Network Working Group C. Huitema
Internet-Draft
Intended status: Standards Track D. Kaiser
Expires: April 30, 2017 University of Konstanz
October 27, 2016
Device Pairing Using Short Authentication Strings
draft-ietf-dnssd-pairing-00.txt
Abstract
This document proposes a device pairing mechanism that establishes a
relationship 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
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Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements . . . . . . . . . . . . . . . . . . . . . . 3
2. Problem Statement and Requirements . . . . . . . . . . . . . 4
2.1. Secure Pairing Over Internet Connections . . . . . . . . 4
2.2. Identity Assurance . . . . . . . . . . . . . . . . . . . 4
2.3. Adequate User Interface . . . . . . . . . . . . . . . . . 4
2.3.1. Short PIN Proved Inadequate . . . . . . . . . . . . . 5
2.3.2. Push Buttons Just Work, But Are Insecure . . . . . . 6
2.3.3. Short Range Communication . . . . . . . . . . . . . . 6
2.3.4. Short Authentication Strings . . . . . . . . . . . . 7
2.4. Resist Cryptographic Attacks . . . . . . . . . . . . . . 7
2.5. Privacy Requirements . . . . . . . . . . . . . . . . . . 10
2.6. Using TLS . . . . . . . . . . . . . . . . . . . . . . . . 11
2.7. QR codes . . . . . . . . . . . . . . . . . . . . . . . . 11
3. Design of the Pairing Mechanism . . . . . . . . . . . . . . . 12
3.1. Discovery . . . . . . . . . . . . . . . . . . . . . . . . 13
3.2. Agreement . . . . . . . . . . . . . . . . . . . . . . . . 13
3.3. Authentication . . . . . . . . . . . . . . . . . . . . . 14
3.4. Intra User Pairing . . . . . . . . . . . . . . . . . . . 14
3.5. Pairing Data Synchronization . . . . . . . . . . . . . . 14
3.6. Public Authentication Keys . . . . . . . . . . . . . . . 14
4. Solution . . . . . . . . . . . . . . . . . . . . . . . . . . 15
4.1. Discovery . . . . . . . . . . . . . . . . . . . . . . . . 15
4.2. Agreement and Authentication . . . . . . . . . . . . . . 15
5. Security Considerations . . . . . . . . . . . . . . . . . . . 18
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 18
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 18
8.1. Normative References . . . . . . . . . . . . . . . . . . 18
8.2. Informative References . . . . . . . . . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 20
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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 be aware of the host's presence. The
necessary relationship 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-
friendliness.
The proposed pairing mechanism consists of three steps needed to
establish a relationship between a host and a peer:
1. Discovery of 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. Authenticate 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 interaction involving an SAS
(short authentication string) to proof the authenticity of the
pairing data.
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].
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2. Problem Statement and Requirements
The general pairing requirement is easy to state: establish a trust
relation between two entities in a secure manner. But details
matter, and in this section we explore the detailed requirements that
guide our design.
2.1. Secure Pairing Over Internet Connections
Many pairing protocols have already been developed, in particular for
the pairing of devices over specific wireless networks. For example,
the current Bluetooth specifications include a pairing protocol that
has evolved over several revisions towards better security and
usability [BTLEPairing]. The Wi-Fi Alliance defined the Wi-Fi
Protected Setup process to ease the setup of security-enabled Wi-Fi
networks in home and small office environments [WPS]. Other wireless
standards have defined or are defining similar protocols, tailored to
specific technologies.
This specification defines a pairing protocol that is independent of
the underlying technology. We simply make the hypothesis that the
two parties engaged in the pairing can discover each other and then
establish connections over IP in order to agree a shared secret.
[[TODO: Should we support certificates besides a shared secret?]]
2.2. Identity Assurance
The parties in the pairing must be able to identify each other. To
put it simply, if Alice believes that she is establishing a pairing
with Bob, she must somehow ensure that the pairing is actually
established with Bob, and not with some interloper like Eve or
Nessie. Providing this assurance requires designing both the
protocol and the user interface (UI) with care.
Consider for example an attack in which Eve tricks Alice into
engaging in a pairing process while pretending to be Bob. Alice must
be able to discover that something is wrong, and refuse to establish
the pairing. The parties engaged in the pairing must at least be
able to verify their identities, respectively.
