wdiff draft-hansen-privacy-terminology-02.txt draft-hansen-privacy-terminology-03.txt

Network Working Group M. Hansen, Ed. Hansen
Internet-Draft ULD Kiel
Intended status: Informational H. Tschofenig
Expires: September 15, 2011 May 1, 2012 Nokia Siemens Networks
March 14,
R. Smith, Ed.
JANET(UK)
October 29, 2011

Privacy Terminology for Talking about
draft-hansen-privacy-terminology-03.txt

Abstract

Privacy by Data Minimization: Anonymity,
Unlinkability, Undetectability, Unobservability, Pseudonymity, is a concept that has been debated and
Identity Management
draft-hansen-privacy-terminology-02.txt

Abstract

This 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", information about an attempt to consolidate terminology individual stored and/or
transmitted electronically in Internet protocols. This context is
highly relevant since a lot of work within the field IETF involves defining
protocols that can potentially transport (either explicitly or
implicitly) personal data.

This document aims to establish a basic lexicon around privacy by data minimization. It motivates and develops definitions
for anonymity/identifiability, (un)linkability, (un)detectability,
(un)observability, pseudonymity, identity, partial identity, digital
identity and identity management. Starting the definitions from so
that IETF contributors who wish to discuss privacy considerations
within their work can do so using terminology consistent across the
anonymity and unlinkability perspective reveals some deeper
structures in this field.
area.

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

Status of This Memo

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Anonymity Context . . . . . . . . . . . . . . . . . . . . . . . . . . 3 . 5
3. Unlinkability Anonymity . . . . . . . . . . . . . . . . . . . . . . . . . . 6
4. Anonymity in Terms of Unlinkability . . . . . . . . . . . . . 8
5. Undetectability and Unobservability . . . . . . . . . . . . . 10
6. Pseudonymity . . . . . . . . . . . . . . . . . . . . . . . 7
5. Undetectability . . 13
7. Identity Management . . . . . . . . . . . . . . . . . . . . . 19
8. Contributors 9
6. Pseudonymity . . . . . . . . . . . . . . . . . . . . . . . . . 20
9. 10
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 21
10. 12
8. Security Considerations . . . . . . . . . . . . . . . . . . . 21
11. 13
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21
12. 14
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 21
12.1. 15
10.1. Normative References . . . . . . . . . . . . . . . . . . 21
12.2. 15
10.2. Informative References . . . . . . . . . . . . . . . . . 21
Appendix A. Overview 15

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 Main Definitions what it actually is. Every individual, every
group, and every culture have their Opposites . . 22
Appendix B. Relationships between Terms . . . . . . . . . . . . . 23

1. Introduction

Early papers 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.

Even within the specific content of computing and computer science,
there are still many facets to privacy. For example, consideration
of privacy in terms of personal information is distinctly different
from consideration of privacy in a geographical information sense: in
the 1980ies about former a loss of privacy might be framed as the uncontrolled
release of personal information without the subject's consent, while
in the latter it might be the ability to compute the location of an
individual beyond a certain degree of accuracy.

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", information about an
individual stored and/or transmitted electronically in Internet
protocols. 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 and can
therefore either, by dint of design decisions when creating them,
enable either privacy protection or result in privacy breaches. In
this specific context, discussions of privacy largely centre around
the collection minimalization, the usage, and release of such
personal data.

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

Data

The core principal of data minimization means is that first of all, the ability for
others to collect any personal data should be minimized. removed. Often,
however, the collection of personal data cannot not be prevented entirely. In
such a case,
entirely, in which case the goal is to minimize the collection amount of
personal data.
The time how long collected personal data is stored should that can be
minimized. collected for a given purpose and to offer
ways to control the dissemination of personal data.

Data minimization is the only generic strategy to enable anonymity, enhance individual
privacy in cases where valid personal information is used since all correct
valid personal data help inherently provides some linkability. Other
techniques have been proposed and implemented that aim to identify if we exclude enhance
privacy by providing misinformation (inaccurate or erroneous
information, provided usually without conscious effort at misleading, deceiving,
or persuading one way to mislead or another)
deceive) or disinformation (deliberately false or distorted
information given out provided in order to mislead or deceive).

Furthermore, data minimization is the only generic strategy to enable
unlinkability, since all correct personal data provide some
linkability if we exclude providing misinformation or disinformation. However, these
techniques are out of scope for this document.

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 aim
attempt to collect define all terms used in the area aspects of
privacy. Even the definition privacy terminology, rather it just
establishes terms to some of the term 'privacy' itself difficult
due most common ideas and concepts.

2. Context

To keep discussion as simple as possible in many cases it is usual to
not distinguish between a human using some software, the contextual nature of it; the understanding of privacy has
changed over time. For software
itself, and the purpose of device on which it is running. In this document we refer to one
fairly well established definition by Alan Westin from 1967 [West67]:

"Privacy case, it is the claim of individuals, groups, or institutions to
determine for themselves when, how, and to what extent information
about them
assumed that there is communicated to others. Viewed in terms of a one-to-one relationship between the
relation of device
running the individual to social participation, privacy software that is the
voluntary and temporary withdrawal scope of a person from Internet protocol
development and the general
society through physical or psychological means, either in a state
of solitude or small-group intimacy or, human using that software.

There are various cases, however, when among larger groups, this human-to-software link is
not one-to-one. Protocols developed in a condition of anonymity or reserve.", see page 7 of [West67].

2. Anonymity

To enable anonymity the IETF typically do not
mandate any specific relationship but typically envision that uses of
a subject, there always has to specific protocol may reveal those relationships. For example,
multiple hosts used by different persons may be attached to an
appropriate set of subjects with potentially single
Internet gateway within a household. From the same attributes.

Definition: Anonymity Internet Service
Provider point of view all these devices belong to a subject means that single person:
the subject is not
identifiable within subscriber with whom a set of subjects, the anonymity set.

