C++ :: Gather Input From User And Encrypt / Decrypt Those Characters?

I have got to create a program that will gather input from the user and then encrypt & decrypt those characters.

I'm not very confident at coding so I'm sure many parts of my code are written poorly and not following the best practice so I have written a simple version of an algorithm where the program simply adds/subtracts a value of 2 to/from the ASCII values but I have discovered the use of the rand() and srand functions but I'm unsure how to go about using them within both my encrypt and decrypt function as a single value (static variable?).

Here is my code in its entirety at the moment.

#include <iostream >
#include <iomanip>
#include <cstdlib>
#include <ctime>
#include <string>
#include <cstring>
#include <fstream>
using namespace std;
char getMenuSelection (char m);
void encrypt (char key[]);
void decrypt (char code[]);

[code]....

This code currently doesn't execute due to ' undefined reference to 'encrypt(char*) '

Question

I am asking how to generate the random number and incorporate this into my encrypt and decrypt functions.

1. Would the use of a static or global variable make this work as is?

2. Would I need to create separate class files for both functions?