[Acme] Revoking certificates issued by an unknown ACME server

Hugo Landau <hlandau@devever.net> Thu, 14 January 2016 15:27 UTC

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Date: Thu, 14 Jan 2016 15:27:47 +0000
From: Hugo Landau <hlandau@devever.net>
To: acme@ietf.org
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Subject: [Acme] Revoking certificates issued by an unknown ACME server
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So while implementing revocation in my ACME client, I came to the
following problem: how do you know which ACME server issued a
certificate?

Given an ACME server URL, one can obtain a certificate, but there is no
reliable way to do the reverse.

If you think about it, it might be desirable to be able to revoke a
certificate possessing nothing but the certificate. For example, suppose
you identify a misissued certificate for a domain you control. Under the
current ACME protocol, if you can prove control of that domain, you can
revoke the certificate; however, this requires you to know what server
issued it.

Not sure what the good solutions to this are. One would be to include
the directory URL as an X.509 or OCSP extension, though that bloats the
certificate/response. Another might be to reuse the OCSP responder URL,
so that given an OCSP endpoint, one can obtain the ACME server URL, or
at least one suitable for revocation.

Something like:

  Normal OCSP Request:
  GET http://ocsp.example.com/ocsp/MFMwUTBPME0wSzAJ
 

  Revocation Location OCSP Request:
  GET http://ocsp.example.com/ocsp/acme-revoker/MFMwUTBPME0wSzAJ

  302 Found
  Location: https://acme-staging.letsencrypt.org/directory


Thoughts?

Hugo Landau