Note:

"not identifiable within contract was established. Unless there
are good reasons to highlight the anonymity set" means that only more complex one-to-many
relationship this document will present scenarios using the information the attacker has at his discretion, the subject is
not distinguishable from the other subjects within simpler
one-to-one relationship, without loss of generality, for editorial
reasons.

When necessary we use the anonymity
set.

In order to underline that there is a possibility term initiator and responder to quantify
anonymity for some applications (instead refer to treating it purely as
the communication interaction of a binary value it protocol. This particular
terminology is possible used to use highlight that many protocols utilize
bidirectional communication where both ends send and receive data.

Finally, we assume that the following variation attacker uses all information available
to infer (probabilities of) his items of
the previous definition: "Anonymity 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.

3. Anonymity

Definition: Anonymity of a subject from an attacker's perspective
means that the attacker cannot sufficiently identify the subject
within a set of subjects, the anonymity set."

The set.

To enable anonymity of a subject, there always has to be an
appropriate set is of subjects with potentially the same attributes.
The set of all possible subjects. 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. With respect to
actors, the anonymity set consists of the subjects who might cause an
action. With respect to actees, the anonymity set consists of the
subjects who might be acted upon.
Therefore, a sender an initiator may be anonymous (sender (initiator anonymity) only
within a set of potential senders,
his/her sender initiators - their initiator anonymity set, set
- which itself may be a subset of all subjects who may send a
message. The same for the recipient means
that Conversely a recipient responder may be anonymous (recipient (responder
anonymity) only within a set of potential recipients, his/her recipient responders - their
responder anonymity set. Both anonymity sets may be disjoint, be the same, or they may overlap.
The anonymity sets
overlap, or may vary over time. Since we assume that the
attacker does not forget anything he knows, the anonymity set cannot
increase w.r.t. a particular IOI. Especially subjects joining the
system in a later stage, do not belong to the anonymity set from the
point of view of an attacker observing the system in an earlier
stage. (Please note that if the attacker cannot decide whether the
joining subjects were present earlier, the anonymity set does not
increase either: It just stays the same.) Due to linkability, cf.
below, be the anonymity set normally can only decrease.

Anonymity of a set of subjects within same.

As an anonymity set means that all
these individual subjects are not identifiable within this anonymity
set. In this definition, "set of subjects" is just taken to describe
that the anonymity property holds for all elements of the set.
Another possible definition would be to example consider the anonymity
property RFC 3325 (P-Asserted-Identity, PAI)
[RFC3325], an extension for the set as a whole. Then a semantically quite different
definition could read: Anonymity of a set S of subjects within a
larger anonymity set A means Session Initiation Protocol (SIP),
that it is not distinguishable whether
the subject S whose anonymity is at stake (and which clearly is
within A) is within S or not.

Anonymity in general as well allows subjects, such as the anonymity of each particular
subject is a concept which is very much context dependent (on, e.g.,
subjects population, attributes, time frame, etc). In order VoIP caller, to
quantify anonymity within concrete situations, one would have to
describe the system in sufficient detail, which is practically instruct an
intermediary he or she trusts not
always possible for large open systems. Besides to populate the quantity of
anonymity provided within a particular setting, there is another
aspect of anonymity: SIP From header
field with its robustness. Robustness of anonymity
characterizes how stable the quantity of anonymity is against changes
in the particular setting, e.g., a stronger attacker or different
probability distributions. We might use quality of anonymity as a
term comprising both quantity authenticated and robustness of anonymity. To keep
this text as simple as possible, we will mainly discuss the quantity
of anonymity in the following, using the wording "strength of
anonymity". verified identity. The above definitions of anonymity and the mentioned measures recipient
of
quantifying anonymity are fine to characterize the status of a
subject in a world call, as it is. If we want to describe changes to the
anonymity of a subject if the world is changed somewhat, e.g., well as any other entity outside the
subject uses user's trust
domain, would therefore only learn that the communication network differently or uses SIP message (typically
a modified
communication network, we need another definition of anonymity
capturing the delta. The simplest way to express this delta is by
the observations of "the" attacker.

Definition: An anonymity delta (regarding SIP INVITE) was sent with a subject's anonymity)
from an attacker's perspective specifies the difference between
the subject's anonymity taking into account the attacker's
observations (i.e., the attacker's a-posteriori knowledge) and header field 'From: "Anonymous"
<sip:anonymous@anonymous.invalid>' rather than the subject's anonymity given the attacker's a-priori knowledge only.

Note:

In some publications, the a-priori knowledge of the attacker
address-of-record, which is
called "background knowledge" and the a-posteriori knowledge typically thought of as the attacker is called "new knowledge".

As we can quantify anonymity in concrete situations, so we can
quantify the anonymity delta. This can be done by just defining:
quantity(anonymity delta) := quantity(anonymity_a-posteriori) -
quantity(anonymity_a-priori)

If anonymity_a-posteriori and anonymity_a-priori are "public
address" of the same, their
quantification user. When PAI is used the same and therefore subject becomes
anonymous within the difference of these
quantifications is 0. If initiator anonymity can only decrease (which usually
is quite a reasonable assumption), the maximum of quantity(anonymity
delta) set that is 0.

Since anonymity cannot increase, the anonymity delta can never be
positive. Having an anonymity delta populated by
every subject making use of zero means that anonymity
stays the same. This means specific intermediary.

