Privacy Extensions for DNS-SD
draft-ietf-dnssd-privacy-03

Versions: (draft-huitema-dnssd-privacy) 00 01 02         Standards Track
          03                                                            
Network Working Group                                         C. Huitema
Internet-Draft                                      Private Octopus Inc.
Intended status: Standards Track                               D. Kaiser
Expires: March 14, 2018                           University of Konstanz
                                                      September 10, 2017


                     Privacy Extensions for DNS-SD
                      draft-ietf-dnssd-privacy-03

Abstract

   DNS-SD (DNS Service Discovery) normally discloses information about
   both the devices offering services and the devices requesting
   services.  This information includes host names, network parameters,
   and possibly a further description of the corresponding service
   instance.  Especially when mobile devices engage in DNS Service
   Discovery over Multicast DNS at a public hotspot, a serious privacy
   problem arises.

   We propose to solve this problem by a two-stage approach.  In the
   first stage, hosts discover Private Discovery Service Instances via
   DNS-SD using special formats to protect their privacy.  These service
   instances correspond to Private Discovery Servers running on peers.
   In the second stage, hosts directly query these Private Discovery
   Servers via DNS-SD over TLS.  A pairwise shared secret necessary to
   establish these connections is only known to hosts authorized by a
   pairing system.

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

   This Internet-Draft will expire on March 14, 2018.






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Copyright Notice

   Copyright (c) 2017 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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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Requirements  . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Privacy Implications of DNS-SD  . . . . . . . . . . . . . . .   4
     2.1.  Privacy Implication of Publishing Service Instance Names    4
     2.2.  Privacy Implication of Publishing Node Names  . . . . . .   5
     2.3.  Privacy Implication of Publishing Service Attributes  . .   5
     2.4.  Device Fingerprinting . . . . . . . . . . . . . . . . . .   6
     2.5.  Privacy Implication of Discovering Services . . . . . . .   7
   3.  Design of the Private DNS-SD Discovery Service  . . . . . . .   7
     3.1.  Device Pairing  . . . . . . . . . . . . . . . . . . . . .   8
     3.2.  Discovery of the Private Discovery Service  . . . . . . .   8
       3.2.1.  Obfuscated Instance Names . . . . . . . . . . . . . .   9
       3.2.2.  Using a Predictable Nonce . . . . . . . . . . . . . .   9
       3.2.3.  Using a Short Proof . . . . . . . . . . . . . . . . .  10
       3.2.4.  Direct Queries  . . . . . . . . . . . . . . . . . . .  12
     3.3.  Private Discovery Service . . . . . . . . . . . . . . . .  12
       3.3.1.  A Note on Private DNS Services  . . . . . . . . . . .  13
     3.4.  Randomized Host Names . . . . . . . . . . . . . . . . . .  14
     3.5.  Timing of Obfuscation and Randomization . . . . . . . . .  14
   4.  Private Discovery Service Specification . . . . . . . . . . .  14
     4.1.  Host Name Randomization . . . . . . . . . . . . . . . . .  15
     4.2.  Device Pairing  . . . . . . . . . . . . . . . . . . . . .  15
     4.3.  Private Discovery Server  . . . . . . . . . . . . . . . .  15
       4.3.1.  Establishing TLS Connections  . . . . . . . . . . . .  15
     4.4.  Publishing Private Discovery Service Instances  . . . . .  16
     4.5.  Discovering Private Discovery Service Instances . . . . .  17
     4.6.  Direct Discovery of Private Discovery Service Instances .  18
     4.7.  Using the Private Discovery Service . . . . . . . . . . .  19
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  19
     5.1.  Attacks Against the Pairing System  . . . . . . . . . . .  19
     5.2.  Denial of Discovery of the Private Discovery Service  . .  19



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     5.3.  Replay Attacks Against Discovery of the Private Discovery
           Service . . . . . . . . . . . . . . . . . . . . . . . . .  20
     5.4.  Denial of Private Discovery Service . . . . . . . . . . .  20
     5.5.  Replay Attacks against the Private Discovery Service  . .  20
     5.6.  Replay attacks and clock synchronization  . . . . . . . .  21
     5.7.  Fingerprinting the number of published instances  . . . .  21
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  21
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  22
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  22
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  22
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  23
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  24

1.  Introduction

   DNS-SD [RFC6763] over mDNS [RFC6762] enables configurationless
   service discovery in local networks.  It is very convenient for
   users, but it requires the public exposure of the offering and
   requesting identities along with information about the offered and
   requested services.  Parts of the published information can seriously
   breach the user's privacy.  These privacy issues and potential
   solutions are discussed in [KW14a] and [KW14b].

   There are cases when nodes connected to a network want to provide or
   consume services without exposing their identity to the other parties
   connected to the same network.  Consider for example a traveler
   wanting to upload pictures from a phone to a laptop when connected to
   the Wi-Fi network of an Internet cafe, or two travelers who want to
   share files between their laptops when waiting for their plane in an
   airport lounge.

   We expect that these exchanges will start with a discovery procedure
   using DNS-SD [RFC6763] over mDNS [RFC6762].  One of the devices will
   publish the availability of a service, such as a picture library or a
   file store in our examples.  The user of the other device will
   discover this service, and then connect to it.

   When analyzing these scenarios in Section 2, we find that the DNS-SD
   messages leak identifying information such as the instance name, the
   host name or service properties.  We review the design constraint of
   a solution in Section 3, and describe the proposed solution in
   Section 4.

