cs50 substitution

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

char char_index(char c);
int main(int argc, string argv[])
{
    //ensure correct amount of args
    if (argc != 2)
    {
        printf("Usage: ./substitution key\n");
        return 1;
    }

    string key = argv[1];
    int key_length = strlen(key);

    //check if key has exactly 26 chars
    if (key_length != 26)
    {
        printf("Key must contain 26 characters.\n");
        return 1;
    }

    //check if key has only alphabetic chars and no repeating char
    for (int i = 0; i < key_length; i++)
    {
        if (!isalpha(key[i]))
        {
            printf("key must be contain only alphabetic characters");
            return 1;
        }

        //double check, literally, if a char has been seen before
        for (int j = 0; j < key_length; j++)
        {
            if (i != j)
            {
                if (key[i] == key[j])
                {
                    printf("key must contain unique characters");
                    return 1;
                }
            }

        }
    }
    string plaintext = get_string("Plaintext: ");
    int length = strlen(plaintext);
    string ciphertext[length];

    printf("ciphertext: ");
    for (int i = 0; i < length; i++)
    {
        //if this letter is a char
        char plain_char = plaintext[i];
        if (isalpha(plain_char))
        {
            int index = char_index(plaintext[i]);
            char cipher_char = key[index];
            if (isupper(plain_char))
            {
                cipher_char = toupper(cipher_char);
            }
            else
            {
                cipher_char = tolower(cipher_char);
            }
            printf("%c", cipher_char);
        }
        else
        {
            printf("%c", plain_char);
        }

    }
    printf("\n");
}

//get the index of each char to be used in the key
char char_index(char c)
{
    int index = 0;
    //determine the positional value of char
    if (isupper(c))
    {
        index = ((int) c) - 65;
    }
    else if (islower(c))
    {
        index = ((int) c) - 97;
    }
    return index;
    //use this to determine the value of cipherchar
    //return cipher
}