Secure group communication for CoAP
draft-tiloca-core-multicast-oscoap-03
The information below is for an old version of the document.
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| Authors | Marco Tiloca , Göran Selander , Francesca Palombini | ||
| Last updated | 2017-07-26 | ||
| Replaced by | draft-ietf-core-oscore-groupcomm, draft-ietf-core-oscore-groupcomm, draft-ietf-core-oscore-groupcomm | ||
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draft-tiloca-core-multicast-oscoap-03
CoRE Working Group M. Tiloca
Internet-Draft RISE SICS AB
Intended status: Standards Track G. Selander
Expires: January 27, 2018 F. Palombini
Ericsson AB
July 26, 2017
Secure group communication for CoAP
draft-tiloca-core-multicast-oscoap-03
Abstract
This document describes a method for protecting group communication
over the Constrained Application Protocol (CoAP). The proposed
approach relies on Object Security of CoAP (OSCOAP) and the CBOR
Object Signing and Encryption (COSE) format. All security
requirements fulfilled by OSCOAP are maintained for multicast OSCOAP
request messages and related unicast OSCOAP response messages.
Source authentication of all messages exchanged within the group is
ensured, by means of digital signatures produced through private keys
of sender devices and embedded in the protected CoAP messages.
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
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 27, 2018.
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
(http://trustee.ietf.org/license-info) in effect on the date of
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publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Prerequisites and Requirements . . . . . . . . . . . . . . . 4
3. Set-up Phase . . . . . . . . . . . . . . . . . . . . . . . . 7
4. Security Context . . . . . . . . . . . . . . . . . . . . . . 8
5. Message Processing . . . . . . . . . . . . . . . . . . . . . 9
5.1. Protecting the Request . . . . . . . . . . . . . . . . . 10
5.2. Verifying the Request . . . . . . . . . . . . . . . . . . 10
5.3. Protecting the Response . . . . . . . . . . . . . . . . . 10
5.4. Verifying the Response . . . . . . . . . . . . . . . . . 11
6. The COSE Object . . . . . . . . . . . . . . . . . . . . . . . 11
7. Security Considerations . . . . . . . . . . . . . . . . . . . 13
7.1. Group-level Security . . . . . . . . . . . . . . . . . . 14
7.2. Management of Group Keying Material . . . . . . . . . . . 14
7.3. Synchronization of Sequence Numbers . . . . . . . . . . . 14
7.4. Provisioning of Public Keys . . . . . . . . . . . . . . . 16
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 17
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 17
10.1. Normative References . . . . . . . . . . . . . . . . . . 17
10.2. Informative References . . . . . . . . . . . . . . . . . 18
Appendix A. Group Joining Based on the ACE Framework . . . . . . 19
Appendix B. List of Use Cases . . . . . . . . . . . . . . . . . 20
Appendix C. No Verification of Signatures . . . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 22
1. Introduction
The Constrained Application Protocol (CoAP) [RFC7252] is a web
transfer protocol specifically designed for constrained devices and
networks [RFC7228].
Group communication for CoAP [RFC7390] addresses use cases where
deployed devices benefit from a group communication model, for
example to reduce latencies and improve performance. Use cases
include lighting control, integrated building control, software and
firmware updates, parameter and configuration updates, commissioning
of constrained networks, and emergency multicast (see Appendix B).
Furthermore, [RFC7390] recognizes the importance to introduce a
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secure mode for CoAP group communication. This specification defines
such a mode.
Object Security of CoAP (OSCOAP)[I-D.ietf-core-object-security]
describes a security protocol based on the exchange of protected CoAP
messages. OSCOAP builds on CBOR Object Signing and Encryption (COSE)
[I-D.ietf-cose-msg] and provides end-to-end encryption, integrity,
and replay protection between a sending endpoint and a receiving
endpoint across intermediary nodes. To this end, a CoAP message is
protected by including payload (if any), certain options, and header
fields in a COSE object, which finally replaces the authenticated and
encrypted fields in the protected message.
This document describes multicast OSCOAP, providing end-to-end
security of CoAP messages exchanged between members of a multicast
group. In particular, the described approach defines how OSCOAP
should be used in a group communication context, while fulfilling the
same security requirements. That is, end-to-end security is assured
for multicast CoAP requests sent by multicaster nodes to the group
and for related unicast CoAP responses sent as reply by multiple
listener nodes. Multicast OSCOAP provides source authentication of
all CoAP messages exchanged within the group, by means of digital
signatures produced through private keys of sender devices and
embedded in the protected CoAP messages. As in OSCOAP, it is still
possible to simultaneously rely on DTLS to protect hop-by-hop
communication between a multicaster node and a proxy (and vice
versa), and between a proxy and a listener node (and vice versa).
1.1. 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.
Readers are expected to be familiar with the terms and concepts
described in CoAP [RFC7252]; group communication for CoAP [RFC7390];
COSE and counter signatures [I-D.ietf-cose-msg].
