Real engineered solution - without new hardware: If this thing is not reprogrammable, and only has an EPROM - do some real enginerering and calculate the ADDITION of bits to set to disable the exploit. Thats the one I would be working on if I worked for Onity.
alternativly, take a mechanical approach to the problem - if you can live without the connector for servicing the lock.
1) De-solder the connector on the board and cut the traces/pads off the board - it won't stop everyone, but enough that have read of the exploit and try to follow through on it without applying any more critical thinking will be thwarted.
2) epoxy over the connector (they kind of did this with the security screw fix, but not really)
3) leave the connector, but add so much resistance between the connector and uP that you have to use a special interface cable to talk to the uP. no one will be able to tell until they pull the lock apart that its not stock.
Even if you fix the vulnerability present in the lock firmware (which you can't do without replacing the Portable Programmer as well), the encryption on the cards is still completely broken.
I've written at length about how this can be fixed; Onity has not yet responded with an effective solution.
Yeah, this is what I found fascinating in your paper(http://demoseen.com/bhpaper.html). I had always wondered how they invalidated the old keys automatically.
Out of curiosity, was that part clear? Writing the section on key rotation and lookaheads took me something like 4 days of editing, and I was never actually happy with it.
If these work like they used to - the connector is used to sync the lock with the key machines at the front desk. It requires a reprogram if the master keys need to change (ie someone is fired), batteries die in the lock, etc. Additionally it provides self test info and obviously if you need to force it open (i.e. Maintenance may send the open command in case of reader malfunction). That's why the mechanical solution doesn't involve physical changes to the lock guts - just the housing.
[D]o some real enginerering and calculate the ADDITION of bits to set to disable the exploit.
That is very unlikely to be possible. The ROM may be a masked ROM, in which case it is not re-programmable at all. Quite likely it is a one-time programmable (OTP) ROM. For a OTP, at best you can flip "1" bits to "0", but you cannot change "0" bits to "1". It would take a large amount of luck to be able to patch "1"s to "0"s (and not need to change any "0"s to "1"s) to vector to patched code fixing the vulnerabilities.
In addition, many programmable memories require special programming voltages and they all need the proper control signals - very often the ROM is not in-circuit programmable or is in-circuit programmable only via a test/programming circuit at the factory, not in the field.
WRT #1 and #2, the reason for the connector is to allow the hotel staff to recover from Bad Things like dead batteries and confused/mis-keyed locks. I know I've been the victim of dead batteries more than once... if the only recourse is to destroy the lock to get into the room, the hotel is going to be very unhappy and the guest isn't going to be very pleased either.
#3 is "security through obscurity", which will be effective briefly until the next security researcher figures out how to defeat the change.
Your comments on the most likely one time programmable ROM are exactly what I was thinking but didn't take the time to write with the clarity you did. We are on exactly the same page there. My train of thought was that if it was possible to patch the boards using only software that would be an awfully nice gesture to the hotel managers to send a technician out for a day and just fix everything at once [although, if you have to rip the lock apart to pull the board to reprogram it, you might as well just drop a new boad in]. My background is in board level manufacturing so the whole special test fixtures to interface and board probing was the easy part to me - solving the problem of hacking a jump instruction to jump to unused memory and then patching the problem there, then jumping back seemed like a really elegant and fun problem to work on.
I didn't look at the exploit in detail, but as daeken reminded us there are problems with more than just the program in the door lock - so even if a patch to the ROM chip could fix the problem, it probably doesn't fix all the problem so it isn't a real solution.
You are right, everything else 1-3 are really just obscurity solutions and not real solutions - thanks for calling me out on that.
Hotels can't even get their internet right. Shit is outsourced to some service company who can't fix on-site problems with their routers, and you just get a shrug of the shoulders from hotel maintenance personnel. How in the unholy fuck do you think a Ramada Inn is going to roll out hundreds of modded door locks?
They don't mod the locks, they call up Onity and say "send us 600 new locks that are not flawed." Onity replies "Sorry, no." Then they fight in court for the next 10 years over whether Onity owes the hotel replacement locks.
Assuming the hotel wins, Onity sends a team in to replace the locks (a relatively simple and already solved problem - that is how the original locks were installed). Then Onity sends the hotel the bill for the replacement service and the hotel says "Sorry, no." Then everybody fights in court over the retrofit bill for another 10 years.
alternativly, take a mechanical approach to the problem - if you can live without the connector for servicing the lock.
1) De-solder the connector on the board and cut the traces/pads off the board - it won't stop everyone, but enough that have read of the exploit and try to follow through on it without applying any more critical thinking will be thwarted.
2) epoxy over the connector (they kind of did this with the security screw fix, but not really)
3) leave the connector, but add so much resistance between the connector and uP that you have to use a special interface cable to talk to the uP. no one will be able to tell until they pull the lock apart that its not stock.