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RFC
6973
:
Privacy Considerations for Internet Protocols
A. Cooper
,
H. Tschofenig
,
B. Aboba
,
J. Peterson
,
J. Morris
,
M. Hansen
,
R. Smith
Informational
Internet Architecture Board (IAB) A. Cooper
Request for Comments: 6973 CDT
Category: Informational H. Tschofenig
ISSN: 2070-1721 Nokia Siemens Networks
B. Aboba
Skype
J. Peterson
NeuStar, Inc.
J. Morris

M. Hansen
ULD
R. Smith
Janet
July 2013
Privacy Considerations for Internet Protocols
Abstract

This document offers guidance for developing privacy considerations
for inclusion in protocol specifications. It aims to make designers,
implementers, and users of Internet protocols aware of privacy-
related design choices. It suggests that whether any individual RFC
warrants a specific privacy considerations section will depend on the
document's content.

Status of This Memo

This document is not an Internet Standards Track specification; it is
published for informational purposes.

This document is a product of the Internet Architecture Board (IAB)
and represents information that the IAB has deemed valuable to
provide for permanent record. It represents the consensus of the
Internet Architecture Board (IAB). Documents approved for
publication by the IAB are not a candidate for any level of Internet
Standard; see
Section 2 of RFC 5741
.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6973
.
Cooper, et al. Informational [Page 1]
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Copyright Notice

Copyright (c) 2013 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.
Cooper, et al. Informational [Page 2]
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Table of Contents
1
. Introduction ....................................................
4
2
. Scope of Privacy Implications of Internet Protocols .............
5
3
. Terminology .....................................................
6
3.1
. Entities ...................................................
7
3.2
. Data and Analysis ..........................................
8
3.3
. Identifiability ............................................
9
4
. Communications Model ...........................................
10
5
. Privacy Threats ................................................
12
5.1
. Combined Security-Privacy Threats .........................
13
5.1.1
. Surveillance .......................................
13
5.1.2
. Stored Data Compromise .............................
14
5.1.3
. Intrusion ..........................................
14
5.1.4
. Misattribution .....................................
14
5.2
. Privacy-Specific Threats ..................................
15
5.2.1
. Correlation ........................................
15
5.2.2
. Identification .....................................
16
5.2.3
. Secondary Use ......................................
16
5.2.4
. Disclosure .........................................
17
5.2.5
. Exclusion ..........................................
17
6
. Threat Mitigations .............................................
18
6.1
. Data Minimization .........................................
18
6.1.1
. Anonymity ..........................................
19
6.1.2
. Pseudonymity .......................................
20
6.1.3
. Identity Confidentiality ...........................
20
6.1.4
. Data Minimization within Identity Management .......
21
6.2
. User Participation ........................................
21
6.3
. Security ..................................................
22
7
. Guidelines .....................................................
23
7.1
. Data Minimization .........................................
24
7.2
. User Participation ........................................
25
7.3
. Security ..................................................
25
7.4
. General ...................................................
26
8
. Example ........................................................
26
9
. Security Considerations ........................................
31
10
. Acknowledgements ..............................................
31
11
. IAB Members at the Time of Approval ...........................
32
12
. Informative References ........................................
32
Cooper, et al. Informational [Page 3]
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1
. Introduction
[
RFC3552
] provides detailed guidance to protocol designers about both
how to consider security as part of protocol design and how to inform
readers of protocol specifications about security issues. This
document intends to provide a similar set of guidelines for
considering privacy in protocol design.

Privacy is a complicated concept with a rich history that spans many
disciplines. With regard to data, often it is a concept applied to
"personal data", commonly defined as information relating to an
identified or identifiable individual. Many sets of privacy
principles and privacy design frameworks have been developed in
different forums over the years. These include the Fair Information
Practices [
FIPs
], a baseline set of privacy protections pertaining to
the collection and use of personal data (often based on the
principles established in [
OECD
], for example), and the Privacy by
Design concept, which provides high-level privacy guidance for
systems design (see [
PbD
] for one example). The guidance provided in
this document is inspired by this prior work, but it aims to be more
concrete, pointing protocol designers to specific engineering choices
that can impact the privacy of the individuals that make use of
Internet protocols.

Different people have radically different conceptions of what privacy
means, both in general and as it relates to them personally [
Westin
].
Furthermore, privacy as a legal concept is understood differently in
different jurisdictions. The guidance provided in this document is
generic and can be used to inform the design of any protocol to be
used anywhere in the world, without reference to specific legal
frameworks.

Whether any individual document warrants a specific privacy
considerations section will depend on the document's content.
Documents whose entire focus is privacy may not merit a separate
section (for example, "Private Extensions to the Session Initiation
Protocol (SIP) for Asserted Identity within Trusted Networks"
[
RFC3325
]). For certain specifications, privacy considerations are a
subset of security considerations and can be discussed explicitly in
the security considerations section. Some documents will not require
discussion of privacy considerations (for example, "Definition of the
Opus Audio Codec" [
RFC6716
]). The guidance provided here can and
should be used to assess the privacy considerations of protocol,
architectural, and operational specifications and to decide whether
those considerations are to be documented in a stand-alone section,
within the security considerations section, or throughout the
Cooper, et al. Informational [Page 4]
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document. The guidance provided here is meant to help the thought
process of privacy analysis; it does not provide specific directions
for how to write a privacy considerations section.

This document is organized as follows.
Section 2
describes the
extent to which the guidance offered here is applicable within the
IETF and within the larger Internet community.
Section 3
explains
the terminology used in this document.
Section 4
reviews typical
communications architectures to understand at which points there may
be privacy threats.
Section 5
discusses threats to privacy as they
apply to Internet protocols.
Section 6
outlines mitigations of those
threats.
Section 7
provides the guidelines for analyzing and
documenting privacy considerations within IETF specifications.
Section 8
examines the privacy characteristics of an IETF protocol to
demonstrate the use of the guidance framework.
2
. Scope of Privacy Implications of Internet Protocols
Internet protocols are often built flexibly, making them useful in a
variety of architectures, contexts, and deployment scenarios without
requiring significant interdependency between disparately designed
components. Although protocol designers often have a particular
target architecture or set of architectures in mind at design time,
it is not uncommon for architectural frameworks to develop later,
after implementations exist and have been deployed in combination
with other protocols or components to form complete systems.

As a consequence, the extent to which protocol designers can foresee
all of the privacy implications of a particular protocol at design
time is limited. An individual protocol may be relatively benign on
its own, and it may make use of privacy and security features at
lower layers of the protocol stack (Internet Protocol Security,
Transport Layer Security, and so forth) to mitigate the risk of
attack. But when deployed within a larger system or used in a way
not envisioned at design time, its use may create new privacy risks.
Protocols are often implemented and deployed long after design time
by different people than those who did the protocol design. The
guidelines in
Section 7
ask protocol designers to consider how their
protocols are expected to interact with systems and information that
exist outside the protocol bounds, but not to imagine every possible
deployment scenario.

Furthermore, in many cases the privacy properties of a system are
dependent upon the complete system design where various protocols are
combined together to form a product solution; the implementation,
which includes the user interface design; and operational deployment
practices, including default privacy settings and security processes
of the company doing the deployment. These details are specific to
Cooper, et al. Informational [Page 5]
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particular instantiations and generally outside the scope of the work
conducted in the IETF. The guidance provided here may be useful in
making choices about these details, but its primary aim is to assist
with the design, implementation, and operation of protocols.

Transparency of data collection and use -- often effectuated through
user interface design -- is normally relied on (whether rightly or
wrongly) as a key factor in determining the privacy impact of a
system. Although most IETF activities do not involve standardizing
user interfaces or user-facing communications, in some cases,
understanding expected user interactions can be important for
protocol design. Unexpected user behavior may have an adverse impact
on security and/or privacy.

