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Versions: 00 01 02 draft-ietf-dprive-eval

Network Working Group                                        A. Mohaisen
Internet-Draft                                                 A. Mankin
Intended status: Informational                             Verisign Labs
Expires: September 10, 2015                                March 9, 2015


             Evaluation of Privacy for DNS Private Exchange
                        draft-am-dprive-eval-00

Abstract

   The set of DNS requests that an individual makes can provide an
   attacker with a large amount of information about that individual.
   DNS Private Exchange (DPRIVE) aims to deprive the attacker of this
   information.  This document describes methods for measuring the
   performance of DNS privacy mechanisms, particularly it provides
   methods for measuring effectiveness in the face of pervasive
   monitoring as defined in RFC7258.  The document includes example
   evaluations for common use cases.

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
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   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
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   time.  It is inappropriate to use Internet-Drafts as reference
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   This Internet-Draft will expire on September 10, 2015.

Copyright Notice

   Copyright (c) 2015 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
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   publication of this document.  Please review these documents
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   to this document.  Code Components extracted from this document must



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   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.  Motivation . . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Privacy Evaluation Definitions . . . . . . . . . . . . . . . .  5
     2.1.  RFC 6973 Definitions - Entities  . . . . . . . . . . . . .  5
     2.2.  RFC 6973 Definitions - Data and Analysis . . . . . . . . .  6
     2.3.  RFC 6973 Definitions - Identifiability . . . . . . . . . .  7
     2.4.  Other Central Definitions and Formalizations . . . . . . .  8
   3.  Assumptions about Quantification of Privacy  . . . . . . . . . 10
   4.  System Model . . . . . . . . . . . . . . . . . . . . . . . . . 11
     4.1.  DNS Resolvers (System Model) . . . . . . . . . . . . . . . 11
     4.2.  System Setup - Putting It Together . . . . . . . . . . . . 11
   5.  Attack Model . . . . . . . . . . . . . . . . . . . . . . . . . 14
     5.1.  Attacker Type-1 - Pervasive Monitor  . . . . . . . . . . . 14
     5.2.  Attacker Type-2 - Malicious Monitor  . . . . . . . . . . . 14
     5.3.  Attackers in the System Setup  . . . . . . . . . . . . . . 15
   6.  Privacy Mechanisms . . . . . . . . . . . . . . . . . . . . . . 16
   7.  Privacy Evaluation . . . . . . . . . . . . . . . . . . . . . . 18
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 23
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 24
   10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 25
   11. Informative References . . . . . . . . . . . . . . . . . . . . 26
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 28























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1.  Motivation

   One of the IETF's core views is that protocols should be designed to
   enable security and privacy while online [RFC3552].  In light of the
   recent reported pervasive monitoring efforts, another goal is to
   design protocols and mechanisms to make such monitoring expensive or
   infeasible to conduct.  As detailed in the DPRIVE problem statement
   [dprive-problem], DNS resolution is an important arena for pervasive
   monitoring, and in some cases may be used for breaching the privacy
   of individuals.  The set of DNS requests that an individual makes can
   provide a large amount of information about that individual.  Not
   only individual requesters reveal information with their sets of DNS
   queries.  In some specific use cases, the sets of DNS requests from a
   DNS recursive resolver or other entity may also provide revealing
   information.  This document describes methods for measuring the
   performance of DNS privacy mechanisms; in particular, it provides
   methods for measuring effectiveness in the face of pervasive
   monitoring as defined in [RFC7258].  The document includes example
   evaluations for common use cases.

   The privacy risks associated with DNS monitoring are not new, however
   they were brought into a greater visibility by the issue described in
   [RFC7258].  The DPRIVE working group was formed to respond and at
   this time has several DNS private exchange mechanisms in
   consideration, including [dns-over-tls], [confidential-dns],
   [phb-dnse], and [privatedns].  There is also related work in other
   working groups, including DNSOP: [qname-minimisation] and
   (potentially) DANE [ipseca].  The recently published [RFC7435] also
   has relevance to DNS private exchange.

   Each effort related to DNS privacy mechanisms asserts some privacy
   assurances and operational relevance.  Metrics for these privacy
   assurances are needed and are in reach based on existing techniques
   from the general field of privacy engineering.  Systematic evaluation
   of DNS privacy mechanisms will enhance the likely operational
   effectiveness of DNS private exchange.

