Privacy Terminology
draft-iab-privacy-terminology-00

Network Working Group                                          M. Hansen
Internet-Draft                                                  ULD Kiel
Intended status: Informational                             H. Tschofenig
Expires: July 13, 2012                            Nokia Siemens Networks
                                                                R. Smith
                                                               JANET(UK)
                                                        January 10, 2012

                          Privacy Terminology
                  draft-iab-privacy-terminology-00.txt

Abstract

   Privacy is a concept that has been debated and argued throughout the
   last few millennia by all manner of people.  Its most striking
   feature is that nobody seems able to agree upon a precise definition
   of what it actually is.  In order to discuss privacy in any
   meaningful way a tightly defined context needs to be elucidated.  The
   specific context of privacy used within this document is that of
   "personal data", any information relating to a data subject; a data
   subject is an identified natural person or a natural person who can
   be identified, directly or indirectly.  This context is highly
   relevant since a lot of work within the IETF involves defining
   protocols that can potentially transport (either explicitly or
   implicitly) personal data.

   This document aims to establish a basic lexicon around privacy so
   that IETF contributors who wish to discuss privacy considerations
   within their work can do so using terminology consistent across the
   area.

   Note: This document is discussed at
   https://www.ietf.org/mailman/listinfo/ietf-privacy

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 http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference

Hansen, et al.            Expires July 13, 2012                 [Page 1]
Internet-Draft             Privacy Terminology              January 2012

   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on July 13, 2012.

Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Anonymity  . . . . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  Unlinkability  . . . . . . . . . . . . . . . . . . . . . . . .  6
   4.  Undetectability  . . . . . . . . . . . . . . . . . . . . . . .  8
   5.  Pseudonymity . . . . . . . . . . . . . . . . . . . . . . . . .  9
   6.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 11
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 12
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 13
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 14
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 14

Hansen, et al.            Expires July 13, 2012                 [Page 2]
Internet-Draft             Privacy Terminology              January 2012

1.  Introduction

   Privacy is a concept that has been debated and argued throughout the
   last few millennia by all manner of people, including philosophers,
   psychologists, lawyers, and more recently, computer scientists.  Its
   most striking feature is that nobody seems able to agree upon a
   precise definition of what it actually is.  Every individual, every
   group, and every culture have their own different views and
   preconceptions about the concept - some mutually complimentary, some
   distinctly different.  However, it is generally (but not
   unanimously!) agreed that the protection of privacy is "A Good Thing"
   and often, people only realize what it was when they feel that they
   have lost it.

   In order to discuss privacy in any meaningful way a tightly defined
   context needs to be elucidated.  The specific context of privacy used
   within this document is that of "personal data", any information
   relating to a data subject; a (data) subject is an identified natural
   person or a natural person who can be identified, directly or
   indirectly.  We A lot of work within the IETF involves defining
   protocols that can potentially transport personal data and can
   therefore either, by dint of design decisions when creating them,
   enable either privacy protection or result in privacy breaches.
   While identifiers and data elements communicated in protocols often
   do not assume a specific association with a human using the software.
   However, a protocol may help or simplify the re-identification of a
   natural person by the choice of their identifiers and other state
   that is established and communicated, particularly when information
   from various sources is combined and analyzed together.

   Work in this area of privacy and privacy protection over the last few
   decades has centered on the idea of data minimization; it uses
   terminologies such as anonymity, unlinkability, unobservability, and
   pseudonymity.  These terms are often used in discussions about the
   privacy properties of systems.

   The core principal of data minimization is that the ability for
   others to collect personal data should be removed or at least
   minimized when this is either not desirable or when it cannot be
   entirely prevented.

   Data minimization is the only generic strategy to enhance individual
   privacy in cases where valid personal information is used since all
   valid personal data inherently provides some linkability.  Other
   techniques have been proposed and implemented that aim to enhance
   privacy by providing misinformation (inaccurate or erroneous
   information, provided usually without conscious effort to mislead or
   deceive) or disinformation (deliberately false or distorted

Hansen, et al.            Expires July 13, 2012                 [Page 3]
Internet-Draft             Privacy Terminology              January 2012

   information provided in order to mislead or deceive).  However, these
   techniques are out of scope for this document.

