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Generating Stable Privacy-Enhanced Addresses With IPv6

Generating Stable Privacy-Enhanced Addresses With IPv6
Posted Mar 31, 2012
Authored by Fernando Gont

This document specifies a method for generating IPv6 Interface Identifiers to be used with IPv6 Stateless Address Autoconfiguration (SLAAC), such that addresses configured using this method are stable within each subnet, but the Interface Identifier changes when hosts move from one network to another. The aforementioned method is meant to be an alternative to generating Interface Identifiers based on IEEE identifiers, such that the benefits of stable addresses can be achieved without sacrificing the privacy of users.

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MD5 | 38b4c27a5caa9d018464a2a4dea98298

Generating Stable Privacy-Enhanced Addresses With IPv6

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IPv6 maintenance Working Group (6man) F. Gont
Internet-Draft UK CPNI
Updates: 4291, 4862 (if approved) March 31, 2012
Intended status: Standards Track
Expires: October 2, 2012


A method for Generating Stable Privacy-Enhanced Addresses with IPv6
Stateless Address Autoconfiguration (SLAAC)
draft-gont-6man-stable-privacy-addresses-01

Abstract

This document specifies a method for generating IPv6 Interface
Identifiers to be used with IPv6 Stateless Address Autoconfiguration
(SLAAC), such that addresses configured using this method are stable
within each subnet, but the Interface Identifier changes when hosts
move from one network to another. The aforementioned method is meant
to be an alternative to generating Interface Identifiers based on
IEEE identifiers, such that the benefits of stable addresses can be
achieved without sacrificing the privacy of users.

Status of this Memo

This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. This document may not be modified,
and derivative works of it may not be created, and it may not be
published except as an Internet-Draft.

Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."

This Internet-Draft will expire on October 2, 2012.

Copyright Notice

Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents



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(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.


Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Design goals . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Algorithm specification . . . . . . . . . . . . . . . . . . . 6
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
5. Security Considerations . . . . . . . . . . . . . . . . . . . 9
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
7. Privacy issues still present with RFC 4941 . . . . . . . . . . 11
7.1. Host tracking . . . . . . . . . . . . . . . . . . . . . . 11
7.1.1. Tracking hosts across networks #1 . . . . . . . . . . 11
7.1.2. Tracking hosts across networks #2 . . . . . . . . . . 12
7.1.3. Revealing the identity of a devices performing
server-like functions . . . . . . . . . . . . . . . . 12
7.2. Host scanning-attacks . . . . . . . . . . . . . . . . . . 12
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
8.1. Normative References . . . . . . . . . . . . . . . . . . . 14
8.2. Informative References . . . . . . . . . . . . . . . . . . 14
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 15























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1. Introduction

[RFC4862] specifies the Stateless Address Autoconfiguration (SLAAC)
for IPv6, which typically results in hosts configuring one or more
"stable" addresses composed of a network prefix advertised by a local
router, and an Interface Identifier (IID) that typically embeds a
hardware address (e.g., using IEEE identifiers) [RFC4291].

Generally, static addresses are thought to simplify network
management, since they simplify ACLs and logging. However, since
IEEE identifiers are typically globally unique, the resulting IPv6
addresses can be leveraged to track and correlate the activity of a
node over time and across multiple subnets and networks, thus
negatively affecting the privacy of users.

The "Privacy Extensions for Stateless Address Autoconfiguration in
IPv6" [RFC4941] were introduced to complicate the task of
eavesdroppers and other information collectors to correlate the
activities of a node, and basically result in temporary (and random)
Interface Identifiers that are typically more difficult to leverage
than those based on IEEE identifiers. When privacy extensions are
enabled, "privacy addresses" are employed for "outgoing
communications", while the traditional IPv6 addresses based on IEEE
identifiers are still used for "server" functions (i.e., receiving
incoming connections).

As noted in [RFC4941], "anytime a fixed identifier is used in
multiple contexts, it becomes possible to correlate seemingly
unrelated activity using this identifier". Therefore, since
"privacy addresses" [RFC4941] do not eliminate the use of fixed
identifiers for server-like functions, they only *partially*
mitigate the correlation of host activities (see Section 7 for
some example attacks that are still possible with privacy
addresses). Therefore, it is vital that the privacy
characteristics of "stable" addresses are improved such that the
ability of an attacker correlating host activities across networks
is reduced.

