There's some chance you've already seen this kind of remote at least once. My garage door openers are branded Craftsman (a Sears brand), but I believe they were actually manufactured by Chamberlain or at least licensed from them. The date code on the back of the remote is "12/03", and that's probably about the right timeframe for when the openers were installed.

If you are interested, you can find some details about this remote by looking up the FCC ID. Although it was manufactured in 2003, the FCC certification was done in 1998. So, it's been around for a while. The Sears model number for the remote is 139.53879. It's pretty easy to find user documentation for this remote or even to buy one today. A lot is known about this sort of remote because the security protocols, marketed as SECURITY+, were described in a patent. Although that specific patent has expired and the original SECURITY+ stuff has moved to a new generation, I haven't chased down whether there are any active patents for the first generation technology. An industrious person has done a Python software implementation of both generations.

For a while, I toyed with the idea of implementing a SECURITY+ v1.0 remote with a microprocessor. I have a pile of ESP8266/ESP32 variants in my cupboard looking for a use case. For the radio signals, I figured they must be using an ISM band frequency, and I do have some microprocessor-compatible 433mHz radio components. Alas, these remotes operate on 390mHz frequency. Maybe I could do something with software-defined radio, but I don't have any SDR transmitters. It was all starting to sound a bit complicated.

The conventional wisdom is to just tear an off-the-shelf remote apart and use its circuit board as the guts of the solution, and that is where I turned.