Re: [Ace] Security of the Communication Between C and RS

Ludwig Seitz <ludwig.seitz@ri.se> Thu, 20 December 2018 07:40 UTC

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To: Jim Schaad <ietf@augustcellars.com>, 'Hannes Tschofenig' <Hannes.Tschofenig@arm.com>, 'Stefanie Gerdes' <gerdes@tzi.de>, <ace@ietf.org>
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From: Ludwig Seitz <ludwig.seitz@ri.se>
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Date: Thu, 20 Dec 2018 08:40:11 +0100
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Subject: Re: [Ace] Security of the Communication Between C and RS
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On 19/12/2018 21:22, Jim Schaad wrote:
> 

>>> In step (4) you tie the expiry of the token to the attacker
>>> getting hold of the key. What happens if the attacker gets hold
>>> of the pop key before the token expires?
>> 
>> If it is detected the AS would revoke the token. Then the client
>> _could_ use client introspection to get that information. Note that
>> this is what the CMU people are looking at.
> 
> I am having a real problem with the idea that we are going to start
> adding the idea of revocation to the entire system.   I would much
> rather make sure that both sides are given an idea of when things are
> going to expire and just make things expire relatively quickly.
> 

It's quite unlike the revocation in PKI, it's just that an AS can mark a 
token as invalid, and this information would then be available to those 
who can do introspection.

>> 
>>> Additionally, if you use DTLS/TLS just having the PoP key still 
>>> requires the attacker to run a new DTLS/TLS handshake with the
>>> RS.
>> 
>> If the pop-key was used as a basis for doing a DTLS-PSK handshake,
>> the attacker should be able to hijack the connection and
>> impersonate either party.
> 
> This depends on the version of PSK that you are doing.  If you use
> PSK+ECDH then the attacker cannot hijack the connection.
>

Right of course. I suspect however that not all constrained devices 
would implement the PSK+ECDH handshake, or are the other PSK handshakes 
deprecated?


>> 
>>> It would also be useful to know where the attacker got the PoP
>>> key from and how you can even detect the compromise.
>> 
>> That is a different story entirely. You could imagine the case of
>> an RS improperly deleting an expired token and the associated
>> pop-key, and then being subject to a physical attack that recovers
>> that information.
> 
> It would be more reasonable to say that if you are doing a physical
> attack, then it would be easy to get an RPK and then you are the RS
> until such a time as the AS is told that the key is no longer
> trusted.  In this case you will just continue getting tokens as a
> client which are still valid and none of this is helpful in any
> event.

Ok my example was perhaps not ideal, since it has an even bigger breach 
as precondition. So under what conditions would an attacker get access 
to a pop-key of an expired token? Steffi any ideas?

/Ludwig



-- 
Ludwig Seitz, PhD
Security Lab, RISE
Phone +46(0)70-349 92 51