   Evaluating an individual mechanism for DNS privacy could be
   accomplished on a one-off basis, presumably as Privacy Considerations
   within each specification, but this will not address as much
   variation of operational contexts nor will it cover using multiple
   mechanisms together (in composition).  Section 2 of [RFC6973]
   discussed both benefits and risks of using multiple mechanisms.

   Definitions required for evaluating the privacy of stand-alone and
   composed design are not limited to privacy notions, but also need to
   include the attacker model and some information about relationships
   among the entities in a given system.  A mechanism for providing



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   privacy to withstand the power and capabilities of a passive
   pervasive monitor may not withstand a more powerful attacker using
   active monitoring by plugging itself into the path of individuals'
   DNS requests as a forwarder, or worse, by controlling a DNS recursive
   server (that is, in what we define later as the malicious attack
   model).  Having some standard attack models, and understanding how
   applicable they are to various designs is a part of evaluating the
   privacy.

   Sections 2 and 3 present privacy terminology and some assumptions.
   Sections 4 and 5 cover the system model or setup and the attack
   models.  In Section 6, we review a list of DNS privacy mechanisms,
   including some which are not in scope of the DPRIVE working group.
   Section 7 tackles how to evaluate privacy mechanisms, in the form of
   templates and outcomes.  Given a specific attack model, the
   guarantees with respect to privacy of an individual or an item of
   interest are quantified.


































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2.  Privacy Evaluation Definitions

   This section provides definitions to be used for privacy evaluation
   of DNS.  [RFC6973] is the verbatim source of most of the definitions.
   Text is added to apply them to the DNS case.  We follow the [RFC6973]
   in classifying the terms.  We have added a new section of terms to
   include several important practical or conventional terms that were
   not included in [RFC6973] such as PII.  For the terms from [RFC6973],
   we include their definitions rather than simply referencing them as
   an aid to readability.

2.1.  RFC 6973 Definitions - Entities

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

   o  Eavesdropper: A type of attacker that passively observes an
      initiator's communications without the initiator's knowledge or
      authorization.  This may include a passive pervasive monitor,
      defined below.

   o  Enabler: A protocol entity that facilitates communication between
      an initiator and a recipient without being directly in the
      communications path.  DNS examples of an enabler in this sense
      include a recursive resolver, a proxy, or a forwarder.

   o  Individual: A human being (or a group of them)

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

   o  Intermediary: A protocol entity that sits between the initiator
      (stub resolver) and the recipient (recursive resolver or authority
      resolver) 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.

   o  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.






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2.2.  RFC 6973 Definitions - Data and Analysis

   o  Attack: An intentional act by which an entity attempts to violate
      an individual's privacy.  See [RFC4949].

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

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

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

   o  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.  In the DNS private
      exchange context, items of interest can be Source IP address, ASN
      of the Source IP address, and the query itself, including the
      qname, and other attributes.

   o  Personal Data: Any information relating to an individual who can
      be identified, directly or indirectly.  Note that when a Subject
      is involved that is not an individual, we will identify Items of
      Interest but not reference this as Personal.

   o  (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.

   o  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].

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

   o  Unlinkability: Within a particular set of information, the
      inability of an observer or attacker to distinguish whether two
      items of interest are related or not (with a high enough degree of
      probability to be useful to the observer or attacker).



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2.3.  RFC 6973 Definitions - Identifiability

   o  Anonymity: The state of being anonymous.

   o  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.

   o  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).

   o  Attribute: A property of an individual or subject.

   o  Identifiability: The extent to which an individual is
      identifiable.  [RFC6973] has the rest of the variations on this
      (Identifiable, Identification, Identified, Identifier, Identity,
      Identity Confidentiality)

   o  Identity Provider: An entity (usually an organization) that is
      responsible for establishing, maintaining, securing, and vouching
      for the identities associated with individuals.

   o  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).  NOTE: The
      reason to import this definition is that some query names that
      cause privacy leakage do so by embedding personal names as
      identifiers of host or other equipment, e.g.
      AllisonMankinMac.example.com.

   o  Pseudonymity: See the formal definition in the next section in
      lieu of [RFC6973].

   NOTE: Identifiability Definitions in [RFC6973] also include some
   material not included here because the distinctions are not major for
   DNS Private Exchange, such as real and official names, and variant
   forms of Pseudonymity in its informal definition.