2.3. Adequate User Interface
Because the pairing protocol is executed without prior knowledge, it
is typically vulnerable to "Man-in-the-middle" attacks. While Alice
is trying to establish a pairing with Bob, Eve positions herself in
the middle. Instead of getting a pairing between Alice and Bob, both
Alice and Bob get paired with Eve. This requires specific features in
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the protocol to detect man-in-the-middle attacks, and if possible
resist them. The reference [NR11] analyzes the various proposals to
solve this problem, and in this document, we present a layman
description of these issues in Section 2.4. The various protocols
proposed in the literature impose diverse constraints on the UI
interface, which we will review here.
2.3.1. Short PIN Proved Inadequate
The initial Bluetooth pairing protocol relied on a four digit PIN,
displayed by one of the devices to be paired. The user would read
that PIN and provide it to the other device. The PIN would then be
used in a Password Authenticated Key Exchange. Wi-Fi Protected Setup
[WPS] offered a similar option. There were various attacks against
the actual protocol; some of the problems were caused by issues in
the protocol, but most were tied to the usage of short PINs.
In the reference implementation, the PIN is picked at random by the
paired device before the beginning of the exchange. But this
requires that the paired device is capable of generating and
displaying a four digit number. It turns out that many devices
cannot do that. For example, an audio headset does not have any
display capability. These limited devices ended up using static
PINs, with fixed values like "0000" or "0001".
Even when the paired device could display a random PIN, that PIN will
have to be copied by the user on the pairing device. It turns out
that users do not like copying long series of numbers, and the
usability thus dictated that the PINs be short -- four digits in
practice. But there is only so much assurance as can be derived from
a four digit key.
It is interesting to note that the latest revisions of the Bluetooth
Pairing protocol [BTLEPairing] do not include the short PIN option
anymore. The PIN entry methods have been superseded by the simple
"just works" method for devices without displays, and by a procedure
based on an SAS (short authentication string) when displays are
available.
A further problem with these PIN based approaches is that -- in
contrast to SASes -- the PIN is a secret instrumental in the security
algorithm. To guarantee security, this PIN had to be transmitted via
a secure out of band channel.
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2.3.2. Push Buttons Just Work, But Are Insecure
Some devices are unable to input or display any code. The industry
more or less converged on a "push button" solution. When the button
is pushed, devices enter a "pairing" mode, during which they will
accept a pairing request from whatever other device connects to them.
The Bluetooth Pairing protocol [BTLEPairing] denotes that as the
"just works" method. It does indeed work, and if the pairing
succeeds the devices will later be able to use the pairing keys to
authenticate connections. However, the procedure does not provide
any protection against MITM attacks during the pairing process. The
only protection is that pushing the button will only allow pairing
for a limited time, thus limiting the opportunities of attacks.
As we set up to define a pairing protocol with a broad set of
applications, we cannot limit ourselves to an insecure "push button"
method. But we probably need to allow for a mode of operation that
works for input-limited and display limited devices.
2.3.3. Short Range Communication
There have been several attempts to define pairing protocols that use
"secure channels." Most of them are based on short range
communication systems, where the short range limits the feasibility
for attackers to access the channels. Example of such limited
systems include for example:
o QR codes, displayed on the screen of one device, and read by the
camera of the other device.
o Near Field Communication (NFC) systems, which provides wireless
communication with a very short range.
o Sound systems, in which one systems emits a sequence of sounds or
ultrasounds that is picked by the microphone of the other system.
A common problem with these solutions is that they require special
capabilities that may not be present in every device. Another
problem is that they are often one-way channels. Yet another problem
is that the side channel is not necessarily secret. QR codes could
be read by third parties. Powerful radios antennas might be able to
interfere with NFC. Sensitive microphones might pick the sounds. We
will discuss the specific case of QR codes in Section 2.7.
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2.3.4. Short Authentication Strings
The evolving pairing protocols seem to converge towards a "display
and compare" method. This is in line with academic studies, such as
[KFR09] or [USK11]. This points to a very simple scenario:
1. Alice initiates pairing
2. Bob selects Alice's device from a list.
3. Alice and Bob compare displayed strings that represent a
fingerprint of the key.
