Comp & Network Security
Comp & Network Security CS 4237
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Mrs. Gerson Lind
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This 0 page Class Notes was uploaded by Alayna Veum on Monday November 2, 2015. The Class Notes belongs to CS 4237 at Georgia Institute of Technology - Main Campus taught by Staff in Fall. Since its upload, it has received 12 views. For similar materials see /class/234115/cs-4237-georgia-institute-of-technology-main-campus in ComputerScienence at Georgia Institute of Technology - Main Campus.
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Date Created: 11/02/15
HHULCUJDQQ DQC QQQQ Introduction to Cryptography CS 4237 Definitions l Process data into unintelligible form reversible without data loss I Usually onetoone not compression l Other services 0 Integrity checking no tampering 0 Authentication not an imposter l Plaintext encryptione ciphertext decryption plaintext 9Q a Computatlonal Difficulty m E l Algorithm needs to be efficient 9 Otherwise only short keys can be used l Most schemes can be broken depends on T Q EG Try all possible keys 739 l Longer key is often more secure b O Encryption ONl 1 O Bruteforce cryptanalysis 02N twice as hard with each additional bit l Cryptanalysis tools i1 9 Specialpurpose hardware 9 Parallel machines 9 Internet coarsegrain parallelism a a n l Secret Key vs Secret Algorithm gs Eu 9 l Secret algonthm add1t10nal hurdle l Hard to keep secret if used widely r5 0 Reverse engineering social engineering l Commercial published g 0 Wide reView trust l Military avoid giving enemy good ideas E EU m 9Q Some Trivial Schemes Q U m g l Caesar c1pher substitution c1pher eruBeE l Captain Midnight Secret Decoder rings 7 9 shift variable by n IBM a HAL or 77 0 letter offset mod 26 gt 9 only 26 possible ways of secret coding 39 7 l Monoalphabetic Cipher O generalization arbitrary mapping of one letter to another a O 26 approximately 4 X 1026 O statistical analysis of letter frequencies l Onetime pad if 9 A random sequence ofO s and 1 s XORed to plaintext a U m Cryptanaly s1s Breaking an El E Encryption Scheme E El g l Ciphertext only 39 O Exhaustive search until recognizable plaintext j 9 Need enough ciphertext r 739 l Known plaintext 9 Secret may be revealed by spy time thus ltciphe1text g plaintextgt pair is obtained 9 Great for monoalphabetic ciphers l Chosen plaintext a 9 Choose text get encrypted 9 Useful if limited set of messages GQQQEQ EviCJQ 71 QEQQQQQQLDUJ Models for Evaluating Security l Unconditional security perfect secrecy 0 Uncertaintyentropy H pH pc l Complexitytheoretic security I Provable security 0 As difficult to break as solving wellknown and supposedly difficult problem I Computational security I Ad hoc security Brute Force Attacks I Number of encryptionsec 1 million to l billionsec l 56bit key broken in 1 week with 120000 processors 67m l 56bit key broken in 1 month with 28000 processors 16m l 64bit key broken in 1 week with 31 x 107 processors 17b l 128bit key broken in 1 week with 56 x 1026 processors LDEZY J Dmgm QQQQ Types of Cryptography l Hash functions no key I Secret key cryptography one key I Public key cryptography two keys public private Secret Key Cryptography l Same key is used for encryption and decryption 0 Symmetric cryptography l Ciphertext approximately the same length as plaintext l Substitution codes DES IDEA l Message transmission 0 Agree on key but how 0 Communicate over insecure channel I Secure storage crypt 9Q a Secret Key Cryptography Cont d El El Ff l Strong authentication prove knowledge of key without revealing it 7 0 Send challenge r verify the returned encrypted f V C 0 Fred can obtain chosen plaintext cihpertext pairs Challenge should chosen from a large pool 4 g l Integrity check xedlength checksum for E3 message 0 Send lVHC along with the message n Public Key Cryptography E E l Asymmetric cryptography Fl l Inventedpublished in 1975 l Two keys private d public 6 0 Encryption public key Decryption private key 0 Signing private key Verification public key I Much slower than secret key cryptography Public Key Cryptography Cont d I Data transmission 0 Alice encrypts ma using e 3 Bob decrypts to ma using db l Storage 0 Can create a safety copy using public key of trusted person I Authentication 0 No need to store secrets only need public keys 0 Secret key cryptography need to share secret key for every person to communicate with Public Key Cryptography Cont d l Digital signatures 0 Encrypt hash hm with private key Authorship Integrity Nonrepudiation can t do with secret key cryptography