Subject: Re: Issue 14: Is CGA-only RD protection useful?

From: Julien Laganier <[email-masked]>
Date: Mon, 1 Sep 2003 08:48:19 +0200

On Monday 25 August 2003 20:57, James Kempf wrote:
> Issue 14 received some discussion in Vienna. The question was raised by one
> of the reviewers whether having CGA-only protection on RAs would be
> sufficient for securing router discovery. This is independent of whether a
> router would use CGA for neighbor discovery.

(...)

> The protection provided by CGAs is fairly simple: a receiver of a message
> signed by the sender and verifiable with the sender's public key has the
> assurance that the message was, in fact, sent from the CGA.
>
> In the absence of any network element or any additional processing between
> the router and the host, CGAs would seem insufficient to address all the
> threats above, though they would address 2, since the victim could clearly
> identify that the bogus RA did not issue from the legitimate default
> router. A malicious host could construct a CGA from its public key and use
> its private key to sign a bogus RA. The RA would be verifiable as issuing
> from the correct IPv6 address, but the RA would contain bogus information
> designed to launch an attack. Since there is no IP network element between
> the host and the router in the standard IP subnet architecture, this would
> suggest that CGA-only RD authentication is not sufficient to address all
> the threats.
>
> If, however, there exists a Layer 2 network element between the host and
> the router that is programmable such that it can perform sufficient Layer 3
> processing to cryptographically verify the RA signature and filter on the
> CGA, then CGA-only RA security may be useful. The Layer 2 network element
> could be programmed with an ACL of CGAs for known good routers, and only
> pass RAs for those whose CGAs match the signatures and the ACL. Depending
> on the Layer 2 technology and the network element, some part of the subnet
> may still be vulnerable. For example, if the Layer 2 technology was
> multi-access prior to the filtering device, some number of hosts may be
> exposed to the bogus RA. Note that such a filtering device would still be
> subject to attack if CGAs were not employed, since the device could not
> verify the RA as issuing from the source address, providing an opening for
> an attacker.

Another benefits of CGA-only protection for RA can be some sort of 
opportunistic security: If there is several on-link routers that send RA's, 
the node has the ability to distinguish them based on their CGA's. Hence it 
may be possible of classifying contradictory routers and try to measure the 
connectivity they provide to select best performers and ignore their 
contradictors. For example if two routers advertize a default route, a node 
may try to communicate with a trusted remote host (its HA for instance) to 
see which of the two routers really forward packets to its HA. It is 
certainly not very secure, but by leveraging on the routing infrastructure it 
may be better than nothing (for example when there is no certs for DCC, like 
in ad-hoc set-up).

Therefore, I suggest that the WG follows the following direction:

> B) As a practical matter, some Layer 2 technologies of interest support
> such filtering devices, and adding the kind of Layer 3 filtering and
> network management functions described above to such devices for CGA-only
> protection is not all that difficult. Since a CGA-only solution may be
> simpler from a deployment and network management perspective in some
> networks than requiring routers to have certificates, the CGA-only solution
> should be fully articulated in a separate section in the draft.

And perhaps mentions that measuring connectivity as a way of using CGA-only 
RA's protection in ad-hoc set-up?

Thanks,

--julien

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