4. If the strings match, Alice and Bob accept the pairing.
Most existing pairing protocols display the fingerprint of the key as
a 6 or 7 digit numbers. Usability studies show that gives good
results, with little risk that users mistakenly accept two different
numbers as matching. However, the authors of [USK11] found that
people had more success comparing computer generated sentences than
comparing numbers. This is in line with the argument in [XKCD936] to
use sequences of randomly chosen common words as passwords. On the
other hand, standardizing strings is more complicated than
standardizing numbers. We would need to specify a list of common
words, and the process to go from a binary fingerprint to a set of
words. We would need to be concerned with internationalization
issues, such as using different lists of words in German and in
English. This could require negotiation of word lists or languages
inside the pairing protocols.
In contrast, numbers are easy to specify, as in "take a 20 bit number
and display it as an integer using decimal notation."
2.4. Resist Cryptographic Attacks
It is tempting to believe that once two peers are connected, they
could create a secret with a few simple steps, such as for example
exchange two nonces, hash the concatenation of these nonces with the
shared secret that is about to be established, display a short
authentication string composed of a short version of that hash on
each device, and verify that the two values match. The sequence of
messages would be something like:
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Alice Bob
g^xA -->
<-- g^xB
nA -->
<-- nB
Computes Computes
s = g^xAxB s = g^xAxB
h = hash(s|nA|nB) h = hash(s|nA|nB)
Displays short Displays short
version of h version of h
If the two short hashes match, Alice and Bob are supposedly assured
that they have computed the same secret, but there is a problem. The
exchange may not deter a smart attacker in the middle. Let's redraw
the same message flow, this time involving Eve:
Alice Eve Bob
g^xA -->
g^xA'-->
<-- g^xB
<--g^xB'
nA -->
nA -->
<-- nB
Picks nB'
smartly
<--nB'
Computes Computes
s' = g^xAxB' s" = g^xA'xB
h' = hash(s|nA|nB') h" = hash(s"|nA|nB)
Displays short Displays short
version of h' version of h"
Let's now assume that to pick the nonce nB' smartly, Eve runs the
following algorithm:
s' = g^xAxB'
s" = g^xA'xB
repeat
pick a new version of nB'
h' = hash(s|nA|nB')
h" = hash(s"|nA|nB)
until the short version of h'
matches the short version of h"
Of course, running this algorithm will require in theory as many
iterations as the possible values of the short hash. But hash
algorithms are fast, and it is possible to try millions of values in
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less than a second. If the short string is made up of fewer than 6
digits, Eve will find a matching nonce quickly, and Alice and Bob
will hardly notice the delay. Even if the matching string is as long
as 8 letters, Eve will probably find a value where the short versions
of h' and h" are close enough, e.g. start and end with the same two
or three letters. Alice and Bob may well be fooled.
The classic solution to such problems is to "commit" a possible
attacker to a nonce before sending it. This commitment can be
realized by a hash. In the modified exchange, Alice sends a secure
hash of her nonce before sending the actual value:
Alice Bob
g^xA -->
<-- g^xB
Computes Computes
s = g^xAxB s = g^xAxB
h_a = hash(s|nA) -->
<-- nB
nA -->
verifies h_a == hash(s|nA)
Computes Computes
h = hash(s|nA|nB) h = hash(s|nA|nB)
Displays short Displays short
version of h version of h
Alice will only disclose nA after having confirmation from Bob that
hash(nA) has been received. At that point, Eve has a problem. She
can still forge the values of the nonces but she needs to pick the
nonce nA' before the actual value of nA has been disclosed. Eve
would still have a random chance of fooling Alice and Bob, but it
will be a very small chance: one in a million if the short
authentication string is made of 6 digits, even fewer if that string
is longer.
Nguyen et al. [NR11] survey these protocols and compare them with
respect to the amount of necessary user interaction and the
computation time needed on the devices. The authors state that such
a protocol is optimal with respect to user interaction if it suffices
for users to verify a single b-bit SAS while having a one-shot attack
success probability of 2^-b. Further, n consecutive attacks on the
protocol must not have a better success probability then n one-shot
attacks.
