So, I’ve decided that the first project I’m planning on doing is a door access control system which I’m calling LIAC because naming things is fun Here’s the current plan… I’ll update this as I progress, hopefully.
LIAC - Low Intrusion (open) Access Control
LIAC has a relatively unique design constraint: it should be able to be installed with as little modification as possible to the door frame, handle, and wall near the door that’s being access controlled, hence, low intrusion. The idea is that it should be able to be installed on rental properties with as little friction as possible.
The system is, for now, going to be focused on interior door access control, i.e. to a bedroom, office, etc. rather than to control an exterior door, which would likely require significantly more modification to be as secure as you’d want it to be. Still, a similar system might be viable with some modification, but that’s not the focus (at least for now). Secondary design goals are to be low cost and use readily available hardware and software while still retaining a relatively high level of security.
Of course, residential interior doors and frames are often going to be vulnerable to brute force physical attacks, but this system should try to reasonably thwart the most common physical and electrical bypass techniques (latch slipping, under/over door attacks, “hotwiring”/wire shorting, data wire sniffing) to serve as a deterrent in the event of a break-in: the vast majority of break-ins last less than 10 minutes, and commonly less than 5, so all we need to do here is not get owned by the lowest common denominator of effort.
High-level system overview
The system is designed to work with HF NFC chips, especially the xDF2/DESFire 2 based chips (it should be relatively trivial to make it work with most 13.56 MHz NFC chips as well, though). The system will be based around controlling an electric strike for access control, and is split into two primary parts: the reader and the controller.
The reader will be a self-contained, custom-designed circuit board and enclosure that goes on the outside of the door. It will contain a PN532 NFC controller and an nRF52810 SoC which will provide encrypted wireless communication to the controller through Bluetooth Low Energy. This will allow the reader to be affixed in a non-destructive manner to a wall or directly onto the outside of the door and will provide secure communication to the controller on the secure side of the door. It will be battery-powered, probably by standard AA or AAA batteries that are easy to replace if they die and should provide power for a long period of time before needing to be replaced.
The controller goes on the secure side of the door and will be in charge of actually controlling the strike. It will be wall-powered and the brains will be a Raspberry Pi Zero W. It will receive data from the reader wirelessly, and will be running a piece of software using libnfc to figure out what to do with the data the reader sends it and decide how to control the strike accordingly.
That’s all for now. Next, I will go into some more detail about the design of both controller and reader as I work it out. Been researching a bunch towards designing the reader PCB, which is really the “fun part” of the project for me as it will be my first custom PCB design