Re: [Acme] Revoking certificates issued by an unknown ACME server

Martin Thomson <martin.thomson@gmail.com> Fri, 15 January 2016 02:55 UTC

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Date: Fri, 15 Jan 2016 13:55:42 +1100
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From: Martin Thomson <martin.thomson@gmail.com>
To: Hugo Landau <hlandau@devever.net>
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Subject: Re: [Acme] Revoking certificates issued by an unknown ACME server
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Is there any reason why you couldn't rely on a search engine for this?
 That is, search for "acme endpoint <intermediate CN>" and thereby
arrive at a value.  That's the "low tech" alternative to packing the
URL in the ee or intermediate cert.

On 15 January 2016 at 02:27, Hugo Landau <hlandau@devever.net> wrote:
> 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
>
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