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
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This Internet-Draft will expire on September 10, 2015.
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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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