# Zac Blohm & Kenny Holtz. ALGORITHMS ARE THE BASIS FOR CRYPTOGRAPHY  The basic idea of Cryptography in Computer Science is to run a message through an.

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Zac Blohm & Kenny Holtz

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

 Monoalphabetic  Caesar Cypher  Polyalphabetic  Transpositional  Compositional

 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

 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…)

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

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

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

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

 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

th is i s a te st m es sa ge

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

 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)

 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

pj to bt ob lw vp ov ki gc oc ra

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

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