Re: [Ace] Review of draft-ietf-ace-key-groupcomm

Jim Schaad <ietf@augustcellars.com> Tue, 23 July 2019 12:55 UTC

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From: Jim Schaad <ietf@augustcellars.com>
To: 'Ludwig Seitz' <ludwig.seitz@ri.se>, <draft-ietf-ace-key-groupcomm@ietf.org>
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Subject: Re: [Ace] Review of draft-ietf-ace-key-groupcomm
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-----Original Message-----
From: Ace <ace-bounces@ietf.org>; On Behalf Of Ludwig Seitz
Sent: Friday, July 19, 2019 7:22 AM
To: draft-ietf-ace-key-groupcomm@ietf.org
Cc: ace@ietf.org
Subject: [Ace] Review of draft-ietf-ace-key-groupcomm

Hello Francesca, Marco,

I have finally managed to read the whole of draft-ietf-ace-key-groupcomm and have a few comments for you:


==
5.2

"If the leaving node wants to be part of a group with fewer roles, it 
does not need to communicate that to the KDC, and can simply stop acting 
according to   such roles."

There are legitimate cases where a node might want to explicitly 
deactivate roles it is currently using (principle of least priviledge) 
and not just stop using them.

[JLS] I trimmed because I only wanted to address this one topic.  I totally agree that there are cases where a node might want to deactivate roles, however in the case of group communication I don't see how this could be done in a reasonable manner.  

If a node says - please stop advertising my public key because I am no longer a publisher, that is reasonable for a KDC to start doing.  However, there are currently no provisions in the protocol for a KDC to advertise that fact to all of the subscribers.   Even if a key roll over were to occur, as the node still is part of the group, it can produce the correct key material and sign a message.  A subscriber with the signature key would successfully validate the signature and accept it the message, only those subscribers which had not yet pulled down a public key would fail to validate the message.

This would require a new mechanism for the purpose of asking if a public key is still associated with a specific key identifier (which is a good reason for the note about keeping the same key ids when rolling keys).  I am not sure that the traffic would be worth the effort for this small gain.

Note that for a gated transmission system such as a pub-sub server, the node can get lesser privilege for the gate system without getting less privilege for the KDC.

Jim


/Ludwig


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