The topic discussed in this chapter are: explore a couple of more examples of shift cipher, continue the discussion on mono-alphabetic cipher with an example will also be discussed, perform a cryptanalysis on a cipher text to reveal the plain text. | Network Security Lecture 12 Presented by: Dr. Munam Ali Shah Part 2 (b) Cryptography as a Network Security Tool Summary of the previous lecture We had overviewed the 3-dimensions of a cryptographic system, ., type of operation to transform the plain text into cipher text, number of keys used and the way in which plain text is processed We also discussed unconditional and computational security. One example of unconditional security is OTP The difference between Cryptanalysis and Brute Force attacks, were also discussed. And lastly, we practically perform cryptography through the Shift Cipher Outlines of today’s lecture We will: Explore a couple of more examples of Shift Cipher Continue the discussion on Mono-alphabetic Cipher with an example will also be discussed Perform a cryptanalysis on a cipher text to reveal the plain text. Objectives You would be able to present an overview of more cryptographic schemes and you will understand the limitation of each scheme. You would be able | Network Security Lecture 12 Presented by: Dr. Munam Ali Shah Part 2 (b) Cryptography as a Network Security Tool Summary of the previous lecture We had overviewed the 3-dimensions of a cryptographic system, ., type of operation to transform the plain text into cipher text, number of keys used and the way in which plain text is processed We also discussed unconditional and computational security. One example of unconditional security is OTP The difference between Cryptanalysis and Brute Force attacks, were also discussed. And lastly, we practically perform cryptography through the Shift Cipher Outlines of today’s lecture We will: Explore a couple of more examples of Shift Cipher Continue the discussion on Mono-alphabetic Cipher with an example will also be discussed Perform a cryptanalysis on a cipher text to reveal the plain text. Objectives You would be able to present an overview of more cryptographic schemes and you will understand the limitation of each scheme. You would be able to break the code (ideally) Concepts A private key cipher is composed of two algorithms encryption algorithm E decryption algorithm D The same key K is used for encryption & decryption K has to be distributed beforehand 6 Caesar Cipher If each letter is assigned a number (a=0, z=25), Encryption/Decryption defined as: C = E(p) = (P + 3) mod (26) P = D(c) = (C – 3) mod (26) Example: meet me after the toga party phhw ph diwhu wkh wrjd sduwb 7 Caesar Cipher: Encryption Example K = 7 P = Rome is the greatest empire C = yvtl pz aol nylhalza ltwpyl Caesar Cipher: Decryption Example K = 7 C = yvtl dhz uva ibpsa pu h khf P = Rome was not built in a day Caesar Cipher: Decryption with Unknown Key C=tfnriujuzvdrepkzdvjsvwfivkyvziuvrkyjkyvmrczrekevmvikrjkvfwuvrkyslkfetv tfnriuj uzv drep kzdvj svwfiv kyvzi uvrkyj; kyv mrczrek evmvi krjkv fw uvrky slk fetv P = Cowards die many times before their deaths; the valiant never taste of death but once. (K = 17) Julius Caesar by William Shakespeare 10 .
