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Superficial Study on the Concept of Cryptography covering different types of ciphers

Superficial Study on the Concept of Cryptography covering different types of ciphers

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- 1. Cryptography: A Brief Overview •Abhishek Gautam •Ashim Gurung •Bikram Koirala • Monil Adhikari
- 2. Agenda Classical Cryptography coverage(esp. Substitution Ciphers): Mono-alphabetic Substitution Ciphers. Homophonic Substitution Ciphers. Poly-graphic Substitution Ciphers. Poly-alphabetic Substitution Ciphers.
- 3. Substitution Ciphers • 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.
- 4. 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
- 5. Caesar Ciphers • This is a video which briefly describes about the working of a Caesar ciphers. (Youtube,2012)
- 6. 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.
- 7. Homophonic Ciphers(Contd.) • • A Homophonic substitution cipher is like a simple substitution cryptosystem, 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 T C = 91 44 56 65 59 33 08 76 28 78
- 8. 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.
- 9. 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.
- 10. 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).
- 11. 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.
- 12. Vigenere(Contd.) • E.g., Message = SEE ME IN MALL • Take keyword as INFOSEC • Vigenère cipher works as follows: SEEME I NMALL I NFOSEC I NFO ------------------------------------A RJ AWMPUNQZ
- 13. Conclusion • We have considered mainly the key aspects of Substitution Ciphers and their workings as well as their algorithms.
- 14. 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-fairprojects.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_Crossoff_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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