Optiboot (an older version) is installed by default on the Arduino Uno and (as of 2018) official Arduino Nano boards. It can be installed on all older mega8, 168 or 328 based Arduinos. As of 2019, ...

Add base64.cpp and base64.hpp to your project folder or library search path, put #include "base64.hpp" in your source, and pass base64.cpp to your compiler ...