Engine Building Lesson 2: An Example MD5 Engine

Coming back after a month and two weeks, it’s time to resume with the next engine lesson, this time building an engine implementing a digest.

It doesn’t matter much what digest algorithm we choose. Being lazy, I’ve chosen one with a well defined reference implementation, MD5 (reference implementation is found in RFC 1321)

Engine building lesson 1: A minimum useless engine

In this lesson, we’re going to explore minimalism, in this case in the form of the most minimal engine possible (without obfuscating it).

The least boilerplate code for an engine looks like this:

#include <openssl/engine.h>

IMPLEMENT_DYNAMIC_BIND_FN(bind)
IMPLEMENT_DYNAMIC_CHECK_FN()

This example isn’t complete, it will not compile. However, it contains the absolute minimum required for those module to even be recognised as an OpenSSL engine.

Engine school, a path to writing standalone engines

For the longest time, it seems that people have wanted to have their diverse engines bundled with the OpenSSL source, as if there was no other way to build it or distribute it. Nothing could be further from the truth. Also, having the engine for some hardware bundled with the OpenSSL source presents a maintainance problem, and the better solution is for those who have an engine to maintain theḿ themselves.