Readers are also expected to be familiar with the terms and concepts
for protection and processing of CoAP messages through OSCOAP, such
as "Security Context", "Master Secret" and "Master Salt", defined in
[I-D.ietf-core-object-security].
Terminology for constrained environments, such as "constrained
device", "constrained-node network", is defined in [RFC7228].
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This document refers also to the following terminology.
o Keying material: data that is necessary to establish and maintain
secure communication among member of a multicast group. This
includes, for instance, keys and IVs [RFC4949].
o Group Manager (GM): entity responsible for creating a multicast
group, establishing and provisioning security contexts among
authorized group members, as well as managing the joining of new
group members and the leaving of current group members. A GM can
be responsible for multiple multicast groups. Besides, a GM is
not required to be an actual group member and to take part in the
group communication. The GM is also responsible for renewing/
updating security contexts and related keying material in the
multicast groups of its competence. Each endpoint in a multicast
group securely communicates with the respective GM.
o Multicaster: member of a multicast group that sends multicast CoAP
messages intended for all members of the group. In a 1-to-N
multicast group, only a single multicaster transmits data to the
group; in an M-to-N multicast group (where M and N do not
necessarily have the same value), M group members are
multicasters.
o Listener: member of a multicast group that receives multicast CoAP
messages when listening to the multicast IP address associated to
the multicast group. A listener may reply back, by sending a
unicast response message to the multicaster which has sent the
multicast message.
o Pure listener: member of a multicast group that is configured as
listener and never replies back to multicasters after receiving
multicast messages.
o Group request: multicast CoAP request message sent by a
multicaster in the group to all listeners in the group through
multicast IP, unless otherwise specified.
o Source authentication: evidence that a received message in the
group originated from a specifically identified group member.
This also provides assurances that the message was not tampered
with either by a different group member or by a non-group member.
2. Prerequisites and Requirements
The following security prerequisites are assumed to be already
fulfilled and are out of the scope of this document.
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o Establishment and management of a security context: a security
context must be established among the group members by the Group
Manager which manages the multicast group. A secure mechanism
must be used to generate, revoke and (re-)distribute keying
material, multicast security policies and security parameters in
the multicast group. The actual establishment and management of
the security context is out of the scope of this document, and it
is anticipated that an activity in IETF dedicated to the design of
a generic key management scheme will include this feature,
preferably based on [RFC3740][RFC4046][RFC4535].
o Multicast data security ciphersuite: all group members MUST agree
on a ciphersuite to provide authenticity, integrity and
confidentiality of messages in the multicast group. The
ciphersuite is specified as part of the security context.
o Backward security: a new device joining the multicast group should
not have access to any old security contexts used before its
joining. This ensures that a new group member is not able to
decrypt confidential data sent before it has joined the group.
The adopted key management scheme should ensure that the security
context is updated to ensure backward confidentiality. The actual
mechanism to update the security context and renew the group
keying material upon a group member's joining has to be defined as
part of the group key management scheme.
o Forward security: entities that leave the multicast group should
not have access to any future security contexts or message
exchanged within the group after their leaving. This ensures that
a former group member is not able to decrypt confidential data
sent within the group anymore. Also, it ensures that a former
member is not able to send encrypted and/or integrity protected
messages to the group anymore. The actual mechanism to update the
security context and renew the group keying material upon a group
member's leaving has to be defined as part of the group key
management scheme.
The following security requirements need to be fulfilled by the
approach described in this document:
o Multicast communication topology: this document considers both
1-to-N (one multicaster and multiple listeners) and M-to-N
(multiple multicasters and multiple listeners) communication
topologies. The 1-to-N communication topology is the simplest
group communication scenario that would serve the needs of a
typical low-power and lossy network (LLN). For instance, in a
typical lighting control use case, a single switch is the only
entity responsible for sending commands to a group of lighting
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devices. In more advanced lighting control use cases, a M-to-N
communication topology would be required, for instance in case
multiple sensors (presence or day-light) are responsible to
trigger events to a group of lighting devices.
o Multicast group size: security solutions for group communication
should be able to adequately support different, possibly large,
group sizes. Group size is the combination of the number of
multicasters and listeners in a multicast group, with possible
overlap (i.e. a multicaster may also be a listener at the same
time). In the use cases mentioned in this document, the number of
multicasters (normally the controlling devices) is expected to be
much smaller than the number of listeners (i.e. the controlled
devices). A security solution for group communication that
supports 1 to 50 multicasters would be able to properly cover the
group sizes required for most use cases that are relevant for this
document. The total number of group members is expected to be in
the range of 2 to 100 devices. Groups larger than that should be
divided into smaller independent multicast groups, e.g. by
grouping lights in a building on a per floor basis.
o Data replay protection: it must be possible to detect a replayed
group request message or response message.