Note that if the attacker has no a-priori
knowledge about the particular subject, having no anonymity delta
implies anonymity. But if the attacker has an a-priori knowledge
covering all actions of the particular subject, having no anonymity
delta does not imply any anonymity at all. If there is no anonymity this example assumes that other personal data cannot be
inferred from the very beginning, even preserving it completely does not yield
any anonymity. To be able to express this conveniently, we use
wordings like "perfect preservation of other SIP protocol payloads, which is a subject's anonymity". It
might be worthwhile to generalize "preservation of anonymity of
single subjects" useful
assumption to "preservation of anonymity of sets of subjects",
in the limiting case all subjects be made in an anonymity set. An important
special case is that the "set of subjects" is the set analysis of subjects
having one or several attribute values A in common. Then the meaning
of "preservation of anonymity of this set of subjects" is that
knowing A does specific protocol
extension but not decrease anonymity. Having a negative anonymity
delta means that anonymity is decreased.

3. for analysis of an entire architecture.

4. Unlinkability

Definition: Unlinkability of two or more items of interest (IOIs,
e.g., Items Of Interest (e.g.,
subjects, messages, actions, ...) from an attacker's perspective
means that within the system (comprising these and
possibly other items), the attacker cannot sufficiently
distinguish whether these IOIs are related or not.

Linkability is the negation of unlinkability:

Definition: Linkability of two or more items a particular set of interest (IOIs,
e.g., subjects, messages, actions, ...) from an attacker's
perspective means that within the system (comprising these and
possibly other items), information, the attacker can sufficiently
cannot distinguish whether these IOIs are related or not.

For example, in a scenario with at least two senders, two messages
sent by subjects within the same anonymity set are unlinkable for an
attacker if for him, the probability that these two messages are sent
by the same sender is sufficiently close to 1/(number of senders).

Definition: An unlinkability delta of two or more items of interest
(IOIs, e.g., subjects, messages, actions, ...) from an attacker's
perspective specifies the difference between the unlinkability of
these IOIs taking into account the attacker's observations and the
unlinkability of these IOIs given the attacker's a-priori
knowledge only.

Since we assume that the attacker does not forget anything,
unlinkability cannot increase. Normally, the attacker's knowledge
cannot decrease (analogously to Shannon's definition of "perfect
secrecy"). An exception of this rule is the scenario where the use
of misinformation (inaccurate or erroneous information, provided
usually without conscious effort at misleading, deceiving, or
persuading one way or another [Wils93]) or disinformation
(deliberately false or distorted information given out in order to
mislead or deceive [Wils93]) leads to a growing uncertainty of the
attacker which information is correct. A related, but different
aspect is that information may become wrong (i.e., outdated) simply
because the state of the world changes over time. Since privacy is
not only about to protect the current state, but the past and history
of (with a data subject as well, we will not make use of this different
aspect in the rest of this document. Therefore, the unlinkability
delta can never be positive. Having an unlinkability delta
high enough degree of zero
means that the probability of those items being related from the
attacker's perspective stays exactly the same before (a-priori
knowledge) and after the attacker's observations (a-posteriori
knowledge of the attacker). If the attacker has no a-priori
knowledge about the particular IOIs, having an unlinkability delta of
zero implies unlinkability. But if the attacker has a-priori
knowledge covering the relationships of all IOIs, having an
unlinkability delta of zero does not imply any unlinkability at all.
If there is no unlinkability from the very beginning, even preserving
it completely does not yield any unlinkability. To be able to
express this conveniently, we use wordings like "perfect preservation
of unlinkability w.r.t. specific items" to express that the
unlinkability delta is zero. It might be worthwhile to generalize
"preservation of unlinkability of two IOIs" to "preservation of
unlinkability of sets of IOIs", in the limiting case all IOIs in the
system.

For example, the unlinkability delta of two messages is sufficiently
small (zero) for an attacker if the probability describing his
a-posteriori knowledge that these two messages are sent by the same
sender and/or received by the same recipient is sufficiently
(exactly) the same as the probability imposed by his a-priori
knowledge. Please note that unlinkability useful).

Unlinkability of two (or more) messages may of course may depend on
whether their content is protected against the attacker considered. attacker. In particular, the
cases where this is not true, messages may only be unlinkable if we
assume that the attacker is not able to get infer information on about the
sender
initiator or recipient responder from the message content. Yet with access to
their content itself. It is worth
noting that even without if the content itself does not betray linkable
information explicitly, deep semantical analysis the attacker of a message
sequence can
notice often detect certain characteristics which link them together - e.g.
together, e.g., similarities in structure, style, use of some words
or phrases, consistent appearance of some grammatical errors, etc. In a sense,
content of messages may play a role as "side channel" in a similar
way as in cryptanalysis - i.e., content of messages may leak some
information on their linkability.

Roughly speaking, no unlinkability delta of items means that the
ability of the attacker to relate these items does not increase by
observing the system or by possibly interacting with it.

The definitions of unlinkability, linkability and unlinkability delta
do not mention any particular set of IOIs they are restricted to.
Therefore, the definitions of unlinkability and unlinkability delta
are very strong, since they cover the whole system. We could weaken
the definitions by restricting them to part of the system:
"Unlinkability of two or more IOIs from an attacker's perspective
means that within an unlinkability set of IOIs (comprising these and
possibly other items), the attacker cannot sufficiently distinguish
whether these IOIs are related or not."

4. Anonymity in Terms of Unlinkability

To describe anonymity in terms of unlinkability, we have to augment
the definitions of anonymity given in Section 2 by making explicit
the attributes anonymity relates to. For example, if we choose the
attribute "having sent a message" then we can define:

A sender s sends a set of messages M anonymously, iff s is anonymous
within the set of potential senders of M, the sender anonymity set of
M.

If the attacker's focus is not on the sender, but on the message, we
can define:

A set of messages M is sent anonymously, iff M property can have been sent by
each set of potential senders, i.e., by any set of subjects within
the cross product of the sender anonymity sets of each message m
within M.