      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



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      semantics of information exchanged between the relying party and
      the identity provider.

2.4.  Other Central Definitions and Formalizations

   Central to the presentation of this document is the definition of
   personally identifiable information (PII), as well as other
   definitions that supplement the definitions listed earlier.  In this
   section, we outline such definitions we further notes on their
   indications.

   o  Personally Identifiable Information (PII): Information
      (attributes) that can be used as is, or along with other side
      information, to identify, locate, and/or contact a single
      individual or subject (c.f. item of interest).

   NOTE: the definition above indicates that PII can be used on its own
   or in context.  In DNS privacy, the items without additional context
   include IP(v4 or v6) address, qname, qtype, timings of queries, etc.
   The additional context includes organization-level attributes, such
   as a network prefix that can be associated with an organization.  The
   definition of PII is complementary to the definition of items of
   interest.

   o  Subject: This term is useful as a parallel term to Individual.
      When the privacy of a group or an organization is of interest to
      an attacker, we can reference the group or organization as Subject
      rather than Individual.

   Often it is desirable to reference alternative identifiers known as
   pseudonyms.  A pseudonym is a name assumed by an individual in some
   context, unrelated to the names or identifiers known by others in
   that context.

   o  Pseudonymity/Pseudonym: a relaxation of the definition of
      anonymity for usability.  In particular, pseudonymity is an
      anonymity feature obtained by using a pseudonym, an identifier
      that is used for establishing a long relationship between two
      entities.

   As an example, in the DNS context, a randomly generated pseudonym
   might identify a set of query data with a shared context, such as
   geographic origin.  Such pseudonymity enables another entity or
   attacker to link multiple queries on a long-term basis.  Pseudonyms
   are assumed long-lived and their uniqueness may be a goal.  There are
   many findings that indicate that pseudonymity is weaker than
   anonymity.




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   o  Unlinkability: Formally, two items of interest are said to be
      unlinkable if the certainty of the attacker concerning those items
      of interest is not affected by observing the system.  This is,
      unlinkability implies that the a-posteriori probability computed
      by the attacker that two items of interest are related is close
      enough to the a-priori probability computed by an attacker based
      on his knowledge.

   Two items of interest are said to be unlinkable if there is a small
   (beta, close to 0) probability that the attacker identifies them as
   associated, and they are linkable if there is a sufficiently large
   probability (referred to as alpha).

   Informally, given two items of interest (user attributes, DNS
   queries, users, etc.), unlinkability is defined as the inability of
   the attacker to sufficiently determine whether those items are
   related to one another.  In the context of DNS, this refers typically
   but not only to an attacker relating queries to the same individual.

   o  Undetectability: a stronger definition of privacy, where an item
      of interest is said to be undetectable if the attacker is not
      sufficiently able to know or tell whether the item exists or not.

   Note that undetectability implies unlinkability.  As explained below,
   a way of ensuring undetectability is to use encryption secure under
   known ciphertext attacks, or randomized encryption.

   o  Unobservability: a stronger definition of privacy that requires
      satisfying both undetectability and anonymity.  Unobservability
      means that an item of interest is undetectable by any uninvolved
      individual, attacker or not.

   In theory, there are many ways of ensuring unobservability by
   fulfilling both requirements.  For example, undetectability requires
   that no party uninvolved in the resolution of a DNS query shall know
   that query has existed or not.  A mechanism to ensure this function
   is encryption secure under known ciphertext attacks, or randomized
   encryption for all other than stub, and pseudonyms for the stub
   resolver.  An alternative mechanism to provide the anonymity property
   would be the use of mix networks for routing DNS queries.











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3.  Assumptions about Quantification of Privacy

   The quantification of privacy is connected with the privacy goals.
   Is the desired privacy property unlinkability only, or is it
   undetectability.  Is pseudonymity a sufficient property?  Parameters
   and entire privacy mechanism choices are affected by the choice of
   privacy goals.

   While a binary measure of privacy is sometimes possible, that is,
   being able to say that the transaction is anonymous, in this
   document, we assume that the binary is not frequently obtainable, and
   therefore we focus on methods for continuous quantification.  Both
   are relevant to DNS Private Exchange.  Another way to state this is
   that the quantification could be exactly the probabilities 1 and 0,
   corresponding to the binary, but the methods prefer to provide
   continuous values instead.