   While we focus on a mDNS-based distribution of the DNS-SD resource
   records, our solution is agnostic about the distribution method and
   also works with other distribution methods, e.g. the classical
   hierarchical DNS.




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1.1.  Requirements

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

2.  Privacy Implications of DNS-SD

   DNS-Based Service Discovery (DNS-SD) is defined in [RFC6763].  It
   allows nodes to publish the availability of an instance of a service
   by inserting specific records in the DNS ([RFC1033], [RFC1034],
   [RFC1035]) or by publishing these records locally using multicast DNS
   (mDNS) [RFC6762].  Available services are described using three types
   of records:

   PTR Record:  Associates a service type in the domain with an
      "instance" name of this service type.

   SRV Record:  Provides the node name, port number, priority and weight
      associated with the service instance, in conformance with
      [RFC2782].

   TXT Record:  Provides a set of attribute-value pairs describing
      specific properties of the service instance.

   In the remaining subsections, we will review the privacy issues
   related to publishing instance names, node names, service attributes
   and other data, as well as review the implications of using the
   discovery service as a client.

2.1.  Privacy Implication of Publishing Service Instance Names

   In the first phase of discovery, the client obtains all the PTR
   records associated with a service type in a given naming domain.
   Each PTR record contains a Service Instance Name defined in Section 4
   of [RFC6763]:

     Service Instance Name = <Instance> . <Service> . <Domain>

   The <Instance> portion of the Service Instance Name is meant to
   convey enough information for users of discovery clients to easily
   select the desired service instance.  Nodes that use DNS-SD over mDNS
   [RFC6762] in a mobile environment will rely on the specificity of the
   instance name to identify the desired service instance.  In our
   example of users wanting to upload pictures to a laptop in an
   Internet Cafe, the list of available service instances may look like:





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   Alice's Images         . _imageStore._tcp . local
   Alice's Mobile Phone   . _presence._tcp   . local
   Alice's Notebook       . _presence._tcp   . local
   Bob's Notebook         . _presence._tcp   . local
   Carol's Notebook       . _presence._tcp   . local

   Alice will see the list on her phone and understand intuitively that
   she should pick the first item.  The discovery will "just work".

   However, DNS-SD/mDNS will reveal to anybody that Alice is currently
   visiting the Internet Cafe.  It further discloses the fact that she
   uses two devices, shares an image store, and uses a chat application
   supporting the _presence protocol on both of her devices.  She might
   currently chat with Bob or Carol, as they are also using a _presence
   supporting chat application.  This information is not just available
   to devices actively browsing for and offering services, but to
   anybody passively listening to the network traffic.

2.2.  Privacy Implication of Publishing Node Names

   The SRV records contain the DNS name of the node publishing the
   service.  Typical implementations construct this DNS name by
   concatenating the "host name" of the node with the name of the local
   domain.  The privacy implications of this practice are reviewed in
   [RFC8117].  Depending on naming practices, the host name is either a
   strong identifier of the device, or at a minimum a partial
   identifier.  It enables tracking of both the device, and, by
   extension, the device's owner.

2.3.  Privacy Implication of Publishing Service Attributes

   The TXT record's attribute-value pairs contain information on the
   characteristics of the corresponding service instance.  This in turn
   reveals information about the devices that publish services.  The
   amount of information varies widely with the particular service and
   its implementation:

   o  Some attributes like the paper size available in a printer, are
      the same on many devices, and thus only provide limited
      information to a tracker.

   o  Attributes that have freeform values, such as the name of a
      directory, may reveal much more information.

   Combinations of attributes have more information power than specific
   attributes, and can potentially be used for "fingerprinting" a
   specific device.




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   Information contained in TXT records does not only breach privacy by
   making devices trackable, but might directly contain private
   information about the user.  For instance the _presence service
   reveals the "chat status" to everyone in the same network.  Users
   might not be aware of that.

   Further, TXT records often contain version information about services
   allowing potential attackers to identify devices running exploit-
   prone versions of a certain service.

2.4.  Device Fingerprinting

   The combination of information published in DNS-SD has the potential
   to provide a "fingerprint" of a specific device.  Such information
   includes:

   o  The list of services published by the device, which can be
      retrieved because the SRV records will point to the same host
      name.

   o  The specific attributes describing these services.

   o  The port numbers used by the services.

   o  The values of the priority and weight attributes in the SRV
      records.

   This combination of services and attributes will often be sufficient
   to identify the version of the software running on a device.  If a
   device publishes many services with rich sets of attributes, the
   combination may be sufficient to identify the specific device.

   A sometimes heard argument is that devices providing services can be
   identified by observing the local traffic, and that trying to hide
   the presence of the service is futile.  This argument, however, does
   not carry much weight because

   1.  proving privacy at the discovery layer is of the essence for
       enabling automatically configured privacy-preserving network
       applications.  Application layer protocols are not forced to
       leverage the offered privacy, but if device tracking is not
       prevented at the deeper layers, including the service discovery
       layer, obfuscating a certain service's protocol at the
       application layer is futile.

   2.  Further, even if the application layer does not protect privacy,
       it is hard to record and analyse the unicast traffic (which most




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       applications will generate) compared to just listening to the
       multicast messages sent by DNS-SD/mDNS.

   The same argument can be extended to say that the pattern of services
   offered by a device allows for fingerprinting the device.  This may
   or may not be true, since we can expect that services will be
   designed or updated to avoid leaking fingerprints.  In any case, the
   design of the discovery service should avoid making a bad situation
   worse, and should as much as possible avoid providing new
   fingerprinting information.