In sum, privacy issues, even those related to protocol development,
go beyond the technical guidance discussed herein. As an example,
consider HTTP [
RFC2616
], which was designed to allow the exchange of
arbitrary data. A complete analysis of the privacy considerations
for uses of HTTP might include what type of data is exchanged, how
this data is stored, and how it is processed. Hence the analysis for
an individual's static personal web page would be different than the
use of HTTP for exchanging health records. A protocol designer
working on HTTP extensions (such as Web Distributed Authoring and
Versioning (WebDAV) [
RFC4918
]) is not expected to describe the
privacy risks derived from all possible usage scenarios, but rather
the privacy properties specific to the extensions and any particular
uses of the extensions that are expected and foreseen at design time.
3
. Terminology
This section defines basic terms used in this document, with
references to pre-existing definitions as appropriate. As in
[
RFC4949
], each entry is preceded by a dollar sign ($) and a space
for automated searching. Note that this document does not try to
attempt to define the term 'privacy' with a brief definition.
Instead, privacy is the sum of what is contained in this document.
We therefore follow the approach taken by [
RFC3552
]. Examples of
several different brief definitions are provided in [
RFC4949
].
Cooper, et al. Informational [Page 6]
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3.1
. Entities
Several of these terms are further elaborated in
Section 4
.

$ Attacker: An entity that works against one or more privacy
protection goals. Unlike observers, attackers' behavior is
unauthorized.

$ Eavesdropper: A type of attacker that passively observes an
initiator's communications without the initiator's knowledge or
authorization. See [
RFC4949
].

$ Enabler: A protocol entity that facilitates communication between
an initiator and a recipient without being directly in the
communications path.

$ Individual: A human being.

$ Initiator: A protocol entity that initiates communications with a
recipient.

$ Intermediary: A protocol entity that sits between the initiator
and the recipient and is necessary for the initiator and recipient
to communicate. Unlike an eavesdropper, an intermediary is an
entity that is part of the communication architecture and
therefore at least tacitly authorized. For example, a SIP
[
RFC3261
] proxy is an intermediary in the SIP architecture.

$ Observer: An entity that is able to observe and collect
information from communications, potentially posing privacy
threats, depending on the context. As defined in this document,
initiators, recipients, intermediaries, and enablers can all be
observers. Observers are distinguished from eavesdroppers by
being at least tacitly authorized.

$ Recipient: A protocol entity that receives communications from an
initiator.
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3.2
. Data and Analysis
$ Attack: An intentional act by which an entity attempts to violate
an individual's privacy. See [
RFC4949
].

$ Correlation: The combination of various pieces of information that
relate to an individual or that obtain that characteristic when
combined.

$ Fingerprint: A set of information elements that identifies a
device or application instance.

$ Fingerprinting: The process of an observer or attacker uniquely
identifying (with a sufficiently high probability) a device or
application instance based on multiple information elements
communicated to the observer or attacker. See [
EFF
].

$ Item of Interest (IOI): Any data item that an observer or attacker
might be interested in. This includes attributes, identifiers,
identities, communications content, and the fact that a
communication interaction has taken place.

$ Personal Data: Any information relating to an individual who can
be identified, directly or indirectly.

$ (Protocol) Interaction: A unit of communication within a
particular protocol. A single interaction may be comprised of a
single message between an initiator and recipient or multiple
messages, depending on the protocol.

$ Traffic Analysis: The inference of information from observation of
traffic flows (presence, absence, amount, direction, timing,
packet size, packet composition, and/or frequency), even if flows
are encrypted. See [
RFC4949
].

$ Undetectability: The inability of an observer or attacker to
sufficiently distinguish whether an item of interest exists
or not.

$ Unlinkability: Within a particular set of information, the
inability of an observer or attacker to distinguish whether two
items of interest are related or not (with a high enough degree of
probability to be useful to the observer or attacker).
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3.3
. Identifiability
$ Anonymity: The state of being anonymous.

$ Anonymity Set: A set of individuals that have the same attributes,
making them indistinguishable from each other from the perspective
of a particular attacker or observer.

$ Anonymous: A state of an individual in which an observer or
attacker cannot identify the individual within a set of other
individuals (the anonymity set).

$ Attribute: A property of an individual.

$ Identifiability: The extent to which an individual is
identifiable.

$ Identifiable: A property in which an individual's identity is
capable of being known to an observer or attacker.

$ Identification: The linking of information to a particular
individual to infer an individual's identity or to allow the
inference of an individual's identity in some context.

$ Identified: A state in which an individual's identity is known.

$ Identifier: A data object uniquely referring to a specific
identity of a protocol entity or individual in some context. See
[
RFC4949
]. Identifiers can be based upon natural names --
official names, personal names, and/or nicknames -- or can be
artificial (for example, x9z32vb). However, identifiers are by
definition unique within their context of use, while natural names
are often not unique.

$ Identity: Any subset of an individual's attributes, including
names, that identifies the individual within a given context.
Individuals usually have multiple identities for use in different
contexts.

$ Identity Confidentiality: A property of an individual where only
the recipient can sufficiently identify the individual within a
set of other individuals. This can be a desirable property of
authentication protocols.

$ Identity Provider: An entity (usually an organization) that is
responsible for establishing, maintaining, securing, and vouching
for the identities associated with individuals.
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$ Official Name: A personal name for an individual that is
registered in some official context (for example, the name on an
individual's birth certificate). Official names are often not
unique.

$ Personal Name: A natural name for an individual. Personal names
are often not unique and often comprise given names in combination
with a family name. An individual may have multiple personal
names at any time and over a lifetime, including official names.
From a technological perspective, it cannot always be determined
whether a given reference to an individual is, or is based upon,
the individual's personal name(s) (see Pseudonym).

$ Pseudonym: A name assumed by an individual in some context,
unrelated to the individual's personal names known by others in
that context, with an intent of not revealing the individual's
identities associated with his or her other names. Pseudonyms are
often not unique.

$ Pseudonymity: The state of being pseudonymous.

$ Pseudonymous: A property of an individual in which the individual
is identified by a pseudonym.

$ Real Name: See Personal Name and Official Name.

$ Relying Party: An entity that relies on assertions of individuals'
identities from identity providers in order to provide services to
individuals. In effect, the relying party delegates aspects of
identity management to the identity provider(s). Such delegation
requires protocol exchanges, trust, and a common understanding of
semantics of information exchanged between the relying party and
the identity provider.
4
. Communications Model
To understand attacks in the privacy-harm sense, it is helpful to
consider the overall communication architecture and different actors'
roles within it. Consider a protocol entity, the "initiator", that
initiates communication with some recipient. Privacy analysis is
most relevant for protocols with use cases in which the initiator
acts on behalf of an individual (or different individuals at
different times). It is this individual whose privacy is potentially
threatened (although in some instances an initiator communicates
information about another individual, in which case both of their
privacy interests will be implicated).
Cooper, et al. Informational [Page 10]
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Communications may be direct between the initiator and the recipient,
or they may involve an application-layer intermediary (such as a
proxy, cache, or relay) that is necessary for the two parties to
communicate. In some cases, this intermediary stays in the
communication path for the entire duration of the communication;
sometimes it is only used for communication establishment, for either
inbound or outbound communication. In some cases, there may be a
series of intermediaries that are traversed. At lower layers,
additional entities involved in packet forwarding may interfere with
privacy protection goals as well.

Some communications tasks require multiple protocol interactions with
different entities. For example, a request to an HTTP server may be
preceded by an interaction between the initiator and an
Authentication, Authorization, and Accounting (AAA) server for
network access and to a Domain Name System (DNS) server for name
resolution. In this case, the HTTP server is the recipient and the
other entities are enablers of the initiator-to-recipient
communication. Similarly, a single communication with the recipient
might generate further protocol interactions between either the
initiator or the recipient and other entities, and the roles of the
entities might change with each interaction. For example, an HTTP
request might trigger interactions with an authentication server or
with other resource servers wherein the recipient becomes an
initiator in those later interactions.

Thus, when conducting privacy analysis of an architecture that
involves multiple communications phases, the entities involved may
take on different -- or opposing -- roles from a privacy
considerations perspective in each phase. Understanding the privacy
implications of the architecture as a whole may require a separate
analysis of each phase.

Protocol design is often predicated on the notion that recipients,
intermediaries, and enablers are assumed to be authorized to receive
and handle data from initiators. As [
RFC3552
] explains, "we assume
that the end systems engaging in a protocol exchange have not
themselves been compromised". However, privacy analysis requires
questioning this assumption, since systems are often compromised for
the purpose of obtaining personal data.

