6TiSCH Working Group M. Vucinic
Internet-Draft Inria
Intended status: Standards Track J. Simon
Expires: September 13, 2017 Linear Technology
K. Pister
University of California Berkeley
M. Richardson
Sandelman Software Works
March 12, 2017
Minimal Security Framework for 6TiSCH
draft-ietf-6tisch-minimal-security-02
Abstract
This document describes the minimal mechanisms required to support
secure enrollment of a pledge, a device being added to an IPv6 over
the TSCH mode of IEEE 802.15.4e (6TiSCH) network. It assumes that
the pledge has been provisioned with a credential that is relevant to
the deployment - the "one-touch" scenario. The goal of this
configuration is to set link-layer keys, and to establish a secure
end-to-end session between each pledge and the join registrar who may
use that to further configure the pledge. Additional security
behaviors and mechanisms may be added on top of this minimal
framework.
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
Task Force (IETF). Note that other groups may also distribute
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 13, 2017.
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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
Provisions Relating to IETF Documents
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. One-Touch Assumptions . . . . . . . . . . . . . . . . . . . . 4
4. Join Overview . . . . . . . . . . . . . . . . . . . . . . . . 4
4.1. Step 1 - Enhanced Beacon . . . . . . . . . . . . . . . . 5
4.2. Step 2 - Neighbor Discovery . . . . . . . . . . . . . . . 6
4.3. Step 3 - Security Handshake . . . . . . . . . . . . . . . 6
4.4. Step 4 - Simple Join Protocol - Join Request . . . . . . 8
4.5. Step 5 - Simple Join Protocol - Join Response . . . . . . 8
5. Architectural Overview and Communication through Join Proxy . 8
5.1. Stateless-Proxy CoAP Option . . . . . . . . . . . . . . . 9
6. Security Handshake . . . . . . . . . . . . . . . . . . . . . 10
6.1. Discovery Message . . . . . . . . . . . . . . . . . . . . 11
7. Simple Join Protocol Specification . . . . . . . . . . . . . 11
7.1. OSCOAP Security Context Instantiation . . . . . . . . . . 12
7.2. Specification of Join Request . . . . . . . . . . . . . . 13
7.3. Specification of Join Response . . . . . . . . . . . . . 13
8. Link-layer Requirements . . . . . . . . . . . . . . . . . . . 15
9. Asymmetric Keys . . . . . . . . . . . . . . . . . . . . . . . 15
10. Rekeying and Rejoin . . . . . . . . . . . . . . . . . . . . . 16
11. Key Derivations . . . . . . . . . . . . . . . . . . . . . . . 16
12. Security Considerations . . . . . . . . . . . . . . . . . . . 16
13. Privacy Considerations . . . . . . . . . . . . . . . . . . . 17
14. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
14.1. CoAP Option Numbers Registry . . . . . . . . . . . . . . 18
15. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 18
16. References . . . . . . . . . . . . . . . . . . . . . . . . . 18
16.1. Normative References . . . . . . . . . . . . . . . . . . 18
16.2. Informative References . . . . . . . . . . . . . . . . . 19
Appendix A. Example . . . . . . . . . . . . . . . . . . . . . . 21
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23
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1. Introduction
This document describes the minimal feature set for a new device,
termed pledge, to securely join a 6TiSCH network. As a successful
outcome of this process, the pledge is able to securely communicate
with its neighbors, participate in the routing structure of the
network or establish a secure session with an Internet host.
When a pledge seeks admission to a 6TiSCH [RFC7554] network, it first
needs to synchronize to the network. The pledge then configures its
link-local IPv6 address and authenticates itself, and also validates
that it is joining the right network. At this point it can expect to
interact with the network to configure its link-layer keying
material. Only then may the node establish an end-to-end secure
session with an Internet host using OSCOAP
[I-D.ietf-core-object-security] or DTLS [RFC6347]. Once the
application requirements are known, the node interacts with its peers
to request additional resources as needed, or to be reconfigured as
the network changes [I-D.ietf-6tisch-6top-protocol].
This document presumes a network as described by [RFC7554],
[I-D.ietf-6tisch-6top-protocol], and [I-D.ietf-6tisch-terminology].
It assumes the pledge pre-configured with either a:
o pre-shared key (PSK),
o raw public key (RPK),
o or a locally-valid certificate and a trust anchor.
As the outcome of the join process, the pledge expects one or more
link-layer key(s) and optionally a temporary network identifier.
2. Terminology
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]. These
words may also appear in this document in lowercase, absent their
normative meanings.
The reader is expected to be familiar with the terms and concepts
defined in [I-D.ietf-6tisch-terminology], [RFC7252],
[I-D.ietf-core-object-security], and
[I-D.ietf-anima-bootstrapping-keyinfra]. The following terms are
imported: pledge, join proxy, join registrar/coordinator, drop ship,
imprint, enrollment, ownership voucher.
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Pledge: the prospective device, which has the identity provided to
at the factory.
Joined Node: the prospective device, after having completed the join
process, often just called a Node.
Join Proxy (JP): a stateless relay that provides connectivity
between the pledge and the join registrar/coordinator.