2.5.  Privacy Implication of Discovering Services

   The consumers of services engage in discovery, and in doing so reveal
   some information such as the list of services they are interested in
   and the domains in which they are looking for the services.  When the
   clients select specific instances of services, they reveal their
   preference for these instances.  This can be benign if the service
   type is very common, but it could be more problematic for sensitive
   services, such as for example some private messaging services.

   One way to protect clients would be to somehow encrypt the requested
   service types.  Of course, just as we noted in Section 2.4, traffic
   analysis can often reveal the service.

3.  Design of the Private DNS-SD Discovery Service

   In this section, we present the design of a two-stage solution that
   enables private use of DNS-SD, without affecting existing users.  The
   solution is largely based on the architecture proposed in [KW14b] and
   [K17], which separates the general private discovery problem in three
   components.  The first component is an offline pairing mechanism,
   which is performed only once per pair of users.  It establishes a
   shared secret over an authenticated channel, allowing devices to
   authenticate using this secret without user interaction at any later
   point in time.  We use the pairing system proposed in
   [I-D.ietf-dnssd-pairing].

   The further two components are online (in contrast to pairing they
   are performed anew each time joining a network) and compose the two
   service discovery stages, namely

   o  Discovery of the Private Discovery Service -- the first stage --
      in which hosts discover the Private Discovery Service (PDS), a
      special service offered by every host supporting our extension.
      After the discovery, hosts connect to the PSD offered by paired
      peers.




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   o  Actual Service Discovery -- the second stage -- is performed
      through the Private Discovery Service, which only accepts
      encrypted messages associated with an authenticated session; thus
      not compromising privacy.

   In other words, the hosts first discover paired peers and then
   directly engage in privacy preserving service discovery.

   The stages are independent with respect to means used for
   transmitting the necessary data.  While in our extension the messages
   for the first stage are transmitted using IP multicast, the messages
   for the second stage are transmitted via unicast.  One could also
   imagine using a Distributed Hash Table for the first stage, being
   completely independent of multicast.

3.1.  Device Pairing

   Any private discovery solution needs to differentiate between
   authorized devices, which are allowed to get information about
   discoverable entities, and other devices, which should not be aware
   of the availability of private entities.  The commonly used solution
   to this problem is establishing a "device pairing".

   Device pairing has to be performed only once per pair of users.  This
   is important for user-friendliness, as it is the only step that
   demands user-interaction.  After this single pairing, privacy
   preserving service discovery works fully automatically.  In this
   document, we utilize [I-D.ietf-dnssd-pairing] as the pairing
   mechanism.

   The pairing yields a mutually authenticated shared secret, and
   optionally mutually authenticated public keys or certificates added
   to a local web of trust.  Public key technology has many advantages,
   but shared secrets are typically easier to handle on small devices.

3.2.  Discovery of the Private Discovery Service

   The first stage of service discovery is to check whether instances of
   compatible Private Discovery Services are available in the local
   scope.  The goal of that stage is to identify devices that share a
   pairing with the querier, and are available locally.  The service
   instances can be browsed using regular DNS-SD procedures, and then
   filtered so that only instances offered by paired devices are
   retained.







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3.2.1.  Obfuscated Instance Names

   The instance names for the Private Discovery Service are obfuscated,
   so that authorized peers can associate the instance with its
   publisher, but unauthorized peers can only observe what looks like a
   random name.  To achieve this, the names are composed as the
   concatenation of a nonce and a proof, which is composed by hashing
   the nonce with a pairing key:

      PrivateInstanceName = <nonce>|<proof>
      proof = hash(<nonce>|<key>)

   The publisher will publish as many instances as it has established
   pairings.

   The discovering party that looks for instances of the service will
   receive lists of advertisements from nodes present on the network.
   For each advertisement, it will parse the instance name, and then,
   for each available pairing key, compares the proof to the hash of the
   nonce concatenated with this pairing key.  If there is no match, it
   discards the instance name.  If there is a match, it has discovered a
   peer.

3.2.2.  Using a Predictable Nonce

   Assume that there are N nodes on the local scope, and that each node
   has on average M pairings.  Each node will publish on average M
   records, and the node engaging in discovery may have to process on
   average N*M instance names.  The discovering node will have to
   compute on average M potential hashes for each nonce.  The number of
   hash computations would scale as O(N*M*M), which means that it could
   cause a significant drain of resource in large networks.

   In order to minimize the amount of computing resource, we suggest
   that the nonce be derived from the current time, for example set to a
   representation of the current time rounded to some period.  With this
   convention, receivers can predict the nonces that will appear in the
   published instances.

   The publishers will have to create new records at the end of each
   rounding period.  If the rounding period is set too short, they will
   have to repeat that very often, which is inefficient.  On the other
   hand, if the rounding period is too long, the system may be exposed
   to replay attacks.  We initially proposed a value of about 5 minutes,
   which would work well for the mDNS variant of DNS-SD.  However, this
   may cause an excessive number of updates for the DNS server based
   version of DNS-SD.  We propose to set a value of about 30 minutes,
   which seems to be a reasonable compromise.



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   Receivers can pre-calculate all the M relevant proofs once per time
   interval and then establish a mapping from the corresponding instance
   names to the pairing data in form of a hash table.  These M relevant
   proofs are the proofs resulting from hashing a host's M pairing keys
   alongside the current nonce.  Each time they receive an instance
   name, they can test in O(1) time if the received service information
   is relevant or not.