   Here is an example of continuous quantification, related to
   identifiability of an individual or item of interest based on
   observing queries.

   o  For an individual A, and a set of observations by an attacker, Y =
      [y1, y2, ... yn], we define the privacy of A as the uncertainty of
      the attacker of knowing that A is itself among many others under
      the observations Y; that is, we define Privacy = 1 - P[A | Y]

   o  For an item of interest r associated with a user A, we similarly
      define the privacy of r as Privacy = 1 - P[r | Y].























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4.  System Model

   A DNS client (a DNS stub resolver) may resolve a domain name or
   address into the corresponding DNS record by contacting the
   authoritative name server responsible for that domain name (or
   address) directly.  However, to improve the operation of DNS
   resolution, and reduce the round trip time required for resolving an
   address, both caching and recursive resolution are implemented.
   Caching is implemented at an intermediary between the stub and the
   authoritative name server.  In practice, many caching servers also
   implement the recursive logic of DNS resolution for finding the name
   server authoritative for a domain, and are thus named DNS recursive
   resolvers.  Another type of entity, forwarders (or proxies) are
   intermediaries between the three named here.  The system model for
   DNS privacy evaluation includes the four entities quickly sketched
   here: stub resolvers, recursive resolvers, authoritative name
   servers, and forwarders.

4.1.  DNS Resolvers (System Model)

   o  Stub resolver (S): a minimal resolver that does not support
      referral, and delegates recursive resolution to a recursive
      resolver.  A stub resolver is a consumer of recursive resolutions.
      Per the terminology of [RFC6973], a stub resolver is an Initiator.

   o  Recursive resolver (R): a resolver that implements the recursive
      function of DNS resolution on behalf of a stub resolver.  Per the
      terminology of [RFC6973], a recursive resolver is an Enabler.

   o  Authoritative resolver (A): is a server that is the origin of a
      DNS record.  A recursive resolver queries the authoritative
      resolver to resolve a domain name or address.  Per the terminology
      of [RFC6973], the authoritative name server is also an Enabler.

   o  Forwarder/proxy (P): between the stub resolver and the
      authoritative resolver there may be more than one DNS-involved
      entity.  These are systems located between S and R (stub resolver
      and recursive), or between R and A (recursive and authoritative),
      which do not play a primary role in the DNS protocol.  Per the
      terminology of [RFC6973], forwarders are Intermediaries.

4.2.  System Setup - Putting It Together

   Evaluating various privacy protection mechanisms in relation to
   attackers such as the pervasive monitoring attackers defined next
   requires understanding links in the System setup.  We define the
   following links.  In relation to [RFC7258] these are the attack
   surface where a monitor (eavesdropper) collects sets of query



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   information.

   o  Stub -> Recursive (S-R): a link between the stub resolver and a
      recursive resolver.  At the time of writing, the scope of DPRIVE
      Working Group privacy mechanisms is supposed to be limited to S-R.

   o  Stub -> Proxy (S-P): a link between the stub resolver and a
      forwarder/ proxy.  The intended function of this link may be
      difficult to analyze.

   o  Proxy -> Recursive (P-R): a link between a proxy and a recursive
      server.

   o  Recursive -> Authoritative (R-A): a link between a recursive and
      an authoritative name server.  Although at the time of writing,
      R-A is not in the DPRIVE scope, we touch on it in evaluations.

   Rather than notating in system setup that an entity is compromised,
   this is covered in the attacker model in Section 6, which has system
   elements as parameters.

   In the System Setup, there is a possibility that S and R exist on a
   single machine.  The concept of the Unlucky Few relates S and R in
   this case.  An attacker can monitor R-A and find the query traffic of
   the initiator individual.  The same concept applies in the case where
   a recursive is serving a relatively small number of individuals.  The
   query traffic of a subject group or organization (c.f.  Subject in
   the definitions) is obtained by the attacker who monitors this
   system's R-A.

   Because R-A is not in the DPRIVE scope, it is for future work to
   examine the Unlucky Few circumstance fully.  The general system setup
   is that PII, the individual's private identifying information, is not
   sent on R-A and is not seen by authoritatives.

