Shift Cipher String Encryption in C!

caesar.c

// this program is run on the command line.  Fist compile the program with clang or C compiler. 
// then run ./<program name> <key> 
// the key is a numerical value greater or equal to zero and is required. 
// The program will then ask you for a plaintext message to encrypt using your key. 
// The program returns ciphertext. 


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

int errorHandle(void);
int numCheck(string str);
int encrypt(int key, string message, int encryptedMessage[], int inputLength);
int encryptAlgo(char c, int key, int count, int cipherArr[]);
string printCipher(int array[], int length);
string getMessage(void);

int main(int argc, string argv[])
{
    string key = argv[1];

    if (argc == 2)
    {
        // is the key a valid number greater or equal to 0?
        numCheck(key);

        // turn string argv to integer
        int keyInt = atoi(key);

        // get plaintext
        string message = getMessage();

        // get the length of the message
        int inputLength = strlen(message);

        // initialize encrypted message array
        const int L = inputLength;
        int encryptedMessage[L];

        // call the encryption function
        encrypt(keyInt, message, encryptedMessage, L);

        // print the encrypted message
        printCipher(encryptedMessage, L);

    }
    // handle errors if argv != 2
    else
    {
        errorHandle();

    }
}



// Utility functions //////////////////////////////////////////////////////////////////////////////////////

// error handler //

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

// numCheck: check the key to see if its all digits //

int numCheck(string str)
{
    int count = 0;
    for (int i = 0; i < strlen(str); i++)
    {
        if (isdigit(str[i]))
        {
            count++;

            if (count == strlen(str))
            {
                return 0;
            }
        }
        else
        {
            errorHandle();
        }
    }
    return 0;
}

// getMessage //

string getMessage(void)
{
    string message = get_string("plaintext: ");
    return message;
}


// encrypt the message //

int encrypt(int key, string message, int array[], int length)
{

    for (int i = 0; i < length; i++)
    {
        if (message[i] >= 'a' && message[i] <= 'z')
        {
            encryptAlgo(message[i], key, i, array);
        }

        else if (message[i] >= 'A' && message[i] <= 'Z')
        {
            encryptAlgo(message[i], key, i, array);
        }

        else if (message[i] == ' ')
        {
            array[i] = message[i];
        }

        else if (message[i] == '?' || message[i] == '!' || message[i] == '.' || message[i] == ',')
        {
            array[i] = message[i];
        }

    }
    return 0;

}

// encrypt the letters

int encryptAlgo(char c, int key, int i, int array[])
{
    // coerce the char to int
    int asciiChar = (int) c;
    // create a sum betewen the ascii value of the char and the key
    int shift = asciiChar + key;
    const int A = 65;
    const int Z = 90;
    const int a = 97;
    const int z = 122;

    // check to see if the char is upper case
    if (asciiChar < 91)
    {
        // if the total shift is less than the upper bound of the uppercase ASCII values
        if (shift <= Z)
        {
            // if shift is under the upper bound then save the character
            int newAsciiLetter = shift;
            array[i] = newAsciiLetter;
        }
        else
        {
            // figure out difference from upper bound to char.
            int toEnd = Z - asciiChar;
            // compute the difference remaining from the key.
            int diffFromKey = key - toEnd - 1;
            // find the modulo of the difference and the set of letters then add to the beginning of the set to find your char
            int newAsciiLetter = ((diffFromKey % 26) + A);
            array[i] = newAsciiLetter;
        }

    }
    // if the char isn't uppcase, its lowercase.
    else
    {

        if (shift <= z)
        {
            int newAsciiLetter = shift;
            array[i] = newAsciiLetter;
        }
        else
        {
            int toEnd = z - asciiChar;
            int diffFromKey = key - toEnd - 1;

            if (diffFromKey > 26)
            {
                int newAsciiLetter = ((diffFromKey % 26) + a);
                array[i] = newAsciiLetter;
            }
            else
            {
                int newAsciiLetter = (a + diffFromKey);
                array[i] = newAsciiLetter;
            }
        }

    }

    return 0;
}

// print the encryption cars

string printCipher(int array[], int length)
{
    printf("ciphertext: ");
    for (int i = 0; i < length; i++)
    {
        char cipherChar = (char) array[i];
        printf("%c", cipherChar);
    }
    printf("\n");
    return 0;
}