Here's some code for cin that encrypts with 3. You probably want the key to be set once and used for every character instead of a different key for each character. That way you don't have to keep save all the keys for decryption.
Code:
#include <iostream>
#include <cctype>


using namespace std;


int main() {
  int key = 3;
  char x;

  while ( cin.get( x ) ) {
    if ( isalnum( x ) ) {
      x += key;
    }

    cout << x;
  }
}
To decrypt you need the same key and just do -= instead of +=.