o Group-level data confidentiality: messages sent within the
multicast group SHALL be encrypted if privacy sensitive data is
exchanged within the group. In fact, some control commands and/or
associated responses could pose unforeseen security and privacy
risks to the system users, when sent as plaintext. This document
considers group-level data confidentiality since messages are
encrypted at a group level, i.e. in such a way that they can be
decrypted by any member of the multicast group, but not by an
external adversary or other external entities.
o Source authentication: messages sent within the multicast group
SHALL be authenticated. That is, it is essential to ensure that a
message is originated by a member of the group in the first place
(group authentication), and in particular by a specific member of
the group (source authentication).
o Message integrity: messages sent within the multicast group SHALL
be integrity protected. That is, it is essential to ensure that a
message has not been tampered with by an external adversary or
other external entities which are not group members.
o Message ordering: it must be possible to determine the ordering of
messages coming from a single sender endpoint. In accordance with
OSCOAP [I-D.ietf-core-object-security], this results in providing
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relative freshness of group requests and absolute freshness of
responses. It is not required to determine ordering of messages
from different sender endpoints.
3. Set-up Phase
An endpoint joins a multicast group by explicitly interacting with
the responsible Group Manager. The actual join process can be based
on the ACE framework [I-D.ietf-ace-oauth-authz] and the OSCOAP
profile of ACE [I-D.seitz-ace-oscoap-profile], as discussed in
Appendix A.
An endpoint registered as member of a group can behave as a
multicaster and/or as a listener. As a multicaster, it can transmit
multicast request messages to the group. As a listener, it receives
multicast request messages from any multicaster in the group, and
possibly replies by transmitting unicast response messages. A pure
listener never replies to multicast request messages. Upon joining
the group, endpoints are not required to know how many and what
endpoints are active in the same group. A number of use cases that
benefit from secure group communication are discussed in Appendix B.
An endpoint is identified by an endpoint ID provided by the Group
Manager upon joining the group, unless configured exclusively as pure
listener. That is, pure listener endpoints are not associated to and
are not provided with an endpoint ID. The Group Manager generates
and manages endpoint IDs in order to ensure their uniqueness within a
same multicast group. That is, within a single multicast group, the
same endpoint ID cannot be associated to more endpoints at the same
time. Endpoint IDs are not necessarily related to any protocol-
relevant identifiers, such as IP addresses.
In order to participate in the secure group communication, an
endpoint needs to maintain a number of information elements, stored
in its own security context. Those include keying material used to
protect and verify group messages, as well as the public keys of
other endpoints in the groups, in order to verify digital signatures
of secure messages and ensure their source authenticity. The Group
Manager provides these pieces of information to an endpoint upon its
joining, through out-of-band means or other pre-established secure
channels. Further details about establishment, revocation and
renewal of the security context and keying material are out of the
scope of this document.
According to [RFC7390], any possible proxy entity is supposed to know
about the multicasters in the group and to not perform aggregation of
response messages. Also, every multicaster expects and is able to
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handle multiple unicast response messages associated to a given
multicast request message.
4. Security Context
To support multicast communication secured with OSCOAP, each endpoint
registered as member of a multicast group maintains a Security
Context as defined in Section 3 of [I-D.ietf-core-object-security].
In particular, each endpoint in a group stores:
1. one Common Context, received from the Group Manager upon joining
the multicast group and shared by all the endpoints in the group.
All the endpoints in the group agree on the same COSE AEAD
algorithm. Besides, in addition to what is defined in
[I-D.ietf-core-object-security], the Common Context stores the
following parameters:
* Context Identifier (Cid). Variable length byte string that
identifies the Security Context. The Cid used in a multicast
group is determined by the responsible Group Manager and does
not change over time. A Cid MUST be unique in the set of all
the multicast groups associated to the same Group Manager.
The choice of the Cid for a given group's Security Context is
application specific. However, Cids MUST be random as well as
long enough so that the probability of collisions is
negligible and Context Identifiers are globally unique. It is
the role of the application to specify how to handle possible
collisions.
* Counter signature algorithm. Value that identifies the
algorithm used for source authenticating messages sent within
the group, by means of a counter signature (see Section 4.5 of
[I-D.ietf-cose-msg]). Its value is immutable once the
security context is established. All the endpoints in the
group agree on the same counter signature algorithm. In the
absence of an application profile standard specifying
otherwise, a compliant application MUST implement the EdDSA
signature algorithm ed25519 [RFC8032].
2. one Sender Context, unless the endpoint is configured exclusively
as pure listener. The Sender Context is used to secure outgoing
messages and is initialized according to Section 3 of
[I-D.ietf-core-object-security], once the endpoint has joined the
multicast group. In practice, the sender endpoint shares the
same symmetric keying material stored in the Sender Context with
all the recipient endpoints receiving its outgoing OSCOAP
messages. The Sender ID in the Sender Context coincides with the
endpoint ID received upon joining the group. As stated in
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Section 3, it is responsibility of the Group Manager to assign
endpoint IDs to new joining endpoints in such a way that uniquess
is ensured within the multicast group. Besides, in addition to
what is defined in [I-D.ietf-core-object-security], the Sender
Context stores also the endpoint's public-private key pair.
