vigenere.c

/**
 * vigenere.c
 *
 * Rafael Beraldo
 * rberaldo@cabaladada.org
 *
 * Encrypts text with the Vigenère cipher 
 */
 
#include <stdio.h>
#include <cs50.h>
#include <stdlib.h>
#include <ctype.h>
#include <string.h>

/* In order for the encryption function to work, we must first get the
alphabetical index of a character by subtracting 65 if it is uppercase and 97
if it is lowercase. That's because in ASCII, they are not 0-indexed. */
#define UPPERCASE_ASCII_SHIFT 65
#define LOWERCASE_ASCII_SHIFT 97
#define SIZE_ALPHABET 26

// Functions
void print_usage(string program);
char encrypt(char letter, int key, int shift);

int main(int argc, string argv[])
{
   // Exits if program has less or more than one argument
   if (argc != 2)
   {
        print_usage(argv[0]);
        return 1;
   }
   
   int key_length = strlen(argv[1]);
   char key[key_length]; 
   
   /** The case of the key shouldn't change anything, so let's make sure it's
    * all lowercase. While we're at that, let's also check there aren't any
    * non-alpha characters.
    */
   for (int i = 0; i < key_length; i++)
   {
       if (!isalpha(argv[1][i]))
       {
            print_usage(argv[0]);
            return 1;
       }
       else
       {
            key[i] = tolower(argv[1][i]);
       }
   }
   
   string message = GetString();
   
   // Cycle though all characters in message
   for (int i = 0, j = 0, message_length = strlen(message); i < message_length; i++)
   {
      // If character isn't alphabetical, we don't need to touch it
      if (!isalpha(message[i]))
      {
         printf("%c", message[i]);
      }
      // If it is alphabetical, it can be either upper or lowercase
      else if (isupper(message[i]))
      {
         printf("%c", encrypt(message[i], key[j % key_length], UPPERCASE_ASCII_SHIFT));
         j++;
      }
      else
      {
         printf("%c", encrypt(message[i], key[j % key_length], LOWERCASE_ASCII_SHIFT));
         j++;
      }
   }
   printf("\n");
   
   return 0;
}

////////////////////////////////////////////

/**
 * Prints usage information
 */
void print_usage(string program)
{
    printf("Usage: %s <keyword>\n", program);
}

/**
 * Encrypts a character using a key, shifting to correct for the ASCII table
 */
char encrypt(char letter, int key, int shift)
{
      letter -= shift;
      return (letter + key - 'a') % SIZE_ALPHABET + shift;
}