Re: [TLS] HTTPS client-certificate-authentication in browsers

Martin Rex <mrex@sap.com> Mon, 25 July 2011 23:51 UTC

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From: Martin Rex <mrex@sap.com>
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To: anders.rundgren@telia.com (Anders Rundgren)
Date: Tue, 26 Jul 2011 01:51:43 +0200 (MEST)
In-Reply-To: <4E2D5C63.3000408@telia.com> from "Anders Rundgren" at Jul 25, 11 02:06:59 pm
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Subject: Re: [TLS] HTTPS client-certificate-authentication in browsers
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Anders Rundgren wrote:
> 
> I don't believe that TLS CCA (Client Certificate Authentication) in the
> form of HTTPS as implemented in current browsers has much of a future.

It works perfectly fine and we've been using it >10 years for
all of our intranet (50.000 employees these days) web servers
accessed via HTTPS, using TLS client certs for Single Sign-On.
(similar to how Kerberos is used by others).


> 
> In fact, quite a bunch of the entities in the EU working with consumer PKI
> have replaced HTTPS CCA with an application level scheme which wasn't such
> a big deal since they anyway were forced writing a browser PKI client more
> or less from scratch since the ones shipped with browsers doesn't support
> PKI as defined by banks and government (like mandatory PIN codes also
> for on-line enrolled keys).

I think you're confusing things.

What you're looking at here is scenarios for individually authenticated
transactions.  That is an entirely different problem domain and not
addressed by TLS at all.  You would have to address that with
a browser plugin that accesses a completely different PKI credential
that has signature qualities, with a clearly defined protocol that
describes what data gets signed, and which requires seperate per-transaction
authorization for every signature operation.


> 
> That the TLS CCA protocol doesn't even support "Logout" haven't made
> it a logical choice for web developers either.

Huh?  I have no clue what you're talking about.

If the server wants to perform a logout operation,
it can delete the TLS session cache entry on the server.

But the Single Sign-On capability of the TLS client cert means
that as long as the client credential is still available to the
TLS client, the client will perform "transparent" reauthentication.


> 
> The button "Clear SSL state" in MSIE is an indication how horribly bad it
> can go when security experts design systems for "people".

Is your intention to get prompted again?
>From a usability standpoint, we prefer the "select automatically"
setting and spare our users the client certificate selection popup.


> 
> There's no way you can hide the fact that TLS CCA is only truly useful
> securing tunnels between "boxes".

The purpose of the TLS CCA is the same as the purpose of Kerberos,
to provide a non-disclosing Single Sign-On convenience.


-Martin