When considering sending and receiving of messages as attributes, the
items of interest (IOIs) are "who has sent or received which
message", then, anonymity of a subject w.r.t. an attribute may be
defined as unlinkability of this subject and this attribute. In the
wording of the definition of unlinkability: a subject s is related to
the attribute value "has sent message m" if s has sent message m. s
is not related to that attribute value if s has not sent message m.
Same for receiving.Unlinkability is a sufficient condition of
anonymity, but it is not a necessary condition. Thus, failing
unlinkability w.r.t. some attribute value(s) does not necessarily
eliminate anonymity considered as defined in Section 2; in specific cases (i.e.,
depending on the attribute value(s)) even the strength of anonymity
may not be affected.

Definition: Sender anonymity of a subject means that to this
potentially sending subject, each message is unlinkable.

Note:

The property unlinkability might be more "fine-grained" than
anonymity,
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. Therefore, 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 - depending on the defined
measures. An subject. As an example might be that the an
attacker, in spite of being able to link, e.g., by timing, link all encrypted messages of in a
set of transactions, does not learn who is doing this transaction.

Correspondingly, recipient anonymity of a subject means that to this
potentially receiving subject, each message is unlinkable.

Relationship anonymity of a pair of subjects, the potentially sending
subject and the potentially receiving subject, means that to this
potentially communicating pair of subjects, each message is
unlinkable. In other words, sender and recipient (or each recipient
in case of multicast) are unlinkable. As sender anonymity of a
message cannot hold against the sender of this message himself nor
can recipient anonymity hold against any identify of the recipients w.r.t.
himself, relationship anonymity subject who
is considered w.r.t. outsiders only,
i.e., attackers being neither the sender nor one of the recipients of the messages under consideration.

Thus, relationship anonymity is a weaker property than each source of sender
anonymity and recipient anonymity: The attacker might know who sends
which messages or he might know who receives which messages (and in
some cases even who sends which messages and who receives which
messages). But as long as for the attacker each message sent and
each message received transactions.

There are unlinkable, he cannot link the respective
senders to recipients and vice versa, i.e., relationship anonymity
holds. The relationship anonymity set can be defined to be the cross
product of two potentially distinct sets, the set several items of potential
senders and terminology heavily related to
unlinkability:

Definition: We use the set of potential recipients or - if it is possible term "profiling" to
exclude some of these pairs - mean learning information
about a subset of this cross product. So the
relationship anonymity set is the set of all possible sender-
recipient(s)-pairs. In case of multicast, the set of potential
recipients is the power set of all potential recipients. If we take particular subject while that subject remains anonymous to
the perspective of attacker. For example, if an attacker concludes that a
subject sending (or receiving) plays a particular
message, specific computer game, reads specific news
article on a website, and uploads certain videos, then the relationship anonymity set becomes
subjects activities have been profiled, even if the set of all
potential recipients (senders) of attacker is
unable to identify that particular message. So fixing
one factor specific subject.

Definition: "Relationship anonymity" of the cross product gives a recipient anonymity set or a
sender anonymity set.

Note:

The following is an explanation pair of the statement made in the
previous paragraph regarding relationship anonymity: For all
attackers it holds subjects means
that sender anonymity implies relationship
anonymity, and recipient anonymity implies relationship anonymity.
This is true if anonymity is taken as a binary property: Either it
holds or it does not hold. If we consider quantities of
anonymity, the validity of the implication possibly depends on the
particular definitions of how to quantify sender anonymity and
recipient anonymity on the one hand, and how to quantify
relationship anonymity on the other. There exists at least one
attacker model, where relationship anonymity does neither imply
sender anonymity nor (or each recipient anonymity. Consider an attacker
who neither controls any senders nor any recipients in case of messages,
but all lines and - maybe - some other stations. If w.r.t. this
attacker relationship anonymity holds, you can neither argue that
against him sender anonymity holds nor that recipient anonymity
holds. 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 when, and who receives messages
when, but cannot figure out who sends is sending messages to whom.

5. Undetectability and Unobservability

In contrast to anonymity and unlinkability, where not the IOI, but
only its relationship to subjects or other IOIs is protected, for
undetectability,

Definition: The term "unlinkable session" refers the IOIs are protected as such. Undetectability can
be regarded as a possible and desirable property ability of steganographic
systems. Therefore it matches the information hiding terminology
(see [Pfit96], [ZFKP98]). In contrast, anonymity, dealing with the
relationship of discernible IOIs
system to subjects, does not directly fit
into that terminology, but independently represents render a different
dimension of properties.

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

If we consider messages as IOIs, this means that messages are not
sufficiently discernible from, e.g., "random noise". A slightly more
precise formulation might be that messages are not discernible from
no message. A quantification of this property might measure the
number of indistinguishable IOIs and/or the probabilities of
distinguishing these IOIs.

Undetectability is maximal iff whether an IOI exists or not is
completely indistinguishable. We call this perfect undetectability.

Definition: An undetectability delta of an item set of interest (IOI) actions by a subject unlinkable from an attacker's perspective specifies the difference between
the undetectability of the IOI taking into account the attacker's
observations and the undetectability of the IOI given the
attacker's a-priori knowledge only.

The undetectability delta is zero iff whether an IOI exists or not is
indistinguishable to exactly the same degree whether the attacker
takes his observations into account or not. We call this "perfect
preservation of undetectability".

Undetectability of an IOI clearly is only possible w.r.t. subjects
being not involved in the IOI (i.e., neither being the sender nor one
of the recipients of a message). Therefore, if we just speak about
undetectability without spelling out
another over a set sequence of IOIs, it goes without
saying that this protocol runs (sessions). This term is
useful for cases where a statement comprising only those IOIs the
attacker is not involved in.

As the definition sequence of undetectability stands, it has nothing to do
with anonymity - it does not mention any relationship interactions between IOIs
and subjects. Even more, for subjects being involved in an IOI,
undetectability of this IOI
initiator and a responder is clearly impossible. Therefore, early
papers describing new mechanisms necessary for undetectability designed the
mechanisms in application logic
rather than a way that if single-shot message. We refer to this as a subject necessarily could detect session.
When doing an
IOI, the other subject(s) involved in that IOI enjoyed anonymity at
least. The rational for analysis with respect to unlinkability we compare
this is session to strive for data minimization: No
subject should get a sequence of sessions to determine linkability.

