Internet DRAFT - draft-hardie-privsec-metadata-insertion
draft-hardie-privsec-metadata-insertion
Network Working Group T. Hardie
Internet-Draft March 27, 2017
Intended status: Informational
Expires: September 28, 2017
Design considerations for Metadata Insertion
draft-hardie-privsec-metadata-insertion-08
Abstract
The IAB has published RFC7624 in response to several revelations of
pervasive attack on Internet communications. This document considers
the implications of protocol designs which associate metadata with
encrypted flows. In particular, it asserts that designs which do so
by explicit actions at the host are preferable to designs in which
middleboxes insert them.
Status of This Memo
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This Internet-Draft will expire on September 18, 2017.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 2
3. Design pattern . . . . . . . . . . . . . . . . . . . . . . . 2
4. Advice . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
5. Deployment considerations . . . . . . . . . . . . . . . . . . 4
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
7. Security Considerations . . . . . . . . . . . . . . . . . . . 5
8. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 6
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
9.1. Normative References . . . . . . . . . . . . . . . . . . 6
9.2. Informative References . . . . . . . . . . . . . . . . . 6
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 7
1. Introduction
To minimize the risks associated with pervasive surveillance, it is
necessary for the Internet technical community to address the
vulnerabilities exploited in the attacks document in [RFC7258] and
the threats described in [RFC7624]. The goal of this document is to
address a common design pattern which emerges from the increase in
encryption: explicit association of metadata which would previously
have been inferred from the plaintext protocol.
2. Terminology
This document makes extensive use of standard security and privacy
terminology; see [RFC4949] and [RFC6973]. Terms used from [RFC6973]
include Eavesdropper, Observer, Initiator, Intermediary, Recipient,
Attack (in a privacy context), Correlation, Fingerprint, Traffic
Analysis, and Identifiability (and related terms). In addition, we
use terms that are specific to the attacks discussed in [RFC7624].
Terms introduced terms from there include: Pervasive Attack, Passive
Pervasive Attack, Active Pervasive Attack, Observation, Inference,
and Collaborator.
3. Design pattern
One of the core mitigations for the loss of confidentiality in the
presence of pervasive surveillance is data minimization, which limits
the amount of data disclosed to those elements absolutely required to
complete the relevant protocol exchange. When data minimization is
in effect, some information which was previously available may be
removed from specific protocol exchanges. The information may be
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removed explicitly (by a browser suppressing cookies during private
modes, as an example) or by other means. As noted in [RFC7624], some
topologies which aggregate or alter the network path also acted to
reduce the ease with which metadata is available to eavesdroppers.
In some cases, other actors within a protocol context will continue
to have access to the information which has been thus withdrawn from
specific protocol exchanges. If those actors attach the information
as metadata to those protocol exchange, the confidentiality effect of
data minimization is lost.
The restoration of information is particularly tempting for systems
whose primary function is not to provide confidentiality. A proxy
providing compression, for example, may wish to restore the identity
of the requesting party; similarly a VPN system used to provide
channel security may believe that origin IP should be restored.
Actors considering restoring metadata may believe that they
understand the relevant privacy considerations or believe that,
because the primary purpose of the service was not privacy-related,
none exist. Examples of this design pattern include [RFC7239] and
[RFC7871].
4. Advice
Avoid inserting metada to restore information which would otherwise
be unavailable to later participants in a protocol exchange. It
contributes to the overall loss of confidentiality for the Internet
and trust in the Internet as a medium. Do not add metadata to flows
at intermediary devices unless a positive affirmation of approval for
restoration has been received from the actor whose data will be
added.
Instead, design the protocol so that the actor can add such metadata
themselves so that it flows end-to-end, rather than requiring the
action of other parties. In addition to improving privacy, this
approach ensures consistent availability between the communicating
parties, no matter what path is taken. (Note that this document does
not attempt to describe how an actor sets policies on providing this
metadata, as the range of systems which might be implied is very
broad).
As an example, RFC 7871 describes a method that had already been
deployed and notes that it is unlikely that a clean-slate design
would result in this mechanism. If a clean-slate design were built
to follow the advice in this document, that design would likely would
not use a core element of RFC 7871: rather than adding metadata at a
proxy, it would provide facilities for end systems to add it to their
initial queries. In the case of RFC 7871, the relevant metadata is
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relatively easy for an end system to derive, as STUN [RFC5389]
provides a method for learning the reflexive transport address from
which a client subnet could be derived. This would allow clients to
populate this data themselves, thus affirming their consent and
providing data at a granularity with which they were comfortable. As
in RFC 7871, the addition of this data would require confirmation
that the upstream DNS resolver understood what to do with it, but the
same negotiation mechanism, an EDNS0 option [RFC6891], could be used.
Because of this negotiation, there would be a new variability in
responses that would change the caching behavior for data supplied by
participating servers. This not a major change from the current
design, however, as the same considerations set out in section 7.3.2
and 7.5 of RFC 7871 would apply to client-supplied subnets as well as
they do for proxy supplied subnets.
From a protocol perspective, in other words, this approach would be a
minor change from RFC 7871, would be as fully featured and would
provide better privacy properties than the on-path update mechanism
RFC 7871 provides. The next section examines why, despite this,
deployment considerations have sometimes trumped cleaner designs.
