I don’t normally write bloggy posts on every version of every source file I check in, but every so often an older project sees some more activity, and I love the opportunity to revisit software I wrote years ago. Sometimes I wonder why I wrote such-and-such thing that way and oh my god what an ugly hack. But usually it’s just refreshing to think about a problem I haven’t thought about in a while.

The AVRFID was a quick but rather unique project, back from when I was on a bit of a 125 kHz RFID binge culminating in the design, construction, and installation of a proxcard reader for my garage door. While I was fidgeting around with such things, it occurred to me that you could (using a whole host of dirty tricks) convince a general-purpose 8-bit microcontroller like the AVR to function as a passive RFID tag.

Since then, I received a couple patches:

• Luke Koops improved the FSK modulation for HID tags, so that the resulting waveform is much more regular.
• Cesar Fernandez described the HID card format in more detail, and implemented a parity calculation. The 45-bit code is actually composed of four distinct fields:
  • A 20-bit manufacturer code or header, constant for all HID cards.
  • An 8-bit site code, unique to the particular security installation.
  • A 16-bit unique ID. These are often printed in decimal on the back of the card, and they seem roughly sequential.
  • An odd parity bit, covering the other 44 bits.

I didn’t have my RFID gear handy, so Cesar was kind enough to verify it with his official HID reader. So I stamped a new version number on it. If you’re interested in building your own HID card emulator, there is now a much better chance it will actually work with your reader

• Grab the latest source code from Subversion.