Definition: We refer as a "linking identifier" to know any (potentially personal) data - except
this is absolutely necessary. This means parameter that

1. Subjects being not involved in the
an attacker can observe about an IOI get and use to know absolutely
nothing.

2. Subjects being involved in the IOI only get link it to know the IOI, but
not the other subjects involved - similar
IOIs. For example, the other subjects may stay
anonymous.

The attributes "sending window size header transmitted in a message" or "receiving
typical HTTP request is a message" are the
only kinds of attributes considered, 1. and 2. together provide data
minimization in this setting in an absolute sense. linking identifier.

5. Undetectability
by uninvolved subjects together with anonymity even if IOIs can
necessarily be detected by the involved subjects has been called
unobservability:

Definition: Unobservability Undetectability of an item of interest (IOI) means

* undetectability of the IOI against all subjects uninvolved in
it and

* anonymity of the subject(s) involved in the IOI even against
the other subject(s) involved in that IOI.

As we had anonymity sets of subjects with respect to anonymity, we
have unobservability sets of subjects with respect to
unobservability. Mainly, unobservability deals with IOIs instead of
subjects only. Though, like anonymity sets, unobservability sets
consist of all subjects who might possibly cause these IOIs, i.e.
send and/or receive messages.

Sender unobservability then from an
attacker's perspective means that it is sufficiently
undetectable whether any sender within the unobservability set sends.
Sender unobservability is perfect iff it is completely undetectable
whether any sender within the unobservability set sends.

Recipient unobservability then means that it is attacker cannot sufficiently
undetectable
distinguish whether any recipient within the unobservability set
receives. Recipient unobservability is perfect iff it is completely
undetectable whether any recipient within exists or not.

In contrast to anonymity and unlinkability, where the unobservability set
receives.

Relationship unobservability then means that it is sufficiently
undetectable whether anything IOI is sent out of a set of could-be
senders to a set
protected indirectly through protection of could-be recipients. In other words, it is
sufficiently undetectable whether within the IOI's relationship
unobservability set of all possible sender-recipient(s)-pairs, a
message is sent in any relationship. Relationship unobservability is
perfect iff it is completely undetectable whether anything is sent
out of a set of could-be senders to
a set of could-be recipients.

All subject or other things being equal, unobservability IOI, undetectability is the stronger, the
larger the respective unobservability set is.

Definition: An unobservability delta direct protection of
an item of interest (IOI)
means

* undetectability delta of the IOI against all subjects
uninvolved in it and

* anonymity delta of the subject(s) involved in the IOI even
against the other subject(s) involved in that IOI.

Since we assume that the attacker does not forget anything,
unobservability cannot increase. Therefore, the unobservability
delta For example, undetectability can never be positive. Having an unobservability delta of zero
w.r.t. an IOI means an undetectability delta of zero of the IOI
against all subjects uninvolved in the IOI regarded as a possible
and an anonymity delta desirable property of
zero against those subjects involved in the IOI. To be able to
express this conveniently, steganographic systems.

If we use wordings like "perfect preservation
of unobservability" to express consider messages as IOIs, then undetectability means that the unobservability delta is
zero.
messages are not sufficiently discernible from, e.g., "random noise".

6. Pseudonymity

Having anonymity of human beings, unlinkability, and maybe
unobservability is superb w.r.t. data minimization, but would prevent
any useful two-way communication. For many applications, we need
appropriate kinds of identifiers:

Definition: A pseudonym is an identifier of a subject other than one
of the subject's real names.

Note:

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. An identifier is defined in
[id] as "a lexical token that names entities".

In our setting 'subject' means sender or recipient.

The term 'real name' is the antonym to "pseudonym". There may be
multiple real names over lifetime, a lifetime -- in particular the legal names,
i.e., for names.
For example, a human being may possess the names which appear on the
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. In the
realm of identifiers, it is tempting to define anonymity as "the
attacker cannot sufficiently determine a real name of the
subject". But despite the simplicity of this definition, it is
severely restricted: It can only deal with subjects which have at
least one real name. It presumes that it is clear who is
authorized to attach real names to subjects. It fails to work if
the relation to real names is irrelevant for the application at
hand. Therefore, we stick to the definitions given in Section 2.
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:

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

Definition: A subject is pseudonymous "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.

So sender

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

In order to be useful in the context of Internet communication we use
the term digital pseudonym and declare it as a pseudonym that is
suitable to be used to authenticate the holder's IOIs.

Defining the process of preparing for the use of pseudonyms, e.g., by
establishing certain rules how and under which conditions civil
identities of holders of pseudonyms will be disclosed by so-called
identity brokers or how to prevent uncovered claims by so-called
liability brokers, leads to the more general notion of pseudonymity,
as defined below.

Note:

Identity brokers have for the pseudonyms they are the identity
broker for the information who is their respective holder.
Therefore, identity brokers can be implemented as a special kind
of certification authorities for pseudonyms. Since anonymity can
be described as a particular kind of unlinkability, cf. Section 4,
the concept of identity broker can be generalized to linkability
broker. A linkability broker is a (trusted) third party that,
adhering to agreed rules, enables linking IOIs for those entities
being entitled to get to know the linking.

To authenticate IOIs relative to pseudonyms usually is not enough to
achieve accountability for IOIs.

Therefore, in many situations, it might make sense to let identity
brokers authenticate digital pseudonyms (i.e., check the civil
identity of the holder of the pseudonym and then issue a digitally
signed statement that this particular identity broker has proof of
the identity of the holder of this digital pseudonym and is willing
to divulge that proof under well-defined circumstances) or both.

