Network Working Group                                          A. Cooper
Internet-Draft                                                       CDT
Intended status: Informational                             H. Tschofenig
Expires: November 23, 2013                        Nokia Siemens Networks
                                                                B. Aboba
                                                   Microsoft Corporation
                                                             J. Peterson
                                                           NeuStar, Inc.
                                                               J. Morris
                                                               M. Hansen
                                                                ULD Kiel
                                                                R. Smith
                                                            May 22, 2013

             Privacy Considerations for Internet Protocols


   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 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
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

   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 November 23, 2013.

Copyright Notice

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

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Scope of Privacy Implications of Internet Protocols . . . . .   4
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
     3.1.  Entities  . . . . . . . . . . . . . . . . . . . . . . . .   5
     3.2.  Data and Analysis . . . . . . . . . . . . . . . . . . . .   6
     3.3.  Identifiability . . . . . . . . . . . . . . . . . . . . .   7
   4.  Communications Model  . . . . . . . . . . . . . . . . . . . .   9
   5.  Privacy Threats . . . . . . . . . . . . . . . . . . . . . . .  10
     5.1.  Combined Security-Privacy Threats . . . . . . . . . . . .  11
       5.1.1.  Surveillance  . . . . . . . . . . . . . . . . . . . .  11
       5.1.2.  Stored Data Compromise  . . . . . . . . . . . . . . .  12
       5.1.3.  Intrusion . . . . . . . . . . . . . . . . . . . . . .  13
       5.1.4.  Misattribution  . . . . . . . . . . . . . . . . . . .  13
     5.2.  Privacy-Specific Threats  . . . . . . . . . . . . . . . .  13
       5.2.1.  Correlation . . . . . . . . . . . . . . . . . . . . .  13
       5.2.2.  Identification  . . . . . . . . . . . . . . . . . . .  14
       5.2.3.  Secondary Use . . . . . . . . . . . . . . . . . . . .  15
       5.2.4.  Disclosure  . . . . . . . . . . . . . . . . . . . . .  15
       5.2.5.  Exclusion . . . . . . . . . . . . . . . . . . . . . .  16
   6.  Threat Mitigations  . . . . . . . . . . . . . . . . . . . . .  16
     6.1.  Data Minimization . . . . . . . . . . . . . . . . . . . .  17
       6.1.1.  Anonymity . . . . . . . . . . . . . . . . . . . . . .  17
       6.1.2.  Pseudonymity  . . . . . . . . . . . . . . . . . . . .  18
       6.1.3.  Identity Confidentiality  . . . . . . . . . . . . . .  18
       6.1.4.  Data Minimization within Identity Management  . . . .  19
     6.2.  User Participation  . . . . . . . . . . . . . . . . . . .  20
     6.3.  Security  . . . . . . . . . . . . . . . . . . . . . . . .  20
   7.  Guidelines  . . . . . . . . . . . . . . . . . . . . . . . . .  22
     7.1.  Data Minimization . . . . . . . . . . . . . . . . . . . .  22
     7.2.  User Participation  . . . . . . . . . . . . . . . . . . .  23
     7.3.  Security  . . . . . . . . . . . . . . . . . . . . . . . .  24
     7.4.  General . . . . . . . . . . . . . . . . . . . . . . . . .  24
   8.  Example . . . . . . . . . . . . . . . . . . . . . . . . . . .  24

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   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  29
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  29
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  29
   12. IAB Members at the Time of Approval . . . . . . . . . . . . .  30
   13. Informative References  . . . . . . . . . . . . . . . . . . .  30
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  33

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 guidance 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

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   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
   document.  The guidance is meant to help the thought process of
   privacy analysis; it does not provide a specific directions for how
   to write a privacy considerations section.

   This document is organized as follows.  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 2 describes the extent to which the guidance
   offered is applicable within the IETF and within the larger Internet
   community.  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.

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   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
   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 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].

3.1.  Entities

   Several of these terms are further elaborated in Section 4.

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

   $ 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

   $ 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 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

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

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      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

   $ 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).

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.

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

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   $ Identifiability:   The extent to which an individual is

   $ 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.

   $ 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.

   $ 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

   $ 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.

   $ Official Name:   A personal name for an individual which is
      registered in some official context.  For example, the name on an
      individual's birth certificate.  Official names are often not

   $ 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).

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   $ 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.)

   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 and
   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 are involved in packet forwarding that 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

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   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
   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

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   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

   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 to the perpetration of
   violence against the individual.  The individual need not be aware of

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   the surveillance for it to impact his or her privacy -- the
   possibility of surveillance may be enough to harm individual

   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 [RFC3552] Section 3) 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].

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.)

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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 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
   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

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   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).