Join Registrar/Coordinator (JRC): central entity responsible for
authentication and authorization of joining nodes.
3. One-Touch Assumptions
This document assumes the one-touch scenario, where devices are
provided with some mechanism by which a secure association may be
made in a controlled environment. There are many ways in which this
might be done, and detailing any of them is out of scope for this
document. But, some notion of how this might be done is important so
that the underlying assumptions can be reasoned about.
Some examples of how to do this could include:
o JTAG interface
o serial (craft) console interface
o pushes of physical buttons simultaneous to network attachment
o unsecured devices operated in a Faraday cage
There are likely many other ways as well. What is assumed is that
there can be a secure, private conversation between the Join
Registrar/Coordinator, and the pledge, and that the two devices can
exchange some trusted bytes of information.
4. Join Overview
This section describes the steps taken by a pledge in a 6TiSCH
network. When a previously unknown device seeks admission to a
6TiSCH [RFC7554] network, the following exchange occurs:
1. The pledge listens for an Enhanced Beacon (EB) frame
[IEEE8021542015]. This frame provides network synchronization
information, and tells the device when it can send a frame to the
node sending the beacons, which plays the role of Join Proxy (JP)
for the pledge, and when it can expect to receive a frame.
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2. The pledge configures its link-local IPv6 address and advertizes
it to Join Proxy (JP).
3. The pledge sends packets to JP in order to securely identify
itself to the network. These packets are directed to the Join
Registrar/Coordinator (JRC), which may be co-located on the JP or
another device.
4. The pledge receives one or more packets from JRC (via the JP)
that sets up one or more link-layer keys used to authenticate
subsequent transmissions to peers.
From the pledge's perspective, minimal joining is a local phenomenon
- the pledge only interacts with the JP, and it need not know how far
it is from the DAG root, or how to route to the JRC. Only after
establishing one or more link-layer keys does it need to know about
the particulars of a 6TiSCH network.
The handshake is shown as a transaction diagram in Figure 1:
+--------+ +-------+ +--------+
| pledge | | JP | | JRC |
| | | | | |
+--------+ +-------+ +--------+
| | |
|<----ENH BEACON (1)-------| |
| | |
|<-Neighbor Discovery (2)->| |
| | |
|<---Sec. Handshake (3)----|---Sec. Handshake (3a)--->|
| | |
.......................................................................
. |-----Join Request (4)-----|------Join Request (4a)-->| .
. | | | Simple Join .
. |<---Join Response (5)-----|-----Join Response (5a)---| Protocol .
. | | | .
.......................................................................
Figure 1: Overview of the join process.
The details of each step are described in the following sections.
4.1. Step 1 - Enhanced Beacon
Due to the channel hopping nature of 6TiSCH, transmissions take place
on physical channels in a circular fashion. For that reason,
Enhanced Beacons (EBs) are expected to be found by listening on a
single channel. However, because some channels may be blacklisted, a
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new pledge must listen for Enhanced Beacons for a certain period on
each of the 16 possible channels. This search process entails having
the pledge keep the receiver portion of its radio active for the
entire period of time.
Once the pledge hears an EB from a JP, it synchronizes itself to the
joining schedule using the cells contained in the EB. The selection
of which beacon to start with is outside the scope of this document.
Implementers SHOULD make use of information such as: whether the L2
address of the EB has been tried before, any Network Identifier
[I-D.richardson-6tisch-join-enhanced-beacon] seen, and the strength
of the signal. The pledge can be configured with the Network
Identifier to seek when it is configured with the PSK.
Once a candidate network has been selected, the pledge can transition
into a low-power duty cycle, waking up only when the provided
schedule indicates shared slots which the pledge may use for the join
process.
At this point the pledge may proceed to step 2, or continue to listen
for additional EBs.
A pledge which receives only Enhanced Beacons containing Network ID
extensions [I-D.richardson-6tisch-join-enhanced-beacon] with the
initiate bit cleared, SHOULD NOT proceed with this protocol on that
network. The pledge SHOULD consider that it is in a network which
manages join traffic, it SHOULD switch to
[I-D.ietf-6tisch-dtsecurity-secure-join].
4.2. Step 2 - Neighbor Discovery
At this point, the pledge forms its link-local IPv6 address based on
EUI64 and may register it at JP, in order to bootstrap the IPv6
neighbor tables. The Neighbor Discovery exchange shown in Figure 1
refers to a single round trip Neighbor Solicitation / Neighbor
Advertisement exchange between the pledge and the JP. The pledge may
further follow the Neighbor Discovery (ND) process described in
Section 5 of [RFC6775].
4.3. Step 3 - Security Handshake
The security handshake between pledge and JRC uses Ephemeral Diffie-
Hellman over COSE (EDHOC) [I-D.selander-ace-cose-ecdhe] to establish
the shared session secret used to encrypt the Simple Join Protocol.
The security handshake step is OPTIONAL in case PSKs are used, while
it is REQUIRED for RPKs and certificates.
