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Date: Thu, 05 Mar 2015 19:23:51 +0100
From: Martin Hecht <>
Subject: Re: Certificate pinning and the browser PKI

Hi Florian,

On 03/05/2015 01:43 PM, Florian Weimer wrote:
> I'm looking for suggestions how to implement certificate pinning.
> Things are relatively straightforward if you are not in the browser PKI
> because you can pin a long-term CA certificate instead, and not the
> server certificate.  Same if you have a dedicated (sub-)CA in the
> browser PKI.
> But if your server has to be in the browser PKI, things get a bit messy.
>  Pinning the CA may not offer much protection (because you are still
> exposed to RA failures at the CA).  
If you implement it yourself, I think you can pin any certificate in the
chain. The more you go up the chain the more you are at risk for RA
failures, but on the other hand the more long-living the certificates
will be. Even if you pin the root CA of the browser PKI, you are still
better off pinning that one, than trusting all of the root CAs in the
browser PKI.

> Pinning the server certificate is
> problematic because the certificates are relatively short-lived, and the
> rollovers have to be coordinated carefully.
> So for the browser PKI case, it may make sense to pin the server public
> key instead (n *and *e), not the entire certificate.  During regular
> rollover, you can keep the public key, and you can have a pre-pinned
> offline copy for emergency rollovers.
It depends if the CA policies allow to re-use the server keys. To my
understanding the concept of certificates with an expiration date is
also based on the assumption that forcing the certificate holders to
renew the certificate improves security (because the use of unnoticed
stolen credentials gets stopped at least at some time). I'm not sure how
many CAs force their clients to renew the keys when they replace the
server certificate.
> Or use SNI, a different endpoint name, and a separate certificate
> outside browser PKI, and pin that.
> Are there other options I'm missing?
A mixture of all that? e.g. pin a hash over the root CA certificate and
the subject(s) of the Sub-CAs in the chain. That would even allow the
intermediate CAs to change their keys but you verify at least the path
which you go down from the root CA.
...and combine the pinning with crls or ocsp
> The pinned certificate magically appears, thanks to the software update
> infrastructure, so that's a solved problem.  It's just synchronizing
> things within the update infrastructure to external events that can be
> tricky, for various reasons.
Well, as you already indicated, if the server certificates are exchanged
quite shortly before they expire, it becomes a problem to propagate the
information to the clients. If you start pinning somewhere higher up in
the chain you usually have one life time of an intermediate CA
certificate for communicating the new one to the clients. But still, for
this time frame you have to trust both of them because you don't know
when exactly the updates are are installed, and probably neither the
exact time when the server certificate is exchanged.

kind regards,

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