Harvard CS50 substitution

substitution.c

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

int main(int argc, string argv[])
{
    // the user must provide one and only argument
    if (argc != 2)
    {
        printf("Your forgot the key, duh.\n");
        return 1;
    }
    // we only accept a key of size 26
    else if (strlen(argv[1]) != 26)
    {
        printf("You must provide a key with a length of 26 characters.\n");
        return 1;
    }
    else
    {
        for (int i = 0, n = strlen(argv[1]); i < n; i++)
        {
            // check if the key is all about alphabetic
            if (!isalpha(argv[1][i]))
            {
                printf("The key should only contain alpha characters.\n");
                return 1;
            }

            // check if every character is only used once
            for (int j = i + 1; j < n; j++)
            {
                if (argv[1][i] == argv[1][j])
                {
                    printf("A key cannot have a same character more than once!\n");
                    return 1;
                }
            }
        }
    }

    // store the key argument
    string key = argv[1];

    // get the text to encrypt
    string plaintext = get_string("plaintext: ");

    // prepare the encrypted variable
    char ciphertext[strlen(plaintext)];

    for (int i = 0, n = strlen(plaintext); i < n; i++)
    {
        // if the character is upper of lower, the alphabet start is not the same
        int alpha_index = plaintext[i] - ((isupper(plaintext[i])) ? 65 : 97);

        // do nothing if the given character is not alphabetic
        if (!isalpha(plaintext[i]))
        {
            ciphertext[i] = plaintext[i];
        }
        else // respect the lower or uppercase of the plaintext character
        {
            ciphertext[i] = islower(plaintext[i]) ? tolower(key[alpha_index]) : key[alpha_index];
        }
    }

    // show result
    printf("ciphertext: %s\n", ciphertext);
}

nul ce code