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When using certificates, the process continues as described in
[I-D.selander-ace-cose-ecdhe], but MAY result in no network key being
returned. In that case, the pledge enters a provisional situation
where it provides access to an enrollment mechanism described in
[I-D.ietf-6tisch-dtsecurity-secure-join].
If using a locally relevant certificate, the pledge will be able to
validate the certificate of the JRC via a local trust anchor. In
that case, the JRC will return networks keys as in the PSK case.
This would typically be the case for a device which has slept so long
that it no longer has valid network keys and must go through a
partial join process again.
In case the handshake step is omitted, the shared secret used for
protection of the Simple Join Protocol in the next step is the PSK.
A consequence is that if the long-term PSK is compromised, keying
material transferred as part of the join response is compromised as
well. Physical compromise of the pledge, however, would also imply
the compromise of the same keying material, as it is likely to be
found in node's memory.
4.3.1. Pre-Shared Symmetric Key
The Diffie-Hellman key exchange and the use of EDHOC is optional,
when using a pre-shared symmetric key. This cuts down on traffic
between JRC and pledge, but requires pre-configuration of the shared
key on both devices.
It is REQUIRED to use unique PSKs for each pledge. If there are
multiple JRCs in the network (such as for redundancy), they would
have to share a database of PSKs.
4.3.2. Asymmetric Keys
The Security Handshake step is required, when using asymmetric keys.
Before conducting the Diffie-Hellman key exchange using EDHOC
[I-D.selander-ace-cose-ecdhe] the pledge and JRC need to receive and
validate each other's public key certificate. As detailed above,
this can only be done for locally relevant (LDevID) certificates.
IDevID certificates require entering a provisional state as described
in [I-D.ietf-6tisch-dtsecurity-secure-join].
When RPKs are pre-configured at pledge and JRC, they can directly
proceed to the handshake.
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4.4. Step 4 - Simple Join Protocol - Join Request
The Join Request that makes part of the Simple Join Protocol is sent
from the pledge to the JP using the shared slot as described in the
EB, and forwarded to the JRC. Which slot the JP uses to transmit to
the JRC is out of scope: some networks may wish to dedicate specific
slots for this join traffic.
The join request is typically authenticated/encrypted end-to-end
using AES-CCM-16-64-128 algorithm from [I-D.ietf-cose-msg] and a key
derived from the shared secret from step 3. Algorithm negotiation is
described in detail in [I-D.selander-ace-cose-ecdhe].
The nonce is derived from the shared secret, the pledge's EUI64 and a
monotonically increasing counter initialized to 0 when first
starting.
4.5. Step 5 - Simple Join Protocol - Join Response
The Join Response that makes part of the Simple Join Protocol is sent
from the JRC to the pledge through JP that serves as a stateless
relay. Packet containing the Join Response travels on the path from
JRC to JP using pre-established routes in the network. The JP
delivers it to the pledge using the slot information from the EB. JP
operates as the application-layer proxy and does not keep any state
to relay the message. It uses information sent in the clear within
the join response to decide where to forward to.
The join response is typically authenticated/encrypted using AES-CCM-
16-64-128 algorithm from [I-D.ietf-cose-msg] and a key derived from
the shared secret from step 3.
The nonce is derived from the shared secret, pledge's EUI64 and a
monotonically increasing counter matching that of the join request.
The join response contains one or more link-layer key(s) that the
pledge will use for subsequent communication. Each key that is
provided by the JRC is associated with an 802.15.4 key identifier.
In other link-layer technologies, a different identifier may be
substituted. Join Response optionally also contains an IEEE 802.15.4
short address [IEEE8021542015] assigned to pledge by JRC.
5. Architectural Overview and Communication through Join Proxy
The protocol in Figure 1 is implemented over Constrained Application
Protocol (CoAP) [RFC7252]. The Pledge plays the role of a CoAP
client, JRC the role of a CoAP server, while JP implements CoAP
forward proxy functionality [RFC7252]. Since JP is also likely a
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constrained device, it does not need to implement a cache but rather
process forwarding-related CoAP options and make requests on behalf
of pledge that is not yet part of the network.
The pledge communicates with a Join Proxy (JP) over link-local IPv6
addresses. The pledge designates a JP as a proxy by including in the
CoAP requests to the JP the Proxy-Scheme option with value "coap"
(CoAP-to-CoAP proxy). The pledge MUST include the Uri-Host option
with its value set to the well-known JRC's alias - "6tisch.arpa".
The pledge does not need to learn the actual IPv6 address of JRC at
any time during the join protocol. The JP knows the address of the
JRC, via a provisioning process that occured when the JP, acting as a
pledge, joined. The initial bootstrap of the DODAG root would
require explicit provisioning of the JRC address.
5.1. Stateless-Proxy CoAP Option
The CoAP proxy by default keeps per-client state information in order
to forward the response towards the originator of the request
(client). This state information comprises CoAP token, but the
implementations also need to keep track of the IPv6 address of the
host, as well as the corresponding UDP source port number. In the
setting where the proxy is a constrained device and there are
potentially many clients, as in the case of JP, this makes it prone
to Denial of Service (DoS) attacks, due to the limited memory.