   There could be one or more proxies between the stub resolver and a
   recursive.  From a functionality point of view they can all be
   consolidated into a single proxy without affecting the system view,
   however, the behavior of such proxies may affect the size and shape
   of the attack surface.  However, we believe that an additional
   treatment is needed for this case and it is not included in the
   discussion.

   We also do not include in discussion proxies that exist along R-A,
   between a recursive and an authoritative name server.  We do so in
   respect for the DPRIVE charter's scope at this time.  According to
   recent work at [openresolverproject.org], there may be multiple
   intermediaries with poorly defined behavior.



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   The system setup here leaves out other realistic considerations for
   simplicity, such as the impact of shared caches in DNS entities.

















































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5.  Attack Model

   The Definitions section defines Attack and Attacker, but not Attack
   Model, which is needed to actually evaluate privacy, so this is now
   defined.

   o  Attack Model: a well-defined set of capabilities indicating how
      much information the attacker has and in what context in order to
      reach a goal of breaching the privacy of an individual or subject
      with respect to a given privacy metric.

   In this document we focus on two attack models, namely a pervasive
   monitor and a malicious monitor.  The first attack model has two
   forms, namely active and passive attack models, which are defined in
   the following.

5.1.  Attacker Type-1 - Pervasive Monitor

   This attacker corresponds to the pervasive monitoring adversary
   described in [RFC7258].  This attacker relies on monitoring
   capabilities to breach the privacy of individuals from the DNS
   traffic.  This attacker is capable of eavesdropping on traffic
   between two end points, including traffic between any of the pairs of
   the entities described in section 2.1.  Per [RFC7258], this type of
   attacker has abilities to eavesdrop pervasively on many links at
   once, which is a powerful form of attack.  Type-1 Attackers are
   passive.  They do not modify traffic or insert traffic.

   o  Type-1A: Pervasive Monitor: an attacker who is able to monitor
      links carrying individual's traffic through access to traffic
      along those links.  This attacker does not specifically target the
      links of a particular individual.  This attacker uses its system
      location to launch attacks and learn as much as possible about all
      individuals using those links.

   o  Type-1B: Directed Monitor: an attacker with the same types of
      capabilities of monitoring links, which selects links in order to
      target specific individuals.  A Type-1B attacker for instance
      might put into place intermediaries in order to obtain traffic on
      specific links.

5.2.  Attacker Type-2 - Malicious Monitor

   This attacker is more powerful than Type-1.  It corresponds to the
   malicious attack model that is widely studied in the security
   community.  Formally, an attacker in the malicious monitor model has
   all of the active pervasive monitor capabilities as well as the
   capability to control one or more infrastructure elements, to modify



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   traffic in both directions.  This type of attacker could have a
   malicious recursive server or forwarder under its control, for
   example.

5.3.  Attackers in the System Setup

   To evaluate the privacy provided by a given mechanism or mechanisms
   in a particular system model, we characterize the attacker with a
   template with parameters from the system model in which the attacker
   is located.  The general template is: Attacker(Type,
   [Compromised_Entities], [Links]).  For example, the template
   Attacker(Type-2, R, S-R) passed as a parameter in the evaluation of a
   privacy mechanism indicates a Type-2 attacker that controls a
   recursive and has the capability of eavesdropping on the link between
   the stub and recursive resolvers.  Other attacker templates include
   the appropriate parameterizations based on the above description of
   those attackers, including attackers that have the capabilities of
   monitoring multiple links and controlling multiple pieces of
   infrastructure.
































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6.  Privacy Mechanisms

   Various mechanisms for enhancing privacy in networks are applicable
   to DNS private exchange.  Some mechanisms common to privacy research
   include mixing networks, dummy traffic, and private information
   retrieval techniques.  Applicable protocol mechanisms include
   encryption-based techniques - encrypting the channel carrying the
   queries using IPSEC [ipseca], TLS [dns-over-tls] or special-purpose
   encryption [confidential-dns]. [privatedns] includes special-purpose
   encryption and also depends on a trusted service broker.

   o  Mixing Networks: in this type of mechanism, the initiator uses a
      mxing network such as Tor to route the DNS queries to the intended
      DNS server entity.  An attacker observing part of the system finds
      it difficult to determine which individual sends which queries,
      and will not be able to tell which individual has sent them
      (ideally, though it is known that attacks exist that allow
      correlation and privacy breaches against mixing networks).  The
      privacy property is unlinkability of the queries; the probability
      that two queries coming from one exit node in the mixing network
      belong to the same individual is uniform among all the individuals
      using the network.

