caesar.c

// Encrypts text with caesar's cypher

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

// prototypes
int error(void);
string cyp_caesar(int key, string plaintext, int n);

int main(int argc, string argv[])
{
    // Check input
    if (argc != 2)
    {
        return error();
    }
    int argLen = strlen(argv[1]);
    for (int i = 0; i < argLen; i++)
    {
        if (isdigit(argv[1][i]) == 0)
        {
            return error();
        }
    }
    
    // Convert key
    int key = atoi(argv[1]);
    key = key % 27;

    // Ask plaintext, Encrypt, Print Cyphertext
    string plain = get_string("plaintext: ");
    int plainLen = strlen(plain);
    plainLen--;
    string cyphertext = cyp_caesar(key, plain, plainLen);
    printf("Cyphertext: %s\n",cyphertext);
}

int error(void)
{
    printf("Usage: ./caesar key\n");
    return 1;
}

//converts plaintext to cypertext
string cyp_caesar(int key, string plaintext, int n)
{
    //stop recursing
    if (n == -1)
    {
        return "";
    }

    cyp_caesar(key, plaintext, n - 1);

    // convert string element if alpha
    char debug = plaintext[n];
    if (isalpha(plaintext[n] != 0))
    {
        int upperCase = isupper(plaintext[n]);
        // int lowerCase = islower(plaintext[n])
        plaintext[n] = (int) plaintext[n] + key;
        
        // Rotate from Z or z back to A or a
        if (upperCase > 0 && plaintext[n] > 90)
        {
            plaintext[n] = plaintext[n] - 27;
        }
        //if (lowerCase > 0 && plaintext[n] > 122)
        //{
        //    plaintext[n] = plaintext[n] - 27;
        //}
        
        else if (plaintext[n] > 122)
        {
            plaintext[n] = plaintext[n] - 27;
        }
    }
    
    return plaintext;
}