3. one Recipient Context for each distinct endpoint from which
messages are received, used to process such incoming secure
messages. The endpoint creates a new Recipient Context upon
receiving an incoming message from another endpoint in the group
for the first time. In practice, the recipient endpoint shares
the symmetric keying material stored in the Recipient Context
with the associated other endpoint from which secure messages are
received. Besides, in addition to what is defined in
[I-D.ietf-core-object-security], each Recipient Context stores
also the public key of the associated other endpoint from which
secure messages are received. Possible approaches to provision
and retrieve public keys of group members are discussed in
Section 7.4.
The Sender Key/IV stored in the Sender Context and the Recipient
Keys/IVs stored in the Recipient Contexts are derived according to
the same scheme defined in Section 3.2 of
[I-D.ietf-core-object-security].
The 3-tuple (Cid, Sender ID, Partial IV) is called Transaction
Identifier (Tid), and SHALL be unique for each Master Secret. The
Tid is used as a unique challenge in the COSE object of the protected
CoAP request. The Tid is part of the Additional Authenticated Data
(AAD, see Section 5.2 of [I-D.ietf-core-object-security]) of the
protected CoAP response message, which is how unicast responses are
bound to multicast requests.
5. Message Processing
Each multicast request message and unicast response message is
protected and processed as specified in
[I-D.ietf-core-object-security], with the modifications described in
the following sections. Furthermore, error handling and processing
of invalid messages are performed according to the same principles
adopted in [I-D.ietf-core-object-security]. In particular, a
receiver endpoint MUST stop processing and reject any message which
is malformed and does not follow the format specified in Section 6.
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5.1. Protecting the Request
A multicaster endpoint transmits a secure multicast request message
as described in Section 7.1 of [I-D.ietf-core-object-security], with
the following modifications:
1. The multicaster endpoint stores the association Token - Cid. That
is, it SHALL be able to find the correct Security Context used to
protect the multicast request and verify the unicast response(s)
by using the CoAP Token used in the message exchange.
2. The multicaster endpoint computes the COSE object as defined in
Section 6 of this specification.
5.2. Verifying the Request
Upon receiving a secure multicast request message, a listener
endpoint proceeds as described in Section 7.2 of
[I-D.ietf-core-object-security], with the following modifications:
1. The listener endpoint retrieves the Context Identifier from the
"gid" parameter of the received COSE object, and uses it to
identify the correct group's Security Context.
2. The listener endpoint retrieves the Sender ID from the header of
the COSE object. Then, the Sender ID is used to retrieve the
correct Recipient Context associated to the multicaster endpoint
and used to process the request message. When receiving a secure
multicast CoAP request message from that multicaster endpoint for
the first time, the listener endpoint creates a new Recipient
Context, initializes it according to Section 3 of
[I-D.ietf-core-object-security], and includes the multicaster
endpoint's public key.
3. The listener endpoint retrieves the corresponding public key of
the multicaster endpoint from the associated Recipient Context.
Then, it verifies the counter signature and decrypts the request
message.
5.3. Protecting the Response
A listener endpoint that has received a multicast request message may
reply with a secure unicast response message, which is protected as
described in Section 7.3 of [I-D.ietf-core-object-security], with the
following modifications:
1. The listener endpoint retrieves the Transaction Identifier (Tid)
as defined in Section 4 of this specification.
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2. The listener endpoint computes the COSE object as defined in
Section 6 of this specification.
5.4. Verifying the Response
Upon receiving a secure unicast response message, a multicaster
endpoint proceeds as described in Section 7.4 of
[I-D.ietf-core-object-security], with the following modifications:
1. The multicaster endpoint retrieves the Security Context
identified by the Token of the received response message.
2. The multicaster endpoint retrieves the Sender ID from the header
of the COSE object. Then, the Sender ID is used to retrieve the
correct Recipient Context associated to the listener endpoint and
used to process the response message. When receiving a secure
CoAP response message from that listener endpoint for the first
time, the multicaster endpoint creates a new Recipient Context,
initializes it according to Section 3 of
[I-D.ietf-core-object-security], and includes the listener
endpoint's public key.
3. The multicaster endpoint retrieves the corresponding public key
of the listener endpoint from the associated Recipient Context.
Then, it verifies the counter signature and decrypts the response
message.
The mapping between unicast response messages from listener endpoints
and the associated multicast request message from a multicaster
endpoint relies on the Transaction Identifier (Tid) associated to the
secure multicast request message. The Tid is used by listener
endpoints as part of the Additional Authenticated Data when
protecting their own response message, as described in Section 4.
6. The COSE Object
When creating a protected CoAP message, an endpoint in the group
computes the COSE object using the untagged COSE_Encrypt0 structure
[I-D.ietf-cose-msg] as defined in Section 5 of
[I-D.ietf-core-object-security], with the following modifications.