   o  Dummy Traffic: a simple mechanism in which the initiator of a DNS
      request will also generate k dummy queries and send the intended
      query along with those queries.  As such, the adversary will not
      be able to tell which query is of interest to the initiator.  For
      a given k, the probability that the adversary will be able to
      detect which query is interest to the initiator is equal to
      1-1/(k+1).  In that sense, and for the proper parameterization of
      the protocol, the attacker is bounded to the undetectability of
      the queries.

   o  Private Information Retrieval: a mechanism that allows a user s to
      retrieve a record r from a database DB on a server without
      allowing the server to learn r.  A trivial solution to the problem
      requires that s downloads the entire DB and then perform the
      queries locally.  While that provides privacy to the queries of
      the user, the solution is communication inefficient at the scale
      of the DNS.  More sophisticated cryptographic solutions are multi-
      round, and thus reduce the communication overhead, but are still
      inefficient for the DNS.

   o  Query Minimization: a mechanism that allows the resolver to
      minimize the amount of information it sends on behalf of a stub
      resolver.  A method of query minimization is specified in
      [qname-minimisation].  Qname minimization deprives a Type-1
      attacker on R-A of information from correlating queries, unless



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      the individuals have an Unfortunate Few problem.

   o  NOTE: queries on R-A generally do not include an identifier of the
      individual making the query, because the source address is that of
      R. With respect R or A themselves, they may have well established
      policies for respecting the sensitivity of queries they process,
      while still using summary analysis of those queries to improve
      security, stability or their business operation.

   o  Encrypted Channel Mechanisms: Using these mechanisms, an initiator
      has an encrypted channel with a corresponding enabler, so that the
      queries are not available to eavesdropping Pervasive Monitor
      attackers.  Examples include [dns-over-tls], [ipseca], and
      [confidential-dns].  Depending on the characteristics of the
      channel, various privacy properties are ensured.  For instance,
      undetectability of queries is ensured for encryption-based
      mechanisms once the encrypted channel is established.
      Unlinkability of the queries may depend on the type of crypto-
      suite; it is provided as long as randomized encryption is used.

   o  Composed (Multiple) Mechanisms: the use of multiple mechanisms is
      a likely scenario and results in varied privacy guarantees.
      Consider a hypothetical system in which mixing networks (for
      unlinkability) and randomized encryption (for undetectability) can
      both be applied, thus providing for unobservability, a stronger
      property than either of the two along.  On the other hand,
      consider another hypothetical system in which mixing networks are
      used to reach a third party broker requiring sign-in and
      authorization.  Depending on the attacker type, this could mean
      that the mixing network unlinkability was cancelled out by the
      linkability due to entrusting the third party with identifying
      information in order to be authorized.



















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7.  Privacy Evaluation

   Now we turn our attention to the evaluation of privacy mechanisms in
   a standard form, given the attacker models and system definitions,
   for some of the example mechanisms.

   An evaluation takes multiple parameters as input.  The output of the
   evaluation template is based on the analysis of the individual
   algorithms, settings, and parameters passed to this evaluation
   mechanism.

   Here is the top level interface of the evaluation template:

   Eval(Privacy_Mechanism(param_1, param_2, ...),
   System_Setting(param_1, param_2, ...), Attacker_Model(param_1,
   param_2,...)

   The output of the function is a privacy guarantee for the given
   settings, expressed through defined properties such as unlinkability
   and unobservability, for the specified system and attacker model.

   7.1 Dummy Traffic Example

   Eval(Dummy_Traffic (k=10, distribution=uniform), System_Setting([S,
   P, R, A], [S-P, P-R, R-A]), Attacker_Model(Type-1A, S-R)).

   The dummy traffic mechanism is not presented as a practical
   mechanism, though there's no way to know if there are deployments of
   this type of mechanism.  This example evaluation uses k=10 to
   indicate that for every one query initiated by an individual, ten
   queries that disguise the query of interest are selected uniformly at
   random from a pool of queries.  In the parameters passed in the
   evaluation function, we indicate that the privacy assurances of
   interest concern the S-R link, with a Passive Pervasive Monitor
   (Type-1A) attacker.

