There are a lot of ways you can do encryption, Just decide which way you want to proceed.. Which mode of operation you want to do. Which sequence you want to follow when you are XOR ing the data. Do you want to add any salt values to the key? Do you want to transform the key to something else? Do you want to follow any standard Algorithm?

Once you decide your strategy, just code that in C++. To start with, i will recommend you to read more on Ecnryption