Another important aspect not mitigated by "Privacy Addresses"
[RFC4941] is that of host scanning. Since IPv6 addresses that
embed IEEE identifiers have specific patterns, an attacker could
leverage such patterns to greatly reduce the search space for
"live" hosts. Since "privacy addresses" do not eliminate the use
of IPv6 addresses that embed IEEE identifiers, host scanning
attacks are still feasible even if "privacy extensions" are
employed [Gont-DEEPSEC2011] [CPNI-IPv6]. This is yet another
motivation to improve the privacy characteristics of "stable"
addresses (currently embedding IEEE identifiers).



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Privacy/temporary addresses can be challenging in a number of areas.
For example, from a network-management point of view, they tend to
increase the complexity of event logging, trouble-shooting, and
enforcing access controls and quality of service, etc. As a result,
some organizations disable the use of privacy addresses even at the
expense of reduced privacy [Broersma]. Also, they result in
increased complexity, which might not be possible or desirable in
some implementations (e.g., some embedded devices).

In scenarios in which "Privacy Extensions" are deliberately not used
(possibly for any of the aforementioned reasons), all a host is left
with is the addresses that have been generated using e.g. IEEE
identifiers, and this is yet another case in which it is also vital
that the privacy characteristics of these stable addresses are
improved.

We note that in most (if not all) of those scenarios in which
"Privacy Extensions" are disabled, there is usually no actual desire
to negatively affect user privacy, but rather a desire to simplify
operation of the network (simplify the use of ACLs, logging, etc.).

This document specifies a method to generate interface identifiers
that are stable/constant within each subnet, but that change as hosts
move from one network to another, thus keeping the "stability"
properties of the interface identifiers specified in [RFC4291], while
still mitigating host-scanning attacks and preventing correlation of
the activities of a node as it moves from one network to another.

For nodes that currently disable "Privacy extensions" [RFC4941] for
some of the reasons stated above, this mechanism provides stable
privacy-enhanced addresses which may already address most of the
privacy concerns related to addresses that embed IEEE identifiers
[RFC4291]. On the other hand, in scenarios in which "Privacy
Extensions" are employed, implementation of the mechanism described
in this document would mitigate host-scanning attacks and also
mitigate correlation of host activities.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].











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2. Design goals

This document specifies a method for selecting interface identifiers
to be used with IPv6 SLAAC, with the following goals:

o The resulting interface identifier remains constant/stable for
each prefix used with SLAAC within each subnet. That is, the
algorithm generates the same interface identifier when configuring
an address belonging to the same prefix within the same subnet.

o The resulting interface identifier does change when addresses are
configured for different prefixes. That is, if different
autoconfiguration prefixes are used to configure addresses for the
same network interface card, the resulting interface identifiers
must be (statistically) different.

o It must be difficult for an outsider to predict the interface
identifiers that will be generated by the algorithm, even with
knowledge of the interface identifiers generated for configuring
other addresses.

o The aforementioned interface identifiers are meant to be an
alternative to those based on IEEE identifiers, as specified in
[RFC4291].

We note that of use of the algorithm specified in this document is
(to a large extent) orthogonal to the use of "Privacy Extensions"
[RFC4941]. Hosts that do not implement/use "Privacy Extensions"
would have the benefit that they would not be subject to the host-
tracking and host scanning issues discussed in the previous section.
On the other hand, in the case of hosts employing "Privacy
Extensions", the method specified in this document would prevent host
scanning attacks and correlation of node activities across networks
(see Section 7).

















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3. Algorithm specification

IPv6 implementations conforming to this specification MUST generate
interface identifiers using the algorithm specified in this section.
The aforementioned algorithm MUST be employed for generating the
interface identifiers for all the IPv6 addresses configured with
SLAAC for a given interface, including IPv6 link-local addresses.
Unless otherwise noted, all of the parameters included in the
expression below MUST be included when generating an Interface ID.

1. Compute a random (but stable) identifier with the expression:

RID = F(Prefix, Modified_EUI64, Network_ID, secret_key)

Where:

RID:
Random (but stable) identifier

F():
A pseudorandom function (PRF) that is not computable from the
outside (without knowledge of the secret key). The PRF could
be implemented as a cryptographic hash of the concatenation of
each of the function parameters .

Prefix:
The prefix to be used for SLAAC, as learned from an ICMPv6
Router Advertisement message.

Modified_EUI64:
The Modified EUI-64 format identifier corresponding to this
network interface.

Network_ID:
Some network specific data that identifies the subnet to which
this interface is attached. For example the IEEE 802.11 SSID
corresponding to the network to which this interface is
associated. This parameter is OPTIONAL.

secret_key:
A secret key that is not known by the attacker. The secret
key MUST be initialized at system installation time to the
concatenation of a pseudo-random number (see [RFC4086] for
randomness requirements for security) and the machine's serial
number. An implementation MAY provide the means for the user
to change the secret key.