   Here is a template format for the example:














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   Eval(Dummy_Traffic (k=10, distribution=uniform),
       System_Setting([S, P, R, A],
       [S-P, P-R, R-A]),
       Attacker_Model(Type-1A, S-R)). {
       Privacy_Mechanism{
           Mechanism_name = Dummy_Traffic
           Parameters{
               Queries = 10
               Query_distribution = uniform
       }
       System_settings{
           Entities = S, P, R and A;
           Links = S-P, P-R, R-A
       }
       Attacker_Model{
           Type = Type-1A
           Compromised_Entities = NA
           Links = S-R
       }
       Privacy_guarantee = undetectability
       Privacy_measure = 1-(1/(queries+1)).

       Return Privacy_guarantee, Privacy_measure

   }

   Undetectability is provided with .91 probability (though we know
   there are other weaknesses for dummy traffic) If the attacker model
   is replaced with Type-2, so that responses to arbitrary requests can
   be injected, and tracked, the undetectability probability is
   decreased.

   7.2 Mixing Network Example

   Here is an input for a mixing network privacy mechanism:

   Eval(mix (u=10, distribution=uniform), System_Setting(link=S-R),
   Attacker_Model(Type-1A)).

   This indicates that the attacker resides between the stub and
   resolver.  While queries are not undetectable, two queries are not
   linkable to the same individual; the provided guarantee is
   unlinkability.  For a given number of individuals in the mixing
   network, indicated by the parameter u, assuming that at any time,
   traffic from these individuals is uniformly random, the probability
   that one query is comes from a given individual is (1/10=0.1).  The
   probability that two queries are issued by the same initiator is
   0.1^2 = 0.01, which represents the linkability probability.  The



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   unlinkability probability is given as 1-0.01 = 0.99.  Thus,

   (unlinkability, 0.99) < Eval(mix (u=10, distribution=uniform),
   System_Setting(link=S-R), Attacker_Model(type-1A)).

   We note that even if there is a Type-2 Attacker in R, the same
   results hold.

   To sum up, the above example is represented in the following
   template:

   Eval(mix (u=10, distribution=uniform),
       System_Setting([S, P, R, A],
           [S-P, P-R, R-A]),
               Attacker_Model(Type-1A, S-R)). {

       Privacy_Mechanism{
           Mechanism_name = mix    //mixing network
           Parameters{
               Users = 10
               Query_distribution = uniform
       }
       System_settings{
           Entities = S, P, R and A;
           Links = S-P, P-R, R-A
       }
       Attacker_Model{
           Type = Type-1A
           Entities = NA
           Links = P-R
       }

       Privacy_guarantee = unlinkability
       Privacy_measure = 1-(1/users)^2.

       Return privacy_guarantee, privacy_measure
   }

   7.3 Encrypted Channel (DNS-over-TLS) Example

   For one of the encryption-based mechanisms, DNS-over-TLS
   [dns-over-tls], we have the following template (TLS parameters are
   from [RFC5246]):








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 Eval(TLS_enc (SHA256, ECDSA, port 53, uniform, NA),
     System_Setting([S, P, R, A],
         [S-P, P-R, RA]),
             Attacker_Model(Type-1B, S-R)). {

     Privacy_Mechanism{
         Mechanism_name = TLS-upgrade-based
         Parameters{
             Users = NA
             Query_distribution = uniform
             Hash_algorithm = SHA256
             Sig_Algorithm = ECDSA
             Port 53
     }
     System_settings{
         Entities = S, P, R and A;
         Links = S-P, P-R, R-A
     }
     Attacker_Model{
         Type = Type-1B
         Entities = NA
         Links = S-R
     }

     Privacy_guarantee = unlinkability, undetectability
     Privacy_measure (unlinkability) = 1
     Privacy_measure (undetectability) = 0 // port 53 indicates DNS used


     Return privacy_guarantee, privacy_measure
 }

   This template features a Directed Monitor attack model (Type-1B) in
   order to show how that the monitor might apply extra resources to an
   encrypted channel.  Undetectability is an issue whether using
   upgrade-based TLS on port 53, or a port-based TLS on a dedicated port
   - both ports indicate the use of DNS.  The source address of the
   individual is exposed in all cases.  If this were a suitably
   parameterized use of [ipseca], the attacker would not be certain that
   all the traffic from S-R was DNS, and undetectability would be
   higher.

