C++ Programming question

You have to make a program that encrypts and decrypts messages. Probably the easiest way of doing this would be to use a Ceaser cipher.  
When the program starts it needs to ask the user if they want to encrypt a message, decrypt a message or to show the stored message. For these parts to work the user will need to know the key to solving the cipher and will need to enter that after selecting an option. 
(Example: If using a Ceaser cipher the user needs to know how many spaces to shift the alphabet so it lines up correctly.) 
So this mean that you have to submit the key to me when you send the code. 
The stored message is, “Enryption or decryption successful. This system is ready for further testing of messages.”
If the user selects the option to show the stored message, after accepting the key from the user, the program should display this message in its encrypted form and in its decrypted form. 
Also I have to see the basic structure of the following items. 
     An Array(s)
     A Class(s)
     Strings
     Recursion
     Inheritance