Usually in a reversible way Lecture 4 Page 6 CS 136, Winter 2010 Encryption Terminology • Encryption is typically described in terms of sending a message – Though it’s used for many other purposes • The sender is S • The receiver is R • The transmission medium is T • And the attacker is O Lecture 4 Page 7 CS 136, Winter 2010 More Terminology • Encryption is the process of making message unreadable/unalterable by O • Decryption is the process of making the encrypted message readable by R • A system performing these transformations is a cryptosystem – Rules for transformation sometimes called a cipher Lecture 4 Page 8 CS 136, Winter 2010 Plaintext and Ciphertext • Plaintext is the original form of the message (often referred to as P ) Transfer $100 to my savings account • Ciphertext is the encrypted form of the message (often referred to as C ) Sqzmredq #099 sn lx rzuhmfr zbbntms Lecture 4 Page 9 CS 136, Winter 2010 Very Basics of Encryption Algorithms • Most algorithms use a key to perform encryption and decryption – Referred to as K • The key is a secret • Without the key, decryption is hard • With the key, decryption is easy Lecture 4 Page 10 CS 136, Winter 2010 Terminology for Encryption Algorithms • The encryption algorithm is referred to as E() • C = E(K,P) • The decryption algorithm is referred to as D() – Sometimes the same algorithm as E() • The decryption algorithm also has a key Lecture 4 Page 11 CS 136, Winter 2010 Symmetric and Asymmetric Encryption Systems • Symmetric systems use the same keys for E and D : P = D(K, C) Expanding, P = D(K, E(K,P)) • Asymmetric systems use different keys for E and D: C = E(K E ,P) P = D(K D ,C) Lecture 4 Page 12 CS 136, Winter 2010 Characteristics of Keyed Encryption Systems • If you change only the key, a given plaintext encrypts to a different ciphertext – Same applies to decryption • Decryption should be hard without knowing the key Lecture 4 Page 13 CS 136, Winter 2010...
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This note was uploaded on 02/08/2010 for the course ENGR 111 taught by Professor King during the Spring '09 term at UCLA.
 Spring '09
 KING
 Computer Security

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