   We use the term 'attacker' in this writeup to refer to an entity that
   violates the privacy expectations of a data subject.  The attacker
   may not only be an entity that is external to the system but, in many
   cases, the attacker is actually one of the communication partners.
   When necessary we use the term initiator and responder to refer to
   the communication interaction of a protocol.  This particular
   terminology is used to highlight that many protocols utilize
   bidirectional communication where both ends send and receive data.
   We assume that the attacker uses all information available to infer
   (probabilities of) his items of interest (IOIs).  These IOIs may be
   attributes (and their values) of personal data, or may be actions
   such as who sent, or who received, which messages.

   This document aims to establish a basic lexicon around privacy so
   that IETF contributors who wish to discuss privacy considerations
   within their work (see [I-D.iab-privacy-considerations]) can do so
   using terminology consistent across areas.  Note that it does not
   attempt to define all aspects of privacy terminology, rather it just
   establishes terms to some of the most common ideas and concepts.

Hansen, et al.            Expires July 13, 2012                 [Page 4]
Internet-Draft             Privacy Terminology              January 2012

2.  Anonymity

   Definition:  Anonymity of a subject means that the attacker cannot
      sufficiently identify the subject within a set of subjects, the
      anonymity set.

   To enable anonymity of a subject, there always has to be an
   appropriate set of subjects with potentially the same attributes.
   The set of all possible subjects is known as the anonymity set, and
   membership of this set may vary over time.

   The set of possible subjects depends on the knowledge of the
   attacker.  Thus, anonymity is relative with respect to the attacker.
   Therefore, an initiator may be anonymous (initiator anonymity) only
   within a set of potential initiators - their initiator anonymity set
   - which itself may be a subset of all subjects who may send a
   message.  Conversely a responder may be anonymous (responder
   anonymity) only within a set of potential responders - their
   responder 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 subjects, such as a VoIP caller, to instruct an
      intermediary he or she trusts not to populate the SIP From header
      field with its authenticated and verified identity.  The recipient
      of the call, as well as any other entity outside the data
      subjects'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 subject's address-of-record, which is typically thought of as
      the "public address" of the user, i.e., the data subject.  When
      PAI is used the subject becomes anonymous within the initiator
      anonymity set that is populated by every subject making use of
      that specific intermediary.

      Note that this example assumes that other personal data cannot be
      inferred from the other SIP protocol payloads, which is a useful
      assumption to be made in the analysis of one specific protocol
      extension but not for analysis of an entire architecture.

Hansen, et al.            Expires July 13, 2012                 [Page 5]
Internet-Draft             Privacy Terminology              January 2012

3.  Unlinkability

   Definition:  Unlinkability of two or more Items Of Interest (e.g.,
      subjects, messages, actions, ...) means that within a particular
      set of information, the attacker cannot distinguish whether these
      IOIs are related or not (with a high enough degree of probability
      to be useful).

   Unlinkability of two (or more) messages may of course depend on
   whether their content is protected against the attacker.  In the
   cases where this is not true, messages may only be unlinkable if we
   assume that the attacker is not able to infer information about the
   initiator or responder from the message content itself.  It is worth
   noting that even if the content itself does not betray linkable
   information explicitly, deep semantical analysis of a message
   sequence can often detect certain characteristics which link them
   together, e.g., similarities in structure, style, use of some words
   or phrases, consistent appearance of some grammatical errors, etc.

   The unlinkability property can be considered as a more "fine-grained"
   version of anonymity since there are many more relations where
   unlinkability might be an issue than just the relation of "anonymity"
   between subjects and IOIs.  As such, it may sometimes be necessary to
   explicitly state to which attributes anonymity refers to (beyond the
   subject to IOI relationship).  An attacker might get to know
   information on linkability of various messages while not necessarily
   reducing anonymity of the particular subject.  As an example an
   attacker, in spite of being able to link all encrypted messages in a
   set of transactions, does not learn the identify of the subject who
   is the source of the transactions.

   There are several items of terminology heavily related to
   unlinkability:

   Definition:  We use the term "profiling" to mean learning information
      about a particular subject while that subject remains anonymous to
      the attacker.  For example, if an attacker concludes that a
      subject plays a specific computer game, reads specific news
      article on a website, and uploads certain videos, then the
      subjects activities have been profiled, even if the attacker is
      unable to identify that specific subject.