The Stateless-Proxy CoAP option (c.f. Figure 2) allows the proxy to
insert within the request the state information necessary for
relaying the response back to the client. Note that the proxy still
needs to keep some state, such as for performing congestion control
or request retransmission, but what is aimed with Stateless-Proxy
option is to free the proxy from keeping per-client state.
Stateless-Proxy option is critical, Safe-to-Forward, not part of the
cache key, not repeatable and opaque. When processed by OSCOAP,
Stateless-Proxy option is neither encrypted nor integrity protected.
+-----+---+---+---+---+-----------------+--------+--------+
| No. | C | U | N | R | Name | Format | Length |
+-----+---+---+---+---+-----------------+--------+--------|
| TBD | x | | x | | Stateless-Proxy | opaque | 1-255 |
+-----+---+---+---+---+-----------------+--------+--------+
C=Critical, U=Unsafe, N=NoCacheKey, R=Repeatable
Figure 2: Stateless-Proxy CoAP Option
Upon reception of a Stateless-Proxy option, the CoAP server MUST echo
it in the response. The value of the Stateless-Proxy option is
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internal proxy state that is opaque to the server. Example state
information includes IPv6 address of the client, its UDP source port,
and the CoAP token. For security reasons, the state information MUST
be authenticated, MUST include a freshness indicator (e.g. a sequence
number or timestamp) and MAY be encrypted. The proxy may use an
appropriate COSE structure [I-D.ietf-cose-msg] to wrap the state
information as the value of the Stateless-Proxy option. The key used
for encryption/authentication of the state information may be known
only to the proxy.
Once the proxy has received the CoAP response with Stateless-Proxy
option present, it decrypts/authenticates it, checks the freshness
indicator and constructs the response for the client, based on the
information present in the option value.
Note that a CoAP proxy using the Stateless-Proxy option is not able
to return 5.04 Gateway Timeout error in case the request to the
server times out. Likewise, if the response to the proxy's request
does not contain the Stateless-Proxy option, for example when the
option is not supported by the server, the proxy is not able to
return the response to the client.
6. Security Handshake
In order to derive a shared session key, pledge and JRC run the EDHOC
protocol [I-D.selander-ace-cose-ecdhe]. During this process, pledge
and JRC mutually authenticate each other and verify authorization
information before proceeding with the Simple Join Protocol. In case
certificates are used for authentication, this document assumes that
a special certificate with role attribute set has been provisioned to
the JRC. This certificate is verified by pledge in order to
authorize JRC to continue with the join process. How such a
certificate is issued to the JRC is out of scope of this document.
Figure 3 details the exchanges between the pledge and JRC that take
place during the execution of the security handshake. Format of
EDHOC messages is specified in [I-D.selander-ace-cose-ecdhe].
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+--------+ +--------+
| pledge | | JRC |
| | | |
+--------+ +--------+
| Discovery Message |
+-------------------------------->|
| |
| Optional ACK |
|< - - - - - - - - - - - - - - - -+
| |
| |
| EDHOC message_1 |
|<--------------------------------+
| |
| EDHOC message_2 |
+-------------------------------->|
| |
| EDHOC message_3 |
|<--------------------------------+
| |
Figure 3: Transaction diagram of the security handshake.
6.1. Discovery Message
Pledge triggers the security handshake by sending a discovery message
to the JRC. This initial message does not make part of the EDHOC
handshake. JRC is the initiator of the EDHOC run and is able to
schedule the execution of many concurrent enrollments at will by
acknowledging the request and sending a separate, delayed response.
The Discovery Message SHALL be mapped to a CoAP request:
o The request method is POST.
o The Proxy-Scheme option is set to "coap".
o The Uri-Host option is set to "6tisch.arpa".
o The Uri-Path option is set to "edhoc".
o The payload is optional and contains pledge's EUI-64.
7. Simple Join Protocol Specification
Simple Join Protocol is a single round trip protocol (c.f. Figure 4)
that facilitates secure enrollment of a pledge, based on a shared
symmetric secret. In case the pledge was provisioned by an
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asymmetric key (certificate or RPK), Simple Join Protocol is preceded
by a security handshake, described in Section 6. When the pledge is
provisioned with a PSK, Simple Join Protocol may be run directly.
Pledge and JRC MUST protect their exchange end-to-end (i.e. through
the proxy) using Object Security of CoAP (OSCOAP)
[I-D.ietf-core-object-security].
+--------+ +--------+
| pledge | | JRC |
| | | |
+--------+ +--------+
| |
| Join Request |
+-------------------------------->|
| |
| Join Response |
|<--------------------------------+
| |
Figure 4: Transaction diagram of the Simple Join Protocol.