Note:

If the holder of the pseudonym is a natural person or a legal
person, civil identity has the usual meaning, i.e. the identity
attributed to that person by a State (e.g., a natural person being
represented by the social security number or the combination of
name, date of birth, and location of birth etc.). If the holder
is, e.g., a computer, it remains to be defined what "civil
identity" should mean. It could mean, for example, exact type and
serial number of the computer (or essential components of it) or
even include the natural person or legal person responsible for
its operation.

If the digitally signed statement of a trusted identity broker is
checked before entering into a transaction with the holder of that
pseudonym, accountability can be realized in spite of anonymity.

Whereas anonymity and accountability are the extremes with respect to
linkability to subjects, pseudonymity is the entire field between and
including these extremes. Thus, pseudonymity comprises all degrees
of linkability to a subject. Ongoing use of the same pseudonym
allows the holder to establish or consolidate a reputation.
Establishing and/or consolidating a reputation under a pseudonym is,
of course, insecure if the pseudonym does not enable to authenticate
messages, i.e., if the pseudonym is not a digital pseudonym. Then,
at any moment, another subject might use this pseudonym possibly
invalidating the reputation, both for the holder of the pseudonym and
all others having to do with this pseudonym. Some kinds of
pseudonyms enable dealing with claims in case of abuse of
unlinkability to holders: Firstly, third parties (identity brokers)
may have the possibility to reveal the civil identity of the holder
in order to provide means for investigation or prosecution. To
improve the robustness of anonymity, chains of identity brokers may
be used [Chau81]. Secondly, third parties may act as liability
brokers of the holder to clear a debt or settle a claim. [BuPf90]
presents the particular case of value brokers.

There are many properties of pseudonyms which may be of importance in
specific application contexts. In order to describe the properties
of pseudonyms with respect to anonymity, we limit our view to two
aspects and give some typical examples:

The knowledge of the linking may not be a constant, but change over
time for some or even all people. Normally, for non-transferable
pseudonyms the knowledge of the linking cannot decrease (with the
exception of misinformation or disinformation, which may blur the
attacker's knowledge.). Typical kinds of such pseudonyms are:

Public Pseudonym: The linking between a public pseudonym and its
holder may be publicly known even from the very beginning. E.g.,
the linking could be listed in public directories such as the
entry of a phone number in combination with its owner.

Initially non-Public Pseudonym: The linking between an initially
non-public pseudonym and its holder may be known by certain
parties, but is not public at least initially. E.g., a bank
account where the bank can look up the linking may serve as a non-
public pseudonym. For some specific non-public pseudonyms,
certification authorities acting as identity brokers could reveal
the civil identity of the holder in case of abuse.

Initially Unlinked Pseudonym: The linking between an initially
unlinked pseudonym and its holder is - at least initially - not
known to anybody with the possible exception of the holder
himself/herself. Examples for unlinked pseudonyms are (non-
public) biometrics like DNA information unless stored in databases
including the linking to the holders.

Public pseudonyms and initially unlinked pseudonyms can be seen as
extremes of the described pseudonym aspect whereas initially non-
public pseudonyms characterize the continuum in between.

Anonymity is the stronger, the less is known about the linking to a
subject. The strength of anonymity decreases with increasing
knowledge of the pseudonym linking. In particular, under the
assumption that no gained knowledge on the linking of a pseudonym
will be forgotten and that the pseudonym cannot be transferred to
other subjects, a public pseudonym never can become an unlinked
pseudonym. In each specific case, the strength of anonymity depends
on the knowledge of certain parties about the linking relative to the
chosen attacker model.

If the pseudonym is transferable, the linking to its holder can
change. Considering an unobserved transfer of a pseudonym to another
subject, a formerly public pseudonym can become non-public again.

With respect to the degree of linkability, various kinds of
pseudonyms may be distinguished according to the kind of context for
their usage:

Person pseudonym: A person pseudonym is a substitute for the
holder's name which is regarded as representation for the holder's
civil identity. It may be used in many different contexts, e.g.,
a number of an identity card, the social security number, DNA, a
nickname, the pseudonym of an actor, or a mobile phone number.

Role pseudonym: The use of role pseudonyms is limited to specific
roles, e.g., a customer pseudonym or an Internet account used for
many instantiations of the same role "Internet user". The same
role pseudonym may be used with different communication partners.
Roles might be assigned by other parties, e.g., a company, but
they might be chosen by the subject himself/herself as well.

Relationship pseudonym: For each communication partner, a different
relationship pseudonym is used. The same relationship pseudonym
may be used in different roles for communicating with the same
partner. Examples are distinct nicknames for each communication
partner. In case of group communication, the relationship
pseudonyms may be used between more than two partners.

Role-relationship pseudonym: For each role and for each
communication partner, a different role-relationship pseudonym is
used. This means that the communication partner does not
necessarily know, whether two pseudonyms used in different roles
belong to the same holder. On the other hand, two different
communication partners who interact with a user in the same role,
do not know from the pseudonym alone whether it is the same user.
As with relationship pseudonyms, in case of group communication,
the role-relationship pseudonyms may be used between more than two
partners.

Transaction pseudonym: Apart from "transaction pseudonym" some
employ the term "one-time-use pseudonym", taking the naming from
"one-time pad". For each transaction, a transaction pseudonym
unlinkable to any other transaction pseudonyms and at least
initially unlinkable to any other IOI is used, e.g., randomly
generated transaction numbers for online-banking. Therefore,
transaction pseudonyms can be used to realize as strong anonymity
as possible. In fact, the strongest anonymity is given when there
is no identifying information at all, i.e., information that would
allow linking of anonymous entities, thus transforming the
anonymous transaction into a pseudonymous one. If the transaction
pseudonym is used exactly once, we have the same strength of
anonymity as if no pseudonym is used at all. Another possibility
to achieve strong anonymity is to prove the holdership of the
pseudonym or specific attribute values (e.g., with zero-knowledge
proofs) without revealing the information about the pseudonym or
more detailed attribute values themselves. Then, no identifiable
or linkable information is disclosed.

