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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 it’s 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 • Let’s say you have a bunch of data to encrypt – Using the same cipher and key...
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 Spring '09
 KING
 Cryptography, Computer Security, Publickey cryptography, public key cryptography

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