Single Sign On with client certificates

In a previous post, I detailed a trick to get complicated webapps working with client certificates.

The problem this solves is that some combination of web sockets, service workers (and perhaps some demonic magic) don’t play nicely with client certificates.  Under some circumstances, the client certificate is just not sent.

The basic idea behind the solution is to instead authenticate by a couple of cookies with an HMAC.  When these cookies aren’t present, you’re required to specify a client certificate.  When a valid client certificate is presented, HMAC cookies are generated and dropped.  If the cookies are present, you’re allowed access, even if you don’t have a client certificate.

This has worked well for me, but I still occasionally ran into issues.  Basically every time I started a new session with something requiring client certs, I’d get some sort of bizarre access error.  I dug in a little, and it seemed like the request to fetch the service worker code was failing because the browser wasn’t sending client certificates.

This led me to double down on the HMAC cookies.

Coming clean

When I call this Single Sign On, please understand that I really only have the vaguest possible understanding of what that means.  If there are standards or something that are implied by this term, I’m not following them.

What I mean is that I have a centralized lua script that I can include in arbitrary nginx server configs, and it handles auth in the same way for all of them.

The nitty gritty

Rather than using HMAC cookies as a fallback auth mechanism and having “ssl_verifiy_client” set to “optional,” I do the following:

  1. If HMAC cookies are not present, nginx redirects to a different subdomain (it’s important that it’s on the same domain).  This server config requires the client certificate.
  2. If the certificate is valid, it generates and drops the appropriate cookies, and the client is redirected to the original URL.  The cookies are configured to be sent for all subdomains of a given domain.
  3. Now that the client has HMAC cookies, it’s allowed access.  If the cookies were present to begin with, the above is skipped.

The setup has a couple of pieces:

  1. An nginx server for an “SSO” domain.  This is the piece responsible for dropping the HMAC cookies.
  2. A lua script which is included everywhere you want to auth using this mechanism.

This is the SSO server config:

And the SSO lua script:

An example of it being used:

 

One thought on “Single Sign On with client certificates

  1. Pingback: Security Hardening Octoprint/Octopi | Chris Mullins

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