7.1. OSCOAP Security Context Instantiation
The OSCOAP security context MUST be derived at pledge and JRC as per
Section 3.2 of [I-D.ietf-core-object-security] using HKDF [RFC5869]
as the key derivation function.
o Master Secret MUST be the secret generated by the run of EDHOC as
per Appendix B of [I-D.selander-ace-cose-ecdhe], or the PSK in
case EDHOC step was omitted.
o Sender ID of the pledge MUST be set to the concatenation of its
EUI-64 and byte string 0x00.
o Recipient ID (ID of JRC) MUST be set to the concatenation of
pledge's EUI-64 and byte string 0x01. The construct uses pledge's
EUI-64 to avoid nonce reuse in the response in the case same PSK
is shared by a group of pledges.
o Algorithm MUST be set to AES-CCM-16-64-128 from
[I-D.ietf-cose-msg]. CoAP messages are therefore protected with
an 8-byte CCM authentication tag and the algorithm uses 13-byte
long nonces.
The hash algorithm that instantiates HKDF MUST be SHA-256 [RFC4231].
The derivation in [I-D.ietf-core-object-security] results in traffic
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keys and static IVs for each side of the conversation. Nonces are
constructed by XOR'ing the static IV with current sequence number.
The context derivation process occurs exactly once.
Implementations MUST ensure that multiple CoAP requests to different
JRCs result in the use of the same OSCOAP context so that sequence
numbers are properly incremented for each request. This may happen
in a scenario where there are multiple 6TiSCH networks present and
the pledge tries to join one network at a time.
7.2. Specification of Join Request
Message Join Request SHALL be mapped to a CoAP request:
o The request method is GET.
o The Proxy-Scheme option is set to "coap".
o The Uri-Host option is set to "6tisch.arpa".
o The Uri-Path option is set to "j".
o The object security option SHALL be set according to
[I-D.ietf-core-object-security] and OSCOAP parameters set as
described above.
7.3. Specification of Join Response
If OSCOAP processing is a success and the pledge is authorized to
join the network, message Join Response SHALL be mapped to a CoAP
response:
o The response Code is 2.05 (Content).
o Content-Format option is set to application/cbor.
o The payload is a CBOR [RFC7049] array containing, in order:
* COSE Key Set, specified in [I-D.ietf-cose-msg], containing one
or more link-layer keys. The mapping of individual keys to
802.15.4-specific parameters is described in Section 7.3.1.
* Optional short address that is assigned to the pledge. The
format of the short address follows Section 7.3.2.
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payload = [
COSE_KeySet,
? short_address,
]
7.3.1. Link-layer Keys Transported in COSE Key Set
Each key in the COSE Key Set [I-D.ietf-cose-msg] SHALL be a symmetric
key. If "kid" parameter of the COSE Key structure is present, the
corresponding keys SHALL belong to an IEEE 802.15.4 KeyIdMode 0x01
class. In that case, parameter "kid" of COSE Key structure SHALL be
used to carry IEEE 802.15.4 KeyIndex value. If the "kid" parameter
is not present in the transported key, the application SHALL consider
the key to be an IEEE 802.15.4 KeyIdMode 0x00 (implicit) key. This
document does not support IEEE 802.15.4 KeyIdMode 0x02 and 0x03 class
keys.
7.3.2. Short Address
Optional "short_address" structure transported as part of the join
response payload represents IEEE 802.15.4 short address assigned to
the pledge. It is encoded as CBOR array object, containing in order:
o Byte string, containing the 16-bit address.
o Optional lease time parameter, "lease_asn". The value of the
"lease_asn" parameter is the 5-byte Absolute Slot Number (ASN)
corresponding to its expiration, carried as a byte string in
network byte order.
short_address = [
address : bstr,
? lease_asn : bstr,
]
It is up to the joined node to request a new short address before the
expiry of its previous address. The mechanism by which the node
requests renewal is the same as during join procedure, as described
in Section 10. The assigned short address is used for configuring
both Layer 2 short address and Layer 3 addresses.
7.3.3. Error Handling
In the case JRC determines that pledge is not supposed to join the
network (e.g. by failing to find an appropriate security context), it
should respond with a 4.01 Unauthorized error. Upon reception of a
4.01 Unauthorized, the pledge SHALL attempt to join the next
advertised 6TiSCH network. If all join attempts have failed at
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pledge, the pledge SHOULD signal to the user by an out-of-band
mechanism the presence of an error condition.
In the case that the JRC determines that the pledge is not (yet)
authorized to join the network, but a further zero-touch process
might permit it, the JRC responds with a 2.05 (Content) code, but the
payload contains the single CBOR string "prov" (for "provisional").
No link-layer keys or short address is returned.
This response is typically only expected when in asymmetric
certificate mode using 802.1AR IDevID certificates. But for reasons
of provisioning or device reuse, this could occur even when a one-
touch PSK authentication process was expected.
8. Link-layer Requirements
In an operational 6TiSCH network, all frames MUST use link-layer
frame security. The frame security options MUST include frame
authentication, and MAY include frame encryption.
Link-layer frames are protected with a 16-byte key, and a 13-byte
nonce constructed from current Absolute Slot Number (ASN) and the
source (the JP for EBs) address, as shown in Figure 5:
+-------------------------------------------+
| Address (8B or 00-padded 2B) | ASN (5B) |
+-------------------------------------------+
Figure 5: Link-layer CCM* nonce construction
The pledge does not initially do any authentication of the EB frames,
as it does not know the K1 key. When sending frames, the pledge
sends unencrypted and unauthenticated frames. JP accepts these
frames (exempt mode in 802.15.4) for the duration of the join
process. How JP learns whether the join process is ongoing is out of
scope of this specification.
