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Removing expiry dates for TOFU

Solderpunk solderpunk at posteo.net

Tue Jul 7 11:14:30 BST 2020

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A proposal: "TOFU-TOTS". You know, like tater tots, but withoutpotatoes in them. Or rather, trust-on-first-use augmented bytrust-over-time-and-space.

Desiderata:

Limit actions of blind faith to a single point in time.* Allow ever more trust to slowly, organically build up after that point.* Permit fully automated regular key rotation on servers.* Don't require client authors to write code to validate signatures.

Assumptions:

MITM attacks limited to a single network for a short duration of time are a plausible threat, but MITM attacks which target an individual user no matter which network their using are extremely hard, and even on a single network (home ISP, say), MITM attacks which last for months or years require dedication and are more likely to be whistleblown.

Quick description:

Certs last for several years* The very first time you connect to a server (or the first time after so long that all your previous knowledge about that server is badly out of date), blindly accept whatever you get.* New keys and certs are generated 1 year before they become valid (consecutive certs have maybe 1 month of overlapping validity to allow for change over)* New certs, or their fingerprint, are served on a well-known end point immediately after generation, i.e. for roughly one year before their validity begins.* TOFU-TOTS clients have a "trust maintenance" procedure which should be run, say, once a month, or maybe every 3 months. Fancy clients can do this automatically in the background on a separate thread. Simple clients can just keep track of when this was last run, check that against the current date on startup and say "Hey, you haven't done trust maintenance in a while, wanna do it now? Y/N" without bothering with threads. Users who are security nerds should be able to run it on demand whenever they like.* Trust maintenance involves: - Removing expired certificates from the TOFU database - If a non-expired cert in the DB has less than 1 year of validity left, hit the server's well known endpoint and, assuming that transaction takes place using a previously trusted cert, store the cert/fingerprint you see at that endpoint, along with a counter of how many times you've observed it. - If the same well known endpoint ever serves up different future certificates while the same current certificate is valid, "warn hard".* When a new cert comes down the line, "warn hard" if it's completely out of the blue. Accept it silently if it's been pre-advertised by the appropriate server at least N times previously (perhaps "warn softly" if it's been advertised less than N times but at least once), and delete that server's previously trusted cert (possibly incrementing a counter of the number of trusted roll-overs this server has achieved, permitting the option to "warn extremely hard" if we ever get an out of the blue cert change for a server which has previously done the right thing multiple times).

Properties:

Meets all desiderata above.* If I steal your private key, I can straightforwardly use this in attacks only until your current cert expires - not forever, as would be the case with 100 year certs.* In order to extend the time I can exploit the stolen key, I need to successfully MITM every possible future victim of my attacks, for multiple executions of their trust maintenance routine, which happen at unpredictable times, over the course of a year, possibly from multiple different networks, and I can't do this if I steal your key in its final year of validity, because victims will have already picked up the true next certificate at least once - this is a big improvement over roll-over schemes where new certificates are signed with the private key of the old certificate, in which case once I've stolen your key it's trivial for me to just sign myself future certificates which anybody will accept - and I can keep doing this, forever, spinning a single key breach out indefinitely despite regular rotation!* Clients are, as always, vulnerable to undetected MITM attacks on that very first connection, but such an attack can be fairly easily discovered after the fact (providing a strong disincentive for e.g. commercial ISPs to do them regularly) unless the attacker commits to a clean exit by MITMing you multiple times in the future to feed you the legitimate next cert.

Right now, I think this provides pretty good robustness against MITMattacks relative to how conceptually complicated it is - nothing isinvolved on the server side except generating stand-alone self-signedcerts and copying them at the right time into the document root, whichis very easily automated, and nothing is involved on the client sideexcept storing hashes of entire certs in a DB and checking expiry dates.Client authors never have to write their own signature validation code,so this can be done even with TLS libraries that don't make that easy.It also has the advantage of keeping TLS network overhead as low aspossible by having servers only ever send a single cert.

But, maybe I've overlooked something. Feedback welcome.

Cheers,Solderpunk