Although recipients, intermediaries, and enablers may not generally
be considered as attackers, they may all pose privacy threats
(depending on the context) because they are able to observe, collect,
process, and transfer privacy-relevant data. These entities are
collectively described below as "observers" to distinguish them from
traditional attackers. From a privacy perspective, one important
Cooper, et al. Informational [Page 11]
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type of attacker is an eavesdropper: an entity that passively
observes the initiator's communications without the initiator's
knowledge or authorization.

The threat descriptions in the next section explain how observers and
attackers might act to harm individuals' privacy. Different kinds of
attacks may be feasible at different points in the communications
path. For example, an observer could mount surveillance or
identification attacks between the initiator and intermediary, or
instead could surveil an enabler (e.g., by observing DNS queries from
the initiator).
5
. Privacy Threats
Privacy harms come in a number of forms, including harms to financial
standing, reputation, solitude, autonomy, and safety. A victim of
identity theft or blackmail, for example, may suffer a financial loss
as a result. Reputational harm can occur when disclosure of
information about an individual, whether true or false, subjects that
individual to stigma, embarrassment, or loss of personal dignity.
Intrusion or interruption of an individual's life or activities can
harm the individual's ability to be left alone. When individuals or
their activities are monitored, exposed, or at risk of exposure,
those individuals may be stifled from expressing themselves,
associating with others, and generally conducting their lives freely.
They may also feel a general sense of unease, in that it is "creepy"
to be monitored or to have data collected about them. In cases where
such monitoring is for the purpose of stalking or violence (for
example, monitoring communications to or from a domestic abuse
shelter), it can put individuals in physical danger.

This section lists common privacy threats (drawing liberally from
[
Solove
], as well as [
CoE
]), showing how each of them may cause
individuals to incur privacy harms and providing examples of how
these threats can exist on the Internet. This threat modeling is
inspired by security threat analysis. Although it is not a perfect
fit for assessing privacy risks in Internet protocols and systems, no
better methodology has been developed to date.

Some privacy threats are already considered in Internet protocols as
a matter of routine security analysis. Others are more pure privacy
threats that existing security considerations do not usually address.
The threats described here are divided into those that may also be
considered security threats and those that are primarily privacy
threats.
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Note that an individual's awareness of and consent to the practices
described below may change an individual's perception of and concern
for the extent to which they threaten privacy. If an individual
authorizes surveillance of his own activities, for example, the
individual may be able to take actions to mitigate the harms
associated with it or may consider the risk of harm to be tolerable.
5.1
. Combined Security-Privacy Threats
5.1.1
. Surveillance
Surveillance is the observation or monitoring of an individual's
communications or activities. The effects of surveillance on the
individual can range from anxiety and discomfort to behavioral
changes such as inhibition and self-censorship, and even to the
perpetration of violence against the individual. The individual need
not be aware of the surveillance for it to impact his or her privacy
-- the possibility of surveillance may be enough to harm individual
autonomy.

Surveillance can impact privacy, even if the individuals being
surveilled are not identifiable or if their communications are
encrypted. For example, an observer or eavesdropper that conducts
traffic analysis may be able to determine what type of traffic is
present (real-time communications or bulk file transfers, for
example) or which protocols are in use, even if the observed
communications are encrypted or the communicants are unidentifiable.
This kind of surveillance can adversely impact the individuals
involved by causing them to become targets for further investigation
or enforcement activities. It may also enable attacks that are
specific to the protocol, such as redirection to a specialized
interception point or protocol-specific denials of service.
Protocols that use predictable packet sizes or timing or include
fixed tokens at predictable offsets within a packet can facilitate
this kind of surveillance.

Surveillance can be conducted by observers or eavesdroppers at any
point along the communications path. Confidentiality protections (as
discussed in
Section 3 of [RFC3552]
) are necessary to prevent
surveillance of the content of communications. To prevent traffic
analysis or other surveillance of communications patterns, other
measures may be necessary, such as [
Tor
].
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5.1.2
. Stored Data Compromise
End systems that do not take adequate measures to secure stored data
from unauthorized or inappropriate access expose individuals to
potential financial, reputational, or physical harm.

Protecting against stored data compromise is typically outside the
scope of IETF protocols. However, a number of common protocol
functions -- key management, access control, or operational logging,
for example -- require the storage of data about initiators of
communications. When requiring or recommending that information
about initiators or their communications be stored or logged by end
systems (see, e.g.,
RFC 6302
[
RFC6302
]), it is important to recognize
the potential for that information to be compromised and for that
potential to be weighed against the benefits of data storage. Any
recipient, intermediary, or enabler that stores data may be
vulnerable to compromise. (Note that stored data compromise is
distinct from purposeful disclosure, which is discussed in
Section 5.2.4
.)
5.1.3
. Intrusion
Intrusion consists of invasive acts that disturb or interrupt one's
life or activities. Intrusion can thwart individuals' desires to be
left alone, sap their time or attention, or interrupt their
activities. This threat is focused on intrusion into one's life
rather than direct intrusion into one's communications. The latter
is captured in
Section 5.1.1
.

Unsolicited messages and denial-of-service attacks are the most
common types of intrusion on the Internet. Intrusion can be
perpetrated by any attacker that is capable of sending unwanted
traffic to the initiator.
5.1.4
. Misattribution
Misattribution occurs when data or communications related to one
individual are attributed to another. Misattribution can result in
adverse reputational, financial, or other consequences for
individuals that are misidentified.

Misattribution in the protocol context comes as a result of using
inadequate or insecure forms of identity or authentication, and is
sometimes related to spoofing. For example, as [
RFC6269
] notes,
abuse mitigation is often conducted on the basis of the source IP
address, such that connections from individual IP addresses may be
prevented or temporarily blacklisted if abusive activity is
determined to be sourced from those addresses. However, in the case
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where a single IP address is shared by multiple individuals, those
penalties may be suffered by all individuals sharing the address,
even if they were not involved in the abuse. This threat can be
mitigated by using identity management mechanisms with proper forms
of authentication (ideally with cryptographic properties) so that
actions can be attributed uniquely to an individual to provide the
basis for accountability without generating false positives.
5.2
. Privacy-Specific Threats
5.2.1
. Correlation
Correlation is the combination of various pieces of information
related to an individual or that obtain that characteristic when
combined. Correlation can defy people's expectations of the limits
of what others know about them. It can increase the power that those
doing the correlating have over individuals as well as correlators'
ability to pass judgment, threatening individual autonomy and
reputation.

Correlation is closely related to identification. Internet protocols
can facilitate correlation by allowing individuals' activities to be
tracked and combined over time. The use of persistent or
infrequently replaced identifiers at any layer of the stack can
facilitate correlation. For example, an initiator's persistent use
of the same device ID, certificate, or email address across multiple
interactions could allow recipients (and observers) to correlate all
of the initiator's communications over time.

As an example, consider Transport Layer Security (TLS) session
resumption [
RFC5246
] or TLS session resumption without server-side
state [
RFC5077
]. In
RFC 5246
[
RFC5246
], a server provides the client
with a session_id in the ServerHello message and caches the
master_secret for later exchanges. When the client initiates a new
connection with the server, it re-uses the previously obtained
session_id in its ClientHello message. The server agrees to resume
the session by using the same session_id and the previously stored
master_secret for the generation of the TLS Record Layer security
association.
RFC 5077
[
RFC5077
] borrows from the session resumption
design idea, but the server encapsulates all state information into a
ticket instead of caching it. An attacker who is able to observe the
protocol exchanges between the TLS client and the TLS server is able
to link the initial exchange to subsequently resumed TLS sessions
when the session_id and the ticket are exchanged in the clear (which
is the case with data exchanged in the initial handshake messages).
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In theory, any observer or attacker that receives an initiator's
communications can engage in correlation. The extent of the
potential for correlation will depend on what data the entity
receives from the initiator and has access to otherwise. Often,
intermediaries only require a small amount of information for message
routing and/or security. In theory, protocol mechanisms could ensure
that end-to-end information is not made accessible to these entities,
but in practice the difficulty of deploying end-to-end security
procedures, additional messaging or computational overhead, and other
business or legal requirements often slow or prevent the deployment
of end-to-end security mechanisms, giving intermediaries greater
exposure to initiators' data than is strictly necessary from a
technical point of view.
5.2.2
. Identification
Identification is the linking of information to a particular
individual to infer an individual's identity or to allow the
inference of an individual's identity. In some contexts, it is
perfectly legitimate to identify individuals, whereas in others,
identification may potentially stifle individuals' activities or
expression by inhibiting their ability to be anonymous or
pseudonymous. Identification also makes it easier for individuals to
be explicitly controlled by others (e.g., governments) and to be
treated differentially compared to other individuals.

