C# stringbuilder | Computer Science homework help


Steganography, as defined by Webster’s Dictionary, is the art or  practice of concealing a message, image, or file within another message,  image, or file.  For this week’s assignment we are going to work with  arrays, strings, and StringBuilder to decode a message that has been  hidden inside another file and sent to us from our friend, Randy  Waterhouse (you have my sincere admiration if you get that reference!).   Randy is sending us a message that he has encoded within a regular data  file.  This means he has taken the file and changed random characters  within that file that will spell out a message if we can figure out  which characters he changed and the ordered in which he changed them. 

The original file looked like this:


Randy then went line by line and changed certain characters  according to the below table.  The lines and columns of the table are  indexed starting at zero, just like our arrays in C#.  The first entry  in the table, “0,5=n” means that Randy changed the character in line  zero at the 5th letter (which is actually the 6th  since we started at zero) to be “n” (it was originally “W”).  Here is  the table of changes that Randy made.  Note that he only changed 5  characters per line and that each line has 60 characters in it.


 And here is the new data with Randy’s changes:


If we know the order that he changed the characters in each line, we  can create the following four strings of five characters each:


If we append all those strings together, we will have the full  message that Randy sent us (apparently Randy is a fan of the movie “A  Christmas Story”):

Drink more Ovaltine!

So how do we figure out the order?  Many cryptographic systems have  two mains components: an equation and a key.  The equation generates a  string of numbers based on the key number that is plugged in to that  equation (that’s a super-simple explanation, but this isn’t a crypto  class, so…).  In this case, Randy kept it simple.  He’s a C# programmer,  after all!  He used the Random class, and he knows that using a certain  number as a “seed” will make Random generate the same sequence of  numbers when “.Next” is called.  If Randy tells you the seed, and you  know that he did his encryption line by line, then you can get the  characters from Randy’s seemingly random data and generate the message.

 So here’s your assignment.  The attached assignment shell has an  array of 22 strings already declared for you that represent the lines in  a data file.  Each line has 60 characters in it, but Randy only changed  5 of those characters.  You need to use nested for loops (i.e. one for  loop inside another) and Random’s “next” method to pull the five  characters from each line and store those in a different array of  strings, where each string is 5 characters long.  Remember, if you seed  your Random with the same number Randy did, then your “next” will  generate the same sequence of numbers.  Those numbers are the indices of  the characters you need to get from each line.  You then need to print  out each 5-character string like I showed you above.  Finally, you’re  going to use a StringBuilder to put all 22 of those 5-character strings  together in order to spell out the message. 

You need to seed your random number generator with 243 because  that’s the number Randy used when encrypting his message (he chose that  because 243 would generate 22 5-character sequences where those 5  characters would not repeat).

Read the comments in the attached shell carefully as they might contain some more insight into how to solve this problem.

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