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I do not mean simply putting the public RSA key of a x.509 certificate into ~/.ssh/authorized_keys - I'm looking for a way to set up a ssh such that x.509 certificates signed by a pre-defined CA will automatically be granted access to the linked user account. RFC 6187 seems to suggest such a functionality, but I can't find any documentation on this, or whether it is implemented in OpenSSH at all.

Here's a more elaborate description of what I want to do:

  • A CA ("SSH-CA") is set up
  • This CA is used to sign user certificates with keyUsage=digitalSignature (and maybe the id-kp-secureShellClient extendedKeyUsage field)
  • This certificate can now be used to log in on a server. The server does not require the public key being present in the authorized_keys. Instead, it is set up to trust the SSH-CA to verify the public key and signature of the certificate (or certificate chain) and the username/UID (probably directly in the subjectAltName field, or maybe using some server-side mapping) before the usual RSA authentication takes place

So, (how) can this be achieved with OpenSSH, and if it requires a patch how can client-side modifications be kept minimal?


As an alternative I guess one could also use any S/MIME certificate plus a username to email-address mapping, without requiring an own CA. The client could also still use only the private RSA key and a certificate server is used obtain a certificate from a public key, additionally offering the possibility to use PGP certificates as well (e.g. via monkeysphere) without the user requiring any knowledge about all this as long as they simply provide a public key.

If it's not natively possible, I guess I could come up with a semi-automatic "implementation" of this by letting a script on the server automatically check a somehow else submitted certificate via openssl (or gnupg) and have the public key be put to the respective user's authorized_keys file - although at that point I am probably more or less re-doing the monkeyshere project...

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2 Answers 2

up vote 9 down vote accepted

OpenSSH does not officially support x.509 certificate based authentication:

The developers have maintained a stance that the complexity of X.509 certificates introduces an unacceptable attack surface for sshd. Instead, they have [recently] implemented an alternative certificate format which is much simpler to parse and thus introduces less risk.

...

OpenSSH just uses the low-level cryptographic algorithms from OpenSSL.

However Roumen Petrov publishes OpenSSH builds that do include X.509 support, and you could try with those.

X.509 certificates can [be] used as "user identity" and/or "host key" in SSH "Public Key" and "Host-Based" authentications.

Roumen Petrov's builds can be downloaded via this page.

Here's a Debian how-to for SSH with authentication key instead of password that might also prove useful in setting up your OpenSSH to accept x509 PKI for user authentication.

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The Debian link is just about the usual key setup, but Roumen Petrov's build sounds like the solution –  Tobias Kienzler Feb 19 '13 at 7:29

Native certificate-based authentication is available in unmodified upstream OpenSSH. It is not, however, based on x.509.

First, generate your CA:

ssh-keygen -f ssh-ca

Next, install your CA key in .authorized_keys with a cert-authority prefix:

echo "cert-authority $(<ssh-ca.pub)" >>.ssh/authorized_keys

From that point, whenever a key is generated by a user:

ssh-keygen -f real-key

...the public portion can be signed by your SSH CA, whereafter it will be trusted by the server without that key itself being added to authorized_keys:

ssh-keygen -s ssh-ca -I identifier_for_your_real_key_goes_here real-key.pub
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Thanks, I never read about this despite it being available since apparently 5.4 from 2011... github.com/cloudtools/ssh-ca might be of interest then. I'll try and check this sometime, in which case you'd deserve the checkmark –  Tobias Kienzler Oct 9 at 4:37

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