   Unix defines a 32 bit time stamp as the number of seconds elapsed
   since January 1st, 1970 not counting leap seconds.  The most
   significant 20 bits of this 32 bit number represent the number of
   2048 seconds intervals since the epoch. 2048 seconds correspond to 34
   minutes and 8 seconds, which is close enough to our design goal of 30
   minutes.  We will thus use this 20 bit number as nonce, which for
   simplicity will be padded zeroes to 24 bits and encoded in 3 octets.

   For coping with time skew, receivers pre-calculate proofs for the
   respective next time interval and store hash tables for the last, the
   current, and the next time interval.  When receiving a service
   instance name, receivers first check whether the nonce corresponds to
   the current, the last or the next time interval, and if so, check
   whether the instance name is in the corresponding hash table.  For
   (approximately) meeting our design goal of 5 min validity, the last
   time interval may only be considered if the current one is less than
   half way over and the next time interval may only be considered if
   the current time interval is more than half way over.

   Publishers will need to compute O(M) hashes at most once per time
   stamp interval.  If records can be created "on the fly", publishers
   will only need to perform that computation upon receipt of the first
   query during a given interval, and cache the computed results for the
   remainder of the interval.  There are however scenarios in which
   records have to be produced in advance, for example when records are
   published within a scope defined by a domain name and managed by a
   "classic" DNS server.  In such scenarios, publishers will need to
   perform the computations and publication exactly once per time stamp
   interval.

3.2.3.  Using a Short Proof

   Devices will have to publish as many instance names as they have
   peers.  The instance names will have to be represented via a text
   string, which means that the binary concatenation of nonce and proof
   will have to be encoded using a binary-to-text conversion such as
   BASE64 ([RFC2045] section 6.8) or BASE32 ([RFC4648] section 6).

   Using long proofs, such as the full output of SHA256 [RFC4055], would
   generate fairly long instance names: 48 characters using BASE64, or



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   56 using BASE32.  These long names would inflate the network traffic
   required when discovering the privacy service.  They would also limit
   the number of DNS-SD PTR records that could be packed in a single
   1500 octet sized packet, to 23 or fewer with BASE64, or 20 or fewer
   with BASE32.

   Shorter proofs lead to shorter messages, which is more efficient as
   long as we do not encounter too many collisions.  A collision will
   happen if the proof computed by the publisher using one key matches a
   proof computed by a receiver using another key.  If a receiver
   mistakenly believes that a proof fits one of its peers, it will
   attempt to connect to the service as explained in section Section 4.5
   but in the absence of the proper pairwise shared key, the connection
   will fail.  This will not create an actual error, but the probability
   of such events should be kept low.

   The following table provides the probability that a discovery agent
   maintaining 100 pairings will observe a collision after receiving
   100000 advertisement records.  It also provides the number of
   characters required for the encoding of the corresponding instance
   name in BASE64 or BASE32, assuming 24 bit nonces.

                 +-------+------------+--------+--------+
                 | Proof | Collisions | BASE64 | BASE32 |
                 +-------+------------+--------+--------+
                 |   24  |  5.96046%  |   8    |   16   |
                 |   32  |  0.02328%  |   11   |   16   |
                 |   40  |  0.00009%  |   12   |   16   |
                 |   48  |  3.6E-09   |   12   |   16   |
                 |   56  |  1.4E-11   |   15   |   16   |
                 +-------+------------+--------+--------+

                                  Table 1

   The table shows that for a proof, 24 bits would be too short. 32 bits
   might be long enough, but the BASE64 encoding requires padding if the
   input is not an even multiple of 24 bits, and BASE32 requires padding
   if the input is not a multiple of 40 bits.  Given that, the desirable
   proof lengths are thus 48 bits if using BASE64, or 56 bits if using
   BASE32.  The resulting instance name will be either 12 characters
   long with BASE64, allowing 54 advertisements in an 1500 byte mDNS
   message, or 16 characters long with BASE32, allowing 47
   advertisements per message.

   In the specification section, we will assume BASE64, and 48 bit
   proofs composed of the first 6 bytes of a SHA256 hash.





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3.2.4.  Direct Queries

   The preceding sections assume that the discovery is performed using
   the classic DNS-SD process, in which a query for all available
   "instance names" of a service provides a list of PTR records.  The
   discoverer will then select the instance names that correspond to its
   peers, and request the SRV and TXT records corresponding to the
   service instance, and then obtain the relevant A or AAAA records.
   This is generally required in DNS-SD because the instance names are
   not known in advance, but for the Private Discovery Service the
   instance names can be predicted, and a more efficient Direct Query
   method can be used.

   At a given time, the node engaged in discovery can predict the nonce
   that its peer will use, since that nonce is composed by rounding the
   current time.  The node can also compute the proofs that its peers
   might use, since it knows the nonce and the keys.  The node can thus
   build a list of instance names, and directly query the SRV records
   corresponding to these names.  If peers are present, they will answer
   directly.

   This "direct query" process will result in fewer network messages
   than the regular DNS-SD query process in some circumstances,
   depending on the number of peers per node and the number of nodes
   publishing the presence discovery service in the desired scope.

   When using mDNS, it is possible to pack multiple queries in a single
   broadcast message.  Using name compression and 12 characters per
   instance name, it is possible to pack 70 queries in a 1500 octet mDNS
   multicast message.  It is also possible to request unicast replies to
   the queries, resulting in significant efficiency gains in wireless
   networks.

3.3.  Private Discovery Service

   The Private Discovery Service discovery allows discovering a list of
   available paired devices, and verifying that either party knows the
   corresponding shared secret.  At that point, the querier can engage
   in a series of directed discoveries.