1. The value of the "Partial IV" parameter in the "unprotected"
field is set to the Sequence Number used to protect the message,
and SHALL always be present in both multicast requests and
unicast responses. Specifically, a multicaster endpoint sets the
value of "Partial IV" to the Sequence Number from its own Sender
Context, upon sending a multicast request message. Furthermore,
unlike described in Section 5 of [I-D.ietf-core-object-security],
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a listener endpoint explicitly sets the value of "Partial IV" to
the Sequence Number from its own Sender Context, upon sending a
unicast response message.
2. The value of the "kid" parameter in the "unprotected" field is
set to the Sender ID of the endpoint and SHALL always be present
in both multicast requests and unicast responses.
3. The "unprotected" field of the "Headers" field SHALL include also
the following parameters:
* gid : its value is set to the Context Identifier (Cid) of the
group's Security Context. This parameter MAY be omitted if
the message is a CoAP response.
* countersign : its value is set to the counter signature of the
COSE object (Appendix C.3.3 of [I-D.ietf-cose-msg]), computed
by the endpoint by means of its own private key as described
in Section 4.5 of [I-D.ietf-cose-msg].
4. The Additional Authenticated Data (AAD) considered to compute the
COSE object is extended. In particular, the "external_aad"
considered for secure response messages SHALL include also the
following parameter:
* gid : bstr, contains the Context Idenfier (Cid) of the
Security Context considered to protect the request message
(which is same as the Cid considered to protect the response
message).
5. The compressed version of COSE defined in Section 8 of
[I-D.ietf-core-object-security] is used, with the following
additions for the encoding of the Object-Security option.
* The three least significant bit of the first byte SHALL NOT
have value 0, since the "Partial IV" parameter is always
present for both multicast requests and unicast responses.
* The fourth least significant bit of the first byte SHALL be
set to 1, to indicate the presence of the "kid" parameter in
the compressed message for both multicast requests and unicast
responses.
* The fifth least significant bit of the first byte is set to 1
if the "gid" parameter is present, or to 0 otherwise. In
order to enable secure group communication as described in
this specification, this bit SHALL be set to 1 for multicast
requests.
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* The sixth least significant bit of the first byte is set to 1
if the "countersign" parameter is present, or to 0 otherwise.
In order to ensure source authentication of group messages as
described in this specification, this bit SHALL be set to 1.
* The following n bytes (n being the value of the Partial IV
size in the first byte) encode the value of the "Partial IV",
which is always present in the compressed message.
* The following byte encodes the size of the "kid" parameter and
SHALL NOT have value 0.
* The following m bytes (m given by the previous byte) encode
the value of the "kid" parameter.
* The following byte encodes the size of the "gid" parameter and
SHALL NOT have value 0.
* The following p bytes (p given by the previous byte) encode
the value of the "gid" parameter.
* The following q bytes (q given by the counter signature
algorithm specified in the Security Context) encode the value
of the "countersign" parameter including the counter signature
of the COSE object.
* The remainining bytes encode the ciphertext.
In particular, "gid" is included as header parameter as defined in
Table 1.
+---------+-------+----------------+------------------+-------------------+
| name | label | value type | value registry | description |
+---------+-------+----------------+------------------+-------------------+
| gid | TBD | bstr | | Identifies the |
| | | | | OSCOAP group |
| | | | | security context |
+---------+-------+----------------+------------------+-------------------+
Table 1: Additional common header parameter for the COSE object
7. Security Considerations
The same security considerations from OSCOAP (Section 10 of
[I-D.ietf-core-object-security]) apply to this specification.
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Furthermore, additional security aspects to be taken into account are
discussed below.
7.1. Group-level Security
The approach described in this document relies on commonly shared
group keying material to protect communication within a multicast
group. This means that messages are encrypted at a group level
(group-level data confidentiality), i.e. they can be decrypted by any
member of the multicast group, but not by an external adversary or
other external entities.
In addition, it is required that all group members are trusted, i.e.
they do not forward the content of group messages to unauthorized
entities. However, in many use cases, the devices in the multicast
group belong to a common authority and are configured by a
commissioner. For instance, in a professional lighting scenario, the
roles of multicaster and listener are configured by the lighting
commissioner, and devices strictly follow those roles.
7.2. Management of Group Keying Material
The presented approach should take into consideration the risk of
compromise of group members. Such a risk is reduced when multicast
groups are deployed in physically secured locations, like lighting
inside office buildings. The adoption of key management schemes for
secure revocation and renewal of security contexts and group keying
material should be considered.
As stated in Section 2, it is RECOMMENDED to adopt a group key
management scheme that updates the security context and keying
material in the group, before a new endpoint joins the group or after
a currently present endpoint leaves the group. This is necessary in
order to preserve backward security and forward security in the
multicast group.