   Definition:  "Relationship anonymity" of a pair of subjects means
      that sender and recipient (or each recipient in case of multicast)
      are unlinkable.  The classical MIX-net [Chau81] without dummy
      traffic is one implementation with just this property: The
      attacker sees who sends messages when, and who receives messages
      when, but cannot figure out who is sending messages to whom.

Hansen, et al.            Expires July 13, 2012                 [Page 6]
Internet-Draft             Privacy Terminology              January 2012

   Definition:  The term "unlinkable session" refers the ability of the
      system to render a set of actions by a subject unlinkable from one
      another over a sequence of protocol runs (sessions).  This term is
      useful for cases where a sequence of interactions between an
      initiator and a responder is necessary for the application logic
      rather than a single-shot message.  We refer to this as a session.
      When doing an analysis with respect to unlinkability we compare
      this session to a sequence of sessions to determine linkability.

   Definition:  We use the term "fingerprinting" to refer to any
      parameter (or set of parameters) that an attacker can observe for
      the purpose of re-identification.  Fingerprinting is a form of
      tracking by associating activities of a communication software at
      different times, potentially with different communication
      partners, but without explicitly sharing state information (as it
      would be the case with cookies [RFC6265]).  For example, the
      Panopticlick project by the Electronic Frontier Foundation uses
      parameters an HTTP-based Web browser shares with sites it visits
      to determine the uniqueness of the browser [panopticlick].

Hansen, et al.            Expires July 13, 2012                 [Page 7]
Internet-Draft             Privacy Terminology              January 2012

4.  Undetectability

   Definition:  Undetectability of an item of interest (IOI) means that
      the attacker cannot sufficiently distinguish whether it exists or
      not.

   In contrast to anonymity and unlinkability, where the IOI is
   protected indirectly through protection of the IOI's relationship to
   a subject or other IOI, undetectability is the direct protection of
   an IOI.  For example, undetectability can be regarded as a possible
   and desirable property of steganographic systems.

   If we consider messages as IOIs, then undetectability means that
   messages are not sufficiently discernible from, e.g., "random noise".

Hansen, et al.            Expires July 13, 2012                 [Page 8]
Internet-Draft             Privacy Terminology              January 2012

5.  Pseudonymity

   Definition:  A pseudonym is an identifier of a subject other than one
      of the subject's real names.  An identifier, as defined in [id],
      is "a lexical token that names entities".

   In the context of IETF protocols almost all identifiers are
   pseudonyms since there is typically no requirement to use real names
   in protocols.  However, in certain scenario it is reasonable to
   assume that real names will be used and it will be worthwhile to
   point out this circumstance.

   For Internet protocols it is important whether protocols allow
   identifiers to be recycled dynamically, what the lifetime of the
   pseudonyms are, to whom they get exposed, how subjects are able to
   control disclosure, and how often they can be changed over time (and
   what the consequences are when they are regularly changed).  These
   aspects are described in [I-D.iab-privacy-considerations].

   Achieving anonymity, unlinkability, and maybe undetectability may
   enable the ideal of data minimization.  Unfortunately, it would also
   prevent a certain class of useful two-way communication scenarios.
   Therefore, for many applications, we need to accept a certain amount
   of linkability and detectability while attempting to retain
   unlinkability between the subject and their transactions.  This is
   achieved through appropriate kinds of pseudonymous identifiers.
   These identifiers are then often used to refer to established state
   or are used for access control purposes, see
   [I-D.iab-identifier-comparison].

   The term 'real name' is the antonym to "pseudonym".  There may be
   multiple real names over a lifetime -- in particular legal names.
   For example, a human being may possess the names which appear on
   their birth certificate or on other official identity documents
   issued by the State; for a legal person the name under which it
   operates and which is registered in official registers (e.g.,
   commercial register or register of associations).  A human being's
   real name typically comprises their given name and a family name.
   Note that from a mere technological perspective it cannot always be
   determined whether an identifier of a subject is a pseudonym or a
   real name.

   Additional useful terms are:

Hansen, et al.            Expires July 13, 2012                 [Page 9]
Internet-Draft             Privacy Terminology              January 2012

   Definition:  The "holder" of the pseudonym is the subject to whom the
      pseudonym refers.

   Definition:  A subject is "pseudonymous" if a pseudonym is used as
      identifier instead of one of its real names.

   Definition:  Pseudonymity is the state of remaining pseudonymous
      through the use of pseudonyms as identifiers.

