#include <stdio.h>
#include <stdlib.h>

#define MODE_ENCRYPT 1
#define BUF_SIZE 1024

void InputError();
void Encrypt(char *text, int key);

int main(int argc, char *argv[])
{
	int i, n, mode, key;
	char input[BUF_SIZE];
	// make sure the argument count was as expected, if not, print error
	if(argc != 3)
	{
		InputError();		// if you pass a code of a string here, you can add a custom error message for each failure case
		return EXIT_FAILURE;
	}
	else
	{
		// now check the work command
		if(strcmp(argv[1], "-e"))	// alternative: check for argv[1][0] == '-', argv[1][1] == 'e', ...
		{
			mode = MODE_ENCRYPT;	// detected encryption mode
		}
		// and now verify that the cipher is a number and then convert it(atoi, sscanf, ...)
		for(i = 0, n = strlen(...); i < n; i++)
		{
			// check each character if it is a number
		}
	}
	printf("Enter text to %s\n", mode == MODE_ENCRYPT ? "encrypt" : "decrypt");
	fgets(input, BUF_SIZE, stdin);
	switch (mode)
	{
		case MODE_ENCRYPT:
			Encrypt(input, key);
			break;
	}
	return EXIT_SUCCESS;
}