lecture_4

# lecture_4 - Introduction to Cryptography CS 136 Computer...

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Lecture 4 Page 1 CS 136, Winter 2010 Introduction to Cryptography CS 136 Computer Security Peter Reiher January 14, 2010

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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

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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

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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

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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

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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)

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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 Cryptanalysis The process of trying to break a cryptosystem Finding the meaning of an encrypted message without being given the key

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