There is still a theoretical problem, if Eve has somehow managed to
"crack" the hash function. We build some "defense in depth" by some
simple measures. In the design presented above, the hash "h_a"
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depends on the shared secret "s", which acts as a "salt" and reduces
the effectiveness of potential attacks based on pre-computed
catalogs. For simplicity, the design used a simple concatenation
mechanism, but we could instead use a keyed-hash message
authentication code (HMAC, [RFC2104], [RFC6151]), using the shared
secret as a key, since the HMAC construct has proven very robust over
time. Then, we can constrain the size of the random numbers to be
exactly the same as the output of the hash function. Hash attacks
often require padding the input string with arbitrary data;
restraining the size limits the likelyhood of such padding.
2.5. Privacy Requirements
Pairing exposes a relation between several devices and their owners.
Adversaries may attempt to collect this information, for example in
an attempt to track devices, their owners, or their "social graph."
It is often argued that pairing could be performed in a safe place,
from which adversaries are assumed absent, but experience shows that
such assumptions are often misguided. It is much safer to
acknowledge the privacy issues and design the pairing process
accordingly.
In order to start the pairing process, devices must first discover
each other. We do not have the option of using the private discovery
protocol [I-D.ietf-dnssd-privacy] since the privacy of that protocol
depends on the pre-existing pairing. In the simplest design, one of
the devices will announce a "friendly name" using DNS-SD.
Adversaries could monitor the discovery protocol, and record that
name. An alternative would be for one device to announce a random
name, and communicate it to the other device via some private
channel. There is an obvious tradeoff here: friendly names are
easier to use but less private than random names. We anticipate that
different users will choose different tradeoffs, for example using
friendly names if they assume that the environment is "safe," and
using random names in public places.
During the pairing process, the two devices establish a connection
and validate a pairing secret. As discussed in Section 2.3, we have
to assume that adversaries can mount MITM attacks. The pairing
protocol can detect such attacks and resist them, but the attackers
will have access to all messages exchanged before validation is
performed. It is important to not exchange any privacy sensitive
information before that validation. This includes, for example, the
identities of the parties or their public keys.
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2.6. Using TLS
The pairing algorithms typically combine the establishment of a
shared secret through an [EC]DH exchange with the verification of
that secret through displaying and comparison of a "short
authentication string" (SAS). As explained in Section 2.4, the
secure comparison requires a "commit before disclose" mechanism.
We have three possible designs: (1) create a pairing algorithm from
scratch, specifying our own crypto exchanges; (2) use an [EC]DH
version of TLS to negotiate a shared secret, export the key to the
application as specified in [RFC5705], and implement the "commit
before disclose" and SAS verification as part of the pairing
application; or, (3) use TLS, integrate the "commit before disclose"
and SAS verification as TLS extensions, and export the verified key
to the application as specified in [RFC5705].
When faced with the same choice, the designers of ZRTP [RFC6189]
chose to design a new protocol integrated in the general framework of
real time communications. We don't want to follow that path, and
would rather not create yet another protocol. We would need to
reinvent a lot of the negotiation capabilities that are part of TLS,
not to mention algorithm agility, post quantum, and all that sort of
things. It is thus pretty clear that we should use TLS.
It turns out that there was already an attempt to define SAS
extensions for TLS ([I-D.miers-tls-sas]). It is a very close match
to our third design option, full integration of SAS in TLS, but the
draft has expired, and there does not seem to be any support for the
SAS options in the common TLS packages.
In our design, we will choose the middle ground option -- use TLS for
[EC]DH, and implement the SAS verification as part of the pairing
application. This minimizes dependencies on TLS packages to the
availability of a key export API following [RFC5705]. We will need
to specify the hash algorithm used for the SAS computation and
validation, which carries some of the issues associated with
"designing our own crypto". One solution would be to use the same
hash algorithm negotiated by the TLS connection, but common TLS
packages do not not always make this algorithm identifier available
through standard APIs. A fallback solution is to specify a state of
the art keyed MAC algorithm.
2.7. QR codes
In Section 2.3.3, we reviewed a number of short range communication
systems that can be used to facilitate pairing. Out of these, QR
codes stand aside because most devices that can display a short
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string can also display the image of a QR code, and because many
pairing scenarios involve cell phones equipped with cameras capable
of reading a QR code.
QR codes could be particularly useful when starting discovery. QR
codes can encode an alphanumeric string, which could for example
encode the selected name of the pairing service. This would enable
automatic discovery, and would be easier to use than reading the
random name of the day and matching it against the results of DNS-SD.
