CS33 -- Laboratory 9

You are encouraged to work with a partner.

Work in a subdirectory called initslab9, where inits are your initials. Once you have completed the C programs and made script files to demonstrate your testing of each, remove any extraneous files so that your directory contains just the .c files and the script files. Then make a tarball of that directory and send it to me as an attachment to the email whose subject line should be CS33 Lab9. Send the email to cfk@cs.swarthmore.edu.

Each program must follow these following guidelines:

• the file should be named using the name provided,
• your code should be adequately commented, and
• your program should follow all input and output guidelines exactly.

LAB 09: C Programming with Functions

1. Write a program (called power.c) which contains this function:

   float raiseRealToPower(float base, int exponent);

   This function will raise base to the power exponent. The base can be any floating point value (negative, positive, zero) and the exponent can be any integer value (negative, positive, zero). You are to write this function yourself using basic C operations and control statements. You may not use math.h functions like exp or pow.

   After you have written this function, test that the function is working by displaying a table of the powers of 0.5 between -15 and +5. Your table should be similar, but necessarily exactly the same as this:

     -15    32768.00000
     -14    16384.00000
     -13     8192.00000
     -12     4096.00000
     -11     2048.00000
     -10     1024.00000
      -9      512.00000
      -8      256.00000
      -7      128.00000
      -6       64.00000
      -5       32.00000
      -4       16.00000
      -3        8.00000
      -2        4.00000
      -1        2.00000
       0        1.00000
       1        0.50000
       2        0.25000
       3        0.12500
       4        0.06250
       5        0.03125
   

2. You will implement several functions to allow for encrypting and decrypting text using a method that is just slightly better than a Caesar Cipher. The idea is that a word or phrase (of all letters) is chosen to construct a key. First remove all duplicates to get a partial key, then append the letters of the alphabet (in order) that are not in the partial key to get the full key. For example, if Trailblazers is the initial phrase, the partial key will be: trailbzes. The full key is:
   trailbzescdfghjkmnopquvwxy. To encrypt, imagine the full alphabet above the full key.

   abcdefghijklmnopqrstuvwxyz trailbzescdfghjkmnopquvwxy To encrypt, each letter of the message is located in the top row and replaced by the letter in the bottom row. To decrypt, do the reverse.

   Your task is to write 3 functions and a main program that tests the functions. Possible prototypes for your functions might be:

   int prepare_key( char key[] ); void encrypt( char data[], char key[] ); void decrypt( char data[], char key[] ); or int prepare_key( char *key ); void encrypt( char *data, char const *key ); void decrypt( char *data, char const *key ); with or without the const. Name the file containing your testing program (with main) cryptdriver.c.
   

   The function prepare_key takes a string as argument which is the initial phrase from which to construct a full key. It removes duplicates and extends the string to a full key. It returns 0 (FALSE) if it is passed an empty string. Otherwise it returns 1 (TRUE).

   The function encrypt takes two arguments. The second argument, key, is a full key as delivered by prepare_key. The first is a string (<= 80 characters) to be encrypted by this method. Nonalphabetic characters of data are not changed. The case of alphabetic characters is to be preserved.

   The function decrypt reconstructs an original message from an encrypted string passed to it in data using the key passed to it in key.

   prepare_key is much harder to write than encrypt and decrypt. So start with the easier functions using one of my full keys shown below. Once you can successfully encrypt and decrypt, then tackle prepare_key.

   You would not ordinarily show the full key or show the plain text when encrypting, but I do so here for illustrative purposes and your driver program should also. In the encrypting and decrypting parts, the lines I entered from the keyboard alternate with the lines output by my test program. Here are a couple of sample runs of my program:

   licorice% a.out This program will encrypt Please enter the key phrase consisting of letters only terminated by the pressing enter. The phrase should be <= 80 letters in length. Trailblazers Original Phrase is: Trailblazers Full key is: trailbzescdfghjkmnopquvwxy Now enter text to encrypt terminating with an empty line. Attack at Dawn. Tpptad tp Itvh. 1 2 buckle my shoe. 3 4 close the door! 1 2 rqadfl gx oejl. 3 4 afjol pel ijjn! Now enter text to decrypt terminating with an empty line. Tpptad tp Itvh. Attack at Dawn. 1 2 rqadfl gx oejl. 3 4 afjol pel ijjn! 1 2 buckle my shoe. 3 4 close the door! licorice% a.out This program will encrypt Please enter the key phrase consisting of letters only terminated by the pressing enter. The phrase should be <= 80 letters in length. MozartRachmaninov Original Phrase is: MozartRachmaninov Full key is: mozartchnivbdefgjklpqsuwxy Now enter text to encrypt terminating with an empty line. While better than a Caesar Cipher, this kind of cipher is Uhnbr orpprk phme m Zmrlmk Znghrk, phnl vnea ft znghrk nl still subject to frequency analysis breaking. lpnbb lqoirzp pf tkrjqrezx membxlnl okrmvnec. Now enter text to decrypt terminating with an empty line. lpnbb lqoirzp pf tkrjqrezx membxlnl okrmvnec. still subject to frequency analysis breaking. licorice% Some c functions I used in my array version were:
   printf
   gets
   exit
   strlen
   islower
   isupper
   tolower
   strchr
   toupper
   
   which were available because I used the preprocessor directives #include <stdlib.h> #include <stdio.h> #include <ctype.h> #include <string.h> I used more string functions in my pointer version. You only have to write one version.

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   Note: As of 1 Oct, the final exam for CS33 has been scheduled by the registrar for Friday 12/17/2010, 9:00am-12:00pm in SC240. Plan to attend.

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