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Poll HN: Do you use SSH certificates (not mere public-key authentication)?

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Re: Poll HN: Do you use SSH certificates (not mere public-key authentication)?

#131

Post by xyzzyz »

cubesnooper wrote: post #119
xyzzyz wrote: post #104

Earlier quoted context omitted.

So anyone on your LAN can visit the URL and download the CA private key? Isn’t it only marginally more secure than just keeping the private key on your workstation in the first place, and foregoing the entire rigamarole with certificates? I mean, if you are worried about your something compromising your workstation and stealing your individual private keys, nothing is stopping whoever compromised your workstation fro…

> So anyone on your LAN can visit the URL and download the CA private key? No, the only files served to the LAN are the certificates, which contain the signatures by the CA of the public keys of other machines. Those are safe to distribute openly because they’re useless without the private key of the public key that was signed.

Okay, so let me make sure I understand it. You have your own personal key, and CA key. Your personal (private) key lives on your workstation, and CA key lives on RPi. There are some other machines that you want to SSH into. They trust CA public key. To log into them, you download client certificates for your personal key, signed by CA key, from the RPi, and present them to remote SSH servers, while proving to them ownership of private key corresponding to the certificate. The remote servers trust the certificate issuer, verify that you own the certificate’s private key, and let you in. Do I get it correct?

If so, how do you issue the certificates that live on RPi? Those certificates have some limited validity period, so that you can worry less about leaking them, which was your concern in the first place. Therefore, you must reissue them on a regular basis. How do you do that? What’s your process for that?

Re: Poll HN: Do you use SSH certificates (not mere public-key authentication)?

#132

Post by fjni »

At work there's a command I run that then requires 2fa auth through our identity provider. As a result, my ssh key gets signed with both a time restriction and an ip restriction. Same mechanism for git interactions and server logins. I believe on the backend it's an HSM which does the signing.

Re: Poll HN: Do you use SSH certificates (not mere public-key authentication)?

#133

Post by cubesnooper »

xyzzyz wrote: post #131
cubesnooper wrote: post #119

Earlier quoted context omitted.

> So anyone on your LAN can visit the URL and download the CA private key? No, the only files served to the LAN are the certificates, which contain the signatures by the CA of the public keys of other machines. Those are safe to distribute openly because they’re useless without the private key of the public key that was signed.

Okay, so let me make sure I understand it. You have your own personal key, and CA key. Your personal (private) key lives on your workstation, and CA key lives on RPi. There are some other machines that you want to SSH into. They trust CA public key. To log into them, you download client certificates for your personal key, signed by CA key, from the RPi, and present them to remote SSH servers, while proving to them ow…

> Your personal (private) key lives on your workstation, and CA key lives on RPi.

Yep.

> There are some other machines that you want to SSH into. They trust CA public key.

Yep, with TrustedUserCAKeys.

> The remote servers trust the certificate issuer, verify that you own the certificate’s private key, and let you in.

Yep. Additionally, the servers themselves present CA‐signed certificates alongside their host keys. My GlobalKnownHostsFile contains the CA public key, so when I connect to a host for the first time, there’s no “unknown host” warning and my user’s known_hosts file is not updated.

> If so, how do you issue the certificates that live on RPi?

The CA has a directory containing the public keys of every user and host that I set up. A cronjob periodically runs ssh-keygen -s against these files, and copies them to htdocs. Each host and user has a cronjob that periodically fetches its certificate and copies it to /etc/ssh or ~/.ssh, respectively.

Re: Poll HN: Do you use SSH certificates (not mere public-key authentication)?

#134

Post by xyzzyz »

cubesnooper wrote: post #133
xyzzyz wrote: post #131

Earlier quoted context omitted.

Okay, so let me make sure I understand it. You have your own personal key, and CA key. Your personal (private) key lives on your workstation, and CA key lives on RPi. There are some other machines that you want to SSH into. They trust CA public key. To log into them, you download client certificates for your personal key, signed by CA key, from the RPi, and present them to remote SSH servers, while proving to them ow…

> Your personal (private) key lives on your workstation, and CA key lives on RPi. Yep. > There are some other machines that you want to SSH into. They trust CA public key. Yep, with TrustedUserCAKeys. > The remote servers trust the certificate issuer, verify that you own the certificate’s private key, and let you in. Yep. Additionally, the servers themselves present CA‐signed certificates alongside their host keys. M…

Ah, now I understand. That makes total sense. Thanks for your detailed explanation.