In addition to the instance name, a QR code could also be leveraged
for authentication. It could encode an SAS or even a longer
authentication string. Transmitting the output of a cryptographic
hash function or HMAC via the OOB channel would make an offline
combinatorial search attack infeasible and thus allow to not sent the
commitment discussed in Section 2.4 saving a message. Further, if a
single device created both QR codes for discovery and verifcation,
respecitvely, and the other device scans these, the users could just
wait while both QRs are scanned subsequently as no user interaction
is necessary between these two scans (but it needs a QR scanner (app)
that support this). This could make the process feel like a single
user interaction.
But still, from a users point of view, scanning QR codes may not be
more efficient than visual verification of a short string. The user
has to take a picture of the QR code, which is arguably not simpler
than just "look at the number on the screen and tell me whether it is
the same as yours".
In the case of a man-in-the-middle attack, the evaluation of the QR
code will fail. The "client" that took the picture will know that,
but the "server" will not. The user will still need to click some
"Cancel" button on the server, which means that the process will not
be completely automated.
3. Design of the Pairing Mechanism
In this section we discuss the design of pairing protocols that use
manually verified short authentication strings (SAS), considering
both security and user experience.
We divide pairing in three parts: discovery, agreement, and
authentication, detailed in the following subsections.
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3.1. 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 a priori
knowledge and without publishing any private information. In
accordance with TLS, we refer to the device initiating the
cryptographic protocol as client, and to the other device as server;
the server has to be discoverable by the client.
Granting privacy during the discovery phase without relying on a
priori knowledge demands another user interaction (besides the SAS
verification during the authentication phase). There are two
possible ways of realizing this user interaction depending on whether
QR codes are supported or not. If 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. Leveraging QR
codes, the discovery proceeds as follows.
1. The server displays a QR code containing the clients A and AAAA
resource records, and further the SRV resource record
corresponding to the pairing service instance. A privacy
preserving instance name is not necessary, because this instance
is never published via an unsecured network.
2. The client scans the QR code retrieving the necessary information
for establishing a connection to the server.
If QR codes are not supported, the discovery proceeds as follows.
1. The server displays its chosen instance name on its screen.
2. The client performs a discovery of all the "pairing" servers
available on the local network. This may result in the discovery
of several servers.
3. Among these available "pairing servers" the client user selects
the name that matches the name displayed by the server.
3.2. Agreement
Once the server has been selected, 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
a cryptographic protocol that yields an SAS. We discussed design
aspects of such protocols in Section 2.4.
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3.3. Authentication
In the authentication phase, the users are asked to validate the
pairing by comparing the SASes -- typically represented by a number
encoded over up to 7 decimal digits. 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".
Depending on whether QR codes are supported, the SAS may also be
represented as QR code. Despite the fact that using QR codes to
represent the authentication string renders using longer
authentication strings feasible, we suggest to always generate an SAS
during the agreement phase, because this makes realizations of the
agreement phase and the authentication phase independent. Devices
may display the "real" name of the other device alongside the SAS.
3.4. Intra User Pairing
Users can pair their own devices in secure (home) networks without
any interaction using a special DNS-SD pairing service. Verification
methods where a single user holds both devices, e.g. synchronously
pressing buttons on both devices a few times, are also suitable.
Further, a secure OOB could be established by connecting two devices
with an USB channel. Pairing via an USB connection is also used by
some Bluetooth devices, e.g. when pairing a controller with a gaming
console.
[[TODO: elaborate]]
3.5. Pairing Data Synchronization
To make it sufficient for users to pair only one of their devices to
one of their friends devices while still being able to engage in
later communication with all of this friend's devices using any of
the own devices, we offer the possibility to synchronize pairing data
among devices of the same user. Pairing data synchronization is
performed via a special DNS-SD service (_pdsync._tls).
[[TODO: elaborate]]
3.6. Public Authentication Keys
[[TODO: Should we discuss public authentication keys whose
fingerprints are verified during pairing?]]
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4. Solution
[[TODO: elaborate on all subsections]]
In the proposed pairing protocol, one of the devices acts as a
"server", and the other acts as a "client". The server will publish
a "pairing service". The client will discover the service instance
during the discovery phase, as explained in Section 4.1. The pairing
service itself is specified in Section 4.2.
