Encrypt & Decrypt input text to Caesar Cipher. (CS50). Input a numeric key and a text to encrypt it into ceaser ciphered code. Send it to anyone. Those who dont know the key cant read your message, those who have the key can decrypt and read it. :)

Caeser Cipher in C.c

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

void caesarEncrypt(char plaintxt[], int key);
void caesarDecrypt(char plaintxt[], int key);

int main(void)
{
    char plaintxt[500];
    char encrypted[500];
    char option;
    int key=1;


    printf("Please input your text: (Maximum 500 characters)\n");

    fgets(plaintxt,500,stdin);

//only integer digits as input will work
     printf("Please input your key: (Numbers only please)\n");
     scanf("%d",&key);

//Option to encrypt or decrypt
    do{
    printf("Enter E to Encrypt & D to Decrypt.\n");
    scanf(" %c",&option);
    }
    while (option != 'E' && option != 'D' && option != 'e' && option != 'd');

    printf("\nYour key is: %d \nYour text length is: %d",key,(int)strlen(plaintxt)-1);


//Now I have string plain  text[], and key

    strcpy(encrypted,plaintxt);

    //Encrypt or Decrypt
    switch(option)
            {
                case 'e':
                case 'E': printf("\nYou chose to ENCRYPT\n");
                          caesarEncrypt(encrypted,key);
                          break;
                case 'd':
                case 'D': printf("\nYou chose to DECRYPT\n");
                          caesarDecrypt(encrypted,key);
                          break;
            }


    printf("\nCaeser cipher complete. \n\nYour secret message: ");
    fputs(encrypted,stdout);
    printf("\n\n\n");


    return 0;
}

void caesarEncrypt(char plaintxt[], int key)
{
    int numAlphas = 26;     //number of alphabets to wrap around
    key=key%26; //modulo rounds the key to below 26, plus 1 makes sure key is not 0

    int i;
    for (i=0;i<(int)strlen(plaintxt);i++)   //Run loop for length of text
        {
            if(isalpha(plaintxt[i]))    //Only encrypt if its alphabetic
            {

            //To keep Uppercase and Lowercase formatting, two separate if else's used.

               if(isupper(plaintxt[i]))     //for Uppercase
                {
                    if(plaintxt[i]+key<='Z')    //Wrap around Z
                    {
                        plaintxt[i] = plaintxt[i]+key;
                    }
                    else        //If exceeds Z, wrap and go to A
                    {
                        plaintxt[i] = plaintxt[i]+(key-numAlphas);
                    }

                }

                //Exactly same as above, but for lowercase

                if(islower(plaintxt[i]))
                {
                    if(plaintxt[i]+key<='z')
                    {
                        plaintxt[i] = plaintxt[i]+key;
                    }
                    else
                    {
                        plaintxt[i] = plaintxt[i]+(key-numAlphas);
                    }

                }
            }
        }

}

void caesarDecrypt(char plaintxt[], int key)
{
    int numAlphas = 26;     //number of alphabets to wrap around
    key=key%26; //modulo rounds the key to below 26, plus 1 makes sure key is not 0

    int i;
    for (i=0;i<(int)strlen(plaintxt);i++)   //Run loop for length of text
        {
            if(isalpha(plaintxt[i]))    //Only encrypt if its alphabetic
            {

            //To keep Uppercase and Lowercase formatting, two separate if else's used.

               if(isupper(plaintxt[i]))     //for Uppercase
                {
                    if(plaintxt[i]-key>='A')    //Wrap around A
                    {
                        plaintxt[i] = plaintxt[i]-key;
                    }
                    else        //If preceeds A, wrap and go to Z
                    {
                        plaintxt[i] = plaintxt[i]-(key-numAlphas);
                    }

                }

                //Exactly same as above, but for lowercase

                if(islower(plaintxt[i]))
                {
                    if(plaintxt[i]-key>='a')
                    {
                        plaintxt[i] = plaintxt[i]-key;
                    }
                    else
                    {
                        plaintxt[i] = plaintxt[i]-(key-numAlphas);
                    }

                }
            }
        }

}