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Re: [Trans] AIA/cRL for logged certificates

Ben Laurie <benl@google.com> Fri, 27 March 2015 12:47 UTC

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Date: Fri, 27 Mar 2015 12:47:43 +0000
Message-ID: <CABrd9SQ1FXEVrLXS-fOHRYpsShX3R-ByBo8E3qxhkoOBMQZY5g@mail.gmail.com>
From: Ben Laurie <benl@google.com>
To: Eran Messeri <eranm@google.com>
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Cc: "trans@ietf.org" <trans@ietf.org>
Subject: Re: [Trans] AIA/cRL for logged certificates
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On 27 March 2015 at 12:45, Eran Messeri <eranm@google.com> wrote:

> I'd like to get opinions from the list on solutions to the following
> problem, which Ben originally pointed out. It applies to Precertificates
> currently, but would apply to X.509 certificates if ticket #4 is accepted.
>
> An "undesirable" certificate is issued and logged (without including
> Authority Information Access / CRL distribution point) and upon discovery
> is revoked - the CRL distribution point in the issuer or one of the
> intermediate certs will list it as revoked.
> That certificate would be signed a second time with the same issuer key,
> but not logged a second time (as the SCT produced for the first certificate
> is valid for the second one). When it is served, it is served together with
> a chain that is different than the one logged, and the issuer or
> intermediates in this chain point to a different AIA/CRL that does *not*
> show list this certificate as revoked (The implied assumption is that the
> attacker controls the private key of the issuer).
>
> Implication: A client believes it has a legitimate certificate by
> validating the SCT and performing an online revocation check.
>
> Potential mitigations:
> - Require that the client only use the AIA/CRL distribution point from the
> chain logged in the CT log (which forces the client to fetch it online,
> before completing the connection).
>

Then its not a potential mitigation!


> - Require the presence of AIA/CRL distribution point in the end-entity
> certificate.
>
> Any other suggestions?
> Eran
>
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