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#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;
}
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