Vigenere's Cipher

// Osman Zakir
// 10 15 2016
// Introduction to Computer Science
// Problem Set 2, vigenere.c
// Take the key for the cipher from the user as a command-line argument and use it to encrypt a message

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

bool isletterstring(string key);

int main(int argc, string argv[])
{
    if (argc == 1 || argc > 2)
    {
        printf("Usage: ./vigenere <key>\n");
        return 1;
    }
    string plaintext = GetString();
    string key = argv[1];
    const int key_size = strlen(key);
    const int plaintext_size = strlen(plaintext);
    char ciphertext[plaintext_size];
    if (isletterstring(key))
    {
        for (int i = 0, j = 0; i < plaintext_size; i++)
        {
            if (j >= key_size)
            {
                j = 0;
            }
            if (isalpha(plaintext[i]))
            {
               int k = toupper(key[j]) % 65;
               int c = plaintext[i] + k;
               if (toupper(c) > 'A')
               {
                   c -= 26;
               }
               ciphertext[i] = c;
               j++;
            }
            else
            {
                ciphertext[i] = plaintext[i];
            }
        }
        printf("%s\n", ciphertext);
    }
    else
    {
        printf("Keyword must only contain letters A-Z and a-z\n");
        return 1;
    }
    return 0;
}

bool isletterstring(string key)
{
    for (int i = 0, n = strlen(key); i < n; i++)
    {
        if (!isalpha(key[i]))
        {
            return false;
        }
    }
    return true;
}