lecture 5 - Crypto

lecture 5 - Crypto - Network Security CS 6823 Lecture 4...

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CS 6823 - Network Security Network Security CS 6823 – Lecture 4 Cryptography Keith O’Brien keith@keithobrien.org 1
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CS 6823 - Network Security Cryptography Overview Symmetric Key Cryptography Public Key Cryptography Message integrity and digital signatures References: - Stamp - Schneier 2
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CS 6823 - Network Security Cryptography basics Cryptography is the process of converting plaintext into ciphertext. - Plaintext – Readable text - Ciphertext – Unreadable or encrypted text Cryptography is used to hide information from unauthorized users Decryption is the process of converting ciphertext back to plaintext 3
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CS 6823 - Network Security History of Cryptography Substitution Cipher - Replaces one letter with another letter based on some key - Example: Julius Ceasar’s Cipher - Key value of 3 - ABCDEFGHIJKLMNOPQRSTUVWXYZ - DEFGHIJKLMNOPQRSTUVWXYZABC 4
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CS 6823 - Network Security History of Cryptography (cont) Cryptanalysis studies the process of breaking encryption algorithms When a new encryption algorithm is developed; cryptanalysts study it and try to break it. - This is an important part of the development cycle of a new encryption algorithm 5
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CS 6823 - Network Security World War I Zimmerman Telegram - Encrypted telegram from foreign secretary of the German empire to German ambassador in Mexico - Intercepted and decrypted by the British - Indicated that unrestricted sub warfare would commence. Proposed an alliance with Mexico to reclaim lost land to US. - Pivotal in US entering WWI 6 Cortesty: Wikipedia
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CS 6823 - Network Security World War II Enigma - Used by the Germans - Replaced letters as they were typed - Substitutions were computed using a key and a set of switches and rotors. 7
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CS 6823 - Network Security Cryptography Issues Confidentiality: only sender, intended receiver should “understand” message contents: - sender encrypts message - receiver decrypts message End-Point Authentication: send, receiver want to confirm identity of each other. Message Integrity: sender, receiver want to ensure message not altered (in transit, or afterwards) without detection. 8
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CS 6823 - Network Security Friends and enemies: Alice, Bob, Eve Well know in network security world Bob, Alice want to communicate securely Trudy(intruder) may intercept, delete, add to message 9
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CS 6823 - Network Security Who might Bob, Alice be? ...well, real-life Bobs and Alices! Web browsers/server for electronic transactions online banking client/server DNS servers routers exchanging routing table updates 10
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CS 6823 - Network Security The Language of Cryptography 11 m plaintext message K A (m) is ciphertext, encrypted with key K A m = K B (K A (m))
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CS 6823 - Network Security Simple Encryption Scheme Substitution Cipher: substituting one thing for another - Mono-alphabetic cipher: substitute one letter for another 12 Key: The mapping from the set of 26 letters to the set of 26 letters
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This note was uploaded on 11/02/2010 for the course CS 393 taught by Professor Staff during the Spring '08 term at NYU Poly.

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lecture 5 - Crypto - Network Security CS 6823 Lecture 4...

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