Linkability across different contexts due to the use of these
pseudonyms can be represented as the lattice that is illustrated in
the following diagram, see Figure 1. The arrows point in direction
of increasing unlinkability, i.e., A -> B stands for "B enables
stronger unlinkability than A". Note that "->" is not the same as
"=>" of Appendix B, which stands for through the implication concerning
anonymity and unobservability.

linkable

+-----------------+ *
Person | | *
/ Pseudonym \ | decreasing | *
// \\ | linkability | *
/ \ | across | *
/ \-+ | contexts | *
+-/ v | | *
v Role Relationship | | *
Pseudonym Pseudonym | | *
-- -- | | *
-- --- | | *
--- ---- | | *
--+ +--- | | *
v v | | *
Role-Relationship | | |*
Pseudonym | | *
| | | *
| | | *
| | | *
| | | *
| | | *
v | | *
Transaction | *
Pseudonym | v

unlinkable

Figure 1: Lattice of pseudonyms according to their use across
different contexts

In general, unlinkability of both role pseudonyms and relationship pseudonyms is stronger than unlinkability of person pseudonyms. The
strength of unlinkability increases with the application of role-
relationship pseudonyms, the use of which is restricted to both the
same role and the same relationship. If a role-relationship
pseudonym is used for roles comprising many kinds of activities, the
danger arises that after a while, it becomes a person pseudonym in
the sense of: "A person pseudonym is a substitute for the holder's
name which is regarded as representation for the holder's civil
identity." This is even more true both for role pseudonyms and
relationship pseudonyms. Ultimate strength of unlinkability is
obtained with transaction pseudonyms, provided that no other
information, e.g., from the context or from the pseudonym itself,
enabling linking is available.

Anonymity is the 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, i.e., from an observer's
perspective unlinkable, chosen pseudonyms are used for new actions.

The amount of information of linked data can be reduced by different
subjects using the same pseudonym (e.g., one after the other when
pseudonyms are transferred or simultaneously with specifically
created group pseudonyms) or by misinformation or disinformation.
The group of pseudonym holders acts as an inner anonymity set within
a, depending on context information, potentially even larger outer
anonymity set.

7. Identity Management

Identity can be explained as an exclusive perception of life,
integration into a social group, and continuity, which is bound to a
body and - at least to some degree - shaped by society. This concept
of identity distinguishes between "I" and "Me" [Mead34] : "I" is the
instance that is accessible only by the individual self, perceived as
an instance of liberty and initiative. "Me" is supposed to stand for
the social attributes, defining a human identity that is accessible
by communications and that is an inner instance of control and
consistency (see [ICPP03] for more information). In this
terminology, we are interested in identity as communicated to others
and seen by them. Therefore, we concentrate on the "Me".

Motivated by identity as an exclusive perception of life, i.e., a
psychological perspective, but using terms defined
actions (i.e., making them, from a computer
science, i.e., a mathematical perspective (as we did in the sections
before), identity can be explained and defined as a property of an
entity in terms of the opposite of anonymity and the opposite of
unlinkability. In a positive wording, identity enables both observer's perspective,
unlinkable)

For Internet protocols it is important whether protocols allow
identifiers to be
identifiable as well as to link IOIs because of some continuity of
life. Here we have the opposite of anonymity (identifiability) and
the opposite of unlinkability (linkability) as positive properties.
So the perspective changes: What is the aim of an attacker w.r.t.

anonymity, now is the aim of the subject under consideration, so the
attacker's perspective becomes recycled dynamically, what the perspective lifetime of the subject. And
again, another attacker (attacker2) might be considered working
against identifiability and/or linkability. I.e., attacker2 might
try
pseudonyms are, to mask different attributes of whom they get exposed, how subjects are able to provide for some kind
of anonymity or attacker2 might spoof some messages to interfere with
the continuity of the subject's life.

Definition: An identity is any subset of attribute values of an
individual person which sufficiently identifies this individual
person within any set of persons. So usually there is no such
thing as "the identity", but several of them.

Definition: Identity management means managing various identities
(usually denoted by pseudonyms) of an individual person, i.e.,
administration of identity attributes including the development
and choice of the partial identity
control disclosure, and pseudonym to how often they can be (re-)used
in a specific context or role. Establishment of reputation is
possible when the individual person re-uses partial identities. A
prerequisite to choose the appropriate partial identity is to
recognize the situation the person is acting in.

Of course, attribute values or even attributes themselves may change
over time. Therefore, if the attacker has no access to the change
history of each particular attribute, the fact whether a particular
subset of attribute values of an individual person is an identity or
not may change changed over time as well. If the attacker has access to the
change history of each particular attribute, any subset forming an
identity will form an identity from his perspective irrespective how
attribute values change. Any reasonable attacker will not just try
to figure out attribute values per se, but the point in time (or even (and
what the time frame) consequences are when they are valid (in), since regularly changed). These
aspects are described in [I-D.iab-privacy-considerations].

7. Acknowledgments

Parts of this change history helps document utilizes content from [anon_terminology],
which had a lot long history starting in linking 2000 and thus inferring further attribute values.
Therefore, it may clarify one's mind whose quality was
improved due to define each "attribute" in a
way that its value cannot get invalid. So instead of the attribute
"location" of a particular individual person, take the set of
attributes "location at time x". Depending on the inferences you are
interested in, refining that set as feedback from a list ordered concerning
"location" or "time" may be helpful.

Identities may number of course comprise particular attribute values like
names, identifiers, digital pseudonyms, and addresses - but they
don't have to.

8. Contributors people. The authors
would like to thank Andreas Pfitzmann for all his work on an earlier
draft version of this document.