   We have considered defining an ad-hoc protocol for the private
   discovery service, but found that just using TLS would be much
   simpler.  The directed Private Discovery Service is just a regular
   DNS-SD service, accessed over TLS, using the encapsulation of DNS
   over TLS defined in [RFC7858].  The main difference with plain DNS
   over TLS is the need for an authentication based on pre-shared keys.





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   We assume that the pairing process has provided each pair of
   authorized client and server with a shared secret.  We can use that
   shared secret to provide mutual authentication of clients and servers
   using "Pre-Shared Key" authentication, as defined in [RFC4279] and
   incorporated in the latest version of TLS [I-D.ietf-tls-tls13].

   One difficulty is the reliance on a key identifier in the protocol.
   For example, in TLS 1.3 the PSK extension is defined as:

      opaque psk_identity<0..2^16-1>;

      struct {
          select (Role) {
              case client:
                  psk_identity identities<2..2^16-1>;

              case server:
                  uint16 selected_identity;
          }
      } PreSharedKeyExtension

   According to the protocol, the PSK identity is passed in clear text
   at the beginning of the key exchange.  This is logical, since server
   and clients need to identify the secret that will be used to protect
   the connection.  But if we used a static identifier for the key,
   adversaries could use that identifier to track server and clients.
   The solution is to use a time-varying identifier, constructed exactly
   like the "proof" described in Section 3.2, by concatenating a nonce
   and the hash of the nonce with the shared secret.

3.3.1.  A Note on Private DNS Services

   Our solution uses a variant of the DNS over TLS protocol [RFC7858]
   defined by the DNS Private Exchange working group (DPRIVE).  DPRIVE
   further published an UDP variant, DNS over DTLS [RFC8094], which
   would also be a candidate.

   DPRIVE and Private Discovery, however, solve two somewhat different
   problems.  While DPRIVE is concerned with the confidentiality of DNS
   transactions addressing the problems outlined in [RFC7626], DPRIVE
   does not address the confidentiality or privacy issues with
   publication of services, and is not a direct solution to DNS-SD
   privacy:

   o  Discovery queries are scoped by the domain name within which
      services are published.  As nodes move and visit arbitrary
      networks, there is no guarantee that the domain services for these
      networks will be accessible using DNS over TLS or DNS over DTLS.



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   o  Information placed in the DNS is considered public.  Even if the
      server does support DNS over TLS, third parties will still be able
      to discover the content of PTR, SRV and TXT records.

   o  Neither DNS over TLS nor DNS over DTLS applies to mDNS.

   In contrast, we propose using mutual authentication of the client and
   server as part of the TLS solution, to ensure that only authorized
   parties learn the presence of a service.

3.4.  Randomized Host Names

   Instead of publishing their actual host names in the SRV records,
   nodes could publish randomized host names.  That is the solution
   argued for in [RFC8117].

   Randomized host names will prevent some of the tracking.  Host names
   are typically not visible by the users, and randomizing host names
   will probably not cause much usability issues.

3.5.  Timing of Obfuscation and Randomization

   It is important that the obfuscation of instance names is performed
   at the right time, and that the obfuscated names change in synchrony
   with other identifiers, such as MAC Addresses, IP Addresses or host
   names.  If the randomized host name changed but the instance name
   remained constant, an adversary would have no difficulty linking the
   old and new host names.  Similarly, if IP or MAC addresses changed
   but host names remained constant, the adversary could link the new
   addresses to the old ones using the published name.

   The problem is handled in [RFC8117], which recommends to pick a new
   random host name at the time of connecting to a new network.  New
   instance names for the Private Discovery Services should be composed
   at the same time.

4.  Private Discovery Service Specification

   The proposed solution uses the following components:

   o  Host name randomization to prevent tracking.

   o  Device pairing yielding pairwise shared secrets.

   o  A Private Discovery Server (PDS) running on each host.

   o  Discovery of the PDS instances using DNS-SD.




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   These components are detailed in the following subsections.

4.1.  Host Name Randomization

   Nodes publishing services with DNS-SD and concerned about their
   privacy MUST use a randomized host name.  The randomized name MUST be
   changed when network connectivity changes, to avoid the correlation
   issues described in Section 3.5.  The randomized host name MUST be
   used in the SRV records describing the service instance, and the
   corresponding A or AAAA records MUST be made available through DNS or
   mDNS, within the same scope as the PTR, SRV and TXT records used by
   DNS-SD.

   If the link-layer address of the network connection is properly
   obfuscated (e.g. using MAC Address Randomization), the Randomized
   Host Name MAY be computed using the algorithm described in section
   3.7 of [RFC7844].  If this is not possible, the randomized host name
   SHOULD be constructed by simply picking a 48 bit random number
   meeting the Randomness Requirements for Security expressed in
   [RFC4075], and then use the hexadecimal representation of this number
   as the obfuscated host name.

4.2.  Device Pairing

   Nodes that want to leverage the Private Directory Service for private
   service discovery among peers MUST share a secret with each of these
   peers.  Each shared secret MUST be a 256 bit randomly chosen number.
   We RECOMMEND using the pairing mechanism proposed in
   [I-D.ietf-dnssd-pairing] to establish these secrets.

4.3.  Private Discovery Server

   A Private Discovery Server (PDS) is a minimal DNS server running on
   each host.  Its task is to offer resource records corresponding to
   private services only to authorized peers.  These peers MUST share a
   secret with the host (see Section 4.2).  To ensure privacy of the
   requests, the service is only available over TLS [RFC5246], and the
   shared secrets are used to mutually authenticate peers and servers.

