At last Friday’s Security Group meeting, we talked about security protocols that are intended to deter or reduce the consquences of theft, and how they go wrong.

Examples include:

• GSM mobile phones have an identifier for the phone (separate from the identifier for the user) that can be blacklisted when the phone is stolen.
• Some car radios will stop working when the battery is disconnected, and only start working again when a numeric code is entered. This is intended to deter theft of the radio.
• In Windows Vista, Bitlocker can be used to encrypt files. One of the intended applications for this is that if someone steals your laptop, it will be difficult for them to gain access to your encrypted files.

Ross told a story of what happened when he needed to disconnect the battery on his car: the radio stopped working, and the code he had been given to reactivate it didn’t work – it was the wrong code.
Ross argues that these reactivation codes are unecessary, because other measures taken by the car manufacturers – such as making radios non-standard sizes, and hence not refittable in other car models – have made them redundant.

I described how the motherboard on a laptop had needed to be replaced recently. The motherboard contains the TPM chip, which contains the encryption keys needed to decrypt files protected with Bitlocker. If you replace the motherboard, the files on your hard disk will become unreadable, even if the disk is physically OK. Domain-joined Vista machines can be configured so that a sysadmin somewhere within your organization is able to recover the keys when this happens.

Both of these situations suffer from classic usability problems: the recovery procedures are invoked rarely (so users may not know what they’re supposed to do), and, if your system is configured incorrectly, you only find out when it is too late: you key in the code to your radio and it remains a doorstop; the admin you hoped was escrowing your keys turns out not to have the private key corresponding to the public key you were encrypting under (or, more subtly: the person with the authority to ask for your laptop’s key to be recovered is not you, because the appropriate admin has the wrong name for the laptop’s owner in their database).

I also described what happens when an XBox 360 is stolen. When you buy XBox downloadable content, you buy two licenses: one that’s valid on any XBox, as long as you’re logged in to XBox live; and one that’s valid on just your XBox, regardless of who’s logged in. If a burglar steals your Xbox, and you buy a new one, you need to get another license of the second type (for all the other people in your household who make use of it). The software makes this awkward, because it knows that you already have a license of the first type, and assumes that you couldn’t possibly want to buy it again. The work-around is to get a new email address, a new Microsoft Live Account, and a new Gamer Tag, and use these to repurchase the license. You can’t just change the gamertag, because XBox live doesn’t let the same Microsoft Live account have two gamertags. And yes, I know, your buddies in the MMORPG you were playing know you by your gamertag, so you don’t want to change it.