Cryptography. Intercepting Information Scenario 1. Tuesday, December 9, December 9, Wireless broadcasts information using radio signals

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Transcription:

Cryptography December 9, 2008 1 Intercepting Information Scenario 1 Wireless broadcasts information using radio signals Any computer on a wireless network CAN listen to any network traffic http://www.geeksquad.com/ uploadedimages/ wwwgeeksquadcom/tools/ Tech_Articles/ going_wireless_1.jpg A computer SHOULD only listen to traffic addressed to it It is impossible to know if anyone is eavesdropping 2

Intercepting Information Scenario 2 Internet Mail server Your laptop Spam filter reads a mail message to decide if it is spam What could an unscrupulous person do when he/she writes a spam filter? 3 Cryptography 4

Caesar Cipher Example Ciphertext: 5 Caesar Cipher Example Ciphertext: P 6

Caesar Cipher Example Ciphertext: PR 7 Caesar Cipher Example Ciphertext: PRX 8

Caesar Cipher Example Ciphertext: PRXQ 9 Caesar Cipher Example Ciphertext: PRXQW 10

Caesar Cipher Example Ciphertext: PRXQW KROBRNH 11 General Letter Substitution Algorithm: Substitute 1 letter for another Key PLAINTEXT LETTER A B C D E F G H I J K L M CIPHERTEXT LETTER G J A O U N E Z Y P H S T PLAINTEXT LETTER N O P Q R S T U V W X Y Z CIPHERTEXT LETTER V W L M I B D C R K F Q X Ciphertext: LUGAU 12

WWII Cryptography Enigma machine Used by Germans in World War II Led to great success for Uboats - sunk 50 ships every month! Broken by British, led by Alan Turing http://upload.wikimedia.org/wikipedia/commons/a/ae/enigma.jpg 13 Enigma Machine Top shows A being typed and encoding to G Typing a letter also moves the rotors. Next time, shown on the bottom, A encodes to C. http://upload.wikimedia.org/wikipedia/commons/thumb/6/6c/enigma-action.svg/ 527px-Enigma-action.svg.png 14

An Enigma Codebook http://upload.wikimedia.org/wikipedia/commons/thumb/8/82/ Kenngruppenheft.jpg/800px-Kenngruppenheft.jpg 15 Alan Turing Group at Bletchley Park in England figured out the algorithm Claim they shortened the war by 2 years! Father of computer science Turing machines Turing test http://encyclozine.com/history/biography/turing/turing.jpg 16

Symmetric Key Cryptography Problem: Sender and receiver must agree on the algorithm and key 17 Public Key Cryptography Keys come in pairs: Public key - anyone can see Private key - must be kept secret Anything encrypted with public key can be decrypted with the private key and vice versa 18

Secret vs. Public Key Shared key Secret key Text Public key Bob s public key Bob s private key 19 Ecommerce Application Your credit card number Your computer Ciphertext Encrypt with Amazon s public key Internet Decrypt with Amazon s private key Your credit card number Amazon s computer 20

Public key challenges What algorithm would use separate keys for encoding and decoding? Why isn t the decoding key guessable if you know what the encoding key is? RSA algorithm depends on the fact that it is very difficult to factor large numbers 21 Client Secure Socket Layer (SSL) Server Server public key CONNECT No encryption Symmetric key Encrypted with server public key Real communication Encrypted with symmetric key 22

Digital Signature Digital signature uses the sender s keys Alice s private key Alice s public key 23 What s next? Quantum computing Computers based on quantum physics Expect that quantum computers will ultimately break RSA Encryption with quantum computers might also be able to detect eavesdropping 24

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