Especially in dynamic, large-scale, multicast groups where endpoints
can join and leave at any time, it is important that the considered
group key management scheme is efficient and highly scalable with the
group size, in order to limit the impact on performance due to the
security context and keying material update.
7.3. Synchronization of Sequence Numbers
Upon joining the multicast group, new listeners are not aware of the
sequence number values currently used by different multicasters to
transmit multicast request messages. This means that, when such
listeners receive a secure multicast request from a given multicaster
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for the first time, they are not able to verify if that request is
fresh and has not been replayed. In order to address this issue, a
listener can perform a challenge-response exchange with a
multicaster, by using the Repeat Option for CoAP described in
Section 2 of [I-D.amsuess-core-repeat-request-tag].
That is, upon receiving a multicast request from a particular
multicaster for the first time, the listener processes the message as
described in Section 5.2 of this specification, but, even if valid,
does not deliver it to the application. Instead, the listener
replies to the multicaster with a 4.03 Forbidden response message
including a Repeat Option, and stores the option value included
therein.
Upon receiving a 4.03 Forbidden response that includes a Repeat
Option and originates from a verified group member, a multicaster
MUST send a group request as a unicast message addressed to the same
listener, echoing the Repeat Option value. In particular, the
multicaster does not necessarily resend the same group request, but
can instead send a more recent one, if the application permits it.
This makes it possible for the multicaster to not retain previously
sent group requests for full retransmission, unless the application
explicitly requires otherwise. In either case, the multicaster uses
the sequence number value currently stored in its own Sender Context.
If the multicaster stores group requests for possible retransmission
with the Repeat Option, it should not store a given request for
longer than a pre-configured time interval. Note that the unicast
request echoing the Repeat Option is correctly treated and processed
as a group message, since the "gid" field including the Context
Identifier of the OSCOAP group's Security Context is still present in
the Object-Security Option as part of the COSE object (see
Section 6).
Upon receiving the unicast group request including the Repeat Option,
the listener verifies that the option value equals the stored and
previously sent value; otherwise, the request is silently discarded.
Then, the listener verifies that the unicast group request has been
received within a pre-configured time interval, as described in
[I-D.amsuess-core-repeat-request-tag]. In such a case, the request
is further processed and verified; otherwise, it is silently
discarded. Finally, the listener updates the Recipient Context
associated to that multicaster, by setting the Sequence Number to the
value included in the unicast group request conveying the Repeat
Option. The listener either delivers the request to the application
if it is an actual retransmission of the original one, or discard it
otherwise. Mechanisms to signal whether the resent request is a full
retransmission of the original one are out of the scope of this
specification.
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In case it does not receive a valid group request including the
Repeat Option within the configured time interval, the listener node
SHOULD perform the same challenge-response upon receiving the next
multicast request from that same multicaster.
A listener SHOULD NOT deliver group request messages from a given
multicaster to the application until one valid group request from
that same multicaster has been verified as fresh, as conveying an
echoed Repeat Option [I-D.amsuess-core-repeat-request-tag]. Also, a
listener MAY perform the challenge-response described above at any
time, if synchronization with sequence numbers of multicasters is
(believed to be) lost, for instance after a device reboot. It is the
role of the application to define under what circumstances sequence
numbers lose synchronization. This can include a minimum gap between
the sequence number of the latest accepted group request from a
multicaster and the sequence number of a group request just received
from the same multicaster. A multicaster MUST always be ready to
perform the challenge-response based on the Repeat Option in case a
listener starts it.
Note that endpoints configured as pure listeners are not able to
perform the challenge-response described above, as they do not store
a Sender Context to secure the 4.03 Forbidden response to the
multicaster. Therefore, pure listeners SHOULD adopt alternative
approaches to achieve and maintain synchronization with sequence
numbers of multicasters.
7.4. Provisioning of Public Keys
Upon receiving a secure CoAP message, a recipient endpoint relies on
the sender endpoint's public key, in order to verify the counter
signature conveyed in the COSE Object.
If not already stored in the Recipient Context associated to the
sender endpoint, the recipient endpoint retrieves the public key from
a trusted key repository. In such a case, the correct binding
between the sender endpoint and the retrieved public key MUST be
assured, for instance by means of public key certificates. Further
details about how this requirement can be fulfilled are out of the
scope of this document.
Alternatively, the Group Manager can be configured to store public
keys of group members and provide them upon request. In such a case,
upon joining a multicast group, an endpoint provides its own public
key to the Group Manager, by means of the same secure channel used to
carry out the join procedure. After that, the Group Manager MUST
verify that the joining endpoint actually owns the associated private
key, for instance by performing a proof-of-possession challenge-
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response. In case of success, the Group Manager stores the received
public key as associated to the joining endpoint and its endpoint ID.
From then on, that public key will be available for secure and
trusted delivery to other endpoints in the multicast group.
Note that a joining endpoint is not required to provide its own
public key to the Group Manager in the following two cases. First,
the endpoint is joining the multicast group exclusively as pure
listener. Second, the endpoint has already provided its own public
key, upon previously joining a multicast group under the same Group
Manager.
