These are percentages

1. The message folders are named "messages0" through "messages9".
   You must decrypt the messages in the folder ending with the same MIDDLE digit as your student ID number.
   

2. Write a program that reads an encrypted message file, computes the frequency (number of occurrences of percentage) of each character and draws a bar graph similar to the one above. The encrypted message above has the bar chart shown on the next page.
   
   
3. Run your program for each of your three encrypted messages. Print a picture of the frequency bar chart for each message. Label the graphs so you know which one goes with each message.
   
   
   
   

   These are number of occurrences.

   
   
   Comparing this bar chart with the one for English, we see that 'E F G' in this graph are a good match for "X Y Z' in the English graph. That is, Z -> G, A -> H and so on. That is, in fact, the cipher that was used.
   
   
   
4. Compare your bar charts with the English one and deduce the shift that was used to encrypt the message and write this, neatly, on the page with the printed bar chart.
   
   
5. Write a second program that:
   
   a. Inputs a text file.
   b. Prompts the user for a shift (0 <= shift<=25)
   c. Writes a new text file that is the old file shifted.
   
   
6. Use your second program and the shifts you found in part 3 to decrypt your three messages. For each decrypted file, do the following:
   
   a. Open the file in teachtext
   b. Adjust the first line to correct the word boundaries. See the example in the box below. Put the encrypted file name at the top, e.g. c16.
   

f.crypt

• A diskette with your two programs, each in its own project, ready to run.
  
  
• Your labeled printouts of the three frequency graphs, showing the shift you deduced in each case.
  
  
• The printouts of your three decrypted files.
  

1. Use an array to count the occurrences of each letter.
   
   typedef FreqArray = int[26];
   FreqArray A; // Remember to set all the values A[k] to 0.
   
   
   When you read a character that is between 'a' and 'z', update its frequency.
   

   if (('a' <= ch) && (ch <= 'z')) 
   	A[ch - 'a'] += 1;
   

   To find percentages of each letter in the text, compute the total number of letters in the message as you read it or by adding up the frequencies. Divide the number of occurences of a letter by the total to get the percentage.
   
   

2. Remember that if 'a' --> 'g' to encrypt, then 'g' --> 'a' to decrypt.
   
   A Template for Reading, Transforming, and Writing:
   The following code outlines the process of opening one file for reading and another for writing. The data is read from InFile, transformed, then written to OutFile.
   

   // Get name of old (encrypted) file
   if (SelectOldFileName(InFileName) // InFileName is of type string
   	// get file name of new file 
   	if (SelectNewFileName(OutFileName) 		
   		if (OpenFile(InFile, InFileName, textread) 
   			if (OpenFile(OutFile, OutFileName, textwrite) {
   				while (InFile.get(ch)) {
   					transform the data item, ch
   					send zero, one, or more new items of data to OutFile
   				}
   
   				{ close both files }
   				InFile.close();
   				OutFile.close();
   			}
   

   Notice that this code asks the user for the name of the old file, then if the user has not clicked CANCEL this code asks for the name of the new file, then if the user has not clicked CANCEL the work on the two files begins.

Last Updated: September 21, 1998 8:22 pm by