As the EB itself cannot be authenticated by pledge, an attacker may
craft a frame that appears to be a valid EB, since the pledge can
neither know the ASN a priori nor verify the address of the JP. This
permits a Denial of Service (DoS) attack at the pledge. Beacon
authentication keys are discussed in [I-D.ietf-6tisch-minimal].
9. Asymmetric Keys
Certificates or pre-configured RPKs may be used to exchange public
keys between the pledge and JRC. The key pair is generated using
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elliptic curve secp256r1, and the certificate containing the public
key is signed using ECDSA. (XXX: would be nice to move to EdDSA)
The certificate itself may be a compact representation of an X.509
certificate, or a full X.509 certificate. Compact representation of
X.509 certificates is out of scope of this specification. The
certificate is signed by a root CA whose certificate is installed on
all nodes participating in a particular 6TiSCH network, allowing each
node to validate the certificate of the JRC or pledge as appropriate.
10. Rekeying and Rejoin
This protocol handles initial keying of the pledge. For reasons such
as rejoining after a long sleep, or expiry of the short address, the
joined node MAY send a new Join Request over the previously
established secure end-to-end session with JRC. JRC responds with
up-to-date keys and a short address. The node may also use the
Simple Join Protocol exchange for node-initiated rekeying. How node
learns that it should be rekeyed is out of scope. Additional work,
such as in [I-D.richardson-6tisch-minimal-rekey] can be used.
11. Key Derivations
When EDHOC is used to derive keys, the cost of the asymmetric
operation can be amortized over any additional connections that may
be required between the node (during or after joining) and the JRC.
Each application SHOULD use a unique session key. EDHOC was designed
with this in mind. In order to accomplish this, the EDHOC key
derivation algorithm can be run with a different label. Other users
of this key MUST define the label.
12. Security Considerations
In case PSKs are used, this document mandates that the pledge and JRC
are pre-configured with unique keys. The uniqueness of generated
nonces is guaranteed under the assumption of unique EUI64 identifiers
for each pledge. Note that the address of the JRC does not take part
in nonce construction. Therefore, even should an error occur, and a
PSK shared by a group of nodes, the nonces constructed as part of the
different responses are unique. The PSK is still important for
authentication of the pledge and authentication of the JRC to the
pledge. Should an attacker come to know the PSK, then a man-in-the-
middle attack is possible. The well known problem with Bluetooth
headsets with a "0000" pin applies here. The design differentiates
between nonces constructed for requests and nonces constructed for
responses by different sender identifiers (0x00 for pledge and 0x01
for JRC).
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Being a stateless relay, JP blindly forwards the join traffic into
the network. While the exchange between pledge and JP takes place
over a shared cell, join traffic is forwarded using dedicated cells
on the JP to JRC path. In case of distributed scheduling, the join
traffic may therefore cause intermediate nodes to request additional
bandwidth. (EDNOTE: this is a problem that needs to be solved)
Because the relay operation of JP is implemented at the application
layer, JP is the only hop on the JP-6LBR path that can distinguish
join traffic from regular IP traffic in the network. It is therefore
recommended to implement stateless rate limiting at JP: a simple
bandwidth (in bytes or packets/second) cap would be appropriate.
The shared nature of the "minimal" cell used for join traffic makes
the network prone to DoS attacks by congesting the JP with bogus
radio traffic. As such an attacker is limited by emitted radio
power, redundancy in the number of deployed JPs alleviates the issue
and also gives the pledge a possibility to use the best available
link for join. How a network node decides to become a JP is out of
scope of this specification.
At the time of the join, the pledge has no means of verifying the
content in the EB and has to accept it at "face value". In case the
pledge tries to join an attacker's network, the join response message
in such cases will either fail the security check or time out. The
pledge may implement a blacklist in order to filter out undesired
beacons and try to join the next seemingly valid network. The
blacklist alleviates the issue but is effectively limited by the
node's available memory. Such bogus beacons will prolong the join
time of the pledge and so the time spent in "minimal"
[I-D.ietf-6tisch-minimal] duty cycle mode.
13. Privacy Considerations
This specification relies on the uniqueness of EUI64 that is
transferred in clear as part of the security context identifier.
(EDNOTE: should we say IID here?) Privacy implications of using such
long-term identifier are discussed in [RFC7721] and comprise
correlation of activities over time, location tracking, address
scanning and device-specific vulnerability exploitation. Since the
join protocol is executed rarely compared to the network lifetime,
long-term threats that arise from using EUI64 are minimal. In
addition, the join response message contains an optional short
address which can be assigned by JRC to the pledge. The short
address is independent of the long-term identifier EUI64 and is
encrypted in the response. For that reason, it is not possible to
correlate the short address with the EUI64 used during the join. Use
of short addresses once the join protocol completes mitigates the
aforementioned privacy risks. In addition, EDHOC may be used for
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identity protection during the join protocol by generating a random
context identifier in place of the EUI64
[I-D.selander-ace-cose-ecdhe].