Furthermore, in simple, less dynamic, multicast groups, it can be
convenient for the Group Manager to provide an endpoint upon its
joining with the public keys associated to the endpoints currently
present in the group.
8. IANA Considerations
TBD. Header parameter 'gid'.
9. Acknowledgments
The authors sincerely thank Rolf Blom, Carsten Bormann, John
Mattsson, Jim Schaad, Stefan Beck, Richard Kelsey, Ludwig Seitz and
Klaus Hartke for their feedback and comments.
10. References
10.1. Normative References
[I-D.amsuess-core-repeat-request-tag]
Amsuess, C., Mattsson, J., and G. Selander, "Repeat And
Request-Tag", draft-amsuess-core-repeat-request-tag-00
(work in progress), July 2017.
[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-04 (work in progress), July 2017.
[I-D.ietf-cose-msg]
Schaad, J., "CBOR Object Signing and Encryption (COSE)",
draft-ietf-cose-msg-24 (work in progress), November 2016.
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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>.
[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>.
[RFC8032] Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital
Signature Algorithm (EdDSA)", RFC 8032,
DOI 10.17487/RFC8032, January 2017,
<http://www.rfc-editor.org/info/rfc8032>.
10.2. Informative References
[I-D.ietf-ace-oauth-authz]
Seitz, L., Selander, G., Wahlstroem, E., Erdtman, S., and
H. Tschofenig, "Authentication and Authorization for
Constrained Environments (ACE)", draft-ietf-ace-oauth-
authz-06 (work in progress), March 2017.
[I-D.seitz-ace-oscoap-profile]
Seitz, L., Palombini, F., and M. Gunnarsson, "OSCOAP
profile of the Authentication and Authorization for
Constrained Environments Framework", draft-seitz-ace-
oscoap-profile-04 (work in progress), July 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-07 (work in progress), July 2017.
[I-D.somaraju-ace-multicast]
Somaraju, A., Kumar, S., Tschofenig, H., and W. Werner,
"Security for Low-Latency Group Communication", draft-
somaraju-ace-multicast-02 (work in progress), October
2016.
[RFC3740] Hardjono, T. and B. Weis, "The Multicast Group Security
Architecture", RFC 3740, DOI 10.17487/RFC3740, March 2004,
<http://www.rfc-editor.org/info/rfc3740>.
[RFC4046] Baugher, M., Canetti, R., Dondeti, L., and F. Lindholm,
"Multicast Security (MSEC) Group Key Management
Architecture", RFC 4046, DOI 10.17487/RFC4046, April 2005,
<http://www.rfc-editor.org/info/rfc4046>.
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[RFC4535] Harney, H., Meth, U., Colegrove, A., and G. Gross,
"GSAKMP: Group Secure Association Key Management
Protocol", RFC 4535, DOI 10.17487/RFC4535, June 2006,
<http://www.rfc-editor.org/info/rfc4535>.
[RFC4944] Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
"Transmission of IPv6 Packets over IEEE 802.15.4
Networks", RFC 4944, DOI 10.17487/RFC4944, September 2007,
<http://www.rfc-editor.org/info/rfc4944>.
[RFC4949] Shirey, R., "Internet Security Glossary, Version 2",
FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,
<http://www.rfc-editor.org/info/rfc4949>.
[RFC6282] Hui, J., Ed. and P. Thubert, "Compression Format for IPv6
Datagrams over IEEE 802.15.4-Based Networks", RFC 6282,
DOI 10.17487/RFC6282, September 2011,
<http://www.rfc-editor.org/info/rfc6282>.
[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>.
[RFC6749] Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
RFC 6749, DOI 10.17487/RFC6749, October 2012,
<http://www.rfc-editor.org/info/rfc6749>.
[RFC7228] Bormann, C., Ersue, M., and A. Keranen, "Terminology for
Constrained-Node Networks", RFC 7228,
DOI 10.17487/RFC7228, May 2014,
<http://www.rfc-editor.org/info/rfc7228>.
[RFC7390] Rahman, A., Ed. and E. Dijk, Ed., "Group Communication for
the Constrained Application Protocol (CoAP)", RFC 7390,
DOI 10.17487/RFC7390, October 2014,
<http://www.rfc-editor.org/info/rfc7390>.
Appendix A. Group Joining Based on the ACE Framework
The join process to register an endpoint as a new member of a
multicast group can be based on the ACE framework
[I-D.ietf-ace-oauth-authz] and the OSCOAP profile of ACE
[I-D.seitz-ace-oscoap-profile]. With reference to the terminology
defined in OAuth 2.0 [RFC6749]:
o The joining endpoint acts as Client;
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o The Group Manager acts as Resource Server, exporting one join-
resource for each multicast group it is responsible for;
o An Authorization Server enables and enforces authorized access of
the joining endpoint to the Group Manager and its join-resources.
