CS283 Lecture 1 - Part 2 - Classical Ciphers - 20090901

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

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

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Classical Ciphers •Terminology •Monoalphabetic ciphers (Shift, Affine) •Vigenere Cipher •Permutation Cipher; •Substitution Cipher and one-time pad
From Schneier 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. ) = C e (P) = C d (C) = P d ( e (P)) = P d ° e = I e one-to-one (mathematically) GWU CS 172/283 - Autumn 2009 Holmblad - Lecture 01 - Rev 20090901 3

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From Schneier: Some terminology - contd. Cipher : is the cryptographical algorithm /mathematical function used to encrypt •A restricted cipher is one whose security depends on eeping the algorithm secret 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 proves the security of a cryptosystem. GWU CS 172/283 - Autumn 2009 Holmblad - Lecture 01 - Rev 20090901 4 improves the security of a cryptosystem.
From Schneier: 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 GWU CS 172/283 - Autumn 2009 Holmblad - Lecture 01 - Rev 20090901 5

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From Stinson Formal definition: cryptosystem A cryptosystem consists of: P set of all plaintexts et of all ciphertexts C set of all ciphertexts K set of all keys E set of encryption rules, e K : P C et of decryption rules d D set of decryption rules d K : C P d K e K (x) = x nd vertible functions and inverses of each other GWU CS 172/283 - Autumn 2009 Holmblad - Lecture 01 - Rev 20090901 6 d K and e K invertible functions and inverses of each other
Typical Scenario • Alice and Bob randomly choose a key, K K when they are unobserved or communicating on a secure hannel 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 here y = e Where y i e K (x i ) x i is a symbol from the alphabet GWU CS 172/283 - Autumn 2009 Holmblad - Lecture 01 - Rev 20090901 7

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hift cipher on the English alphabet Shift cipher on the English alphabet A Classical Substitution Cipher ymbols (aka alphabet) 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 ncryption Key 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 8
hift cipher on English alphabet Shift cipher on English alphabet Classical Substitution Cipher (Cont’d) ncryption example (k = 10) 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 d e f g h pq y g j GWU CS 172/283 - Autumn 2009 Holmblad - Lecture 01 - Rev 20090901 9

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

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