14. IANA Considerations
Note to RFC Editor: Please replace all occurrences of "[[this
document]]" with the RFC number of this specification.
This document allocates a well known name under the .arpa name space
according to the rules given in: [RFC3172]. The name "6tisch.arpa"
is requested. No subdomains are expected. No A, AAAA or PTR record
is requested.
14.1. CoAP Option Numbers Registry
The Stateless-Proxy option is added to the CoAP Option Numbers
registry:
+--------+-----------------+-------------------+
| Number | Name | Reference |
+--------+-----------------+-------------------+
| TBD | Stateless-Proxy | [[this document]] |
+--------+-----------------+-------------------+
15. Acknowledgments
The work on this document has been partially supported by the
European Union's H2020 Programme for research, technological
development and demonstration under grant agreement No 644852,
project ARMOUR.
The authors are grateful to Thomas Watteyne and Goeran Selander for
reviewing the draft. The authors would also like to thank Francesca
Palombini and Ludwig Seitz for participating in the discussions that
have helped shape the document.
16. References
16.1. Normative References
[I-D.ietf-core-object-security]
Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
"Object Security of CoAP (OSCOAP)", draft-ietf-core-
object-security-01 (work in progress), December 2016.
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[I-D.ietf-cose-msg]
Schaad, J., "CBOR Object Signing and Encryption (COSE)",
draft-ietf-cose-msg-24 (work in progress), November 2016.
[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>.
[RFC3172] Huston, G., Ed., "Management Guidelines & Operational
Requirements for the Address and Routing Parameter Area
Domain ("arpa")", BCP 52, RFC 3172, DOI 10.17487/RFC3172,
September 2001, <http://www.rfc-editor.org/info/rfc3172>.
[RFC7049] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049,
October 2013, <http://www.rfc-editor.org/info/rfc7049>.
[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252,
DOI 10.17487/RFC7252, June 2014,
<http://www.rfc-editor.org/info/rfc7252>.
16.2. Informative References
[I-D.ietf-6tisch-6top-protocol]
Wang, Q. and X. Vilajosana, "6top Protocol (6P)", draft-
ietf-6tisch-6top-protocol-03 (work in progress), October
2016.
[I-D.ietf-6tisch-dtsecurity-secure-join]
Richardson, M., "6tisch Secure Join protocol", draft-ietf-
6tisch-dtsecurity-secure-join-01 (work in progress),
February 2017.
[I-D.ietf-6tisch-minimal]
Vilajosana, X., Pister, K., and T. Watteyne, "Minimal
6TiSCH Configuration", draft-ietf-6tisch-minimal-21 (work
in progress), February 2017.
[I-D.ietf-6tisch-terminology]
Palattella, M., Thubert, P., Watteyne, T., and Q. Wang,
"Terminology in IPv6 over the TSCH mode of IEEE
802.15.4e", draft-ietf-6tisch-terminology-08 (work in
progress), December 2016.
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[I-D.ietf-anima-bootstrapping-keyinfra]
Pritikin, M., Richardson, M., Behringer, M., Bjarnason,
S., and K. Watsen, "Bootstrapping Remote Secure Key
Infrastructures (BRSKI)", draft-ietf-anima-bootstrapping-
keyinfra-04 (work in progress), October 2016.
[I-D.richardson-6tisch-join-enhanced-beacon]
Dujovne, D. and M. Richardson, "IEEE802.15.4 Informational
Element encapsulation of 6tisch Join Information", draft-
richardson-6tisch-join-enhanced-beacon-01 (work in
progress), March 2017.
[I-D.richardson-6tisch-minimal-rekey]
Richardson, M., "Minimal Security rekeying mechanism for
6TiSCH", draft-richardson-6tisch-minimal-rekey-01 (work in
progress), February 2017.
[I-D.selander-ace-cose-ecdhe]
Selander, G., Mattsson, J., and F. Palombini, "Ephemeral
Diffie-Hellman Over COSE (EDHOC)", draft-selander-ace-
cose-ecdhe-04 (work in progress), October 2016.
[IEEE8021542015]
IEEE standard for Information Technology, ., "IEEE Std
802.15.4-2015 Standard for Low-Rate Wireless Personal Area
Networks (WPANs)", 2015.
[RFC4231] Nystrom, M., "Identifiers and Test Vectors for HMAC-SHA-
224, HMAC-SHA-256, HMAC-SHA-384, and HMAC-SHA-512",
RFC 4231, DOI 10.17487/RFC4231, December 2005,
<http://www.rfc-editor.org/info/rfc4231>.
[RFC5869] Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
Key Derivation Function (HKDF)", RFC 5869,
DOI 10.17487/RFC5869, May 2010,
<http://www.rfc-editor.org/info/rfc5869>.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
January 2012, <http://www.rfc-editor.org/info/rfc6347>.
[RFC6775] Shelby, Z., Ed., Chakrabarti, S., Nordmark, E., and C.
