General Letter Substitution Algorithm: Substitute 1 letter for - - PowerPoint PPT Presentation

General Letter Substitution

Algorithm: Substitute 1 letter for another Key Ciphertext: LUGAU

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

WWII Cryptography

http:/ /upload.wikimedia.org/wikipedia/commons/a/ae/Enigma.jpg 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

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Enigma Machine

• Top shows A being

typed and encoding to G

• Typing a letter also

moves the rotors.

• Next time, shown
• n the bottom, A

encodes to C. http://upload.wikimedia.org/wikipedia/commons/thumb/6/6c/Enigma-action.svg/ 527px-Enigma-action.svg.png

An Enigma Codebook

http:/ /upload.wikimedia.org/wikipedia/commons/thumb/8/82/ Kenngruppenheft.jpg/800px-Kenngruppenheft.jpg

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Alan Turing

http:/ / encyclozine.com/History/Biography/Turing/turing.jpg 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

Symmetric Key Cryptography

Problem: Sender and receiver must agree on the algorithm and key

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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

Symmetric

• vs. Public Key

Text Symmetric key Public key Shared key Bob’ s public key Bob’ s private key

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Internet Amazon’ s computer Your computer

Ecommerce Application

Your credit card number Ciphertext Encrypt with Amazon’ s public key Your credit card number Decrypt with Amazon’ s private key

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

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Secure Socket Layer (SSL)

Client CONNECT Server public key Symmetric key Real communication Server No encryption Encrypted with server public key Encrypted with symmetric key

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