ATmega328 (Arduino Uno compatible) MD5 optimized assembly implementation

Disclaimer: the things I am writing about are new to me. Although I do my best to have a solid (well, decent in this case) understanding of the covered topics, some inaccuracies might have slipped through. Feel free to point them out in the comments.

Hey guys, or anyone who still happens to visit this blog. First of all, thanks for the Pwnie Awards nomination!

I’ve been recently toying with the idea of learning some electronics that’s not necesarilly related to desktop PCs or the Intel X86(-64) architecture. Finally, the choice fell on programming AVR microcontrollers, or specifically playing with the Arduino Uno board (including an ATmega328 unit) and its many optional shields which can make a project physically functional. As you can see, there’s totally nothing out of ordinary going on, but it’s a start and I’ll probably take the chance to share anything that takes a few hours to learn :-) My first idea for the very initial project was to write an optimized MD5 hash function implementation and create a simplistic bruteforce password cracker. Because of the very limited CPU resources (16MHz) and the fact that avr-gcc 4.3.3 turned out not to be so clever about generating efficient code, I decided to learn some of the AVR architecture, make a first bunch of silly mistakes and write the code in assembly.

As a consequence, I created a very basic md5 library, with the public interface (md5_init, md5_update, …) in C and the computation-heavy part calculating the hash itself (md5_transform) in assembly; it is the subject of this post. As far as I can tell, the code should work on any of ATmega48PA, ATmega88PA, ATmega168PA, ATmega328P MCU and potentially some others. It’s been thoroughly tested both manually and automatically against another third-party implementation, but I do not guarantee that it would always work as expected. Use at your own risk.

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