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February 8, 2020

Short Take: Why Trust-On-First-Use Doesn't Work (Even for SSH)

Considering all the progress that has been made over the last decade making SSL certificates on the Web easy, free, automated, and transparent, it's a bit jarring to see someone arguing in 2020 that trust-on-first-use (TOFU) would be better for the Web:

Unpopular opinion. Most people would be better off with a Trust On First Use system for accessing sites. Like SSH, perhaps with some unique (per user) OOB addition to it. Would we really design it this way of starting again?

— Nick Hutton @nickdothutton, Feb 6, 2020

First, be wary of any comparison with SSH, because in the grand scheme of things, very few people use SSH. *nix sysadmins do, obviously. Many, but not all, software developers do. Some people in engineering/science fields might. But that's a drop in the bucket compared to the Web, which basically everyone uses. So just because something appears to work for SSH doesn't mean it will work for the Web.

And I would argue that TOFU actually doesn't work very well for SSH, and the only reason we put up with it is because of SSH's low deployment. SSH server host keys rarely change (which is bad for post-compromise security, so this is nothing to celebrate), but when they do, SSH handles it very poorly. The user gets a big scary message about a possible man-in-the-middle attack. And then what do you think they do? They do this:

Hi all,

It appears that as of midnight last night, SSH and login are working. However, there were a couple students last night who were getting errors such as “REMOTE HOST IDENTIFICATION HAS CHANGED!” or “POSSIBLE DNS SPOOFING DETECTED!” when trying to SSH in.

To fix this, you can run `ssh-keygen -R [REDACTED]` then try to SSH in again. I believe someone else mentioned last night that you could also just delete the entire ~/.ssh/known_hosts file as well to fix the issue, but this seems to be a less destructive solution.

That's from a real email that I once received. I would not be at all surprised if TOFU actually devolves to opportunistic encryption in practice, because users just bypass any man-in-the-middle error they receive.

You could make it really hard to bypass man-in-the-middle errors, but then people would brick their servers, as happened with HTTP public key pinning, which is one of the reasons why that technology is now extinct.

Proponents of TOFU might say that even if TOFU devolves to opportunistic encryption, the man-in-the-middle errors at least make attacks noisy. True, but the errors are seen by people who generally don't know what they mean and even if they did, can't evaluate whether an error is a legitimate key change or an actual attack. In contrast, a PKI with Certificate Transparency (i.e. the system currently deployed on the Web) also makes attacks noisy, but alerts about new certificates go to server operators, who actually know whether a new certificate is legitimate or not. They just need to be monitoring Certificate Transparency logs.

So yes, I do believe we would design the Web this way if starting again.


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