only works for capital letters. Look at an ascii table and modify if statement to make it work for lowercase.

rot13.cpp

#include <cstring>
#include <iostream>
#include <string>

using namespace std;

char input[80];
int counter = 1;
string sInput;

int main()
{
        cin >> sInput;
        if(sInput.length() > 80){
                cout << "Input is greater than 80 characters..." << endl;
                return 0;
        }
        strcpy(input, sInput.c_str());

        for(int i = 0; i < sInput.length(); i++){
                if( input[i] >= 65 && input[i] <= 77){
                        input[i] +=  13;
                }else if(input[i] >= 78 && input[i] <= 90){
                        input[i] -= 13;
                }
                cout << input[i];
        }

        cout << endl;
        return 0;
}