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•Abhishek Gautam•Ashim Gurung •Bikram Koirala• Monil Adhikari

Cryptography: A Brief

Overview

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Agenda Classical Cryptography coverage(esp. Substitution Ciphers):

Mono-alphabetic Substitution Ciphers.Homophonic Substitution Ciphers.Poly-graphic Substitution Ciphers.Poly-alphabetic Substitution Ciphers.

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• Introduction

Substitution Ciphers are the ciphers where a alphabet is replaced by another different ciphers. This ciphers mainly uses the alphabets to encrypt messages for confidentiality in communication.

Substitution Ciphers

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Mono-alphabetic Ciphers• The Mono-alphabetic Substitution cipher

is where each character in the plaintext is replaced by a corresponding one from a cipher alphabet. There are 3 types of Mono-alphabetic Ciphers.– 1) Caesar Ciphers– 2) Affine Ciphers– 3) Atbash Ciphers

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Caesar Ciphers• This is a video

which briefly describes about the working of a Caesar ciphers.

(Youtube,2012)

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Homophonic Ciphers• A homophonic substitution cipher maps each

plaintext letter of a message to one or more cipher text symbols. Homophonic ciphers hide letter frequencies of plaintext messages which are mapped to one or more cipher text units called homophones.

• This method is based on the mono alphabetic substitution. The original text is not only encrypted by one but is also replaced with multiple secret symbols.

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Homophonic Ciphers(Contd.)• A Homophonic substitution cipher is like a simple substitution crypto-

system, in which a single character of plaintext can map to one of several characters of cipher text. A plaintext message M=m1m2... is enciphered as C=c1c2..., where each ci is picked at random from the set of homophones f(mi).

• Letter Homophones A 17 19 34 41 56 60 67 83 I 08 22 53 65 88 90

L 03 44 76 N 02 09 15 27 32 40 59 O 01 11 23 28 42 54 70 80

P 33 91 T 05 10 20 29 45 58 64 78 99 One possible encipherment of the message is:

M = P L A I N P I L O TC = 91 44 56 65 59 33 08 76 28 78

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Poly-graphic Ciphers

• “In a polygraphic substitution cipher plaintext letters are substituted in large groups instead of substituting letters individually”.(wordIQ, n.d)

• The first advantage is that the frequency distribution is much flatter than that of individual letter.

• Second , the large numbers of symbols requires correspondingly more cipher text to productively analyze letter frequencies.

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Playfair Ciphers• The playfair cipher encrypts pair of

letters(diagraph), instead of single letters.• It was invented in 1854 by Charles Wheatstone

for telegraph secrecy. • It is the earliest practical Polygraphic

Substitution ciphers.• The cipher used a 5 by five table containing a

key word.

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Poly-alphabetic Ciphers

• In poly-alphabetic cipher, each plaintext character may be replaced by more than one character

• Since there are only 26 alphabets this process will require using a different representation than the alphabets

• Alphabets ‘A’ through ‘Z’ are replaced by 00, 01, 02, …, 25

• We need two digits in this representation since we need to know how to reverse the process at the decryption side (Babylon,1997).

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Vignere Ciphers

• The most common method used as a type of Poly-Alphabetic Cipher is Vigenère cipher

• Vigenère cipher starts with a 26 x 26 matrix of alphabets in sequence. First row starts with ‘A’, second row starts with ‘B’, etc.

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Vigenere(Contd.)

• E.g., Message = SEE ME IN MALL

• Take keyword as INFOSEC• Vigenère cipher works as

follows:

S E E M E I N M A L L

I N F O S E C I N F O

-------------------------------------

A R J A W M P U N Q Z

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Conclusion• We have considered mainly the key

aspects of Substitution Ciphers and their workings as well as their algorithms.

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References

• Brown, 1996, Introduction to Cryptography, [online], Available at: http://williamstallings.com/Extras/Security-Notes/lectures/classical.html [Accessed at 1st Jan 2012]

• All Science Projects, 2011, [online], Available at: http://www.all-science-fair-projects.com/science_fair_projects_encyclopedia/Caesar_cipher [Accessed at 1 Jan 2012]

•  Chargrill, 2006,PlayFair Cipher ,[online] http://www.perlmonks.org/?node_id=554404 accessed on Jan 11

• WordIQ, n.d, Substitution cipher- Definition, [online] http://www.wordiq.com/definition/Substitution_cipher#Polygraphic_substitution accessed on Jan 11

• University of Adelaide,2012,[online],Available at : https://www.eleceng.adelaide.edu.au/personal/dabbott/wiki/index.php/Cipher_Cross-off_List#Homophonic_Substitution_Ciphers,[Accessed on 10 Jan 2012]

• Mrgeorged, 2009. Top Gear The Stig revealed Full. [video online],Available at:<http://www.youtube.com/watch#!v=eTapK5dRaw4>[Accessed 23 June 2009].

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