4.1. Discovery
The discovery uses DNS-SD [RFC6763] over mDNS [RFC6762]. The pairing
service is identified in DNS SD as "_pairing._tcp". When the pairing
service starts, the server starts publishing the chosen instance
name. The client will discover that name and the corresponding
connection parameters.
If QR code scanning is available as OOB channel, the discovery data
is directly transmitted via QR codes instead of DNS-SD over mDNS.
[[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 fix name
as instance name.]]
4.2. Agreement and Authentication
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;
Devices implementing the service MUST support TLS 1.2 [RFC5246], and
SHOULD support TLS 1.3 as soon as 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 SHOULD be able to negotiate
the cipher suite TLS_DH_anon_WITH_AES_256_CBC_SHA256.
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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"
Context value: empty.
Length value: 32 bytes (256 bits).
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 SHOULD be the same algorithm as negotiated for building the PRF
in the TLS connection. If there is no suitable API to retrieve that
algorithm, the client MAY use SHA256 instead. 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".
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:
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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.
Both client and server display the SAS as a decimal integer, and ask
the user to compare the values. If the values do not match, the user
cancels the pairing. Otherwise, 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.
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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.
5. 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 4.1 and
the authentication procedures specified in Section 4.2 are
specifically designed to mitigate such attacks.
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
mitigate such attacks, nodes MUST store the secret safely, and MUST
be able to quickly revoke a compromised pairing. This is however not
sufficient, as the compromise of the pairing key could remain
undetected for a long time. For further safety, nodes SHOULD assign
a time limit to the validity of pairings, discard the corresponding
keys when the time has passed, and establish new pairings.
6. IANA Considerations
This draft does not require any IANA action.
7. Acknowledgments
TODO
8. References
8.1. Normative References
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[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://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,
<http://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, <http://www.rfc-editor.org/info/rfc5705>.
[RFC6762] Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762,
DOI 10.17487/RFC6762, February 2013,
<http://www.rfc-editor.org/info/rfc6762>.
[RFC6763] Cheshire, S. and M. Krochmal, "DNS-Based Service
Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013,
<http://www.rfc-editor.org/info/rfc6763>.
8.2. Informative References
[BTLEPairing]
Bluetooth SIG, "Bluetooth Low Energy Security Overview",
2016,
<https://developer.bluetooth.org/TechnologyOverview/Pages/
LE-Security.aspx>.
[I-D.ietf-dnssd-privacy]
Huitema, C. and D. Kaiser, "Privacy Extensions for DNS-
SD", draft-ietf-dnssd-privacy-00 (work in progress),
October 2016.
[I-D.miers-tls-sas]
Miers, I., Green, M., and E. Rescorla, "Short
Authentication Strings for TLS", draft-miers-tls-sas-00
(work in progress), February 2014.
[KFR09] Kainda, R., Flechais, I., and A. Roscoe, "Authentication
protocols based on low-bandwidth unspoofable channels: a
comparative survey", 2009.
[NR11] Nguyen, L. and A. Roscoe, "Authentication protocols based
on low-bandwidth unspoofable channels: a comparative
survey", 2011.
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[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104,
DOI 10.17487/RFC2104, February 1997,
<http://www.rfc-editor.org/info/rfc2104>.
[RFC6151] Turner, S. and L. Chen, "Updated Security Considerations
for the MD5 Message-Digest and the HMAC-MD5 Algorithms",
RFC 6151, DOI 10.17487/RFC6151, March 2011,
<http://www.rfc-editor.org/info/rfc6151>.
[RFC6189] Zimmermann, P., Johnston, A., Ed., and J. Callas, "ZRTP:
Media Path Key Agreement for Unicast Secure RTP",
RFC 6189, DOI 10.17487/RFC6189, April 2011,
<http://www.rfc-editor.org/info/rfc6189>.
[USK11] Uzun, E., Saxena, N., and A. Kumar, ". Pairing devices for
social interactions: a comparative usability evaluation",
2009.
[WPS] Wi-Fi Alliance, "Wi-Fi Protected Setup", 2016,
<http://www.wi-fi.org/discover-wi-fi/
wi-fi-protected-setup>.
[XKCD936] Munroe, R., "XKCD: Password Strength", 2011,
<https://www.xkcd.com/936/>.
Authors' Addresses
Christian Huitema
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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