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
Device Pairing Using Short Authentication Strings
draft-ietf-dnssd-pairing-03
Abstract
This document proposes a device pairing mechanism that establishes a
relation between two devices by agreeing on a secret and manually
verifying the secret's authenticity using an SAS (short
authentication string). Pairing has to be performed only once per
pair of devices, as for a re-discovery at any later point in time,
the exchanged secret can be used for mutual authentication.
The proposed pairing method is suited for each application area where
human operated devices need to establish a relation that allows
configurationless and privacy preserving re-discovery at any later
point in time. Since privacy preserving applications are the main
suitors, we especially care about privacy.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on March 14, 2018.
Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Document Organization . . . . . . . . . . . . . . . . . . 3
2. Protocol Specification . . . . . . . . . . . . . . . . . . . 4
2.1. Discovery . . . . . . . . . . . . . . . . . . . . . . . . 4
2.2. Agreement on a Shared Secret . . . . . . . . . . . . . . 5
2.3. Authentication . . . . . . . . . . . . . . . . . . . . . 6
3. Optional Use of QR Codes . . . . . . . . . . . . . . . . . . 8
3.1. Discovery Using QR Codes . . . . . . . . . . . . . . . . 8
3.2. Agreement with QR Codes . . . . . . . . . . . . . . . . . 9
3.3. Authentication with QR Codes . . . . . . . . . . . . . . 9
4. Security Considerations . . . . . . . . . . . . . . . . . . . 9
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
7.1. Normative References . . . . . . . . . . . . . . . . . . 10
7.2. Informative References . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
To engage in secure and privacy preserving communication, hosts need
to differentiate between authorized peers, which must both know about
the host's presence and be able to decrypt messages sent by the host,
and other peers, which must not be able to decrypt the host's
messages and ideally should not obtain information that could be used
to identify the host. The necessary relation between host and peer
can be established by a centralized service, e.g. a certificate
authority, by a web of trust, e.g. PGP, or -- without using global
identities -- by device pairing.
This document proposes a device pairing mechanism that provides human
operated devices with pairwise authenticated secrets, allowing mutual
automatic re-discovery at any later point in time along with mutual
private authentication. We especially care about privacy and user-
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friendliness. This pairing system can provide the pairing secrets
used in DNSSD Privacy Extensions [I-D.ietf-dnssd-privacy].
The proposed pairing mechanism consists of three steps needed to
establish a relationship between a host and a peer:
1. Discovering the peer device. The host needs a means to discover
network parameters necessary to establish a connection to the
peer. During this discovery process, neither the host nor the
peer must disclose its presence.
2. Agreeing on pairing data. The devices have to agree on pairing
data, which can be used by both parties at any later point in
time to generate identifiers for re-discovery and to prove the
authenticity of the pairing. The pairing data can e.g. be a
shared secret agreed upon via a Diffie-Hellman key exchange.
3. Authenticating pairing data. Since in most cases the messages
necessary to agree upon pairing data are send over an insecure
channel, means that guarantee the authenticity of these messages
are necessary; otherwise the pairing data is in turn not suited
as a means for a later proof of authenticity. For the proposed
pairing mechanism we use manual authentication involving an SAS
(short authentication string) to proof the authenticity of the
pairing data.
The design of this protocol is based on the analysis of pairing
protocols issues presented in [I-D.ietf-dnssd-pairing-info] and in
[K17].
Many pairing scenarios involve cell phones equipped with cameras
capable of reading a QR code. In these scenarios, scanning QR codes
might be more user friendly than selecting names or reading short
authentication strings from on screen menus. An optional use of QR
codes in pairing protocols is presented is Section 3.
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].
1.2. Document Organization
NOTE TO RFC EDITOR: remove or rewrite this section before
publication.
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The original version of this document was organized in two parts.
The first part presented the pairing need, the list of requirements
that shall be met. This first part was informational in nature. The
second part composed the actual specification of the protocol.
In his early review, Steve Kent observed that the style of the first
part seems inappropriate for a standards track document, and
suggested that the two parts should be split into two documents, the
first part becoming an informational document, and the second
focusing on standard track specification of the protocol, making
reference to the informational document as appropriate.
The DNS-SD working group approved this split during its meeting in
Prague in July 2017. This version of the document implements the
split, only retaining the specification part.
2. Protocol Specification
In the proposed pairing protocol, we will consider the device that
initiates the pairing as the "client" and the device that responds as
the "server". The server will publish a "pairing service". The
client will discover the service instance during the discovery phase,
as explained in Section 2.1. The pairing service itself is specified
in Section 2.3.
We divide pairing in three parts: discovery, agreement, and
authentication, detailed in the following subsections.
2.1. Discovery
The goal of the discovery phase is establishing a connection, which
is later used to exchange the pairing data between the two devices
that are about to be paired in an IP network without any prior
knowledge and without publishing any private information.
