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Unformatted text preview: Lecture 5 Page 1 CS 136, Winter 2010 More on Cryptography CS 136 Computer Security Peter Reiher January 19, 2010 Lecture 5 Page 2 CS 136, Winter 2010 Outline Desirable characteristics of ciphers Stream and block ciphers Cryptographic modes Uses of cryptography Symmetric and asymmetric cryptography Digital signatures Lecture 5 Page 3 CS 136, Winter 2010 Desirable Characteristics of Ciphers Well matched to requirements of application Amount of secrecy required should match labor to achieve it Freedom from complexity The more complex algorithms or key choices are, the worse Lecture 5 Page 4 CS 136, Winter 2010 More Characteristics Simplicity of implementation Seemingly more important for hand ciphering But relates to probability of errors in computer implementations Errors should not propagate Lecture 5 Page 5 CS 136, Winter 2010 Yet More Characteristics Ciphertext size should be same as plaintext size Encryption should maximize confusion Relation between plaintext and ciphertext should be complex Encryption should maximize diffusion Plaintext information should be distributed throughout ciphertext Lecture 5 Page 6 CS 136, Winter 2010 Stream and Block Ciphers Stream ciphers convert one symbol of plaintext immediately into one symbol of ciphertext Block ciphers work on a given sized chunk of data at a time Lecture 5 Page 7 CS 136, Winter 2010 Stream Ciphers Plaintext Ciphertext Key Encryption fsnarT fsnar T S S fsna r q qS fsn a z zqS fs n m mzqS f s r rmzqS f e ermzqS Lecture 5 Page 8 CS 136, Winter 2010 Advantages of Stream Ciphers + Speed of encryption and decryption Each symbol encrypted as soon as its available + Low error propagation Errors affect only the symbol where the error occurred Lecture 5 Page 9 CS 136, Winter 2010 Disadvantages of Stream Ciphers Low diffusion Each symbol separately encrypted Each ciphertext symbol only contains information about one plaintext symbol Susceptible to insertions and modifications Not good match for many common uses of cryptography Some disadvantages can be mitigated by use of proper cryptographic mode Lecture 5 Page 10 CS 136, Winter 2010 Block Ciphers Plaintext Ciphertext Key Encryption T r a n s f e r $ 1 0 T s r f $ a e 1 n r 0 T r a n s f e r $ 1 0 T s r f $ a e 1 n r 0 Lecture 5 Page 11 CS 136, Winter 2010 Advantages of Block Ciphers + Good diffusion Easier to make a set of encrypted characters depend on each other + Immunity to insertions Encrypted text arrives in known lengths Most common Internet crypto done with block cyphers Lecture 5 Page 12 CS 136, Winter 2010 Disadvantages of Block Ciphers Slower Need to wait for block of data before encryption/decryption starts Worse error propagation Errors affect entire blocks Lecture 5 Page 13 CS 136, Winter 2010 Cryptographic Modes Lets say you have a bunch of data to encrypt Using the same cipher and key...
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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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