Many protocols provide functionality to convey the idea that some
means has been provided to validate that entities are who they claim
to be. Often, this is accomplished with cryptographic
authentication. Furthermore, many protocol identifiers, such as
those used in SIP or the Extensible Messaging and Presence Protocol
(XMPP), may allow for the direct identification of individuals.
Protocol identifiers may also contribute indirectly to identification
via correlation. For example, a web site that does not directly
authenticate users may be able to match its HTTP header logs with
logs from another site that does authenticate users, rendering users
on the first site identifiable.

As with correlation, any observer or attacker may be able to engage
in identification, depending on the information about the initiator
that is available via the protocol mechanism or other channels.
5.2.3
. Secondary Use
Secondary use is the use of collected information about an individual
without the individual's consent for a purpose different from that
for which the information was collected. Secondary use may violate
people's expectations or desires. The potential for secondary use
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can generate uncertainty as to how one's information will be used in
the future, potentially discouraging information exchange in the
first place. Secondary use encompasses any use of data, including
disclosure.

One example of secondary use would be an authentication server that
uses a network access server's Access-Requests to track an
initiator's location. Any observer or attacker could potentially
make unwanted secondary uses of initiators' data. Protecting against
secondary use is typically outside the scope of IETF protocols.
5.2.4
. Disclosure
Disclosure is the revelation of information about an individual that
affects the way others judge the individual. Disclosure can violate
individuals' expectations of the confidentiality of the data they
share. The threat of disclosure may deter people from engaging in
certain activities for fear of reputational harm, or simply because
they do not wish to be observed.

Any observer or attacker that receives data about an initiator may
engage in disclosure. Sometimes disclosure is unintentional because
system designers do not realize that information being exchanged
relates to individuals. The most common way for protocols to limit
disclosure is by providing access control mechanisms (discussed in
Section 5.2.5
). A further example is provided by the IETF
geolocation privacy architecture [
RFC6280
], which supports a way for
users to express a preference that their location information not be
disclosed beyond the intended recipient.
5.2.5
. Exclusion
Exclusion is the failure to allow individuals to know about the data
that others have about them and to participate in its handling and
use. Exclusion reduces accountability on the part of entities that
maintain information about people and creates a sense of
vulnerability in relation to individuals' ability to control how
information about them is collected and used.

The most common way for Internet protocols to be involved in
enforcing exclusion is through access control mechanisms. The
presence architecture developed in the IETF is a good example where
individuals are included in the control of information about them.
Using a rules expression language (e.g., presence authorization rules
[
RFC5025
]), presence clients can authorize the specific conditions
under which their presence information may be shared.
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Exclusion is primarily considered problematic when the recipient
fails to involve the initiator in decisions about data collection,
handling, and use. Eavesdroppers engage in exclusion by their very
nature, since their data collection and handling practices are
covert.
6
. Threat Mitigations
Privacy is notoriously difficult to measure and quantify. The extent
to which a particular protocol, system, or architecture "protects" or
"enhances" privacy is dependent on a large number of factors relating
to its design, use, and potential misuse. However, there are certain
widely recognized classes of mitigations against the threats
discussed in
Section 5
. This section describes three categories of
relevant mitigations: (1) data minimization, (2) user participation,
and (3) security. The privacy mitigations described in this section
can loosely be mapped to existing privacy principles, such as the
Fair Information Practices, but they have been adapted to fit the
target audience of this document.
6.1
. Data Minimization
Data minimization refers to collecting, using, disclosing, and
storing the minimal data necessary to perform a task. Reducing the
amount of data exchanged reduces the amount of data that can be
misused or leaked.

Data minimization can be effectuated in a number of different ways,
including by limiting collection, use, disclosure, retention,
identifiability, sensitivity, and access to personal data. Limiting
the data collected by protocol elements to only what is necessary
(collection limitation) is the most straightforward way to help
reduce privacy risks associated with the use of the protocol. In
some cases, protocol designers may also be able to recommend limits
to the use or retention of data, although protocols themselves are
not often capable of controlling these properties.

However, the most direct application of data minimization to protocol
design is limiting identifiability. Reducing the identifiability of
data by using pseudonyms or no identifiers at all helps to weaken the
link between an individual and his or her communications. Allowing
for the periodic creation of new or randomized identifiers reduces
the possibility that multiple protocol interactions or communications
can be correlated back to the same individual. The following
sections explore a number of different properties related to
identifiability that protocol designers may seek to achieve.
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Data minimization mitigates the following threats: surveillance,
stored data compromise, correlation, identification, secondary use,
and disclosure.
6.1.1
. Anonymity
To enable anonymity of an individual, there must exist a set of
individuals that appear to have the same attribute(s) as the
individual. To the attacker or the observer, these individuals must
appear indistinguishable from each other. The set of all such
individuals is known as the anonymity set, and membership of this set
may vary over time.

The composition of the anonymity set depends on the knowledge of the
observer or attacker. Thus, anonymity is relative with respect to
the observer or attacker. An initiator may be anonymous only within
a set of potential initiators -- its initiator anonymity set -- which
itself may be a subset of all individuals that may initiate
communications. Conversely, a recipient may be anonymous only within
a set of potential recipients -- its recipient anonymity set. Both
anonymity sets may be disjoint, may overlap, or may be the same.

As an example, consider
RFC 3325
(P-Asserted-Identity (PAI))
[
RFC3325
], an extension for the Session Initiation Protocol (SIP)
that allows an individual, such as a Voice over IP (VoIP) caller, to
instruct an intermediary that he or she trusts not to populate the
SIP From header field with the individual's authenticated and
verified identity. The recipient of the call, as well as any other
entity outside of the individual's trust domain, would therefore only
learn that the SIP message (typically a SIP INVITE) was sent with a
header field 'From: "Anonymous" <sip:anonymous@anonymous.invalid>'
rather than the individual's address-of-record, which is typically
thought of as the "public address" of the user. When PAI is used,
the individual becomes anonymous within the initiator anonymity set
that is populated by every individual making use of that specific
intermediary.

Note that this example ignores the fact that the recipient may infer
or obtain personal data from the other SIP payloads (e.g., SIP Via
and Contact headers, the Session Description Protocol (SDP)). The
implication is that PAI only attempts to address a particular threat,
namely the disclosure of identity (in the From header) with respect
to the recipient. This caveat makes the analysis of the specific
protocol extension easier but cannot be assumed when conducting
analysis of an entire architecture.
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6.1.2
. Pseudonymity
In the context of Internet protocols, almost all identifiers can be
nicknames or pseudonyms, since there is typically no requirement to
use personal names in protocols. However, in certain scenarios it is
reasonable to assume that personal names will be used (with vCard
[
RFC6350
], for example).

Pseudonymity is strengthened when less personal data can be linked to
the pseudonym; when the same pseudonym is used less often and across
fewer contexts; and when independently chosen pseudonyms are more
frequently used for new actions (making them, from an observer's or
attacker's perspective, unlinkable).

For Internet protocols, the following are important considerations:
whether protocols allow pseudonyms to be changed without human
interaction, the default length of pseudonym lifetimes, to whom
pseudonyms are exposed, how individuals are able to control
disclosure, how often pseudonyms can be changed, and the consequences
of changing them.
6.1.3
. Identity Confidentiality
An initiator has identity confidentiality when any party other than
the recipient cannot sufficiently identify the initiator within the
anonymity set. The size of the anonymity set has a direct impact on
identity confidentiality, since the smaller the set is, the easier it
is to identify the initiator. Identity confidentiality aims to
provide a protection against eavesdroppers and intermediaries rather
than against the intended communication endpoints.

