Everybody seems to have an opinion about the new TouchID fingerprint sensor on Apple’s iPhone 5S. I suppose I do, as well.

Critics object to the idea that a fingerprint sensor, no matter how good, should be used to safeguard critical data. Dustin Kirkland makes the case (via John Moltz) that biometric information is inherently bad as a substitute for a password, because it cannot be “independently chosen, changed, and rotated.”

I take his points seriously, and they seem well reasoned from a security point of view, but they are based upon the premise that passwords are the end-all be-all of security, when in fact common sense proves they are not. The oldest, most trusted, and most widely deployed method of authentication on the planet is in fact “biometric”: the human ability to recognize a familiar face. The fact that my appearance could technically be spoofed does not change the fact that arriving at the home of a childhood friend after 20 years of separation will still earn me an invitation to the dinner table, if not a bed for the night.

So fingerprints make lousy passwords. Who cares? Their use in practice need not replace other authentication schemes, it only needs to augment other schemes in a manner that increases overall security.

Most authentication systems in society are scaled appropriately for the context in which they are deployed. When I travel by airplane, I am asked to show a government ID to get through the security gate, but thereafter, a simple piece of printed paper will get me on a plane. The government takes it for granted that once I’m in the boarding area, the odds of somebody getting hold of my scrap of paper, or my getting hold of theirs, and neither one of us subsequently complaining, are marginally small. Furthermore, if the two of us mutually agree to swap tickets and travel to the other’s destination, we haven’t really caused any significant harm, except perhaps to the egos of the folks in charge at the TSA.

Boarding passes are little scraps of paper that make lousy identification cards. I can’t use them to reserve a hotel, file a police report, or obtain a marriage license. Yet in the right context, I can use one to travel from Boston to Shanghai without a single person batting an eye or thinking twice about verifying my identity.

In this sense the airline boarding pass is like an access token. On the web, an access token is something obtained by stronger authentication that permits continued access with weaker authentication. For example when I allow a Twitter client to connect to my Twitter account, I must first visit Twitter.com and possibly enter my full account credentials. After the access token has been vended however, it serves much like a boarding pass, allowing free access until and unless I or Twitter registers a complaint.

There’s another sort of abstract access token that has been available to users iOS devices since day one: your continuous use of the device. If you have in your possession an iOS device and you abstain from turning it off or letting it sit idle, you retain free access to the various data on the phone. From a security point of view, this token is even worse than a fingerprint: anybody, including the family cat, can sustain it if they are so inclined. Leave a phone on a table for 5 seconds, somebody else picks it up, they have your “activity token,” and they didn’t even need to scan your fingerprint.

I view the fingerprint sensor on the iPhone 5S and other devices as an opportunity for extending this kind of implicit authentication. It’s not a substitute for a password, but rather a convenient token for obtaining streamlined, continued access to protected resources. It’s the boarding pass that prevents you needing to take out your ID, and go through a body scanner or pat-down again, just to get on the damned plane.

We can argue about whether Apple has chosen the right boundaries for where a fingerprint should be traded for full authentication, but as a technology it stands to fill that gap between the frighteningly insecure “unlocked while active,” and frustratingly unusable “full authentication required when inactive.”

In short, I would like to see fingerprint authentication deployed in a way that pays respect both to the relative convenience and the relative insecurity of a fingerprint. If I can for example configure my phone to require a fingerprint unlock after 1 minute of inactivity, but to require a passcode unlock after 30 minutes of inactivity, my concerns about fingerprint security would be effectively put to rest. Could a malicious person steal a high quality impression of my thumb print, construct a prosthetic, fleshy representation of it, and use it to unlock my phone? Perhaps. But if they can’t do it within half an hour of stealing my phone, they better get to work on cracking the passcode.