We expect to keep an eye on biometric authentication technologies as they continue to become cheaper and more reliable, but at this time we believe that the cost/reliability tradeoff is not where it needs to be for our target users. We expect these devices to be covered in our users' fingerprints, so a low-cost fingerprint scanner could actually increase the likelihood of compromise. We were able to break into one device that used facial recognition authentication software just by holding it up to the user's photo. Bluetooth adds a whole new software stack to our login/screenlocker code that could potentially be buggy, and the security of the pairing protocol has been criticized in the past. Smart cards and USB crypto tokens are an interesting technology, but we don't want our users to have to keep track of a physically distinct item just to use their device.