Internet DRAFT - draft-am-dprive-eval
draft-am-dprive-eval
Network Working Group A. Mohaisen
Internet-Draft SUNY Buffalo
Intended status: Informational A. Mankin
Expires: April 20, 2016 Verisign Labs
October 18, 2015
Evaluation of Privacy for DNS Private Exchange
draft-am-dprive-eval-02
Abstract
The set of DNS requests that an individual makes can provide a
monitor with a large amount of information about that individual.
DNS Private Exchange (DPRIVE) aims to deprive this actor 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 April 20, 2016.
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to this document. Code Components extracted from this document must
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Table of Contents
1. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Privacy Evaluation Definitions . . . . . . . . . . . . . . . 4
2.1. Entities . . . . . . . . . . . . . . . . . . . . . . . . 4
2.2. Data and Analysis . . . . . . . . . . . . . . . . . . . . 5
2.3. Identifiability . . . . . . . . . . . . . . . . . . . . . 5
2.4. Other Central Definitions and Formalizations . . . . . . 6
3. Assumptions about Quantification of Privacy . . . . . . . . . 8
4. System Model . . . . . . . . . . . . . . . . . . . . . . . . 8
4.1. DNS Resolvers (System Model) . . . . . . . . . . . . . . 9
4.2. System Setup - Putting It Together . . . . . . . . . . . 9
5. Risk Model . . . . . . . . . . . . . . . . . . . . . . . . . 10
5.1. Risk Type-1 - Passive Pervasive Monitor . . . . . . . . . 11
5.2. Risk Type-2 - Active Monitor . . . . . . . . . . . . . . 11
5.3. Risks in the System Setup . . . . . . . . . . . . . . . . 11
6. Privacy Mechanisms . . . . . . . . . . . . . . . . . . . . . 12
7. Privacy Evaluation . . . . . . . . . . . . . . . . . . . . . 13
8. Other evaluation . . . . . . . . . . . . . . . . . . . . . . 20
9. Security Considerations . . . . . . . . . . . . . . . . . . . 20
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 20
12. Informative References . . . . . . . . . . . . . . . . . . . 20
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 22
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
[RFC7626], 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. 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.
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The privacy threats associated with DNS monitoring are not new,
however they were made more obvious 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 RFC on
opportunistic security [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 risk model and some information about relationships among
the entities in a given system. A mechanism for providing privacy to
withstand the power and capabilities of a passive pervasive monitor
may not withstand a more powerful actor using active monitoring by
plugging itself into the path of individuals' DNS requests as a
forwarder . Having some standard 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 risk models
of interest. 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 risk 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. The verbatim source of most of those definitions is from
[RFC6973], which are included as an aid in reasability. In some
definitions, text is added to apply them to the DNS case. Also, a
new section of terms has been added 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. Entities
o Attacker: An entity that works against one or more privacy
protection goals. Unlike observers, attackers' behavior is
unauthorized, in a way similar to that of an eavesdropper.
o Eavesdropper: A type of attacker that passively observes an
initiator's communications without the initiator's knowledge or
authorization. DNS example 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
(DNS example of stub resolver) and the recipient (DNS example of
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 risks,
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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o We note that while the definition of an observer may include an
initiator in the risk model, an initiator of a request is excluded
in the context of this document, because it corresponds to the
subject of interest being studied. Similar to the definition in
[RFC7258], we note that an attacker is broader than an observer.
While [RFC7258] claim that an attack does not consider the motive
of the actor, the given context of DNS implies a motive if the
term attacker is used to characterize the risk.
2.2. Data and Analysis
We assume the following definitions related to data and analysis from
[RFC4949]: attacker, correlation, fingerprint, fingerprinting, item
of interest (IOI), personal data, interaction, traffic analysis,
undetectability, and unlinkability. We augment some of those
definitions later in this document.
from [RFC4949], we relax the definition of IOI to exclude "the fact
that a communication interaction has taken place" as this does not
suite the evaluated context of DNS.
2.3. Identifiability
We assume the following definitions related to identifiability from
[RFC4949]: anonymity, anonymity set, anonymous, attribute, identity
provider, personal name, and relying party.
The following definitions are modified for the context of this
document from those defined in [RFC4949]
o Identifiability: The extent to which an individual is
identifiable. [RFC6973] includes the rest of the variations on
this (Identifiable, Identification, Identified, Identifier,
Identity, Identity Confidentiality)
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.
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o Pseudonymity: See the formal definition in section 2.4 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.