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2. The Interface Identifier is finally obtained by taking the
leftmost 64 bits of the RID value computed in the previous step,
and and setting bit 6 (the leftmost bit is numbered 0) to zero.
This creates an interface identifier with the universal/local bit
indicating local significance only.

Note that the result of F() in the algorithm above is no more secure
than the secret key. If an attacker is aware of the PRF that is
being used by the victim (which we should expect), and the attacker
can obtain enough material (i.e. addresses configured by the victim),
the attacker may simply search the entire secret-key space to find
matches. To protect against this, the secret key should be of a
reasonable length. Key lengths of at least 128 bits should be
adequate. The secret key is initialized at installation time to the
concatenation of a pseudo-random number and the machine's serial
number. This allows this mechanism to be enabled/used automatically,
without user intervention.

The machine's serial number is concatenated to the pseudo-random
number, such that the entropy of the key is increased (since at
installation time the entropy of the Pseudo-Random Number
Generator might be reduced).

Including the SLAAC prefix in the PRF computation causes the
Interface ID to vary across networks that employ different prefixes,
thus mitigating host-tracking attacks and any other attacks that
benefit from predictable Interface IDs (such as host scanning).

Including the optional Network_ID parameter when computing the RID
value above would cause the algorithm to produce a different
Interface Identifier when connecting to different networks, even when
configuring addresses belonging to the same prefix. This means that
a host would employ a different Interface ID as it moves from one
network to another even for IPv6 link-local addresses or Unique Local
Addresses (ULAs).

Note that there are a number of ways in which these addresses
might leak out. For example, an attacker could use ICMPv6 Node
Information queries [RFC4620] to obtain such addresses.












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4. IANA Considerations

There are no IANA registries within this document. The RFC-Editor
can remove this section before publication of this document as an
RFC.














































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5. Security Considerations

This document specifies an algorithm for generating interface
identifiers to be used with IPv6 Stateless Address Autoconfiguration
(SLAAC), replacing e.g. the Modified EUI-64 format identifiers. When
compared to modified EUI-64 format identifiers, the identifiers
specified in this document have a number of advantages:

o They prevent trivial host-tracking, since when a host moves from
one network to another the network prefix used for
autoconfiguration and/or the Network ID (e.g., IEEE 802.11 SSID)
will typically change, and hence the resulting interface
identifier will also change (see Section 7.

o They mitigate host-scanning techniques which leverage predictable
interface identifiers (e.g., known Organizational Unique
Identifiers).

We note that this algorithm is meant to replace Modified EUI-64
format identifiers, but not the temporary-addresses such as those
specified in [RFC4941]. Clearly, temporary addresses may help to
mitigate the correlation of activities of a node within the same
network, and may also reduce the attack exposure window (since the
lifetime of privacy/temporary IPv6 address is reduced when compared
to that of addresses generated with the method specified in this
document). We note that implementation of this algorithm would still
benefit those hosts employing "Privacy Addresses", since it would
mitigate host-tracking vectors still present when privacy addresses
are used (Section 7, and would also mitigate host-scanning techniques
that leverage patterns in IPv6 addresses that embed IEEE identifiers.

Finally, we note that the method described in this document may
mitigate most of the privacy concerns arising from the use of IPv6
addresses that embed IEEE identifiers, without the use of temporary
addresses, thus possibly offering an interesting trade-off for those
scenarios in which the use of temporary addresses is not feasible.















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6. Acknowledgements

The author would like to thank (in alphabetical order) Steven
Bellovin, Dominik Elsbroek, Ray Hunter, Jong-Hyouk Lee, and Michael
Richardson, for providing valuable comments on earlier versions of
this document.

This document is based on the technical report "Security Assessment
of the Internet Protocol version 6 (IPv6)" [CPNI-IPv6] authored by
Fernando Gont on behalf of the UK Centre for the Protection of
National Infrastructure (CPNI).

Fernando Gont would like to thank CPNI (http://www.cpni.gov.uk) for
their continued support.





































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7. Privacy issues still present with RFC 4941

This aims to clarify the motivation of using the method specified in
this document even when privacy/temporary addresses are employed. It
has been incorporated in the document to clarify a number of
questions that arose during the presentation of this document at IETF
83 (Paris). This entire section might be removed prior to
publication of this document as an RFC.

