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Zac Blohm & Kenny Holtz

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ALGORITHMS ARE THE BASIS FOR CRYPTOGRAPHY The basic idea of Cryptography in Computer Science is to run a message through an algorithm to receive an encrypted text which can safely be sent to be decrypted with another algorithm

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Monoalphabetic Caesar Cypher Polyalphabetic Transpositional Compositional

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Simple substitutions One of the earliest used forms of cryptography Easily cracked by statistical analysis (ex. how many times each character occurs) and trial and error Most famous example is the Caesar Cypher which simply replaces each character with one “K” places further in the alphabet

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If K=3, then A becomes C, B becomes D etc… Therefore the plaintext “This is a message” Is encrypted to say “Vjku ku c oguucig” Notice that the number of characters and any patterns between them are shared (repeated characters, the standalone vowel etc…)

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Multiple alphabets to disguise patterns Biggest difference between them is how many alphabets and what determines a change of alphabet

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KeyA1A1 A2A2 A3A3 AXCL BYDM CZEN D FO EAGP FBHQ GCIR HDJS …………

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PLAINTEXT This is a message CYPHERTEXT Pjtobtoblwopoulcg The key changed alphabets after each character (spaces were incorporated into each alphabet to conceal word length)

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Still direct substitutions The change of alphabets can be recognized, which then reduces the problem to a series of monoalphabetic problems

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Changes the arrangement of the plaintext to disguise the message Immune to the frequency analysis that defeats substitution cyphers Pure transpositional cyphers produce same amount of each letter as present in plaintext A common example involves reading into a matrix one way, and reading out the other

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th is i s a te st m es sa ge

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PLAINTEXT This is a test message # of t’s: 3 # of h’s: 1 # of i’s: 2 # of s’s: 5 # of spaces: 4 # of a’s: 2 # of e’s: 3 # of m’s: 1 # of g’s: 1 CYPHERTEXT Ti sats esghsi etmsae # of t’s: 3 # of h’s: 1 # of i’s: 2 # of s’s: 5 # of spaces: 4 # of a’s: 2 # of e’s: 3 # of m’s: 1 # of g’s: 1

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The matrix example never changes the letter in the first position or the last, and requires the key to contain the size of the matrix needed for decryption Creates an anagram (meaning some messages are easily decrypted just by rearranging the letters to make the most probable words, longer messages make this harder)

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Combining both makes a much stronger cypher as you can eliminate most of the apparent patterns in your cyphertext An example would be taking “this is a test message” through the previously used polyalphabetic cypher to get “pjtobtoblwvpovkigcocra” and then reading it through a 2x11 matrix as before

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pj to bt ob lw vp ov ki gc oc ra

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PLAINTEXT “This is a test message” CYPHERTEXT “ptbolvokgorjotbwpvicca” No direct correlation between the position or frequency of each character

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