Classical-Cryptography - Classical Cryptography...

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Introduction to Cryptography cryptography is the study of secret (crypto-) writing (-graphy) concerned with developing algorithms which may be used to: conceal the context of some message from all except the sender and recipient (privacy or secrecy), and/or verify the correctness of a message to the recipient ( authentication ) form the basis of many technological solutions to computer and communications security problems for a good overview paper see: W Diffie, M E Hellman, "Privacy and Authentication: An Introduction to Cryptography", in Proc. IEEE, Vol 67(3) Mar 1979, pp 397-427 Basic Concepts cryptography the art or science encompassing the principles and methods of transforming an intelligible message into one that is unintelligible, and then retransforming that message back to its original form plaintext the original intelligible message ciphertext the transformed message cipher an algorithm for transforming an intelligible message into one that is unintelligible by transposition and/or substitution methods key 9/1/2010 Classical Cryptography williamstallings.com/…/classical.html 1/20
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encipher (encode) the process of converting plaintext to ciphertext using a cipher and a key decipher (decode) the process of converting ciphertext back into plaintext using a cipher and a key cryptanalysis the study of principles and methods of transforming an unintelligible message back into an intelligible message without knowledge of the key. Also called codebreaking cryptology both cryptography and cryptanalysis code an algorithm for transforming an intelligible message into an unintelligible one using a code-book Concepts Encryption C = E_(K)(P) Decryption P = E_(K)^(-1)(C) E _(K) is chosen from a family of transformations known as a cryptographic system. The parameter that selects the individual transformation is called the key K, selected from a keyspace K More formally a cryptographic system is a single parameter family of invertible transformations E_(K) ; K in K : P -> C with the inverse algorithm E_(K)^(-1) ; K in K : C -> P such that the inverse is unique Usually assume the cryptographic system is public, and only the key is secret information Private-Key Encryption Algorithms a private-key (or secret-key, or single-key) encryption algorithm is one where the sender and the recipient share a common, or closely related, key all traditional encryption algorithms are private-key overview of a private-key encryption system and attacker 9/1/2010 Classical Cryptography williamstallings.com/…/classical.html 2/20
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Cryptanalytic Attacks have several different types of attacks ciphertext only only have access to some enciphered messages use statistical attacks only known plaintext know (or strongly suspect) some plaintext-ciphertext pairs use this knowledge in attacking cipher chosen plaintext can select plaintext and obtain corresponding ciphertext use knowledge of algorithm structure in attack
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This note was uploaded on 02/05/2011 for the course CS 2105 taught by Professor Ana during the Fall '09 term at National University of Singapore.

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Classical-Cryptography - Classical Cryptography...

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