Re: Poll HN: Do you use SSH certificates (not mere public-key authentication)?

#135

Post by dotwaffle »

gunapologist99 wrote: post #114
tptacek wrote: post #98

Earlier quoted context omitted.

Ok, these are words, but again I'm not talking about DNS security here, I'm talking about SSH key distribution. Why would you elect to have your key distribution controlled by the DNS PKI? What's the upside? The downside is, an actor with control over the DNS PKI (there are many of those; see, for instance, every DOJ seizure of a domain) gets a degree of control over your SSH authorized keys. Seems... bad?

Agreed completely. The threat model for DNSSEC is vast; why would you diminish the security of a perfectly good end-to-end model in SSH keys or certificates, as long as you control 100% of that infrastructure. Introducing any outside actors at all objectively diminishes the security of the whole model by becoming another link in the chain, even if that link isn't necessarily the weakest (and I certainly believe it wo…

You'd have to compromise not only the root keys just to get into my infrastructure, but sign all the zones from there downwards, and intercept all DNS traffic leaving that server in the first place. That's secure enough for me.

Re: Poll HN: Do you use SSH certificates (not mere public-key authentication)?

#136

Post by dotwaffle »

candiddevmike wrote: post #49
dotwaffle wrote: post #20

I store my authorized_keys in DNS TXT records, that are DNSSEC signed, with a validating resolver on the box. I then just use "/usr/bin/hesinfo %u ssh" as my AuthorizedKeysCommand in OpenSSH. I wrote a little tool that allowed you to "#include" other DNS records etc, but "hesinfo" is generally easily installable/available so it's just easier.

This method puts a lot of reliance on DNSSEC working and trusting that it is preventing spoofing. I personally wouldn't rely on this in production, there are too many stories about DNSSEC cutovers rendering the domain unresolvable for hours+. Imagine not being able to get to your servers too...!

My DNS zones are not hosted on those servers, Google Cloud DNS does the dnssec signing, and there is a breakglass key installed on there too that when used automatically sends alerts out.

Re: Poll HN: Do you use SSH certificates (not mere public-key authentication)?

#137

Post by dotwaffle »

aliqot wrote: post #30
dotwaffle wrote: post #20

I store my authorized_keys in DNS TXT records, that are DNSSEC signed, with a validating resolver on the box. I then just use "/usr/bin/hesinfo %u ssh" as my AuthorizedKeysCommand in OpenSSH. I wrote a little tool that allowed you to "#include" other DNS records etc, but "hesinfo" is generally easily installable/available so it's just easier.

Is there some way I can purchase you a pint?

It would be rude of me to decline such an offer ;)

Re: Poll HN: Do you use SSH certificates (not mere public-key authentication)?

#138

Post by dotwaffle »

egberts1 wrote: post #113
dotwaffle wrote: post #20

I store my authorized_keys in DNS TXT records, that are DNSSEC signed, with a validating resolver on the box. I then just use "/usr/bin/hesinfo %u ssh" as my AuthorizedKeysCommand in OpenSSH. I wrote a little tool that allowed you to "#include" other DNS records etc, but "hesinfo" is generally easily installable/available so it's just easier.

Yes. Hesiod is safer than DNS TXT once secured by DNSSEC, because I envision mass blocking of DNS TXT in the near future.

Good luck with that. DNS TXT records are used for a lot of infrastructure right now, from DMARC/SPF/DKIM to DNS-01 validation through LetsEncrypt afaik.

Re: Poll HN: Do you use SSH certificates (not mere public-key authentication)?

#139

Post by dotwaffle »

chociej wrote: post #110
snowstormsun wrote: post #44

Earlier quoted context omitted.

doesn't this approach run into caching problems?

With a short TTL I imagine it's quite workable.

My TTL is lower than most people's SSH certificate validity duration, for sure :)

Re: Poll HN: Do you use SSH certificates (not mere public-key authentication)?

#140

Post by bongobingo1 »

chociej wrote: post #111
bongobingo1 wrote: post #89

Earlier quoted context omitted.

I think, 1) You don't have to ssh-copy-id to new boxes, which is nice. 2) You can de-auth a key for all machines by changing the DNS record. This would depend on some propagation time but perhaps you can point the resolver at your nameserver directly which would avoid that.

You could simply choose a short TTL, or your tool could check for e.g. "some-name._sshkeys.whatever.tld" as well as "_revoked.some-name._sshkeys.whatever.tld" to handle revocation instantly

You could also just stick it behind a https GET and probably skip a bunch of bother.
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