As an example, consider the network access authentication procedures
utilizing the Extensible Authentication Protocol (EAP) [
RFC3748
].
EAP includes an identity exchange where the Identity Response is
primarily used for routing purposes and selecting which EAP method to
use. Since EAP Identity Requests and Identity Responses are sent in
cleartext, eavesdroppers and intermediaries along the communication
path between the EAP peer and the EAP server can snoop on the
identity, which is encoded in the form of the Network Access
Identifier (NAI) as defined in
RFC 4282
[
RFC4282
]. To address this
threat, as discussed in
RFC 4282
[
RFC4282
], the username part of the
NAI (but not the realm part) can be hidden from these eavesdroppers
and intermediaries with the cryptographic support offered by EAP
methods. Identity confidentiality has become a recommended design
criteria for EAP (see [
RFC4017
]). The EAP method for 3rd Generation
Authentication and Key Agreement (EAP-AKA) [
RFC4187
], for example,
protects the EAP peer's identity against passive adversaries by
utilizing temporal identities. The EAP-Internet Key Exchange
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Protocol version 2 (EAP-IKEv2) method [
RFC5106
] is an example of an
EAP method that offers protection against active attackers with
regard to the individual's identity.
6.1.4
. Data Minimization within Identity Management
Modern systems are increasingly relying on multi-party transactions
to authenticate individuals. Many of these systems make use of an
identity provider that is responsible for providing AAA functionality
to relying parties that offer some protected resources. To
facilitate these functions, an identity provider will usually go
through a process of verifying the individual's identity and issuing
credentials to the individual. When an individual seeks to make use
of a service provided by the relying party, the relying party relies
on the authentication assertions provided by its identity provider.
Note that in more sophisticated scenarios the authentication
assertions are traits that demonstrate the individual's capabilities
and roles. The authorization responsibility may also be shared
between the identity provider and the relying party and does not
necessarily need to reside only with the identity provider.

Such systems have the ability to support a number of properties that
minimize data collection in different ways:

In certain use cases, relying parties do not need to know the real
name or date of birth of an individual (for example, when the
individual's age is the only attribute that needs to be
authenticated).

Relying parties that collude can be prevented from using an
individual's credentials to track the individual. That is, two
different relying parties can be prevented from determining that
the same individual has authenticated to both of them. This
typically requires identity management protocol support as well as
support by both the relying party and the identity provider.

The identity provider can be prevented from knowing which relying
parties an individual interacted with. This requires, at a
minimum, avoiding direct communication between the identity
provider and the relying party at the time when access to a
resource by the initiator is made.
6.2
. User Participation
As explained in
Section 5.2.5
, data collection and use that happen
"in secret", without the individual's knowledge, are apt to violate
the individual's expectation of privacy and may create incentives for
misuse of data. As a result, privacy regimes tend to include
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provisions to require informing individuals about data collection and
use and involving them in decisions about the treatment of their
data. In an engineering context, supporting the goal of user
participation usually means providing ways for users to control the
data that is shared about them. It may also mean providing ways for
users to signal how they expect their data to be used and shared.
Different protocol and architectural designs can make supporting user
participation (for example, the ability to support a dialog box for
user interaction) easier or harder; for example, OAuth-based services
may have more natural hooks for user input than AAA services.

User participation mitigates the following threats: surveillance,
secondary use, disclosure, and exclusion.
6.3
. Security
Keeping data secure at rest and in transit is another important
component of privacy protection. As they are described in
Section 2
of [RFC3552]
, a number of security goals also serve to enhance
privacy:

o Confidentiality: Keeping data secret from unintended listeners.

o Peer entity authentication: Ensuring that the endpoint of a
communication is the one that is intended (in support of
maintaining confidentiality).

o Unauthorized usage: Limiting data access to only those users who
are authorized. (Note that this goal also falls within data
minimization.)

o Inappropriate usage: Limiting how authorized users can use data.
(Note that this goal also falls within data minimization.)

Note that even when these goals are achieved, the existence of items
of interest -- attributes, identifiers, identities, communications,
actions (such as the sending or receiving of a communication), or
anything else an attacker or observer might be interested in -- may
still be detectable, even if they are not readable. Thus,
undetectability, in which an observer or attacker cannot sufficiently
distinguish whether an item of interest exists or not, may be
considered as a further security goal (albeit one that can be
extremely difficult to accomplish).

Detection of the protocols or applications in use via traffic
analysis may be particularly difficult to defend against. As with
the anonymity of individuals, achieving "protocol anonymity" requires
that multiple protocols or applications exist that appear to have the
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same attributes -- packet sizes, content, token locations, or
inter-packet timing, for example. An attacker or observer will not
be able to use traffic analysis to identify which protocol or
application is in use if multiple protocols or applications are
indistinguishable.

Defending against the threat of traffic analysis will be possible to
different extents for different protocols, may depend on
implementation- or use-specific details, and may depend on which
other protocols already exist and whether they share similar traffic
characteristics. The defenses will also vary relative to what the
protocol is designed to do; for example, in some situations
randomizing packet sizes, timing, or token locations will reduce the
threat of traffic analysis, whereas in other situations (real-time
communications, for example) holding some or all of those factors
constant is a more appropriate defense. See "Guidelines for the Use
of Variable Bit Rate Audio with Secure RTP" [
RFC6562
] for an example
of how these kinds of trade-offs should be evaluated.

By providing proper security protection, the following threats can be
mitigated: surveillance, stored data compromise, misattribution,
secondary use, disclosure, and intrusion.
7
. Guidelines
This section provides guidance for document authors in the form of a
questionnaire about a protocol being designed. The questionnaire may
be useful at any point in the design process, particularly after
document authors have developed a high-level protocol model as
described in [
RFC4101
].

Note that the guidance provided in this section does not recommend
specific practices. The range of protocols developed in the IETF is
too broad to make recommendations about particular uses of data or
how privacy might be balanced against other design goals. However,
by carefully considering the answers to each question, document
authors should be able to produce a comprehensive analysis that can
serve as the basis for discussion of whether the protocol adequately
protects against privacy threats. This guidance is meant to help the
thought process of privacy analysis; it does not provide specific
directions for how to write a privacy considerations section.

The framework is divided into four sections: three sections that
address each of the mitigation classes from
Section 6
, plus a general
section. Security is not fully elaborated, since substantial
guidance already exists in [
RFC3552
].
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7.1
. Data Minimization
a. Identifiers. What identifiers does the protocol use for
distinguishing initiators of communications? Does the protocol
use identifiers that allow different protocol interactions to be
correlated? What identifiers could be omitted or be made less
identifying while still fulfilling the protocol's goals?

b. Data. What information does the protocol expose about
individuals, their devices, and/or their device usage (other than
the identifiers discussed in (a))? To what extent is this
information linked to the identities of the individuals? How
does the protocol combine personal data with the identifiers
discussed in (a)?

c. Observers. Which information discussed in (a) and (b) is exposed
to each other protocol entity (i.e., recipients, intermediaries,
and enablers)? Are there ways for protocol implementers to
choose to limit the information shared with each entity? Are
there operational controls available to limit the information
shared with each entity?

d. Fingerprinting. In many cases, the specific ordering and/or
occurrences of information elements in a protocol allow users,
devices, or software using the protocol to be fingerprinted. Is
this protocol vulnerable to fingerprinting? If so, how? Can it
be designed to reduce or eliminate the vulnerability? If not,
why not?

e. Persistence of identifiers. What assumptions are made in the
protocol design about the lifetime of the identifiers discussed
in (a)? Does the protocol allow implementers or users to delete
or replace identifiers? How often does the specification
recommend deleting or replacing identifiers by default? Can the
identifiers, along with other state information, be set to
automatically expire?

f. Correlation. Does the protocol allow for correlation of
identifiers? Are there expected ways that information exposed by
the protocol will be combined or correlated with information
obtained outside the protocol? How will such combination or
correlation facilitate fingerprinting of a user, device, or
application? Are there expected combinations or correlations
with outside data that will make users of the protocol more
identifiable?
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g. Retention. Does the protocol or its anticipated uses require
that the information discussed in (a) or (b) be retained by
recipients, intermediaries, or enablers? If so, why? Is the
retention expected to be persistent or temporary?
7.2
. User Participation
a. User control. What controls or consent mechanisms does the
protocol define or require before personal data or identifiers
are shared or exposed via the protocol? If no such mechanisms or
controls are specified, is it expected that control and consent
will be handled outside of the protocol?

