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 1 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 2 Friday, November 18, 2011
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 3 An Enigma Codebook http:/ /upload.wikimedia.org/wikipedia/commons/thumb/8/82/ Kenngruppenheft.jpg/800px-Kenngruppenheft.jpg 4 Friday, November 18, 2011
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 5 Symmetric Key Cryptography Problem: Sender and receiver must agree on the algorithm and key 6 Friday, November 18, 2011
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 7 Symmetric Shared key vs. Public Key Symmetric key Text Public key Bob’ s public key Bob’ s private key 8 Friday, November 18, 2011
Ecommerce Application Your computer Your credit card number Encrypt with Amazon’ s public key Internet Ciphertext Decrypt with Amazon’ s private key Amazon’ s Your credit card number computer 9 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 10 Friday, November 18, 2011
Secure Socket Layer (SSL) Client Server CONNECT No encryption Server public key Encrypted with server Symmetric key public key Encrypted with Real symmetric key communication 11 Friday, November 18, 2011
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