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assign2

# assign2 - McGill University School of Computer Science...

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McGill University School of Computer Science COMP-206 Software Systems Assignment #2 Due: Monday October 12, 2009 on Web CT at 23:55 The assignments in this course lead you, by the end of the course, to develop a web store. As we do that we also learn how to properly engineer a program using standard available tools, like GNU and the Unix environment. This assignment continues in that theme. Every good web site needs a secure login page. A secure login page implies that information is encrypted. Our first question is just that, an encryption algorithm in C. It will be eventually used in your web site. The second question asks you to create a bash program that helps you compile complex programs, while providing you some sound automated engineering procedures. Question 1: C Programming A semi-advanced cryptographic method is called the Grid Caesar Cipher. This method uses an integer number and a two dimensional array to encrypt messages. This method combines Simple Caesar Cipher with a transposition matrix. Simple Caesar Cipher asks the user for an input message and an integer number. Each character of the message is shifted by N characters, where N is the inputted integer number. The shifted characters are wrapped back to the first letter of the alphabet if the shift goes past the last letter of the alphabet. For example: if our message is ZAP and our inputted integer is 2, the resulting cipher would be BCR. A Grid Cipher is when we have a message saved in a 2D array but we write out the message by performing a matrix transposition of the array. For example, let us say we have a 10X3 matrix containing the following information: 0 1 2 3 4 5 6 7 8 9 0H I T HERE 1MR B I L L 2B Y E The transposition output of the matrix would be: HMB IRY E TB HI EL RL E (but without any extra spaces – the extra spaces are shown here to help you see what is going on). The Grid Caesar Cipher takes input text and stores it into an array. It then performs the standard Simple Caesar Cipher on all the characters of the array but writes out the array in its transposition.

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