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IETF I-D On "Stable Privacy Addresses"

IETF I-D On "Stable Privacy Addresses"
Posted Dec 16, 2011
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 same manageability benefits can be achieved without sacrificing the privacy of users.

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IETF I-D On "Stable Privacy Addresses"

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IPv6 maintenance Working Group (6man) F. Gont
Internet-Draft UK CPNI
Updates: 4862 (if approved) December 15, 2011
Intended status: Standards Track
Expires: June 17, 2012


A method for Generating Stable Privacy-Enhanced Addresses with IPv6
Stateless Address Autoconfiguration (SLAAC)
draft-gont-6man-stable-privacy-addresses-00

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 same manageability benefits 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and derivative works of it may not be created, and it may not be
published except as an Internet-Draft.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 June 17, 2012.

Copyright Notice

Copyright (c) 2011 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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Internet-Draft Stable Privacy Addresses December 2011


(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. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
7.1. Normative References . . . . . . . . . . . . . . . . . . . 11
7.2. Informative References . . . . . . . . . . . . . . . . . . 11
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 12






























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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 (using IEEE identifiers) [RFC4291].

Static addresses are generally considered 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, thus negatively affecting the privacy of users.

The "Privacy Extensions for Stateless Address Autoconfiguration in
IPv6" [RFC4941] were introduced to difficult 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). Some flavor of these "Privacy Extensions"
have been implemented in a variety of systems, some of which (notably
Microsoft Windows Vista and Microsoft Windows 7) enable them by
default.

Privacy 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 enforcing access controls and event
logging. 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 such scenarios in which "Privacy Extensions" are deliberately not
used, addresses are generated using e.g. IEEE identifiers, and are
subject to the privacy issues discussed above.

We note that even in those scenarios in which "Privacy Extensions"
are not used, there is still no need or desire to negatively affect
user privacy. As a result, 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 preventing to correlate the
activities of a node as it moves from one network to another.



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On the other hand, even in scenarios in which "Privacy Extensions"
are employed, IPv6 addresses based on IEEE identifiers are still
typically used for performing "server" functions. In such scenarios,
implementation of the mechanism described in this document would
still be desirable, such that the "stable" addresses used by hosts
for "server" functions are not easily predictable (and hence
difficult host-scanning).

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 implment/use "Privacy Extensions" would
have the benefit that they would not be subject to the host-tracking
issues discussed in the previous section. On the other hand, since
hosts implementing "Privacy Extensions" still make use of IEEE-
derived identifiers (mostly for performing "server" functions),
implementation of this algorithm would still benefit such
implementations, since it would prevent leakage of the Organizational
Unique Identifier (OUI), which would be of help to attackers for
host-scanning purposes [Gont-DEEPSEC2011] [CPNI-IPv6].















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3. Algorithm specification

IPv6 implementations conforming to this specification MUST generate
interface identifiers with 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.

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.

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 PRF is being used by



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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 128 bits should be adequate. The
secret key can either be a true random number [RFC4086], or some per-
host secret.

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.





































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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), in replacement of 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 prefix used for autoconfiguration will
typically change, and hence the resulting interface identifier
will also change.

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

Finally, we note that this algorithm is meant to be an alternative
for e.g. the Modified EUI-64 format identifiers, but not for
temporary-address methods such as that specified in [RFC4941].
Clearly, temporary addresses can help reduce the attack exposure
window, since the lifetime of each IPv6 address is reduced when
compared to that of addresses generated with the method specified in
this document. Additionally, they may be of help to correlate
different activities performed by the same host while attached to the
same network. However, we note that implementation of this algorithm
would still benefit those hosts employing "Privacy Addresses", since
it would prevent leakage of the IEEE Organizational Unique Identifier
(OUI) when IEEE-identifier-derived addresses are used for serve-like
functions, which can be of help to attackers for host-scanning
purposes.



















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6. Acknowledgements

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















































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7. References

7.1. 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.

7.2. Informative References

[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












































Gont Expires June 17, 2012 [Page 12]

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