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 or modify
them for the context of this document. 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) addresses, 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, 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
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interested in breaching the privacy 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.
o Unlinkability: Formally, two items of interest are said to be
unlinkable if the certainty of an actor concerning those items of
interest is not affected by observing the system. This is,
unlinkability implies that the a-posteriori probability computed a
monitor that two items of interest are related is close enough to
the a-priori probability computed by a monitor based on their
knowledge.
Two items of interest are said to be unlinkable if there is a small
chance (beta, close to 0 probability) that the monitor 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 monitor to sufficiently determine whether those items are related
to one another. In the context of DNS, this refers typically but not
only to a monitor 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 monitor 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 that is 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 not involved
individual, monitor or not.
In theory, there are many ways of ensuring unobservability by
fulfilling both requirements. For example, undetectability requires
that no party that is not invovled 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 resolvers, 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 a monitor Y,
where Y = [y1, y2, ... yn], we define the privacy of A as the
uncertainty of the monitor 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].
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 called DNS recursive
resolvers. Another type of entity, DNS forwarders (or proxies), acts
as intermediaries between stub resolvers and recursive resolvers, and
recursive resolvers and authoritative resolvers. The system model
for DNS privacy evaluation includes the four entities quickly
sketched here: stub resolvers, recursive resolvers, authoritative
name servers, and forwarders.
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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 one or more 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
monitors such as the pervasive monitors 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) can collect sets of query 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 discuss it in the evaluation.
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Rather than notating in system setup that an entity is compromised,
this is covered in the monitor 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. A monitor 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 monitor 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 authoritative name server.
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.
The system setup here leaves out other realistic considerations for
simplicity, such as the impact of shared caches in DNS entities.
5. Risk Model
The Definitions section defines observer, attack and monitor, but not
a Risk Model, which is needed to actually evaluate privacy.
For consistency, we note that the only difference between an attacker
and an obeserver is that an attacker is an unauthorized observer with
all the capabilities it may have. We also stress that for the
context of DNS privacy, the term attacker may implicitly assume an
intent. To that end, active and passive observers are collectively
referred to as actors.
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o Risk Model: a well-defined set of capabilities indicating how much
information an observer (or eavesdropper) 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 risk models, namely a pervasive
monitor and a malicious monitor.
5.1. Risk Type-1 - Passive Pervasive Monitor
This risk corresponds to the passive pervasive monitoring model
described in [RFC7258]. This model relies on monitoring capabilities
to breach the privacy of individuals from the DNS traffic at scale
without decimation. An actor causing this risk is capable of
eavesdropping or observing traffic between two end points, including
traffic between any of the pairs of entities described in section
2.1. Per [RFC7258], this type of actor has the ability to eavesdrop
pervasively on many links at once, which is a powerful form of
attack. Type-1 monitors are passive. They do not modify traffic or
insert traffic.
5.2. Risk Type-2 - Active Monitor
This risk corresponds to an actor with the same types of capabilities
of monitoring links. Additionally, this model can select links in
order to target specific individuals. A Type-2 monitor for instance
might put into place intermediaries in order to obtain traffic on
specific links.
Note that we exclude malicious monitoring from this document since,
by definition, a malicious actor has an intent associated with his
actions.
5.3. Risks in the System Setup
To evaluate the privacy provided by a given mechanism or mechanisms
in a particular system model, we characterize the risk using a
template where parameters from the system model in which the risk
actor (eavesdropper or observer as monitors) is located are used.
The general template is: Risk(Type, [Entities], [Links]). For
example, the template Risk(Type-2, R, S-R) passed as a parameter in
the evaluation of a privacy mechanism indicates a Type-2 monitor that
controls a recursive resolver and has the capability of eavesdropping
on the link between the stub and recursive resolvers (S-R). Other
risk templates include the appropriate parameterizations based on the
above description of those monitors, including monitors 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 mix 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 but also depends on a trusted service broker.
o Mix Networks: in this type of mechanism, the initiator uses a mix
network such as Tor to route the DNS queries to the intended DNS
server entity. A monitor 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 mix networks). The privacy property is
unlinkability of the queries; the probability that two queries
coming from one exit node in the mix 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 of interest to the initiator is equal to
1-1/(k+1). In that sense, and for the proper parameterization of
the protocol, the monitor 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 the recordr. A trivial solution to
the problem requires that s downloads the entire database DB and
then perform the queries locally. While this 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 risk
on R-A of information from correlating queries, unless the
individuals have an Unfortunate Few problem.
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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 of 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 risk.