   The Private Name Server SHOULD support DNS push notifications
   [I-D.ietf-dnssd-push], e.g. to facilitate an up-to-date contact list
   in a chat application without polling.

4.3.1.  Establishing TLS Connections

   The PDS MUST only answer queries via DNS over TLS [RFC7858] and MUST
   use a PSK authenticated TLS handshake [RFC4279].  The client and
   server SHOULD negotiate a forward secure cipher suite such as DHE-PSK



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   or ECDHE-PSK when available.  The shared secret exchanged during
   pairing MUST be used as PSK.  To guarantee interoperability,
   implementations of the Private Name Server MUST support
   TLS_PSK_WITH_AES_256_GCM_SHA384.

   When using the PSK based authentication, the "psk_identity" parameter
   identifying the pre-shared key MUST be identical to the "Instance
   Identifier" defined in Section 4.4, i.e. 24 bit nonce and 48 bit
   proof encoded in BASE64 as 12 character string.  The server will use
   the pairing key associated with this instance identifier.

4.4.  Publishing Private Discovery Service Instances

   Nodes that provide the Private Discovery Service SHOULD advertise
   their availability by publishing instances of the service through
   DNS-SD.

   The DNS-SD service type for the Private Discovery Service is
   "_pds._tcp".

   Each published instance describes one server and one pairing.  In the
   case where a node manages more than one pairing, it should publish as
   many instances as necessary to advertise the PDS to all paired peers.

   Each instance name is composed as follows:

      pick a 24 bit nonce, set to the 20 most significant bits of the
      32 bit Unix GMT time padded with 4 zeroes.

         For example, on August 22, 2017 at 20h 4 min and 54 seconds
         international time, the Unix 32 bit time had the
         hexadecimal value 0x599C8E68. The corresponding nonce
         would be set to the 24 bits: 0x599C80.

      compute a 48 bit proof:
         proof = first 48 bits of HASH(<nonce>|<pairing key>)

      set the 72 bit binary identifier as the concatenation
      of nonce and proof

      set instance_name = BASE64(binary identifier)

   In this formula, HASH SHOULD be the function SHA256 defined in
   [RFC4055], and BASE64 is defined in section 6.8 of [RFC2045].  The
   concatenation of a 24 bit nonce and 48 bit proof result in a 72 bit
   string.  The BASE64 conversion is 12 characters long per [RFC6763].





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4.5.  Discovering Private Discovery Service Instances

   Nodes that wish to discover Private Discovery Service Instances
   SHOULD issue a DNS-SD discovery request for the service type
   "_pds._tcp".  They MAY, as an alternative, use the Direct Discovery
   procedure defined in Section 4.6.  When using the Direct Discovery
   procedure over mDNS, nodes SHOULD always set the QU-bit (unicast
   response requested, see [RFC6762] Section 5.4) because responses
   related to a "_pds._tcp" instance are only relevant for the querying
   node itself.

   When nodes send a DNS-SD discovery request, they will receive in
   response a series of PTR records, each providing the name of one of
   the instances present in the scope.

   For each time interval, the querier SHOULD pre-calculate a hash table
   mapping instance names to pairings according to the following
   conceptual algorithm:

     nonce = 20 bit rounded time stamp of the \
       respective next time interval padded to \
       24 bits with four zeroes
     for each available pairing
       retrieve the key Xj of pairing number j
       compute F = first 48 bits of hash(nonce, Xj)
       construct the binary instance_name as described \
         in the previous section
       instance_names[nonce][instance_name] = Xj;

   The querier SHOULD store the hash tables for the previous, the
   current, and the next time interval.

   The querier SHOULD examine each instance to see whether it
   corresponds to one of its available pairings, according to the
   following conceptual algorithm:
















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      for each received instance_name:
         convert the instance name to binary using BASE64
         if the conversion fails,
            discard the instance.
         if the binary instance length is not 72 bits,
            discard the instance.

         nonce = first 24 bits of binary.

         Check that the 4 least significant bits of the nonce
         have the value 0, and that the 20 most significant
         bits of the nonce match the first 20 bits of
         the current time, or the previous interval (20 bit number
         minus 1) if the current interval is less than half over,
         or the next interval (20 bit number plus 1) if the
         current interval is more than half over. If the
         nonce does not match an acceptable value, discard
         the instance.

         if ((Xj = instance_names[nonce][instance_name]) != null)
           mark the pairing number j as available

   The check of the current time is meant to mitigate replay attacks,
   while not mandating a time synchronization precision better than 15
   minutes.

   Once a pairing has been marked available, the querier SHOULD try
   connecting to the corresponding instance, using the selected key.
   The connection is likely to succeed, but it MAY fail for a variety of
   reasons.  One of these reasons is the probabilistic nature of the
   proof, which entails a small chance of "false positive" match.  This
   will occur if the hash of the nonce with two different keys produces
   the same result.  In that case, the TLS connection will fail with an
   authentication error or a decryption error.

4.6.  Direct Discovery of Private Discovery Service Instances

   Nodes that wish to discover Private Discovery Service Instances MAY
   use the following Direct Discovery procedure instead of the regular
   DNS-SD Discovery explained in Section 4.5.

   To perform Direct Discovery, nodes should compose a list of Private
   Discovery Service Instances Names.  There will be one name for each
   pairing available to the node.  The Instance name for each name will
   be composed of a nonce and a proof, using the algorithm specified in
   Section 4.4.