5. Deployment considerations
There are a few common tensions associated with the deployment of
systems which restore metadata. The first is the trade-off in speed
of deployment for different actors. The Forwarded HTTP Extension in
[RFC7239] provides an example of this. When used with a proxy, it
restores information related to the original requesting party, thus
allowing a responding server to tailor responses according to the
original party's region, network, or other characteristics associated
with the identity. It would, of course, be possible for the
originating client to add this data itself, after using STUN
[RFC5389] or a similar mechanism to first determine the information
to declare. This would require, however, full specification and
adoption of this mechanism by the end systems. It would not be
available at all during this period, and would thereafter be limited
to those systems which have been upgraded to include it. The long
tail of browser deployments indicates that many systems might go
without upgrades for a significant period of time. The proxy
infrastructure, in contrast, is commonly under more active management
and represents a much smaller number of elements; this impacts both
the general deployment difficulty and the number of systems which the
origin server must trust.
The second common tension is between the metadata minimization and
the desire to tailor content responses. For origin servers whose
content is common across users, the loss of metadata may have limited
impact on the system's functioning. For other systems, which
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commonly tailor content by region or network, the loss of metadata
may imply a loss of functionality. Where the user desires this
functionality, restoration can commonly be achieved by the use of
other identifiers or login procedures. Where the user does not
desire this functionality, but it is a preference of the server or a
third party, adjustment is more difficult. At the extreme, content
blocking by network origin may be a regulatory requirement. Trusting
a network intermediary to provide accurate data is, of course,
fragile in this case, but it may be a part of the regulatory
framework.
There are also tensions with latency of operation. For example,
where the end system does not initially know the information which
would be added by on-path devices, it must engage the protocol
mechanisms to determine it. Determining a public IP address to
include in a locally supplied header might require a STUN exchange,
and the additional latency of this exchange discourages deployment of
host-based solutions. To minimize this latency, engaging those
mechanisms may need to be done in parallel with or in advance of the
core protocol exchanges with which this metadata would be supplied.
These tensions do not change the basic recommendation, but they
suggest that the parties who are introducing encryption and data
minimization for existing protocols consider carefully whether the
work also implies introducing mechanisms for the end-to-end
provisioning of metadata when a user has actively consented to
provide it.
6. IANA Considerations
This memo makes no request of IANA.
7. Security Considerations
This memorandum describes a design pattern related emerging from
responses to the attacks described in [RFC7258]. Continued use of
this design pattern, which uses mid-flow devices to restore metadat,
lowers the impact of mitigations to that attack.
Note that some emergency service recipients, notably PSAPs (Public
Safety Answering Points) may prefer data provided by a network to
data provided by end system, because an end system could use false
data to attack others or consume resources. While this has the
consequence that the data available to the PSAP is often more coarse
than that available to the end system, the risk of false data being
provided involved a risk to the lives of those targeted.
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8. Contributors
This document is derived in part from the work initially done on the
Perpass mailing list and at the [STRINT] workshop. It has been
discussed with the IAB's Privacy and Security program, whose review
and input is gratefully acknowledged. The document also benefited
from an extensive review by Mohamed Boucadair.
9. References
9.1. Normative References
[RFC4949] Shirey, R., "Internet Security Glossary, Version 2",
FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,
<http://www.rfc-editor.org/info/rfc4949>.
[RFC6973] Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
Morris, J., Hansen, M., and R. Smith, "Privacy
Considerations for Internet Protocols", RFC 6973,
DOI 10.17487/RFC6973, July 2013,
<http://www.rfc-editor.org/info/rfc6973>.
[RFC7258] Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
2014, <http://www.rfc-editor.org/info/rfc7258>.
[RFC7624] Barnes, R., Schneier, B., Jennings, C., Hardie, T.,
Trammell, B., Huitema, C., and D. Borkmann,
"Confidentiality in the Face of Pervasive Surveillance: A
Threat Model and Problem Statement", RFC 7624,
DOI 10.17487/RFC7624, August 2015,
<http://www.rfc-editor.org/info/rfc7624>.
9.2. Informative References
[RFC5389] Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
"Session Traversal Utilities for NAT (STUN)", RFC 5389,
DOI 10.17487/RFC5389, October 2008,
<http://www.rfc-editor.org/info/rfc5389>.
[RFC6891] Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
for DNS (EDNS(0))", STD 75, RFC 6891,
DOI 10.17487/RFC6891, April 2013,
<http://www.rfc-editor.org/info/rfc6891>.
[RFC7239] Petersson, A. and M. Nilsson, "Forwarded HTTP Extension",
RFC 7239, DOI 10.17487/RFC7239, June 2014,
<http://www.rfc-editor.org/info/rfc7239>.
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[RFC7871] Contavalli, C., van der Gaast, W., Lawrence, D., and W.
Kumari, "Client Subnet in DNS Queries", RFC 7871,
DOI 10.17487/RFC7871, May 2016,
<http://www.rfc-editor.org/info/rfc7871>.
[STRINT] S Farrell, ., "Strint Workshop Report", April 2014,
<https://www.w3.org/2014/strint/draft-iab-strint-
report.html>.
Author's Address
Ted Hardie
Email: ted.ietf@gmail.com
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