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 mix 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 mix networks are
used to reach a third party broker requiring sign-in and
authorization. Depending on the risk type, this could mean that
the mix network unlinkability was cancelled out by the linkability
due to entrusting the third party with identifying information in
order to be authorized.
7. Privacy Evaluation
Now we turn our attention to the evaluation of privacy mechanisms in
a standard form, given the risk 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(params),
System_Setting(params),
Risk_Model(params)
)
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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 risk 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]),
Risk_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 randomly
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) risk.
Here is a template format for the example:
Eval(Dummy_Traffic (k=10, distribution=uniform),
System_Setting([S, P, R, A], [S-P, P-R, R-A]),
Risk_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
}
Risk_Model{
Type = Type-1A
Links = S-R
}
Privacy_guarantee = undetectability
Privacy_measure = 1-(1/(queries+1)).
Return Privacy_guarantee, Privacy_measure
}
Undetectability is provided with 0.91 probability (though we know
there are other weaknesses for dummy traffic) If the threat model is
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replaced with Type-2, so that responses to arbitrary requests can be
injected, and tracked, the undetectability probability is decreased.
7.2 Mix Network Example
Here is an input for a mix network privacy mechanism:
Eval(Mix (u=10, distribution=uniform), System_Setting(link=S-R),
threat_Model(Type-1A))
This indicates that the monitor 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 mix 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 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), Risk_Model(type-1)).
We note that even if there is a Type-2 Risk in recursive resolver R,
the same results hold.
To sum up, the above example is represented in the following
template:
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Eval(Mix (u=10, distribution=uniform),
System_Setting([S, P, R, A], [S-P, P-R, R-A]),
Risk_Model(Type-1A, S-R)) {
Privacy_Mechanism{
Mechanism_name = mix //mix network
Parameters{
Users = 10
Query_distribution = uniform
}
System_settings{
Entities = S, P, R and A;
Links = S-P, P-R, R-A
}
Risk_Model{
Type = Type-1A
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),
System_Setting([S, P, R, A], [S-P, P-R, RA]),
Risk_Model(Type-1B, S-R)) {
Privacy_Mechanism{
Mechanism_name = TLS-upgrade-based
Parameters{
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
}
Risk_Model{
Type = Type-1B
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 an Active Monitor risk model (Type-2) 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 monitor would not be certain that
all the traffic from S-R was DNS, and undetectability would be
higher.
7.4 Encrypted Channel (IPSec) Example
In the following, we use the same template above to characterize the
encryption capabilities provided by IPSec, as a potential mechanisms
for enabling privacy in DNS exchanges.
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Eval(IPSEc_enc([...]),
System_Setting([S, P, R, A], [S-P, P-R, RA]),
Risk_Model(Type-1B, S-R)) {
Privacy_Mechanism{
Mechanism_name = IPSec
Parameters{
Query_distribution = uniform
}
System_settings{
Entities = S, P, R and A;
Links = S-P, P-R, R-A
}
Risk_Model{
Type = 2
Links = S-R
}
Privacy_guarantee = unlinkability, undetectability
Privacy_measure (unlinkability) = 1
Privacy_measure (undetectability) = 1
Return privacy_guarantee, privacy_measure
}
We note that IPSec can provide better guarantees with respect to
studied privacy notions. However, whether the technique itself is
widely deployable or not is worth further investigation.
7.5 QName Minimization Example (R-A) Example
Analyzing the privacy assurances of QName minimization is a non-
trivial problem, given that the notions introduced in this document
are techniques that do not alter items of interest. This is, the
notions of privacy as outlined above are concerned with a certain IOI
that is modified by this technique. To this end, we modify the
aforementioned notions in ordered to be able to analyze the
technique. For example, we define linkability as the ability of an
adversary to link two labels of (minimized) queries to each other,
and relate them to original source of query. Assuming a reasonable
use of a recursive resolver that minimizes queries on behalf of
users, this task is non-trivial, although quantifying the probability
would depend on the number of labels in queries, the number of
queries issued, and the number of users using the studied recursive
resolvers. The following template captures QName minimization as a
template
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Eval(Qname_minimisation ([...],
System_Settings([S, P, R, A], [R-A]),
Risk_Model(Type=2){
Privacy_Mechanism{
Mechanism_name = Qname_minimisation
Parameters{
Qtype_used = NS
}
System_settings{
Entities = S, P, R and A;
Links = R-A
}
Risk_model{
Type = 2
Links = R-A
}
Privacy_guarantee = unlinkability
Privacy_measure = analytical
Return privacy_guarantee, privacy_measure
}
Note that QName minimization does not solve the problem of the
privacy for a monitoring risk between the stub and recursive.