b. Control over sharing with individual recipients. Does the
protocol provide ways for initiators to share different
information with different recipients? If not, are there
mechanisms that exist outside of the protocol to provide
initiators with such control?

c. Control over sharing with intermediaries. Does the protocol
provide ways for initiators to limit which information is shared
with intermediaries? If not, are there mechanisms that exist
outside of the protocol to provide users with such control? Is
it expected that users will have relationships that govern the
use of the information (contractual or otherwise) with those who
operate these intermediaries?

d. Preference expression. Does the protocol provide ways for
initiators to express individuals' preferences to recipients or
intermediaries with regard to the collection, use, or disclosure
of their personal data?
7.3
. Security
a. Surveillance. How do the protocol's security considerations
prevent surveillance, including eavesdropping and traffic
analysis? Does the protocol leak information that can be
observed through traffic analysis, such as by using a fixed token
at fixed offsets, or packet sizes or timing that allow observers
to determine characteristics of the traffic (e.g., which protocol
is in use or whether the traffic is part of a real-time flow)?

b. Stored data compromise. How do the protocol's security
considerations prevent or mitigate stored data compromise?
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c. Intrusion. How do the protocol's security considerations prevent
or mitigate intrusion, including denial-of-service attacks and
unsolicited communications more generally?

d. Misattribution. How do the protocol's mechanisms for identifying
and/or authenticating individuals prevent misattribution?
7.4
. General
a. Trade-offs. Does the protocol make trade-offs between privacy
and usability, privacy and efficiency, privacy and
implementability, or privacy and other design goals? Describe
the trade-offs and the rationale for the design chosen.

b. Defaults. If the protocol can be operated in multiple modes or
with multiple configurable options, does the default mode or
option minimize the amount, identifiability, and persistence of
the data and identifiers exposed by the protocol? Does the
default mode or option maximize the opportunity for user
participation? Does it provide the strictest security features
of all the modes/options? If the answer to any of these
questions is no, explain why less protective defaults were
chosen.
8
. Example
The following section gives an example of the threat analysis and
threat mitigations recommended by this document. It covers a
particularly difficult application protocol, presence, to try to
demonstrate these principles on an architecture that is vulnerable to
many of the threats described above. This text is not intended as an
example of a privacy considerations section that might appear in an
IETF specification, but rather as an example of the thinking that
should go into the design of a protocol when considering privacy as a
first principle.

A presence service, as defined in the abstract in [
RFC2778
], allows
users of a communications service to monitor one another's
availability and disposition in order to make decisions about
communicating. Presence information is highly dynamic and generally
characterizes whether a user is online or offline, busy or idle, away
from communications devices or nearby, and the like. Necessarily,
this information has certain privacy implications, and from the start
the IETF approached this work with the aim of providing users with
the controls to determine how their presence information would be
shared. The Common Profile for Presence (CPP) [
RFC3859
] defines a
set of logical operations for delivery of presence information. This
abstract model is applicable to multiple presence systems. The SIP
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for Instant Messaging and Presence Leveraging Extensions (SIMPLE)
presence system [
RFC3856
] uses CPP as its baseline architecture, and
the presence operations in the Extensible Messaging and Presence
Protocol (XMPP) have also been mapped to CPP [
RFC3922
].

The fundamental architecture defined in
RFC 2778
and
RFC 3859
is a
mediated one. Clients (presentities in
RFC 2778
terms) publish their
presence information to presence servers, which in turn distribute
information to authorized watchers. Presence servers thus retain
presence information for an interval of time, until it either changes
or expires, so that it can be revealed to authorized watchers upon
request. This architecture mirrors existing pre-standard deployment
models. The integration of an explicit authorization mechanism into
the presence architecture has been widely successful in involving the
end users in the decision-making process before sharing information.
Nearly all presence systems deployed today provide such a mechanism,
typically through a reciprocal authorization system by which a pair
of users, when they agree to be "buddies", consent to divulge their
presence information to one another. Buddylists are managed by
servers but controlled by end users. Users can also explicitly block
one another through a similar interface, and in some deployments it
is desirable to provide "polite blocking" of various kinds.

From a perspective of privacy design, however, the classical presence
architecture represents nearly a worst-case scenario. In terms of
data minimization, presentities share their sensitive information
with presence services, and while services only share this presence
information with watchers authorized by the user, no technical
mechanism constrains those watchers from relaying presence to further
third parties. Any of these entities could conceivably log or retain
presence information indefinitely. The sensitivity cannot be
mitigated by rendering the user anonymous, as it is indeed the
purpose of the system to facilitate communications between users who
know one another. The identifiers employed by users are long-lived
and often contain personal information, including personal names and
the domains of service providers. While users do participate in the
construction of buddylists and blacklists, they do so with little
prospect for accountability: the user effectively throws their
presence information over the wall to a presence server that in turn
distributes the information to watchers. Users typically have no way
to verify that presence is being distributed only to authorized
watchers, especially as it is the server that authenticates watchers,
not the end user. Moreover, connections between the server and all
publishers and consumers of presence data are an attractive target
for eavesdroppers and require strong confidentiality mechanisms,
though again the end user has no way to verify what mechanisms are in
place between the presence server and a watcher.
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Additionally, the sensitivity of presence information is not limited
to the disposition and capability to communicate. Capabilities can
reveal the type of device that a user employs, for example, and since
multiple devices can publish the same user's presence, there are
significant risks of allowing attackers to correlate user devices.
An important extension to presence was developed to enable the
support for location sharing. The effort to standardize protocols
for systems sharing geolocation was started in the GEOPRIV working
group. During the initial requirements and privacy threat analysis
in the process of chartering the working group, it became clear that
the system would require an underlying communication mechanism
supporting user consent to share location information. The
resemblance of these requirements to the presence framework was
quickly recognized, and this design decision was documented in
[
RFC4079
]. Location information thus mingles with other presence
information available through the system to intermediaries and to
authorized watchers.

Privacy concerns about presence information largely arise due to the
built-in mediation of the presence architecture. The need for a
presence server is motivated by two primary design requirements of
presence: in the first place, the server can respond with an
"offline" indication when the user is not online; in the second
place, the server can compose presence information published by
different devices under the user's control. Additionally, to
facilitate the use of URIs as identifiers for entities, some service
must operate a host with the domain name appearing in a presence URI,
and in practical terms no commercial presence architecture would
force end users to own and operate their own domain names. Many end
users of applications like presence are behind NATs or firewalls and
effectively cannot receive direct connections from the Internet --
the persistent bidirectional channel these clients open and maintain
with a presence server is essential to the operation of the protocol.

One must first ask if the trade-off of mediation for presence is
worthwhile. Does a server need to be in the middle of all
publications of presence information? It might seem that end-to-end
encryption of the presence information could solve many of these
problems. A presentity could encrypt the presence information with
the public key of a watcher and only then send the presence
information through the server. The IETF defined an object format
for presence information called the Presence Information Data Format
(PIDF), which for the purposes of conveying location information was
extended to the PIDF Location Object (PIDF-LO) -- these XML objects
were designed to accommodate an encrypted wrapper. Encrypting this
data would have the added benefit of preventing stored cleartext
presence information from being seized by an attacker who manages to
compromise a presence server. This proposal, however, quickly runs
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into usability problems. Discovering the public keys of watchers is
the first difficulty, one that few Internet protocols have addressed
successfully. This solution would then require the presentity to
publish one encrypted copy of its presence information per authorized
watcher to the presence service, regardless of whether or not a
watcher is actively seeking presence information -- for a presentity
with many watchers, this may place an unacceptable burden on the
presence server, especially given the dynamism of presence
information. Finally, it prevents the server from composing presence
information reported by multiple devices under the same user's
control. On the whole, these difficulties render object encryption
of presence information a doubtful prospect.