Bormann, "Neighbor Discovery Optimization for IPv6 over
Low-Power Wireless Personal Area Networks (6LoWPANs)",
RFC 6775, DOI 10.17487/RFC6775, November 2012,
<http://www.rfc-editor.org/info/rfc6775>.
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[RFC7554] Watteyne, T., Ed., Palattella, M., and L. Grieco, "Using
IEEE 802.15.4e Time-Slotted Channel Hopping (TSCH) in the
Internet of Things (IoT): Problem Statement", RFC 7554,
DOI 10.17487/RFC7554, May 2015,
<http://www.rfc-editor.org/info/rfc7554>.
[RFC7721] Cooper, A., Gont, F., and D. Thaler, "Security and Privacy
Considerations for IPv6 Address Generation Mechanisms",
RFC 7721, DOI 10.17487/RFC7721, March 2016,
<http://www.rfc-editor.org/info/rfc7721>.
Appendix A. Example
Figure 6 illustrates a join protocol exchange in case PSKs are used.
Pledge instantiates the OSCOAP context and derives the traffic keys
and nonces from the PSK. It uses the instantiated context to protect
the CoAP request addressed with Proxy-Scheme option and well-known
host name of JRC in the Uri-Host option. The example assumes a JP
that is already aware of JRC's IPv6 address and does not need to
resolve the well-known "6tisch.arpa" host name. Triggered by the
presence of Proxy-Scheme option, JP forwards the request to the JRC
and adds the Stateless-Proxy option with value set to the internally
needed state, authentication tag, and a freshness indicator. Once
JRC receives the request, it looks up the correct context based on
the Sender ID (sid) parameter. It reconstructs OSCOAP's external
Additional Authenticated Data (AAD) needed for verification based on:
o Version field of the received CoAP header.
o Code field of the received CoAP header.
o Algorithm being the AES-CCM-16-64-128 from [I-D.ietf-cose-msg].
o Request ID being set to pledge's EUI-64 concatenated with 0x00.
o Request Sequence number set to the value of "Partial IV" of the
received COSE object.
Replay protection is ensured by OSCOAP and the tracking of sequence
numbers at each side. In the example below, the response contains
sequence number 7 meaning that there have already been some attempts
to join under a given context, not coming from the pledge. Once JP
receives the response, it authenticates the Stateless-Proxy option
before deciding where to forward. JP sets its internal state to that
found in the Stateless-Proxy option. Note that JP does not posses
the key to decrypt the COSE object present in the payload so the
join_response object is opaque to it. The response is matched to the
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request and verified for replay protection at pledge using OSCOAP
processing rules.
<--E2E OSCOAP-->
Client Proxy Server
Pledge JP JRC
| | |
+----->| | Code: [0.01] (GET)
| GET | | Token: 0x8c
| | | Proxy-Scheme: [coap]
| | | Uri-Host: [6tisch.arpa]
| | | Object-Security: [sid:EUI-64 | 0, seq:1,
| | | {Uri-Path:"j"},
| | | <Tag>]
| | | Payload: -
| | |
| +----->| Code: [0.01] (GET)
| | GET | Token: 0x7b
| | | Uri-Host: [6tisch.arpa]
| | | Object-Security: [sid:EUI-64 | 0, seq:1,
| | | {Uri-Path:"j"},
| | | <Tag>]
| | | Stateless-Proxy: opaque state
| | | Payload: -
| | |
| |<-----+ Code: [2.05] (Content)
| | 2.05 | Token: 0x7b
| | | Object-Security: -
| | | Stateless-Proxy: opaque state
| | | Payload: [ seq:7,
| | | {join_response}, <Tag>]
| | |
|<-----+ | Code: [2.05] (Content)
| 2.05 | | Token: 0x8c
| | | Object-Security: -
| | | Payload: [ seq:7,
| | | {join_response}, <Tag>]
| | |
Figure 6: Example of a join protocol exchange with a PSK. {} denotes
encryption and authentication, [] denotes authentication.
Where join_response is as follows.
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join_response:
[
[ / COSE Key Set array with a single key /
{
1 : 4, / key type symmetric /
2 : h'01', / key id /
-1 : h'e6bf4287c2d7618d6a9687445ffd33e6' / key value /
}
],
[
h'af93' / assigned short address /
]
]
Encodes to
h'8281a301040241012050e6bf4287c2d7618d6a9687445ffd33e68142af93' with
a size of 30 bytes.
Authors' Addresses
Malisa Vucinic
Inria
2 Rue Simone Iff
Paris 75012
France
Email: malisa.vucinic@inria.fr
Jonathan Simon
Linear Technology
32990 Alvarado-Niles Road, Suite 910
Union City, CA 94587
USA
Email: jsimon@linear.com
Kris Pister
University of California Berkeley
512 Cory Hall
Berkeley, CA 94720
USA
Email: pister@eecs.berkeley.edu
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Michael Richardson
Sandelman Software Works
470 Dawson Avenue
Ottawa, ON K1Z5V7
Canada
Email: mcr+ietf@sandelman.ca
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