[jonathanlydall]

Blizzard (I suspect) did exactly this with the original Warcraft III.

To play on an emulated Battle.net server you had to apply a patch to the client .exe or it would refuse to connect.

I remember chatting with one of the emulator developers on IRC and they said it was practically impossible to reverse engineer the key, so that sounded to me like public/private key encryption.

I suspect Blizzard reasoned that it made the emulated servers less legitimate and that people uncomfortable downloading "cracks" would be put off. It didn't have to be fool proof, just raise the bar high enough to encourage more sales.