A program implementing Caesar's cipher to encrypt plaintext.

caesar.c

/*
    A program to encrypt messages using a Caesar cipher.
    
    Usage: ./caesar key
    
        - Where key is the number of places to move your letters by.
*/


// Include libraries
#include <cs50.h>
#include <ctype.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>

// Declare functions
char encrypt(char alpha, int key);
void extract_alpha(string plaintext, int key);

// Main program
int main(int argc, string argv[])
{
    string key = argv[1];
    
    // Check if there are more or less than 2 command-line arguments
    if (argc != 2)
    {
        printf("Usage: ./caesar key\n");

        return 1;
    }
    else
    {
        for (int i = 0; i < strlen(key); i++)
        {
            // Check if key is not a number
            if ((int) key[i] > 57 || (int) key[i] < 48)
            {
                printf("Usage: ./caesar key\n");
                return 1;
            }
            else
            {
                continue;
            }
        }
    }
    // Convert key to integer
    int num = atoi(argv[1]);

    string input = get_string("plaintext: ");
    
    // Extract alphabets from plaintext
    extract_alpha(input, num);
}

// Function to extract alphabets from plaintext
void extract_alpha(string plaintext, int key)
{
    int txtlength = strlen(plaintext);
    char cipherarray[txtlength];
    
    for (int i = 0; i < txtlength; i++)
    {
        // Check for symbols, spaces and digits
        if (ispunct(plaintext[i]) || isblank(plaintext[i]) || isdigit(plaintext[i]))
        {
            cipherarray[i] = plaintext[i];
        }
        // Check for alphabets
        else if (isalpha(plaintext[i]))
        {
            // Encrypt plaintext
            char newchar = encrypt(plaintext[i], key);
            cipherarray[i] = newchar;
        }
    }
    printf("ciphertext: ");
    for (int i = 0; i < txtlength; i++)
    {
        printf("%c", cipherarray[i]);
    }
    printf("\n");
}
// Function to encrypt plaintext to ciphertext
char encrypt(char alpha, int key)
{
    int places = key % 26;
    int position = places + (int) alpha;
    
    // Check if uppercase
    if (isupper(alpha))
    {
        // Check if encrypted char exceeds ASCII uppercase alphabets
        if (position > 90)
        {
            int p3 = position - 90;
            char final = (char)(p3 + 64);
            
            return final;
        }
        else
        {
            return (char) position;
        }
    }
    else
    {
        // Check if encrypted char exceeds ASCII lowercase alphabets
        if (position > 122)
        {
            int p3 = position - 122;
            char final = (char)(p3 + 96);
            
            return final;
        }
        else
        {
            return (char) position;
        }
    }
}

// VICTORY!!!