   Sender pseudonymity is defined as the sender being pseudonymous,
   recipient pseudonymity is defined as the recipient being
   pseudonymous.

   Anonymity through the use of pseudonyms is stronger where ...

   o  the less personal data of the pseudonym holder can be linked to
      the pseudonym;

   o  the less often and the less context-spanning pseudonyms are used
      and therefore the less data about the holder can be linked;

   o  the more often independently chosen pseudonyms are used for new
      actions (i.e., making them, from an observer's perspective,
      unlinkable)

Hansen, et al.            Expires July 13, 2012                [Page 10]
Internet-Draft             Privacy Terminology              January 2012

6.  Acknowledgments

   Parts of this document utilizes content from [anon_terminology],
   which had a long history starting in 2000 and whose quality was
   improved due to the feedback from a number of people.  The authors
   would like to thank Andreas Pfitzmann for his work on an earlier
   draft version of this document.

   Within the IETF a number of persons had provided their feedback to
   this document.  We would like to thank Scott Brim, Marc Linsner,
   Bryan McLaughlin, Nick Mathewson, Eric Rescorla, Alissa Cooper, Scott
   Bradner, Nat Sakimura, Bjoern Hoehrmann, David Singer, Dean Willis,
   Christine Runnegar, Lucy Lynch, Trend Adams, Mark Lizar, Martin
   Thomson, Josh Howlett, and Mischa Tuffield.

Hansen, et al.            Expires July 13, 2012                [Page 11]
Internet-Draft             Privacy Terminology              January 2012

7.  Security Considerations

   This document introduces terminology for talking about privacy within
   IETF specifications.  Since privacy protection often relies on
   security mechanisms then this document is also related to security in
   its broader context.

Hansen, et al.            Expires July 13, 2012                [Page 12]
Internet-Draft             Privacy Terminology              January 2012

8.  IANA Considerations

   This document does not require actions by IANA.

Hansen, et al.            Expires July 13, 2012                [Page 13]
Internet-Draft             Privacy Terminology              January 2012

9.  References

9.1.  Normative References

   [I-D.iab-privacy-considerations]  Cooper, A., Tschofenig, H., Aboba,
                                     B., Peterson, J., and J. Morris,
                                     "Privacy Considerations for
                                     Internet Protocols",
                                     draft-iab-privacy-considerations-01
                                     (work in progress), October 2011.

   [id]                              "Identifier - Wikipeadia",
                                     Wikipedia , URL: http://
                                     en.wikipedia.org/wiki/Identifier,
                                     2011.

9.2.  Informative References

   [Chau81]                          Chaum, D., "Untraceable Electronic
                                     Mail, Return Addresses, and Digital
                                     Pseudonyms", Communications of the
                                     ACM , 24/2, 84-88, 1981.

   [I-D.iab-identifier-comparison]   Thaler, D., "Issues in Identifier
                                     Comparison for Security Purposes",
                                     draft-iab-identifier-comparison-00
                                     (work in progress), July 2011.

   [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.

   [RFC6265]                         Barth, A., "HTTP State Management
                                     Mechanism", RFC 6265, April 2011.

   [anon_terminology]                Pfitzmann, A. and A. Pfitzmann, "A
                                     terminology for talking about
                                     privacy by data minimization:
                                     Anonymity, Unlinkability,
                                     Undetectability, Unobservability,
                                     Pseudonymity, and Identity
                                     Management", URL: http://
                                     dud.inf.tu-dresden.de/literatur/
                                     Anon_Terminology_v0.34.pdf ,
                                     version 034, 2010.

Hansen, et al.            Expires July 13, 2012                [Page 14]
Internet-Draft             Privacy Terminology              January 2012

   [panopticlick]                    Eckersley, P., "How Unique Is Your
                                     Web Browser?", Electronig Frontier
                                     Foundation , URL: https://
                                     panopticlick.eff.org/
                                     browser-uniqueness.pdf, 2009.

Hansen, et al.            Expires July 13, 2012                [Page 15]
Internet-Draft             Privacy Terminology              January 2012

Authors' Addresses

   Marit Hansen
   ULD Kiel

   EMail: marit.hansen@datenschutzzentrum.de

   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

   Rhys Smith
   JANET(UK)

   EMail: rhys.smith@ja.net

Hansen, et al.            Expires July 13, 2012                [Page 16]