Some protocols that support presence information, such as SIP, can
operate intermediaries in a redirecting mode rather than a publishing
or proxying mode. Instead of sending presence information through
the server, in other words, these protocols can merely redirect
watchers to the presentity, and then presence information could pass
directly and securely from the presentity to the watcher. It is
worth noting that this would disclose the IP address of the
presentity to the watcher, which has its own set of risks. In that
case, the presentity can decide exactly what information it would
like to share with the watcher in question, it can authenticate the
watcher itself with whatever strength of credential it chooses, and
with end-to-end encryption it can reduce the likelihood of any
eavesdropping. In a redirection architecture, a presence server
could still provide the necessary "offline" indication without
requiring the presence server to observe and forward all information
itself. This mechanism is more promising than encryption but also
suffers from significant difficulties. It too does not provide for
composition of presence information from multiple devices -- it in
fact forces the watcher to perform this composition itself. The
largest single impediment to this approach is, however, the
difficulty of creating end-to-end connections between the
presentity's device(s) and a watcher, as some or all of these
endpoints may be behind NATs or firewalls that prevent peer-to-peer
connections. While there are potential solutions for this problem,
like Session Traversal Utilities for NAT (STUN) and Traversal Using
Relays around NAT (TURN), they add complexity to the overall system.

Consequently, mediation is a difficult feature of the presence
architecture to remove. It is hard to minimize the data shared with
intermediaries, especially due to the requirement for composition.
Control over sharing with intermediaries must therefore come from
some other explicit component of the architecture. As such, the
presence work in the IETF focused on improving user participation in
the activities of the presence server. This work began in the
GEOPRIV working group, with controls on location privacy, as location
Cooper, et al. Informational [Page 29]
RFC 6973
Privacy Considerations July 2013
of users is perceived as having especially sensitive properties.
With the aim of meeting the privacy requirements defined in
[
RFC2779
], a set of usage indications, such as whether retransmission
is allowed or when the retention period expires, have been added to
the PIDF-LO such that they always travel with the location
information itself. These privacy preferences apply not only to the
intermediaries that store and forward presence information but also
to the watchers who consume it.

This approach very much follows the spirit of Creative Commons [
CC
],
namely the usage of a limited number of conditions (such as 'Share
Alike' [
CC-SA
]). Unlike Creative Commons, the GEOPRIV working group
did not, however, initiate work to produce legal language or design
graphical icons, since this would fall outside the scope of the IETF.
In particular, the GEOPRIV rules state a preference on the retention
and retransmission of location information; while GEOPRIV cannot
force any entity receiving a PIDF-LO object to abide by those
preferences, if users lack the ability to express them at all, we can
guarantee their preferences will not be honored. The GEOPRIV rules
can provide a means to establish accountability.

The retention and retransmission elements were envisioned as the most
essential examples of preference expression in sharing presence. The
PIDF object was designed for extensibility, and the rulesets created
for the PIDF-LO can also be extended to provide new expressions of
user preference. Not all user preference information should be bound
into a particular PIDF object, however; many forms of access control
policy assumed by the presence architecture need to be provisioned in
the presence server by some interface with the user. This
requirement eventually triggered the standardization of a general
access control policy language called the common policy framework
(defined in [
RFC4745
]). This language allows one to express ways to
control the distribution of information as simple conditions,
actions, and transformation rules expressed in an XML format. Common
Policy itself is an abstract format that needs to be instantiated:
two examples can be found with the presence authorization rules
[
RFC5025
] and the Geolocation Policy [
RFC6772
]. The former provides
additional expressiveness for presence-based systems, while the
latter defines syntax and semantics for location-based conditions and
transformations.

Ultimately, the privacy work on presence represents a compromise
between privacy principles and the needs of the architecture and
marketplace. While it was not feasible to remove intermediaries from
the architecture entirely or prevent their access to presence
information, the IETF did provide a way for users to express their
preferences and provision their controls at the presence service. We
have not had great successes in the implementation space with privacy
Cooper, et al. Informational [Page 30]
RFC 6973
Privacy Considerations July 2013
mechanisms thus far, but by documenting and acknowledging the
limitations of these mechanisms, the designers were able to provide
implementers, and end users, with an informed perspective on the
privacy properties of the IETF's presence protocols.
9
. Security Considerations
This document describes privacy aspects that protocol designers
should consider in addition to regular security analysis.
10
. Acknowledgements
We would like to thank Christine Runnegar for her extensive helpful
review comments.

We would like to thank Scott Brim, Kasey Chappelle, Marc Linsner,
Bryan McLaughlin, Nick Mathewson, Eric Rescorla, Scott Bradner, Nat
Sakimura, Bjoern Hoehrmann, David Singer, Dean Willis, Lucy Lynch,
Trent Adams, Mark Lizar, Martin Thomson, Josh Howlett, Mischa
Tuffield, S. Moonesamy, Zhou Sujing, Claudia Diaz, Leif Johansson,
Jeff Hodges, Stephen Farrell, Steven Johnston, Cullen Jennings, Ted
Hardie, Dave Thaler, Klaas Wierenga, Adrian Farrel, Stephane
Bortzmeyer, Dave Crocker, and Hector Santos for their useful feedback
on this document.

Finally, we would like to thank the participants for the feedback
they provided during the December 2010 Internet Privacy workshop
co-organized by MIT, ISOC, W3C, and the IAB.

Although John Morris is currently employed by the U.S. Government, he
participated in the development of this document in his personal
capacity, and the views expressed in the document may not reflect
those of his employer.
Cooper, et al. Informational [Page 31]
RFC 6973
Privacy Considerations July 2013
11
. IAB Members at the Time of Approval
Bernard Aboba
Jari Arkko
Marc Blanchet
Ross Callon
Alissa Cooper
Spencer Dawkins
Joel Halpern
Russ Housley
Eliot Lear
Xing Li
Andrew Sullivan
Dave Thaler
Hannes Tschofenig
12
. Informative References
[
CC-SA
] Creative Commons, "Share Alike", 2012,
<
http://wiki.creativecommons.org/Share_Alike
>.

[
CC
] Creative Commons, "Creative Commons", 2012,
<
http://creativecommons.org/
>.

[
CoE
] Council of Europe, "Recommendation CM/Rec(2010)13 of the
Committee of Ministers to member states on the protection
of individuals with regard to automatic processing of
personal data in the context of profiling", November 2010,
<
https://wcd.coe.int/ViewDoc.jsp?Ref=CM/Rec%282010%2913
>.

[
EFF
] Electronic Frontier Foundation, "Panopticlick", 2013,
<
http://panopticlick.eff.org
>.

[
FIPs
] Gellman, B., "Fair Information Practices: A Basic
History", 2012,
<
http://bobgellman.com/rg-docs/rg-FIPShistory.pdf
>.

[
OECD
] Organisation for Economic Co-operation and Development,
"OECD Guidelines on the Protection of Privacy and
Transborder Flows of Personal Data", (adopted 1980),
September 2010, <
http://www.oecd.org/
>.

[
PbD
] Office of the Information and Privacy Commissioner,
Ontario, Canada, "Privacy by Design", 2013,
<
http://privacybydesign.ca/
>.
Cooper, et al. Informational [Page 32]
RFC 6973
Privacy Considerations July 2013
[
RFC2616
] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1",
RFC 2616
, June 1999.

[
RFC2778
] Day, M., Rosenberg, J., and H. Sugano, "A Model for
Presence and Instant Messaging",
RFC 2778
, February 2000.

[
RFC2779
] Day, M., Aggarwal, S., Mohr, G., and J. Vincent, "Instant
Messaging / Presence Protocol Requirements",
RFC 2779
,
February 2000.

[
RFC3261
] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol",
RFC 3261
,
June 2002.

[
RFC3325
] Jennings, C., Peterson, J., and M. Watson, "Private
Extensions to the Session Initiation Protocol (SIP) for
Asserted Identity within Trusted Networks",
RFC 3325
,
November 2002.

[
RFC3552
] Rescorla, E. and B. Korver, "Guidelines for Writing RFC
Text on Security Considerations",
BCP 72
,
RFC 3552
,
July 2003.

[
RFC3748
] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
Levkowetz, "Extensible Authentication Protocol (EAP)",
RFC 3748
, June 2004.

[
RFC3856
] Rosenberg, J., "A Presence Event Package for the Session
Initiation Protocol (SIP)",
RFC 3856
, August 2004.

