The original LEGO Mindstorms had a handshaking mechanism like this. The firmware for the RCX brick was stored in battery-backed RAM, so you had to re-upload it

The original LEGO Mindstorms had a handshaking mechanism like this. The firmware for the RCX brick was stored in battery-backed RAM, so you had to re-upload it via infrared whenever you changed the batteries. But the ROM bootloader would only accept a firmware image if it contained the string: > "Do you byte, when I knock?" After uploading, the host computer would send an "unlock request" with the string "LEGO®", at which point the RCX would return the other half of the rhyme: > "Just a bit off the block!" I assume the only reason for jumping through these hoops was to have a legal excuse to discourage other companies from making interoperable products. I have no idea whether this strategy would have actually held up in court; in any case, there were plenty of homebrew tools that just embedded the magic strings anyway.