Encrypting the channel between the recursive and the stub, utilizing
other techniques such as TDNS or IPSec, can marginalize such risk.
Furthermore, note that the risk on the link between the recursive and
authority name servers is always mitigated by the fact that recursive
name servers act as a mixer of queries, even when they are sent in
full to the authority name servers.
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.
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8. Other evaluation
This document does not address a lot of the evaluation aspects not
associated with privacy. For example, some of the mechanisms
discussed in the working group are built of well-understood and
standardized technologies, whereas others use other non-standard and
less widely deployed techniques. A comprehensive evaluation of such
mechanisms should take into account such facts.
9. 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 monitors 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 [RFC7525] are in use.
10. IANA Considerations
No requests are made to IANA.
11. Acknowledgements
We wish to thank Scott Hollenbeck, Burt Kaliski, Minsuk Kang, Paul
Livesay and Eric Osterweil for reviewing early versions. We wish to
thank Stephane Bortzmeyer and Tim Wicinski for their detailed review
and feedback on the previous versions of this document. 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.
12. Informative References
[confidential-dns]
Wijngaards, W. and G. Wiley, "Confidential DNS", draft-
wijngaards-dnsop-confidentialdns-03 (work in progress),
March 2015.
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[dns-over-tls]
Zhu, L., Hu, Z., Heidemann, J., Wessels, D., Mankin, A.,
and P. Hoffman, "TLS for DNS: Initiation and Performance
Considerations", draft-ietf-dprive-dns-over-tls-01.txt
(work in progress), February 2015.
[ipseca] Osterweil, E., Wiley, G., Okubo, T., Lavu, R., and A.
Mohaisen, "Opportunistic Encryption with DANE Semantics
and IPsec: IPSECA", draft-osterweil-dane-ipsec-03 (work in
progress), March 2015.
[openresolverproject.org]
Mauch, J., "The Open Resolver Project", April 2015.
[phb-dnse]
Hallam-Baker, P., "DNS Privacy and Censorship: Use Cases
and Requirements", draft-hallambaker-dnse-02 (work in
progress), November 2014.
[privatedns]
Hallam-Baker, P., "Private-DNS", draft-hallambaker-
privatedns-01 (work in progress), November 2014.
[qname-minimisation]
Bortzmeyer, S., "DNS query name minimisation to improve
privacy", draft-ietf-dnsop-qname-minimisation-07 (work in
progress), March 2015.
[RFC3552] Rescorla, E. and B. Korver, "Guidelines for Writing RFC
Text on Security Considerations", BCP 72, RFC 3552, DOI
10.17487/RFC3552, July 2003,
<http://www.rfc-editor.org/info/rfc3552>.
[RFC4949] Shirey, R., "Internet Security Glossary, Version 2", FYI
36, RFC 4949, DOI 10.17487/RFC4949, August 2007,
<http://www.rfc-editor.org/info/rfc4949>.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, DOI 10.17487/
RFC5246, August 2008,
<http://www.rfc-editor.org/info/rfc5246>.
[RFC6973] Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
Morris, J., Hansen, M., and R. Smith, "Privacy
Considerations for Internet Protocols", RFC 6973, DOI
10.17487/RFC6973, July 2013,
<http://www.rfc-editor.org/info/rfc6973>.
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[RFC7258] Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
2014, <http://www.rfc-editor.org/info/rfc7258>.
[RFC7435] Dukhovni, V., "Opportunistic Security: Some Protection
Most of the Time", RFC 7435, DOI 10.17487/RFC7435,
December 2014, <http://www.rfc-editor.org/info/rfc7435>.
[RFC7525] Sheffer, Y., Holz, R., and P. Saint-Andre,
"Recommendations for Secure Use of Transport Layer
Security (TLS) and Datagram Transport Layer Security
(DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May
2015, <http://www.rfc-editor.org/info/rfc7525>.
[RFC7626] Bortzmeyer, S., "DNS Privacy Considerations", RFC 7626,
DOI 10.17487/RFC7626, August 2015,
<http://www.rfc-editor.org/info/rfc7626>.
Authors' Addresses
Aziz Mohaisen
SUNY Buffalo
323 Davis Hall
Buffalo, NY 14260
US
Phone: +1 716 645-1592
Email: mohaisen@buffalo.edu
Allison Mankin
Verisign Labs
12061 Bluemont Way
Reston, VA 20190
US
Phone: +1 703 948-3200
Email: amankin@verisign.com
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