Playfair Cipher
1.not even the large number of keys in a monoalphabetic cipher provides security
2.one approach to improving security was to encrypt multiple letters
3.the Playfair Cipher is an example
4.invented by Charles Wheatstone in 1854,but named after his friend Baron Playfair
Playfair Key Matrix
1.a 5X5 matrix of letters based on a keyword
2.fill in letters of keyword (sans duplicates)
3.fill rest of matrix with other letters
Encrypting and Decrypting
-plaintext is encrypted two letters at a time
1. if a pair is a repeated letter, insert filler like X’
2. if both letters fall in the same row, replace each with letter to right (wrapping back to start from end)
3. if both letters fall in the same column, replace each with the letter below it (again wrapping to top from
bottom)
4. otherwise each letter is replaced by the letter in the same row and in the column of the other letter of the pair
Heapsort
1.A heap is a binary tree satisfying the followingconditions:
-This tree is completely balanced.
-If the height of this binary tree is h, then leaves can be at level h or level h-1.
-All leaves at level h are as far to the left as possible.
-The data associated with all descendants of a node are smaller than the datum associated with this node.
Implementation
1.using a linear array not a binary tree.
-The sons of A(h) are A(2h) and A(2h+1).
2.time complexity: O(n log n)
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Very hard to design reusable Software
Design must be specific to problem on hand
Design must be general enough to address future problems and requirements
Almost impossible to do it right the first time
Experienced designers don’t reinvent the wheel
They use good solutions again and again
Find & use recurring patterns of classes &
Very hard to design reusable Software
Design must be specific to problem on hand
Design must be general enough to address future problems and requirements
Almost impossible to do it right the first time
Experienced designers don’t reinvent the wheel
They use good solutions again and again
Find & use recurring patterns of classes &
Very hard to design reusable Software
Design must be specific to problem on hand
Design must be general enough to address future problems and requirements
Almost impossible to do it right the first time
Experienced designers don’t reinvent the wheel
They use good solutions again and again
Find & use recurring patterns of classes &
Very hard to design reusable Software
Design must be specific to problem on hand
Design must be general enough to address future problems and requirements
Almost impossible to do it right the first time
Experienced designers don’t reinvent the wheel
They use good solutions again and again
Find & use recurring patterns of classes &
