I have reviewed draft-gont-6man-stable-privacy-addresses and have
already sent some nits direct to the author.
I like this draft.
One of my biggest criticisms of RFC4941 today is that end nodes act
unilaterally, and that due consideration was not made of the needs of
organizations (such as Enterprises or ISPs), where logging is perfectly
legal, desirable, and has compliance issues.
If I were to write the design goal for interface identifier privacy
myself I'd write:
o It should be easy for an authorized person to be able to reliably
predict the interface
identifiers that will be generated by the algorithm, provided
that the authorized person has
access to the algorithm and the secret keying material used in
generating the interface identifiers, but not otherwise.
The idea being that authorized persons e.g. law enforcement and network
managers SHOULD be able to correlate activity at a later date (for legal
compliance, logging, fault finding etc.) whilst an attacker or
unauthorized person SHOULD NOT.
I have no objection per se to the time variance of temporary addresses
of RFC4941, provided that the rotation of addresses happens in a
controlled manner. Again, a common criticism of 4941 is that long-lived
sessions may be broken, and there's no way of knowing when authorized
persons should relearn/re-correlate private addresses to native
EUI64's/machines.
I can think of 2 mechanisms to achieve controlled time-variance using
your draft:
1. If controlled renumbering is ever widely implemented, it would be
possible for a network manager to regularly rotate the /64 subnet
prefixes in a controlled manner (add new prefix, deprecate old prefix)
to generate new Random Interface Identifiers (RIDs), rather than this
being an unilateral choice of the end nodes as in 4941. An end node
would generate a new RID and use this for new sessions, whilst existing
sessions would continue to use the RID generated from the deprecated
prefix. This would mean that the time-variant aspect of user privacy in
4941 could be maintained, whilst end node activity could still also be
tracked and correlated by the authorized person/network manager (but not
as easily by an attacker who does not have access to the secret key).
Some might say that this is a weakness, in that a node connected to a
3rd party LAN could be tricked into giving away a few bits of
information every time a RID is regenerated. However, I don't consider
that a major issue, because the 3rd party LAN manager could anyway track
usage directly, either based on the /64 parent prefix or some other
means like EUI64 media address. A remote passive attacker might also
have difficulty working out exactly how a new /64 prefix is correlated
to the existing /64 prefix (provided there was enough variance in users
and all other things/information leaks being equal.)
2. If the implementation provided a mechanism for the secret_key to be
changed, it would also be possible for a network manager to regularly
update the secret_keys in a controlled manner so that an end node would
generate a new RID when an interface was re-initialised.
For these reasons, I feel that all implementations SHOULD include a
mechanism for the secret_key to be updated (to repair a potentially
compromised node where the secret_key has been learned by an
unauthorized person, complicate long-term snooping of RID changes when
prefixes change, and to allow a manager to trigger a RID rotation per node)
I make no comment on the mechanism by which the secret_key should be
updated. Some environments may require specialist hardware, or use of an
existing Security Association to create a secure channel to a secure
node, whilst others might consider a DHCPv6 option sufficient. These
mechanisms could be defined in related drafts.
This secret_key could then also be stored somewhere in a central
(secure) log for use in cross-referencing obfuscated addresses from
various logs at a later date.
Some switches or other devices that limit access to a single IPv6 link
local address per port may have problems with RIDs. RFC4941 Sections
3.6 & 4 are certainly pertinent.
regards,
RayH
Fernando Gont wrote:
Folks,
Probably the only objection that I got for
draft-gont-6man-stable-privacy-addresses is that the prefix shouldn't be
included in F() (the hash function).
I'd like to clarify the motivation for that, and also trigger some
discussion on the topic such that I can produce a revision of this
docuement that addresses any remaining concerns.
* Motivation for including the SLAAC Prefix in F() (the hash function)
The essential rationale for including the prefix in the hash function is
that, if we don't, then we'd be leaking more information than we should.
If we don't include the prefix as part of the data to be hashed, then
the Interface-ID will be exactly the same no matter whether the devices
moves from one network to another, and hence the node could be tracked,
even if it implements RFC 4941.
For example, think about the following scenario:
Attack scenario #1
A host configures the stable addresses without including the Prefix in
the hash function. Let's say the host employs any application that needs
to perform a server-like function (P2P, or whatever). As a result of
that, an attacker attacker participating in that application (e.g., P2P)
would learn the Interface-ID used for the stable address.
Now the same host moves to a completely different network, and uses the
same P2P application, probably even with a different user. The attacker
now interacts with the same node again, and hence can learn the "new"
stable address. Since the interface ID is the same as the one above, he
can infer that it is communicating with the same device as before.
Had the host included the Prefix when computing the Interface-ID (with
the hash function F()), the Interface-ID would have been different, and
this wouldn't have been made possible.
This is just *one* possible attack scenario, which should remind us that
one shouldn't disclose more than it is really needed (and a static
Interface-ID across many different networks does exactly that).
* Use of draft-gont-6man-stable-privacy-addresses without RFC 4941
Temporary addresses are painful to manage. They make painful to
correlate the activities of your own nodes. FOr example, one host gets
infeceted will malware, you logged the IPv6 address, but then you don't
know which node was using that address at that point in time.
This is the reason for which in many deployments temporary addresses
(RFC 4941) are disabled (e.g., look at any presentation by Ron Broersma).
When you disable temporary addresses, all you're left with is whatever
stable addresses your host uses. If you don't include the Prefix in F()
(the hash function) then, while you may be resistant to host scanning
attacks (your addresses is not predictable anymore), you're still
trackable, since your interface ID doesn't change as you move from one
network to another.
So yes, there are cases in which you may want privacy, without
necessarily be willing to deal with the burden of temporary addresses.
And if you're just going to have stable addresses, you do not really
want the Interface ID to be constant across networks.
* Concerns about CPU impact
Other folks have expressed concerns (in corridor meetings) that if the
Prefix is included in the hash function, they may need to recompute the
hash every time they attach to a network (assuming the SLAAC prefix
changes), and that would be expensive. I personally think this shouldn't
be an issue for most implementations (for instance, we do MD5 for every
single new TCP connection! -- see RFC 6528). And, in any case, we do not
really specify which hash function to use. -- If you're concerned about
CPU, you may want to use a (CPU-wise) cheaper function.
That said, I'd have no problem with making the inclusion of the Prefix a
SHOULD (i.e., do it, unless you have very good reasons not to do so
(such as CPU-constraints)), or even a MAY (if inclusion of the Prefix in
the hash function turns out to be the show-stopper here).
If you have any feedback/comments, I'd be glad to hear. I'm planning to
rev the document asap, clarifying and incorporating many of the things
explained throughout this e-mail. So if you send your feedback shortly,
the next rev could reflect your comments.
Thanks,
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