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   The querier will issue SRV record queries for each of these names.
   The queries will only succeed if the corresponding instance is
   present, in which case a pairing is discovered.  After that, the
   querier SHOULD try connecting to the corresponding instance, as
   explained in Section 4.4.

4.7.  Using the Private Discovery Service

   Once instances of the Private Discovery Service have been discovered,
   peers can establish TLS connections and send DNS requests over these
   connections, as specified in DNS-SD.

5.  Security Considerations

   This document specifies a method for protecting the privacy of nodes
   that offer and query for services.  This is especially useful when
   operating in a public space.  Hiding the identity of the publishing
   nodes prevents some forms of "targeting" of high value nodes.
   However, adversaries can attempt various attacks to break the
   anonymity of the service, or to deny it.  A list of these attacks and
   their mitigations are described in the following sections.

5.1.  Attacks Against the Pairing System

   There are a variety of attacks against pairing systems, which may
   result in compromised pairing secrets.  If an adversary manages to
   acquire a compromised key, the adversary will be able to perform
   private service discovery according to Section 4.5.  This will allow
   tracking of the service.  The adversary will also be able to discover
   which private services are available for the compromised pairing.

   Attacks on pairing systems are detailed in [I-D.ietf-dnssd-pairing].

5.2.  Denial of Discovery of the Private Discovery Service

   The algorithm described in Section 4.5 scales as O(M*N), where M is
   the number of pairings per node and N is the number of nodes in the
   local scope.  Adversaries can attack this service by publishing
   "fake" instances, effectively increasing the number N in that scaling
   equation.

   Similar attacks can be mounted against DNS-SD: creating fake
   instances will generally increase the noise in the system and make
   discovery less usable.  Private Discovery Service discovery SHOULD
   use the same mitigations as DNS-SD.

   The attack could be amplified if the clients needed to compute proofs
   for all the nonces presented in Private Discovery Service Instance



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   names.  This is mitigated by the specification of nonces as rounded
   time stamps in Section 4.5.  If we assume that timestamps must not be
   too old, there will be a finite number of valid rounded timestamps at
   any time.  Even if there are many instances present, they would all
   pick their nonces from this small number of rounded timestamps, and a
   smart client will make sure that proofs are only computed once per
   valid time stamp.

5.3.  Replay Attacks Against Discovery of the Private Discovery Service

   Adversaries can record the service instance names published by
   Private Discovery Service instances, and replay them later in
   different contexts.  Peers engaging in discovery can be misled into
   believing that a paired server is present.  They will attempt to
   connect to the absent peer, and in doing so will disclose their
   presence in a monitored scope.

   The binary instance identifiers defined in Section 4.4 start with 24
   bits encoding the most significant bits of the "UNIX" time.  In order
   to protect against replay attacks, clients SHOULD verify that this
   time is reasonably recent, as specified in Section 4.5.

5.4.  Denial of Private Discovery Service

   The Private Discovery Service is only available through a mutually
   authenticated TLS connection, which provides state-of-the-art
   protection mechanisms.  However, adversaries can mount a denial of
   service attack against the service.  In the absence of shared
   secrets, the connections will fail, but the servers will expend some
   CPU cycles defending against them.

   To mitigate such attacks, nodes SHOULD restrict the range of network
   addresses from which they accept connections, matching the expected
   scope of the service.

   This mitigation will not prevent denial of service attacks performed
   by locally connected adversaries; but protecting against local denial
   of service attacks is generally very difficult.  For example, local
   attackers can also attack mDNS and DNS-SD by generating a large
   number of multicast requests.

5.5.  Replay Attacks against the Private Discovery Service

   Adversaries may record the PSK Key Identifiers used in successful
   connections to a private discovery service.  They could attempt to
   replay them later against nodes advertising the private service at
   other times or at other locations.  If the PSK identifier is still
   valid, the server will accept the TLS connection, and in doing so



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   will reveal being the same server observed at a previous time or
   location.

   The PSK identifiers defined in Section 4.3.1 start with the 24 most
   significant bits of the "UNIX" time.  In order to mitigate replay
   attacks, servers SHOULD verify that this time is reasonably recent,
   and fail the connection if it is too old, or if it occurs too far in
   the future.

   The processing of timestamps is however affected by the accuracy of
   computer clocks.  If the check is too strict, reasonable connections
   could fail.  To further mitigate replay attacks, servers MAY record
   the list of valid PSK identifiers received in a recent past, and fail
   connections if one of these identifiers is replayed.

5.6.  Replay attacks and clock synchronization

   The mitigation of replay attacks relies on verification of the time
   encoded in the nonce.  This verification assumes that the hosts
   engaged in discovery have a reasonably accurate sense of the current
   time.

5.7.  Fingerprinting the number of published instances

   Adversaries could monitor the number of instances published by a
   particular device, which in the absence of mitigations will reflect
   the number of pairings established by that device.  This number will
   probably vary between 1 and maybe 100, providing the adversary with
   maybe 6 or 7 bits of input in a fingerprinting algorithm.

   Devices MAY protect against this fingerprinting by publishing a
   number of "fake" instances in addition to the real ones.  The fake
   instance identifiers will contain the same nonce as the genuine
   instance identifiers, and random bits instead of the proof.  Peers
   should be able to quickly discard these fake instances, as the proof
   will not match any of the values that they expect.  One plausible
   padding strategy is to ensure that the total number of published
   instances, either fake or genuine, matches one of a few values such
   as 16, 32, 64, or higher powers of 2.