When the pairing service starts, the server will advertise the
pairing service according to DNS-SD [RFC6763] over mDNS [RFC6762].
In conformance with DNS-SD, the service is described by an SRV record
and by and empty TXT record. These records will be organized as
follows:
1. The pairing service is identified in DNS-SD as "_pairing._tcp".
2. The instance name will be a text chosen by the server. It MAY be
a random string if the server does not want to advertise its
identity in the local environment, or the user friendly name of
the server in other cases.
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3. The priority and weight fields of the SRV record SHOULD be set
according to [RFC6763].
4. The host name MUST be set to the host name advertised by the
server in mDNS. The server MAY use a randomized host name as
explained in [I-D.ietf-dnssd-privacy], provided that this name is
properly published in mDNS.
5. The port number MUST be set to the number at which the server is
listening for the pairing service. This port number SHOULD be
randomly picked by the server.
The discovery proceeds as follows:
1. The server advertises an instance of the above described pairing
service and displays its instance name on the server's screen.
2. The client discovers all the instances of the pairing service
available on the local network. This may result in the discovery
of several instance names.
3. Among these available instance names, the client's user selects
the name that matches the name displayed by the server.
4. Per DNS-SD, the client then retrieves the SRV record of the
selected instance, retrieves the corresponding server's A (or
AAAA) record, and establishes the connection.
2.2. Agreement on a Shared Secret
Once the server has been selected at the end of the discovery phase,
the client connects to it without further user intervention. Client
and server use this connection for exchanging data that allows them
to agree on a shared secret by using TLS and a key exporter.
Devices implementing the service MUST support TLS 1.2 [RFC5246], and
MAY negotiate TLS 1.3 when it becomes available. When using TLS, the
client and server MUST negotiate a ciphersuite providing forward
secrecy (PFS), and strong encryption (256 bits symmetric key). All
implementations using TLS 1.2 MUST be able to negotiate the cipher
suite TLS_DH_anon_WITH_AES_256_CBC_SHA256.
Once the TLS connection has been established, each party extracts the
pairing secret S_p from the connection context per [RFC5705], using
the following parameters:
Disambiguating label string: "PAIRING SECRET"
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Context value: empty.
Length value: 32 bytes (256 bits).
The secret "S_p" will be authenticated in the authentication part of
the protocol.
2.3. Authentication
The pairing protocol implemented on top of TLS allows the users to
authenticate the shared secret established in the "Agreement" phase,
and to minimize the risk of interference by a third party like a
"man-in-the-middle". The pairing protocol is built using TLS. The
following description uses the presentation language defined in
section 4 of [RFC5246]. The protocol uses five message types,
defined in the following enum:
enum {
ClientHash(1),
ServerRandom(2),
ClientRandom(3),
ServerSuccess(4),
ClientSuccess(5)
} PairingMessageType;
Once S_p has been obtained, the client picks a random number R_c,
exactly 32 bytes long. The client then selects a hash algorithm,
which MUST be the same algorithm as negotiated for building the PRF
in the TLS connection. The client then computes the hash value H_c
as:
H_c = HMAC_hash(S_p, R_c)
Where "HMAC_hash" is the HMAC function constructed with the
selected algorithm.
The client transmits the selected hash function and the computed
value of H_c in the Client Hash message, over the TLS connection:
struct {
PairingMessageType messageType;
hashAlgorithm hash;
uint8 hashLength;
opaque H_c[hashLength];
} ClientHashMessage;
messageType: Set to "ClientHash".
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hash: The code of the selected hash algorithm, per definition of
HashAlgorithm in section 7.4.1.1.1 of [RFC5246].
hashLength: The length of the hash H_c, which MUST be consistent
with the selected algorithm "hash".
H_c: The value of the client hash.
Upon reception of this message, the server stores its value. The
server picks a random number R_s, exactly 32 bytes long, and
transmits it to the client in the server random message, over the TLS
connection:
struct {
PairingMessageType messageType;
opaque R_s[32];
} ServerRandomMessage;
messageType Set to "ServerRandom".
R_s: The value of the random number chosen by the server.
Upon reception of this message, the client discloses its own random
number by transmitting the client random message:
struct {
PairingMessageType messageType;
opaque R_c[32];
} ClientRandomMessage;
messageType Set to "ClientRandom".
R_c: The value of the random number chosen by the client.
Upon reception of this message, the server verifies that the number
R_c hashes to the previously received value H_c. If the number does
not match, the server MUST abandon the pairing attempt and abort the
TLS connection.
At this stage, both client and server can compute the short hash SAS
as:
SAS = first 20 bits of HMAC_hash(S_p, R_c || R_s)
Where "HMAC_hash" is the HMAC function constructed with the hash
algorithm selected by the client in the ClientHashMessage.
