FWIW, I'm working on an Arduino/Particle port [1] of the HAP protocol, trying to emulate the Home client using 32-bit C/C++ crypto libraries from the OSS WolfSSL [2]. The folks from Apple Home have been supportive, although their spec needs some help [3]...

[1] https://github.com/moflo/homekit-accessory-emulator [2] https://github.com/wolfSSL/wolfssl [3] https://developer.apple.com/homekit/