A simple string encryption in C++

First off i should say that i dont want to use any windows api cause it needs to be platform independent. Does anybody know of a way to code in c++ a simple program that can take a string input & a cipher value & then encrypt that message using the cipher code. For instance if the cipher code is 2 then it will take each character of the text inputted & jump 2 letters to the next letter in our alphabet system to encrypt the text.
For example: Cipher = 2, Text="this works"
Encrypted text: vjku yqtu

Thanks in advance.