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Both client and server display the SAS as a 7 digit decimal integer,
including leading zeroes, and ask the user to compare the values. If
the SASes match, each user enters an agreement, for example by
pressing a button labeled "OK", which results in the pairing being
remembered. If they do not match, each user should cancel the
pairing, for example by pressing a button labeled "CANCEL".
If the values do match and both users agree, the protocol continues
with the exchange of names, both server and client announcing their
own preferred name in a Success message
struct {
PairingMessageType messageType;
uint8 nameLength;
opaque name[nameLength];
} ClientSuccessMessage;
messageType: Set to "ClientSuccess" if transmitted by the client,
"ServerSuccess" if by the server.
nameLength: The length of the string encoding the selected name.
name: The selected name of the client or the server, encoded as a
string of UTF8 characters.
After receiving these messages, client and servers can orderly close
the TLS connection, terminating the pairing exchange.
3. Optional Use of QR Codes
When QR codes are supported, the discovery process can be independent
of DNS-SD, because QR codes allow the transmission of a sufficient
amount of data. The agreement process can also be streamlined by the
scanning of a second QR code.
3.1. Discovery Using QR Codes
If QR code scanning is available as out-of-band channel, the
discovery data is directly transmitted via QR codes instead of DNS-SD
over mDNS. Leveraging QR codes, the discovery proceeds as follows:
1. The server displays a QR code containing the connection data
otherwise found in the SRV and A or AAAA records: IPv4 or IPv6
address, port number, and optionally host name.
2. The client scans the QR code retrieving the necessary information
for establishing a connection to the server.
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[[TODO: We should precisely specify the data layout of this QR code.
It could either be the wire format of the corresponding resource
records (which would be easier for us), or a more efficient
representation. If we chose the wire format, we could use a fixed
name as instance name.]]
3.2. Agreement with QR Codes
When QR codes are available, the agreement on a shared secret
proceeds exactly as in the general case.
3.3. Authentication with QR Codes
The availability of QR codes does not change the required network
messages or the computation of the SAS, which will performed exactly
as specified in Section 2.3, but when QR codes are supported, the SAS
may also be represented as QR code.
In the general case, both client and server display the SAS as a
decimal integer, and ask the user to compare the values. If the
server supports QR codes, the server displays a QR code encoding the
decimal string representation of the SAS. If the client is capable
of scanning QR codes, it may scan the value and compare it to the
locally computed value.
Once user agreement has been obtained, the protocol continues as in
the general case presented in Section 2.3.
4. Security Considerations
We need to consider two types of attacks against a pairing system:
attacks that occur during the establishment of the pairing relation,
and attacks that occur after that establishment.
During the establishment of the pairing system, we are concerned with
privacy attacks and with MitM attacks. Privacy attacks reveal the
existence of a pairing between two devices, which can be used to
track graphs of relations. MitM attacks result in compromised
pairing keys. The discovery procedures specified in Section 2.1 and
the authentication procedures specified in Section 2.3 are
specifically designed to mitigate such attacks, assuming that the
client and user are in close, physical proximity and thus a human
user can visually acquire and verify the pairing information.
The establishment of the pairing results in the creation of a shared
secret. After the establishment of the pairing relation, attackers
who compromise one of the devices could access the shared secret.
This will enable them to either track or spoof the devices. To
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mitigate such attacks, nodes MUST store the secret safely, and MUST
be able to quickly revoke a compromised pairing.
5. IANA Considerations
This draft does not require any IANA action.
6. Acknowledgments
We would like to thank Steve Kent and Ted Lemon for their detailed
reviews of this document, and for their advice on how to improve it.
7. References
7.1. Normative References
[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>.
[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>.
[RFC5705] Rescorla, E., "Keying Material Exporters for Transport
Layer Security (TLS)", RFC 5705, DOI 10.17487/RFC5705,
March 2010, <https://www.rfc-editor.org/info/rfc5705>.
[RFC6762] Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762,
DOI 10.17487/RFC6762, February 2013,
<https://www.rfc-editor.org/info/rfc6762>.
[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>.
7.2. Informative References
[I-D.ietf-dnssd-pairing-info]
Kaiser, D. and C. Huitema, "Device Pairing Design Issues",
draft-ietf-dnssd-pairing-info-00 (work in progress),
September 2017.
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[I-D.ietf-dnssd-privacy]
Huitema, C. and D. Kaiser, "Privacy Extensions for DNS-
SD", draft-ietf-dnssd-privacy-02 (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>.
Authors' Addresses
Christian Huitema
Private Octopus Inc.
Friday Harbor, WA 98250
U.S.A.
Email: huitema@huitema.net
Daniel Kaiser
University of Konstanz
Konstanz 78457
Germany
Email: daniel.kaiser@uni-konstanz.de
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