9. Acknowledgments

Before this document was submitted to

Within the IETF it already had a long
history starting at 2000 and a number of people helped to improve the
quality of the document with their feedback. A number of persons
contributed had provided their feedback to the original writeup
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 they are acknowledged in
http://dud.inf.tu-dresden.de/Anon_Terminology.shtml.

10. Mischa Tuffield.

8. Security Considerations

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

11.

9. IANA Considerations

This document does not require actions by IANA.

12.

10. References

12.1.

10.1. Normative References

12.2. Informative References

[BuPf90] Buerk, H. and A. Pfitzmann, "Value Exchange Systems
Enabling Security

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

[id] "Identifier - Wikipeadia",
Wikipedia , 9/8, 715-721, January 1990. 2011.

10.2. Informative References

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

[ICPP03] Independent Centre for Privacy Protection & Studio Notarile
Genghini, "Identity Management Systems (IMS):
Identification and Comparison Study", Study commissioned by
the Joint Research Centre Seville, Spain , http://
www.datenschutzzentrum.de/projekte/idmanage/study.htm,
September 2003.

[Mead34] Mead, G., "Mind, Self and Society", Chicago Press , 1934.

[Pfit96] Pfitzmann, B., "Information Hiding Terminology -- Results
of an informal plenary meeting and additional proposals",
Information Hiding , NCS 1174, Springer, Berlin 1996, 347-
350, 1996.

[ReRu98] Reiter, M. and A. Rubin, "Crowds: Anonymity for Web
Transactions", ACM Transactions on Information and System
Security , 1(1), 66-92, November 1998.

[West67] Westin, A., "Privacy and Freedom", Atheneum, New York ,
1967.

[Wils93] Wilson, K., "The Columbia Guide to Standard American
English", Columbia University Press, New York , 1993.

[ZFKP98] Zoellner,

[RFC3325] Jennings, C., Peterson, J., Federrath, H., Klimant, H., Pfitzmann, A.,
Piotraschke, R., Westfeld, A., Wicke, G., and G. Wolf,
"Modeling the security of steganographic systems", 2nd
Workshop on Information Hiding , LNCS 1525, Springer,
Berlin 1998, 345-355, 1998.

[id] "Identifier - Wikipeadia", Wikipedia , 2011.

Appendix A. Overview of Main Definitions and their Opposites

+---------------------------------+---------------------------------+
| Definition | Negation |
+---------------------------------+---------------------------------+
| Anonymity of a subject from an | Identifiability of a subject |
| attacker's perspective means | from an attacker's perspective |
| that the attacker cannot | means that the attacker can |
| sufficiently identify the | sufficiently identify the |
| subject within a set of | subject within a set of |
| subjects, the anonymity set. | subjects, the identifiability |
| | set. |
| ------------------------------- | ------------------------------- |
| Unlinkability of two or more | Linkability of two or more |
| items of interest (IOIs, e.g., | items of interest (IOIs, e.g., |
| subjects, messages, actions, | subjects, messages, actions, |
| ...) from an attacker's | ...) from an attacker's |
| perspective means that within | perspective means that within |
| the system (comprising these | the system (comprising these |
| and possibly other items), the | and possibly other items), the |
| attacker cannot sufficiently | attacker can sufficiently |
| distinguish whether these IOIs | distinguish whether these IOIs |
| are related or not. | are related or not. |
| ------------------------------- | ------------------------------- |
| Undetectability of an item of | Detectability of an item of |
| interest (IOI) from an | interest (IOI) from an |
| attacker's perspective means | attacker's perspective means |
| that the attacker cannot | that the attacker can |
| sufficiently distinguish | sufficiently distinguish |
| whether it exists or not. | whether it exists or not. |
| ------------------------------- | ------------------------------- |
| Unobservability of an item of | Observability of an item of |
| interest (IOI) means | interest (IOI) means "many |
| undetectability of the IOI | possibilities to define the |
| against all subjects uninvolved | semantics". |
| in it and anonymity of the | |
| subject(s) involved in the IOI | |
| even against the other | |
| subject(s) involved in that | |
| IOI. | |
+---------------------------------+---------------------------------+

Appendix B. Relationships between Terms

With respect M.
Watson, "Private Extensions to the same attacker, unobservability reveals always
only a subset of the information anonymity reveals. [ReRu98] propose
a continuum
Session Initiation Protocol (SIP)
for describing the strength of anonymity. They give
names: "absolute privacy" (the attacker cannot perceive the presence
of communication, i.e., unobservability) - "beyond suspicion" -
"probable innocence" - "possible innocence" - "exposed" - "provably
exposed" (the attacker can prove the sender, recipient, or their
relationship to others). Although we think that the terms "privacy" Asserted Identity within
Trusted Networks", RFC 3325,
November 2002.

[anon_terminology] Pfitzmann, A. and "innocence" are misleading, the spectrum is quite useful. We
might use the shorthand notation

unobservability => anonymity A. Pfitzmann, "A
terminology for that (=> reads "implies"). Using the same argument talking about
privacy by data minimization:
Anonymity, Unlinkability,
Undetectability, Unobservability,
Pseudonymity, and notation,
we have

sender unobservability => sender anonymity

recipient unobservability => recipient anonymity

relationship unobservability => relationship anonymity

As noted above, we have

sender anonymity => relationship anonymity

recipient anonymity => relationship anonymity
sender unobservability => relationship unobservability

recipient unobservability => relationship unobservability

With respect to the same attacker, unobservability reveals always
only a subset of the information undetectability reveals

unobservability => undetectability Identity
Management", URL: http://
dud.inf.tu-dresden.de/literatur/
Anon_Terminology_v0.34.pdf ,
version 034, 2010.

Authors' Addresses

Marit Hansen (editor)
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 (editor)
JANET(UK)

EMail: rhys.smith@ja.net