Comment by AnthonyMouse
7 hours ago
If the driver's license can generate new anonymous tokens itself then anyone can hook up a driver's license to a computer and set up a service to sign for everybody. If it can't, whenever you want to prove your age to a service you need to get a new token from a third party, and then there is a timing correlation because you're asking for the token right before you use the service.
The article proposes a hypothetical solution where you get some finite number of tokens at once, but then the obvious problem is, what happens when you run out? First, it brings back the timing correlation when you ask for more just before you use one, and the number of times you have to correlate in order to be unique is so small it could still be a problem. Second, there are legitimate reasons to use an arbitrarily large number of tokens (e.g. building a search index of the web, content filters that want to scan the contents of links), but "finite number of tokens" was the thing preventing someone from setting up the service to provide tokens to anyone.
Blocking said search indexes is probably a good thing.
I'm thinking perhaps a system where you feed it a credential, a small program runs and maintains a pool of tokens that has some reasonably finite lifespan. The server that issues the tokens restricts the number of uses of the credential. Timing attacks are impossible because your token requests are normally not associated with your uses of the tokens.
And when you use a token the site gives back a session key, further access just replays the session key (so long as it's HTTPS the key is encrypted, hard to do a replay attack) up to whatever time and rate limits the website permits.