Design considerations for Metadata Insertion
draft-hardie-privsec-metadata-insertion-01
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draft-hardie-privsec-metadata-insertion-01
Network Working Group T. Hardie, Ed.
Internet-Draft March 07, 2016
Intended status: Informational
Expires: September 8, 2016
Design considerations for Metadata Insertion
draft-hardie-privsec-metadata-insertion-01
Abstract
The IAB has published [RFC7624] in response to several revelations of
pervasive attack on Internet communications. In this document we
consider the implications of protocol designs which associate
metadata with encrypted flows.
In particular, we assert that designs which do so by explicit actions
of the end system are preferable to designs in which middleboxes
insert them.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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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 September 8, 2016.
Copyright Notice
Copyright (c) 2016 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
(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
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 2
3. Design patterns . . . . . . . . . . . . . . . . . . . . . . . 4
4. Advice . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
5. Deployment considerations . . . . . . . . . . . . . . . . . . 5
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
7. Security Considerations . . . . . . . . . . . . . . . . . . . 6
8. Contributors {Contributors} . . . . . . . . . . . . . . . . . 6
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
9.1. Normative References . . . . . . . . . . . . . . . . . . 6
9.2. Informative References . . . . . . . . . . . . . . . . . 7
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction
To ensure that the Internet can be trusted by users, 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 a few terms that are specific to the attacks discussed in this
document. Note especially that "passive" and "active" below do not
refer to the effort used to mount the attack; a "passive attack" is
any attack that accesses a flow but does not modify it, while an
"active attack" is any attack that modifies a flow. Some passive
attacks involve active interception and modifications of devices,
rather than simple access to the medium. The introduced terms are:
Pervasive Attack: An attack on Internet communications that makes
use of access at a large number of points in the network, or
otherwise provides the attacker with access to a large amount of
Internet traffic; see [RFC7258].
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Passive Pervasive Attack: An eavesdropping attack undertaken by a
pervasive attacker, in which the packets in a traffic stream
between two endpoints are intercepted, but in which the attacker
does not modify the packets in the traffic stream between two
endpoints, modify the treatment of packets in the traffic stream
(e.g. delay, routing), or add or remove packets in the traffic
stream. Passive pervasive attacks are undetectable from the
endpoints. Equivalent to passive wiretapping as defined in
[RFC4949]; we use an alternate term here since the methods
employed are wider than those implied by the word "wiretapping",
including the active compromise of intermediate systems.
Active Pervasive Attack: An attack undertaken by a pervasive
attacker, which in addition to the elements of a passive pervasive
attack, also includes modification, addition, or removal of
packets in a traffic stream, or modification of treatment of
packets in the traffic stream. Active pervasive attacks provide
more capabilities to the attacker at the risk of possible
detection at the endpoints. Equivalent to active wiretapping as
defined in [RFC4949].
Observation: Information collected directly from communications by
an eavesdropper or observer. For example, the knowledge that
<alice@example.com> sent a message to <bob@example.com> via SMTP
taken from the headers of an observed SMTP message would be an
observation.
Inference: Information derived from analysis of information
collected directly from communications by an eavesdropper or
observer. For example, the knowledge that a given web page was
accessed by a given IP address, by comparing the size in octets of
measured network flow records to fingerprints derived from known
sizes of linked resources on the web servers involved, would be an
inference.
Collaborator: An entity that is a legitimate participant in a
communication, and provides information about that communication
to an attacker. Collaborators may either deliberately or
unwittingly cooperate with the attacker, in the latter case
because the attacker has subverted the collaborator through
technical, social, or other means.
Key Exfiltration: The transmission of cryptographic keying material
for an encrypted communication from a collaborator, deliberately
or unwittingly, to an attacker.
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Content Exfiltration: The transmission of the content of a
communication from a collaborator, deliberately or unwittingly, to
an attacker.
Data Minimization: With respect to protocol design, refers to the
practice of only exposing the minimum amount of data or metadata
necessary for the task supported by that protocol to the other
endpoint(s) and/or devices along the path.
3. Design patterns
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
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
[I-D.ietf-dnsop-edns-client-subnet].
4. Advice
Avoid this design pattern. 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
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parties. In addition to improving privacy, this approach ensures
consistent availability between the communicating parties, no matter
what path is taken.
5. Deployment considerations
There are two 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 "Forward-for" method cited
above provides an example of this. When used with a proxy,
Forwarded-for restores the original identity of the 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, using STUN [RFC5389]
or a similar mechanism to first determine the identity 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
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.
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.
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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 lowers the impact of mitigations to that attack.
8. Contributors {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
is gratefully acknowledged.
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[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>.
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9.2. Informative References
[I-D.ietf-dnsop-edns-client-subnet]
Contavalli, C., Gaast, W., tale, t., and W. Kumari,
"Client Subnet in DNS Queries", draft-ietf-dnsop-edns-
client-subnet-06 (work in progress), December 2015.
[RFC2015] Elkins, M., "MIME Security with Pretty Good Privacy
(PGP)", RFC 2015, DOI 10.17487/RFC2015, October 1996,
<http://www.rfc-editor.org/info/rfc2015>.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, DOI 10.17487/RFC4301,
December 2005, <http://www.rfc-editor.org/info/rfc4301>.
[RFC4306] Kaufman, C., Ed., "Internet Key Exchange (IKEv2)
Protocol", RFC 4306, DOI 10.17487/RFC4306, December 2005,
<http://www.rfc-editor.org/info/rfc4306>.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008,
<http://www.rfc-editor.org/info/rfc5246>.
[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>.
[RFC5750] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
Mail Extensions (S/MIME) Version 3.2 Certificate
Handling", RFC 5750, DOI 10.17487/RFC5750, January 2010,
<http://www.rfc-editor.org/info/rfc5750>.
[RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
of Named Entities (DANE) Transport Layer Security (TLS)
Protocol: TLSA", RFC 6698, DOI 10.17487/RFC6698, August
2012, <http://www.rfc-editor.org/info/rfc6698>.
[RFC6962] Laurie, B., Langley, A., and E. Kasper, "Certificate
Transparency", RFC 6962, DOI 10.17487/RFC6962, June 2013,
<http://www.rfc-editor.org/info/rfc6962>.
[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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[STRINT] S Farrell, ., "Strint Workshop Report", April 2014,
<https://www.w3.org/2014/strint/draft-iab-strint-
report.html>.
Author's Address
Ted Hardie (editor)
Email: ted.ietf@gmail.com
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