week2 - SIT281 Introduction to Cryptography WEEK 2 Lecture...

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7/15/2010 1 SIT281 Introduction to Cryptography WEEK 2 Lecture objectives In this lecture: > We look at examples of substitution ciphers. > We see how block ciphers work. > We are introduced to linear shift registers.

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7/15/2010 2 Substitution ciphers > This is a slightly more general idea than the affine cipher. > In the substitution cipher, we replace each letter by another letter. > This is by definition a 1-1 map and so decrypting is easy as long as you know the substitution used. The codeword puzzle in the Saturday Age is an example of a substitution cipher cipher. In this example, 17 -> N
7/15/2010 3 Substitution ciphers cont’d Let’s apply a permutation to some plaintext: Plaintext: meet me at the pier The only letters here are: a,e,h,i,m,p,r,t. A permutation on this set can be written in a nice concise way eg. (h,i,t,m,p,r,a,e) which can produce a substitution interpreted by: interpreted by: h is replaced by i, i is replaced by t, t by m, and so on, and finally, e is replaced by h. So the ciphertext is: phhmphemmihrmht Substitution ciphers cont’d > With enough text, a frequency analysis also works on these. > In the Age codeword puzzle, you are also given the lengths of all the words and this helps a lot to determine the other letters. As does a good understanding of word construction in English. > When you do not have the word lengths you also > When you do not have the word lengths, you also need frequencies of common letter pairs and even triples in English.

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7/15/2010 4 Substitution ciphers cont’d > For instance, the pair ‘th’ is the most common digram occurring in English. > The triple ‘the’ is the most common trigram. Also useful is that: > the letter n is usually preceded by a vowel (80% of the time). > you will never see jj or xx together in English, except possibly at the end of one word and beginning of the next. The Playfair and ADFGX ciphers These ciphers were used in World War 1 by the British and Germans respectively. > The Playfair system was invented around 1854 by Charles Wheatstone who named it after his friend Baron Playfair. > As in the Vigenere method, the key is again a word; remove any repeated letters from it remove any repeated letters from it. > Begin with the letters of the key and with the remaining letters of the alphabet, complete a 5x5 matrix.
7/15/2010 5 The Playfair cipher Use the keyword ‘monarchy’. Treating i and j as one letter (since j is rare), we get: MO N A R CH Y BD E F G I/J K L P Q S T U V W X Z Preparing the plaintext > If the plaintext has two adjacent letters the same, put an x between them. > Then break the plaintext into pairs. > If there is an odd number of plaintext letters, add an x at the end. Eg if the plaintext is ‘meet me at the pier’ we would Eg, if the plaintext is meet me at the pier , we would rewrite this as me xe tm ea tx th ep ie rx

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7/15/2010 6 The Playfair cipher cont’d For each pair, encrypt as follows: 1. If both letters fall in the same row, replace each with letter to right (wrapping back to start from end) 2. If both letters fall in the same column, replace
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This note was uploaded on 08/12/2010 for the course CRYOPTOGRA 3232 taught by Professor Jhhgjhgjh during the Spring '10 term at Trinity College, Hartford.

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week2 - SIT281 Introduction to Cryptography WEEK 2 Lecture...

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