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Aboutvar me = new mikael();
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Joined devRant on 6/3/2016
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Let's talk a bit about CA-based SSH and TOFU, because this is really why I hate the guts out of how SSH works by default (TOFU) and why I'm amazed that so few people even know about certificate-based SSH.
So for a while now I've been ogling CA-based SSH to solve the issues with key distribution and replacement. Because SSH does 2-way verification, this is relevant to both the host key (which changes on e.g. reinstallation) and user keys (ever replaced one? Yeah that's the problem).
So in my own network I've signed all my devices' host keys a few days ago (user keys will come later). And it works great! Except... Because I wanted to "do it right straight away" I signed only the ED25519 keys on each host, because IMO that's what all the keys should be using. My user keys use it, and among others the host keys use it too. But not by default, which brings me back to this error message.
If you look closely you'd find that the host key did not actually change. That host hasn't been replaced. What has been replaced however is the key this client got initially (i.e. TOFU at work) and the key it's being presented now. The key it's comparing against is ECDSA, which is one of the host key types you'd find in /etc/ssh. But RSA is the default for user keys so God knows why that one is being served... Anyway, the SSH servers apparently prefer signed keys, so what is being served now is an ED25519 key. And TOFU breaks and generates this atrocity of a warning.
This is peak TOFU at its worst really, and with the CA now replacing it I can't help but think that this is TOFU's last scream into the void, a climax of how terrible it is. Use CA's everyone, it's so much better than this default dumpster fire doing its thing.
PS: yes I know how to solve it. Remove .ssh/known_hosts and put the CA as a known host there instead. This is just to illustrate a point.
Also if you're interested in learning about CA-based SSH, check out https://ibug.io/blog/2019/... and https://dmuth.org/ssh-at-scale-cas-... - these really helped me out when I started deploying the CA-based authentication model.
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