6.  IANA Considerations

   This draft does not require any IANA action.








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7.  Acknowledgments

   This draft results from initial discussions with Dave Thaler, and
   encouragements from the DNS-SD working group members.  We would like
   to thank Stephane Bortzmeyer and Ted Lemon for their detailed reviews
   of the working draft.

8.  References

8.1.  Normative References

   [RFC2045]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part One: Format of Internet Message
              Bodies", RFC 2045, DOI 10.17487/RFC2045, November 1996,
              <https://www.rfc-editor.org/info/rfc2045>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC4055]  Schaad, J., Kaliski, B., and R. Housley, "Additional
              Algorithms and Identifiers for RSA Cryptography for use in
              the Internet X.509 Public Key Infrastructure Certificate
              and Certificate Revocation List (CRL) Profile", RFC 4055,
              DOI 10.17487/RFC4055, June 2005,
              <https://www.rfc-editor.org/info/rfc4055>.

   [RFC4075]  Kalusivalingam, V., "Simple Network Time Protocol (SNTP)
              Configuration Option for DHCPv6", RFC 4075,
              DOI 10.17487/RFC4075, May 2005,
              <https://www.rfc-editor.org/info/rfc4075>.

   [RFC4279]  Eronen, P., Ed. and H. Tschofenig, Ed., "Pre-Shared Key
              Ciphersuites for Transport Layer Security (TLS)",
              RFC 4279, DOI 10.17487/RFC4279, December 2005,
              <https://www.rfc-editor.org/info/rfc4279>.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,
              <https://www.rfc-editor.org/info/rfc5246>.

   [RFC6763]  Cheshire, S. and M. Krochmal, "DNS-Based Service
              Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013,
              <https://www.rfc-editor.org/info/rfc6763>.





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8.2.  Informative References

   [I-D.ietf-dnssd-pairing]
              Huitema, C. and D. Kaiser, "Device Pairing Using Short
              Authentication Strings", draft-ietf-dnssd-pairing-02 (work
              in progress), July 2017.

   [I-D.ietf-dnssd-push]
              Pusateri, T. and S. Cheshire, "DNS Push Notifications",
              draft-ietf-dnssd-push-12 (work in progress), July 2017.

   [I-D.ietf-tls-tls13]
              Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", draft-ietf-tls-tls13-21 (work in progress),
              July 2017.

   [K17]      Kaiser, D., "Efficient Privacy-Preserving
              Configurationless Service Discovery Supporting Multi-Link
              Networks", 2017,
              <http://nbn-resolving.de/urn:nbn:de:bsz:352-0-422757>.

   [KW14a]    Kaiser, D. and M. Waldvogel, "Adding Privacy to Multicast
              DNS Service Discovery", DOI 10.1109/TrustCom.2014.107,
              2014, <http://ieeexplore.ieee.org/xpl/
              articleDetails.jsp?arnumber=7011331>.

   [KW14b]    Kaiser, D. and M. Waldvogel, "Efficient Privacy Preserving
              Multicast DNS Service Discovery",
              DOI 10.1109/HPCC.2014.141, 2014,
              <http://ieeexplore.ieee.org/xpl/
              articleDetails.jsp?arnumber=7056899>.

   [RFC1033]  Lottor, M., "Domain Administrators Operations Guide",
              RFC 1033, DOI 10.17487/RFC1033, November 1987,
              <https://www.rfc-editor.org/info/rfc1033>.

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
              <https://www.rfc-editor.org/info/rfc1034>.

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <https://www.rfc-editor.org/info/rfc1035>.

   [RFC2782]  Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
              specifying the location of services (DNS SRV)", RFC 2782,
              DOI 10.17487/RFC2782, February 2000,
              <https://www.rfc-editor.org/info/rfc2782>.



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   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
              <https://www.rfc-editor.org/info/rfc4648>.

   [RFC6762]  Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762,
              DOI 10.17487/RFC6762, February 2013,
              <https://www.rfc-editor.org/info/rfc6762>.

   [RFC7626]  Bortzmeyer, S., "DNS Privacy Considerations", RFC 7626,
              DOI 10.17487/RFC7626, August 2015,
              <https://www.rfc-editor.org/info/rfc7626>.

   [RFC7844]  Huitema, C., Mrugalski, T., and S. Krishnan, "Anonymity
              Profiles for DHCP Clients", RFC 7844,
              DOI 10.17487/RFC7844, May 2016,
              <https://www.rfc-editor.org/info/rfc7844>.

   [RFC7858]  Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D.,
              and P. Hoffman, "Specification for DNS over Transport
              Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May
              2016, <https://www.rfc-editor.org/info/rfc7858>.

   [RFC8094]  Reddy, T., Wing, D., and P. Patil, "DNS over Datagram
              Transport Layer Security (DTLS)", RFC 8094,
              DOI 10.17487/RFC8094, February 2017,
              <https://www.rfc-editor.org/info/rfc8094>.

   [RFC8117]  Huitema, C., Thaler, D., and R. Winter, "Current Hostname
              Practice Considered Harmful", RFC 8117,
              DOI 10.17487/RFC8117, March 2017,
              <https://www.rfc-editor.org/info/rfc8117>.

Authors' Addresses

   Christian Huitema
   Private Octopus Inc.
   Friday Harbor, WA  98250
   U.S.A.

   Email: huitema@huitema.net
   URI:   http://privateoctopus.com/










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   Daniel Kaiser
   University of Konstanz
   Konstanz  78457
   Germany

   Email: daniel.kaiser@uni-konstanz.de













































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