[
RFC3859
] Peterson, J., "Common Profile for Presence (CPP)",
RFC 3859
, August 2004.

[
RFC3922
] Saint-Andre, P., "Mapping the Extensible Messaging and
Presence Protocol (XMPP) to Common Presence and Instant
Messaging (CPIM)",
RFC 3922
, October 2004.

[
RFC4017
] Stanley, D., Walker, J., and B. Aboba, "Extensible
Authentication Protocol (EAP) Method Requirements for
Wireless LANs",
RFC 4017
, March 2005.

[
RFC4079
] Peterson, J., "A Presence Architecture for the
Distribution of GEOPRIV Location Objects",
RFC 4079
,
July 2005.
Cooper, et al. Informational [Page 33]
RFC 6973
Privacy Considerations July 2013
[
RFC4101
] Rescorla, E. and IAB, "Writing Protocol Models",
RFC 4101
,
June 2005.

[
RFC4187
] Arkko, J. and H. Haverinen, "Extensible Authentication
Protocol Method for 3rd Generation Authentication and Key
Agreement (EAP-AKA)",
RFC 4187
, January 2006.

[
RFC4282
] Aboba, B., Beadles, M., Arkko, J., and P. Eronen, "The
Network Access Identifier",
RFC 4282
, December 2005.

[
RFC4745
] Schulzrinne, H., Tschofenig, H., Morris, J., Cuellar, J.,
Polk, J., and J. Rosenberg, "Common Policy: A Document
Format for Expressing Privacy Preferences",
RFC 4745
,
February 2007.

[
RFC4918
] Dusseault, L., "HTTP Extensions for Web Distributed
Authoring and Versioning (WebDAV)",
RFC 4918
, June 2007.

[
RFC4949
] Shirey, R., "Internet Security Glossary, Version 2",
RFC 4949
, August 2007.

[
RFC5025
] Rosenberg, J., "Presence Authorization Rules",
RFC 5025
,
December 2007.

[
RFC5077
] Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig,
"Transport Layer Security (TLS) Session Resumption without
Server-Side State",
RFC 5077
, January 2008.

[
RFC5106
] Tschofenig, H., Kroeselberg, D., Pashalidis, A., Ohba, Y.,
and F. Bersani, "The Extensible Authentication Protocol-
Internet Key Exchange Protocol version 2 (EAP-IKEv2)
Method",
RFC 5106
, February 2008.

[
RFC5246
] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2",
RFC 5246
, August 2008.

[
RFC6269
] Ford, M., Boucadair, M., Durand, A., Levis, P., and P.
Roberts, "Issues with IP Address Sharing",
RFC 6269
,
June 2011.

[
RFC6280
] Barnes, R., Lepinski, M., Cooper, A., Morris, J.,
Tschofenig, H., and H. Schulzrinne, "An Architecture for
Location and Location Privacy in Internet Applications",
BCP 160
,
RFC 6280
, July 2011.

[
RFC6302
] Durand, A., Gashinsky, I., Lee, D., and S. Sheppard,
"Logging Recommendations for Internet-Facing Servers",
BCP 162
,
RFC 6302
, June 2011.
Cooper, et al. Informational [Page 34]
RFC 6973
Privacy Considerations July 2013
[
RFC6350
] Perreault, S., "vCard Format Specification",
RFC 6350
,
August 2011.

[
RFC6562
] Perkins, C. and JM. Valin, "Guidelines for the Use of
Variable Bit Rate Audio with Secure RTP",
RFC 6562
,
March 2012.

[
RFC6716
] Valin, JM., Vos, K., and T. Terriberry, "Definition of the
Opus Audio Codec",
RFC 6716
, September 2012.

[
RFC6772
] Schulzrinne, H., Tschofenig, H., Cuellar, J., Polk, J.,
Morris, J., and M. Thomson, "Geolocation Policy: A
Document Format for Expressing Privacy Preferences for
Location Information",
RFC 6772
, January 2013.

[
Solove
] Solove, D., "Understanding Privacy", March 2010.

[
Tor
] The Tor Project, Inc., "Tor", 2013,
<
https://www.torproject.org/
>.

[
Westin
] Kumaraguru, P. and L. Cranor, "Privacy Indexes: A Survey
of Westin's Studies", December 2005,
<
http://reports-archive.adm.cs.cmu.edu/anon/isri2005/
CMU-ISRI-05-138.pdf
>.

Authors' Addresses

Alissa Cooper
CDT
1634 Eye St. NW, Suite 1100
Washington, DC 20006
US

Phone: +1-202-637-9800
EMail: acooper@cdt.org
URI:
http://www.cdt.org/
Hannes Tschofenig
Nokia Siemens Networks
Linnoitustie 6
Espoo 02600
Finland

Phone: +358 (50) 4871445
EMail: Hannes.Tschofenig@gmx.net
URI:
http://www.tschofenig.priv.at
Cooper, et al. Informational [Page 35]
RFC 6973
Privacy Considerations July 2013
Bernard Aboba
Skype

EMail: bernard_aboba@hotmail.com

Jon Peterson
NeuStar, Inc.
1800 Sutter St. Suite 570
Concord, CA 94520
US

EMail: jon.peterson@neustar.biz

John B. Morris, Jr.

EMail: ietf@jmorris.org

Marit Hansen
ULD

EMail: marit.hansen@datenschutzzentrum.de

Rhys Smith
Janet

EMail: rhys.smith@ja.net

Cooper, et al. Informational [Page 36]
RFC
6973
: Privacy Considerations for Internet Protocols
Informational
Contents
About this RFC
Errata
In this section
1. Introduction
2. Scope of Privacy Implications of Internet Protocols
3. Terminology
3.1. Entities
3.2. Data and Analysis
3.3. Identifiability
4. Communications Model
5. Privacy Threats
5.1. Combined Security-Privacy Threats
5.1.1. Surveillance
5.1.2. Stored Data Compromise
5.1.3. Intrusion
5.1.4. Misattribution
5.2. Privacy-Specific Threats
5.2.1. Correlation
5.2.2. Identification
5.2.3. Secondary Use
5.2.4. Disclosure
5.2.5. Exclusion
6. Threat Mitigations
6.1. Data Minimization
6.1.1. Anonymity
6.1.2. Pseudonymity
6.1.3. Identity Confidentiality
6.1.4. Data Minimization within Identity Management
6.2. User Participation
6.3. Security
7. Guidelines
7.1. Data Minimization
7.2. User Participation
7.3. Security
7.4. General
8. Example
9. Security Considerations
10. Acknowledgements
11. IAB Members at the Time of Approval
12. Informative References
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Domain Name System
Internationalized Domain Name ,IDN,"IDNs are domain names that include characters used in the local representation of languages that are not written with the twenty-six letters of the basic Latin alphabet ""a-z"". An IDN can contain Latin letters with diacritical marks, as required by many European languages, or may consist of characters from non-Latin scripts such as Arabic or Chinese. Many languages also use other types of digits than the European ""0-9"". The basic Latin alphabet together with the European-Arabic digits are, for the purpose of domain names, termed ""ASCII characters"" (ASCII = American Standard Code for Information Interchange). These are also included in the broader range of ""Unicode characters"" that provides the basis for IDNs. The ""hostname rule"" requires that all domain names of the type under consideration here are stored in the DNS using only the ASCII characters listed above, with the one further addition of the hyphen ""-"". The Unicode form of an IDN therefore requires special encoding before it is entered into the DNS. The following terminology is used when distinguishing between these forms: A domain name consists of a series of ""labels"" (separated by ""dots""). The ASCII form of an IDN label is termed an ""A-label"". All operations defined in the DNS protocol use A-labels exclusively. The Unicode form, which a user expects to be displayed, is termed a ""U-label"". The difference may be illustrated with the Hindi word for ""test"" — परीका — appearing here as a U-label would (in the Devanagari script). A special form of ""ASCII compatible encoding"" (abbreviated ACE) is applied to this to produce the corresponding A-label: xn--11b5bs1di. A domain name that only includes ASCII letters, digits, and hyphens is termed an ""LDH label"". Although the definitions of A-labels and LDH-labels overlap, a name consisting exclusively of LDH labels, such as""icann.org"" is not an IDN."