   7.4 Encrypted Channel (IPSec) Example

   Template TODO

   7.5 QName Minimization Example (R-A) Example




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   Template TODO

   7.6 Encrypted Channel (S-R), QName Minimization (R-A) Example

   Template TODO

   7.7 Private-DNS (S-R) Example

   The template for [privatedns] takes note of deployments in which in
   addition to S, R and A, there is another entity in the system, the
   function that authenticates the individual using S prior to
   permitting an encrypted channel to be formed to R or A. If the
   Private-DNS connection is with R, then identifiability of S as an
   individual may be similar to the identifiability of S from source
   address, or it may be stronger, depending on the nature of the
   account information required.  If the Private-DNS connection is with
   A, source address PII is provided to A, and linkability of the
   queries from S has probability 1.

   Template TODO































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8.  Security Considerations

   The purpose of this document is to provide methods for those
   deploying or using DNS private exchange to assess the effectiveness
   of privacy mechanisms in depriving attackers of access to private
   information.  Protecting privacy is one of the dimensions of an
   overall security strategy.

   It is possible for privacy-enhancing mechanisms to be deployed in
   ways that are vulnerable to security risks, with the result of not
   achieving security gains.  For the purposes of privacy evaluation, it
   is important for the person making an evaluation to also ensure close
   attention to the content of the Security Considerations section of
   each mechanism being evaluated, for instance, to ensure if TLS is
   used for encryption of a link against surveillance, that TLS best
   security practices [uta-tls-bcp] are in use.



































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9.  IANA Considerations

   No requests are made to IANA.
















































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10.  Acknowledgements

   We thank Scott Hollenbeck, Burt Kaliski, Paul Livesay and Eric
   Osterweil for reviewing early versions.  We also wish to thank those
   who commented on presentations of this work ahead of publication,
   including Simson Garfinkel, Cathy Meadows, Paul Syverson, and
   Christine Task.












































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11.  Informative References

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

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

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

   [RFC6973]  Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
              Morris, J., Hansen, M., and R. Smith, "Privacy
              Considerations for Internet Protocols", RFC 6973,
              July 2013.

   [RFC7258]  Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
              Attack", BCP 188, RFC 7258, May 2014.

   [RFC7435]  Dukhovni, V., "Opportunistic Security: Some Protection
              Most of the Time", RFC 7435, December 2014.

   [confidential-dns]
              Wijngaards, W. and G. Wiley, "Confidential DNS",
              draft-wijngaards-dnsop-confidentialdns-03 (work in
              progress).

   [dns-over-tls]
              Zhu, L., Hu, Z., Heidemann, J., Wessels, D., Mankin, A.,
              and P. Hoffman, "TLS for DNS: Initiation and Performance
              Considerations",
              draft-hzhwm-dprive-start-tls-for-dns-01.txt (work in
              progress).

   [dprive-problem]
              Bortzmeyer, S., "DNS privacy considerations",
              draft-ietf-dprive-problem-statement-01 (work in progress).

   [ipseca]   Osterweil, E., Wiley, G., Mitchell, D., and A. Newton,
              "Opportunistic Encryption with DANE Semantics and IPsec:
              IPSECA".

   [openresolverproject.org]
              Mauch, J., "The Open Resolver Project".

   [phb-dnse]
              Hallam-Baker, P., "DNS Privacy and Censorship: Use Cases



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              and Requirements", draft-hallambaker-dnse-02 (work in
              progress).

   [privatedns]
              Hallam-Baker, P., "Private-DNS",
              draft-hallambaker-wsconnect-08 (work in progress).

   [qname-minimisation]
              Bortzmeyer, S., "DNS query name minimisation to improve
              privacy", draft-ietf-dnsop-qname-minimisation-02 (work in
              progress).

   [uta-tls-bcp]
              Sheffer, Y., Holz, R., and P. StAndre, "Recommendations
              for Secure Use of TLS and DTLS", draft-ietf-uta-tls-bcp-11
              (work in progress).



































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Authors' Addresses

   Aziz Mohaisen
   Verisign Labs
   12061 Bluemont Way
   Reston, VA  20190
   US

   Phone: +1 703 948-3200
   Email: amohaisen@verisign.com


   Allison Mankin
   Verisign Labs
   12061 Bluemont Way
   Reston, VA  20190
   US

   Phone: +1 703 948-3200
   Email: amankin@verisign.com































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