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# 09 - Chapter 9 Basic Cryptography Classical Cryptography...

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June 1, 2004 Computer Security: Art and Science ©2002-2004 Matt Bishop Slide #9-1 Chapter 9: Basic Cryptography Classical Cryptography Public Key Cryptography Cryptographic Checksums

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June 1, 2004 Computer Security: Art and Science ©2002-2004 Matt Bishop Slide #9-2 Overview Classical Cryptography Caesar cipher Vigènere cipher DES Public Key Cryptography Diffie-Hellman RSA Cryptographic Checksums HMAC
June 1, 2004 Computer Security: Art and Science ©2002-2004 Matt Bishop Slide #9-3 Cryptosystem Quintuple ( E , D , M , K , C ) M set of plaintexts K set of keys C set of ciphertexts E set of encryption functions e : M × K C D set of decryption functions d : C × K M

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June 1, 2004 Computer Security: Art and Science ©2002-2004 Matt Bishop Slide #9-4 Example Example: Caesar cipher M = { sequences of letters } K = { i | i is an integer and 0 i 25 } E = { E k | k K and for all letters m , E k ( m ) = ( m + k ) mod 26 } D = { D k | k K and for all letters c , D k ( c ) = (26 + c k ) mod 26 } C = M
June 1, 2004 Computer Security: Art and Science ©2002-2004 Matt Bishop Slide #9-5 Attacks Opponent whose goal is to break cryptosystem is the adversary Assume adversary knows algorithm used, but not key Three types of attacks: ciphertext only : adversary has only ciphertext; goal is to find plaintext, possibly key known plaintext : adversary has ciphertext, corresponding plaintext; goal is to find key chosen plaintext : adversary may supply plaintexts and obtain corresponding ciphertext; goal is to find key

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June 1, 2004 Computer Security: Art and Science ©2002-2004 Matt Bishop Slide #9-6 Basis for Attacks Mathematical attacks Based on analysis of underlying mathematics Statistical attacks Make assumptions about the distribution of letters, pairs of letters (digrams), triplets of letters (trigrams), etc. Called models of the language Examine ciphertext, correlate properties with the assumptions.
June 1, 2004 Computer Security: Art and Science ©2002-2004 Matt Bishop Slide #9-7 Classical Cryptography Sender, receiver share common key Keys may be the same, or trivial to derive from one another Sometimes called symmetric cryptography Two basic types Transposition ciphers Substitution ciphers Combinations are called product ciphers

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June 1, 2004 Computer Security: Art and Science ©2002-2004 Matt Bishop Slide #9-8 Transposition Cipher Rearrange letters in plaintext to produce ciphertext Example (Rail-Fence Cipher) Plaintext is HELLO WORLD Rearrange as HLOOL ELWRD Ciphertext is HLOOL ELWRD
June 1, 2004 Computer Security: Art and Science ©2002-2004 Matt Bishop Slide #9-9 Attacking the Cipher Anagramming If 1-gram frequencies match English frequencies, but other n -gram frequencies do not, probably transposition Rearrange letters to form n -grams with highest frequencies

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09 - Chapter 9 Basic Cryptography Classical Cryptography...

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