CS283 Lecture 1 - Part 2 - Classical Ciphers - 20090901

# CS283 Lecture 1 - Part 2 - Classical Ciphers - 20090901 -...

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CSCI 172/283 Fall 2009 GWU Lecture 1 – Part 2 Classical Ciphers

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Classical Ciphers Terminology Monoalphabetic ciphers (Shift, Affine) Vigenere Cipher Permutation Cipher; Substitution Cipher and one-time pad
GWU CS 172/283 - Autumn 2009 Holmblad - Lecture 01 - Rev 20090901 3 Some terminology A sender encrypts a plaintext message to get ciphertext which is sent to the receiver who decrypts it to obtain the plaintext. For the application of secret communication between two parties, it should not be possible for an eavesdropper to decrypt the message. i.e d should be easy for the (legitimate) receiver, not for anyone else. e (P) = C d (C) = P d ( e (P)) = P d ° e = I e one-to-one (mathematically) From Schneier

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GWU CS 172/283 - Autumn 2009 Holmblad - Lecture 01 - Rev 20090901 4 Some terminology - contd. Cipher : is the cryptographical algorithm /mathematical function used to encrypt A restricted cipher is one whose security depends on keeping the algorithm secret. Security through obscurity is generally considered inadequate because doing so does not provide a systematic way of simulated attack/vulnerability analysis by external experts - which typically improves the security of a cryptosystem. From Schneier:
GWU CS 172/283 - Autumn 2009 Holmblad - Lecture 01 - Rev 20090901 5 Some terminology - contd. A key is used as a parameter in some ciphers. The security of ciphers that use keys is based on keeping the key(s), and not the cipher itself, secret. e K1 (P) = C; d K2 (C) = P Keyspace : set of all possible keys. Cryptosystem : algorithm + all ciphertexts + all plaintexts + all keys From Schneier:

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GWU CS 172/283 - Autumn 2009 Holmblad - Lecture 01 - Rev 20090901 6 Formal definition: cryptosystem A cryptosystem consists of: P set of all plaintexts C set of all ciphertexts K set of all keys E set of encryption rules, e K : P C D set of decryption rules d K : C P d K e K (x) = x d K and e K invertible functions and inverses of each other From Stinson
GWU CS 172/283 - Autumn 2009 Holmblad - Lecture 01 - Rev 20090901 7 Typical Scenario Alice and Bob randomly choose a key, K when they are unobserved or communicating on a secure channel If Alice wants to send Bob a message, x 1 x 2 x 3 x 4 …x n Alice sends: y 1 y 2 y 3 y 4 …y n Where y i = e K (x i ) x i is a symbol from the alphabet

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GWU CS 172/283 - Autumn 2009 Holmblad - Lecture 01 - Rev 20090901 8 Shift cipher on the English alphabet A Classical Substitution Cipher Symbols (aka alphabet) A B C D E F G H I J K L M N 0 1 2 3 4 5 6 7 8 9 10 11 12 13 O P Q R S T U V W X Y Z 14 15 16 17 18 19 20 21 22 23 24 25 Encryption Key Key = k (add 10, so A goes to 10, i.e. k)
GWU CS 172/283 - Autumn 2009 Holmblad - Lecture 01 - Rev 20090901 9 Shift cipher on English alphabet Classical Substitution Cipher (Cont’d) Encryption example (k = 10) Plaintext A B C D E F G H I J K L M N O P Q R S T U V W X Y Z Ciphertext K l m n o p q r s t u v w x y z a b c d e f g h i j

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CS283 Lecture 1 - Part 2 - Classical Ciphers - 20090901 -...

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