vigenere_final.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
#include <cs50.h>

#define UPPER_ALPHA_INDEX 65
#define LOWER_ALPHA_INDEX 97
#define ALPHABET_MAX 26

char encipher(char index, int cipher_key);
int convert_key(char index);

int main(int argc, string argv[])
{
    // Get the key from the user
    string cipher_key;
    // Guard against the user not entering the correct number of command line arguments
    if(argc != 2)
    {
        printf("Error: Incorrect number of arguments passed!\n");
        return 1;
    }
    
    for(int i = 0; i < strlen(argv[1]); i++)
    {
        if ( isupper(argv[1][i]) || islower(argv[1][i]) )
            continue;
        else
        {
            printf("Error: Something in the cipher_key was not a valid character\n");
            return 1;
        }
    }
    
    cipher_key = argv[1];
    
    // Get the plain text they want to encode
    string cipher_text = "";
    do
    {
        printf("\nNOTE: Only uppercase and lowercase letters will be encoded\n");
        cipher_text = get_string("Please enter the text to encode to continue: ");
    }
    while(strlen(cipher_text) == 0);
    
    
    // Encipher
    printf("ciphertext: ");
    
    int key_len = strlen(cipher_key);
    int converted_key;
    for (int i = 0, j = 0; i < strlen(cipher_text); i++)
    {
        if(isupper(cipher_text[i]) || islower(cipher_text[i]))
        {
            converted_key = convert_key( cipher_key[j % key_len]);
            j++;
        }
        else
        {
            converted_key = 0;
        }
        printf("%c", encipher(cipher_text[i], converted_key) );
    }
        
        
    printf("\n");
}


// The way to encipher is by understanding the ascii indicies for each letter.
// Because lowercase letters and uppercase letters have arbitrary indicies
// we must first adjust the indicies so 'a' and 'A' start at index 0
// This is done by subtracting either UPPER_ALPHA_INDEX or LOWER_ALPHA_INDEX

// once the index is in the correct mapping we can add or subtract the cipher key
// and use the modulo operator to get the new index
// finally we re-adjust back up to the ascii index
// by adding back UPPER_ALPHA_INDEX or LOWER_ALPHA_INDEX

// Anything that isn't a character is returned back to the caller

char encipher(char index, int cipher_key)
{
        if ( isupper(index) )
        {
            index -= UPPER_ALPHA_INDEX;
            index += cipher_key;
            index %= ALPHABET_MAX;
            index += UPPER_ALPHA_INDEX;
            return index;
        }
        else if ( islower(index) )
        {
            index -= LOWER_ALPHA_INDEX;
            index += cipher_key;
            index %= ALPHABET_MAX;
            index += LOWER_ALPHA_INDEX;
            return index;
        }
        else
            return index;
}

int convert_key(char index)
{
    
    if ( isupper(index) )
        return index - UPPER_ALPHA_INDEX;
    else if ( islower(index) )
        return index - LOWER_ALPHA_INDEX;
    else
        return 1;
}