7.1. Host tracking

Some 6man participants questioned the inclusion of the SLAAC prefix
in PRF function, and noted that the ID of "stable" addresses need not
change across networks, since privacy/temporary addresses already
mitigate host tracking. This section describes one possible attack
scenario that illustrates that host-tracking may still be possible
when privacy/temporary addresses are employed.

7.1.1. Tracking hosts across networks #1

A host configures the stable addresses without including the Prefix
in the F() (the PRF). The aforementioned host now runs any
application that needs to perform a server-like function (e.g. a
peer-to-peer application). As a result of that, an attacker/user
participating in the same application (e.g., P2P) would learn the
Interface-ID used for the stable address.

Some time later, 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 host again,
and hence can learn the "new" stable address. Since the interface ID
is the same as the one used before, the attacker 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()) as RECOMMENDED in this document, the
Interface-ID would have been different, and this privacy attack would
not have been possible.

This is just *one* possible attack scenario, which should remind us
that one should not disclose more than it is really needed for
achieving a specific goal (and an Interface-ID that is constant
across different networks does exactly that: it discloses more
information than is needed for providing a stable address).







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7.1.2. Tracking hosts across networks #2

Once an attacker learns the fixed Interface-ID employed by the victim
host for its stable address, the attacker is able to "probe" a
network for the presence of such host at any given network.

See Section 7.1.1 for just one example of how an attacker could
learn such prefix. Other examples include being able to share the
same network segment at some point in time (think about sharing a
conference network with 1000+ peers), etc.

For example, if an attacker learns that in one network the victim
used the prefix 1111:2222:3333:4444 for its stable addresses, then we
could subsequently probe for the presence of such device in the
network 2011:db8::/64 by sending a probe packet (ICMPv6 Echo Request,
or your favourite probe packet) to the address 2001:db8::1111:2222:
3333:4444.

7.1.3. Revealing the identity of a devices performing server-like
functions

Some devices may typically perform server-like functions and may be
usually moved from one network to another (e.g., from storage devices
to printers). Such devices are likely to simply disable (or not even
implement) privacy/temporary addresses [RFC4941]. If the
aforementioned devices employ Interface-IDs that are constant across
networks, it would be trivial for an attacker to tell whether the
same device is being used across networks by simply looking at the
Interface ID. Clearly, performing server-like should not imply that
a device discloses its identity (i.e., that the attacker can tell
whether it is the same device providing some function in two
different networks, at two different points in time.

The scheme proposed in this document prevents such information
leakage by causing nodes to generate different Interface-IDs when
moving to one network to another, thus mitigating this kind of
privacy attack.

7.2. Host scanning-attacks

While it is usually assumed that host-scanning attacks are
unfeasible, an attack can leverage patterns in IPv6 address
generation to greatly reduce the search space.

As noted earlier in this document, privacy/temporary addresses do not
eliminate the use of IPv6 addresses that embed IEEE identifiers, and
hence such hosts would still be vulnerable to host-scanning attacks
unless they eliminate the patterns introduced by embedding IEEE



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identifiers in the Interface-ID. The method specified in this
document would mitigate the aforementioned host-scanning attacks.

















































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8. References

8.1. Normative References

[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

[RFC4086] Eastlake, D., Schiller, J., and S. Crocker, "Randomness
Requirements for Security", BCP 106, RFC 4086, June 2005.

[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, February 2006.

[RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
Address Autoconfiguration", RFC 4862, September 2007.

[RFC4941] Narten, T., Draves, R., and S. Krishnan, "Privacy
Extensions for Stateless Address Autoconfiguration in
IPv6", RFC 4941, September 2007.

8.2. Informative References

[RFC4620] Crawford, M. and B. Haberman, "IPv6 Node Information
Queries", RFC 4620, August 2006.

[Gont-DEEPSEC2011]
Gont, "Results of a Security Assessment of the Internet
Protocol version 6 (IPv6)", DEEPSEC 2011 Conference,
Vienna, Austria, November 2011, <http://
www.si6networks.com/presentations/deepsec2011/
fgont-deepsec2011-ipv6-security.pdf>.

[Broersma]
Broersma, R., "IPv6 Everywhere: Living with a Fully IPv6-
enabled environment", Australian IPv6 Summit 2010,
Melbourne, VIC Australia, October 2010,
<http://www.ipv6.org.au/summit/talks/Ron_Broersma.pdf>.

[CPNI-IPv6]
Gont, F., "Security Assessment of the Internet Protocol
version 6 (IPv6)", UK Centre for the Protection of
National Infrastructure, (available on request).






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Author's Address

Fernando Gont
UK CPNI

Email: fgont@si6networks.com
URI: http://www.cpni.gov.uk












































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