   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

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   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 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
   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

   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

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   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 about 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.

   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 chapter
   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.

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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.

   Data minimization mitigates the following threats: surveillance,
   stored data compromise, correlation, identification, secondary use,

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.

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   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 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

   Note that this example ignores the fact that the recipient may infer
   or obtain personal data from the other SIP protocol payloads (e.g.,
   SIP Via and Contact headers, 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

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 it is important 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

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   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 end points.

   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 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) 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]).  EAP-AKA [RFC4187], for example,
   protects the EAP peer's identity against passive adversaries by
   utilizing temporal identities.  EAP-IKEv2 [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 authentication,
   authorization, and accounting 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

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      individual's age is the only attribute that needs to be

      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 and as
      well as support by both the relying party and the identity

      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 happens
   "in secret," without the individual's knowledge, is 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
   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 Authentication,
   Authorization, and Accounting (AAA) services.

   User participation mitigates the following threats: surveillance,
   secondary use, disclosure, exclusion

6.3.  Security

   Keeping data secure at rest and in transit is another important
   component of privacy protection.  As they are described in [RFC3552]
   Section 2, a number of security goals also serve to enhance privacy:

   o  Confidentiality: Keeping data secret from unintended listeners.

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   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

   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
   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 depending on 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 tradeoffs should be evaluated.

   By providing proper security protection the following threats can be
   mitigated: surveillance, stored data compromise, misattribution,
   secondary use, disclosure, intrusion

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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 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.  The 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 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

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,

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      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

      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
      to delete or replace identifiers by default?  Can the identifiers,
      along with other state information, be set to automatically

      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

      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?

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      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?

      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

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 any of these answers are no, explain
      why less protective defaults were chosen.

8.  Example

   The following section gives an example of the threat analysis and
   threat mitigation 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

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   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-
   based SIMPLE presence system [RFC3261] 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

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   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.  Connections between the server and all publishers
   and consumers of presence data are moreover 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.

   Moreover, 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

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   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
   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

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   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 STUN and TURN, they add
   complexity to the overall system.

   Consequently, mediation is a difficult feature of the presence
   architecture to remove.  Especially due to the requirement for
   composition, it is hard to minimize the data shared with
   intermediaries.  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 the user participation in the activities of the presence
   server.  This work began in the GEOPRIV working group, with controls
   on location privacy, as location 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 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 nor to
   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 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 (defined in
   [RFC4745]) framework.  This language allows one to express ways to

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   control the distribution of information as simple conditions,
   actions, and transformations rules expressed in an XML format.
   Common Policy itself is an abstract format which 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 semantic 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, nor to 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
   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.  IANA Considerations

   This document does not require actions by IANA.

11.  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 Farrel, Steven Johnston, Cullen Jennings, Ted
   Hardie, Dave Thaler, Klaas Wierenga, Adrian Farrell, 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.

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12.  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

13.  Informative References

   [CC-SA]    Creative Commons, "Share Alike", 2012.

   [CC]       Creative Commons, "Creative Commons", 2012.

   [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", available at
              (November 2010) ,
              Rec%282010%2913, 2010.

   [EFF]      Electronic Frontier Foundation, "Panopticlick", 2011.

   [FIPs]     Gellman, B., "Fair Information Practices: A Basic
              History", 2012.

   [OECD]     Organisation for Economic Co-operation and Development,
              "OECD Guidelines on the Protection of Privacy and

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              Transborder Flows of Personal Data", available at
              (September 2010) ,,,EN-
              document-0-nodirectorate-no-24-10255-0,00.html, 1980.

   [PbD]      Office of the Information and Privacy Commissioner,
              Ontario, Canada, "Privacy by Design", 2011.

   [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

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

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   [RFC4079]  Peterson, J., "A Presence Architecture for the
              Distribution of GEOPRIV Location Objects", RFC 4079, July

   [RFC4101]  Rescorla, E. 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

   [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.

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   [RFC6302]  Durand, A., Gashinsky, I., Lee, D., and S. Sheppard,
              "Logging Recommendations for Internet-Facing Servers", BCP
              162, RFC 6302, June 2011.

   [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

   [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.J., "Understanding Privacy", 2010.

   [Tor]      The Tor Project, Inc., "Tor", 2011.

   [Westin]   Kumaraguru, P. and L. Cranor, "Privacy Indexes: A Survey
              of Westin's Studies", 2005.

Authors' Addresses

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

   Phone: +1-202-637-9800

   Hannes Tschofenig
   Nokia Siemens Networks
   Linnoitustie 6
   Espoo  02600

   Phone: +358 (50) 4871445

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   Bernard Aboba
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA  98052


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


   John B. Morris, Jr.


   Marit Hansen
   ULD Kiel


   Rhys Smith


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