Then, in accordance with [I-D.seitz-ace-oscoap-profile], the joining
endpoint and the Group Manager rely on OSCOAP
[I-D.ietf-core-object-security] for secure communication and can use
Ephemeral Diffie-Hellman Over COSE (EDHOC)
[I-D.selander-ace-cose-ecdhe] as a possible method to establish key
material.
The joining endpoint sends to the Group Manager an OSCOAP request to
access the join-resource associated to the multicast group to join.
The Group Manager replies with an OSCOAP response including the
Common Context associated to that group (see Section 4). In case the
Group Manager is configured to store the public keys of group
members, the joining endpoint additionally provides the Group Manager
with its own public key, and MAY obtain from the Group Manager the
public keys of the endpoints currently present in the group (see
Section 7.4).
Both the joining endpoint and the Group Manager MUST adopt secure
communication also for any message exchange with the Authorization
Server. To this end, different alternatives are possible, including
OSCOAP and DTLS [RFC6347].
Appendix B. List of Use Cases
Group Communication for CoAP [RFC7390] provides the necessary
background for multicast-based CoAP communication, with particular
reference to low-power and lossy networks (LLNs) and resource
constrained environments. The interested reader is encouraged to
first read [RFC7390] to understand the non-security related details.
This section discusses a number of use cases that benefit from secure
group communication. Specific security requirements for these use
cases are discussed in Section 2.
o Lighting control: consider a building equipped with IP-connected
lighting devices, switches, and border routers. The devices are
organized into groups according to their physical location in the
building. For instance, lighting devices and switches in a room
or corridor can be configured as members of a single multicast
group. Switches are then used to control the lighting devices by
sending on/off/dimming commands to all lighting devices in a
group, while border routers connected to an IP network backbone
(which is also multicast-enabled) can be used to interconnect
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routers in the building. Consequently, this would also enable
logical multicast groups to be formed even if devices in the
lighting group may be physically in different subnets (e.g. on
wired and wireless networks). Connectivity between ligthing
devices may be realized, for instance, by means of IPv6 and
(border) routers supporting 6LoWPAN [RFC4944][RFC6282]. Group
communication enables synchronous operation of a group of
connected lights, ensuring that the light preset (e.g. dimming
level or color) of a large group of luminaires are changed at the
same perceived time. This is especially useful for providing a
visual synchronicity of light effects to the user. Devices may
reply back to the switches that issue on/off/dimming commands, in
order to report about the execution of the requested operation
(e.g. OK, failure, error) and their current operational status.
o Integrated building control: enabling Building Automation and
Control Systems (BACSs) to control multiple heating, ventilation
and air-conditioning units to pre-defined presets. Controlled
units can be organized into multicast groups in order to reflect
their physical position in the building, e.g. devices in the same
room can be configured as members of a single multicast group.
Furthermore, controlled units are expected to possibly reply back
to the BACS issuing control commands, in order to report about the
execution of the requested operation (e.g. OK, failure, error)
and their current operational status.
o Software and firmware updates: software and firmware updates often
comprise quite a large amount of data. This can overload a LLN
that is otherwise typically used to deal with only small amounts
of data, on an infrequent base. Rather than sending software and
firmware updates as unicast messages to each individual device,
multicasting such updated data to a larger group of devices at
once displays a number of benefits. For instance, it can
significantly reduce the network load and decrease the overall
time latency for propagating this data to all devices. Even if
the complete whole update process itself is secured, securing the
individual messages is important, in case updates consist of
relatively large amounts of data. In fact, checking individual
received data piecemeal for tampering avoids that devices store
large amounts of partially corrupted data and that they detect
tampering hereof only after all data has been received. Devices
receiving software and firmware updates are expected to possibly
reply back, in order to provide a feedback about the execution of
the update operation (e.g. OK, failure, error) and their current
operational status.
o Parameter and configuration update: by means of multicast
communication, it is possible to update the settings of a group of
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similar devices, both simultaneously and efficiently. Possible
parameters are related, for instance, to network load management
or network access controls. Devices receiving parameter and
configuration updates are expected to possibly reply back, to
provide a feedback about the execution of the update operation
(e.g. OK, failure, error) and their current operational status.
o Commissioning of LLNs systems: a commissioning device is
responsible for querying all devices in the local network or a
selected subset of them, in order to discover their presence, and
be aware of their capabilities, default configuration, and
operating conditions. Queried devices displaying similarities in
their capabilities and features, or sharing a common physical
location can be configured as members of a single multicast group.
Queried devices are expected to reply back to the commissioning
device, in order to notify their presence, and provide the
requested information and their current operational status.
o Emergency multicast: a particular emergency related information
(e.g. natural disaster) is generated and multicast by an emergency
notifier, and relayed to multiple devices. The latters may reply
back to the emergency notifier, in order to provide their feedback
and local information related to the ongoing emergency.
Appendix C. No Verification of Signatures
Some application scenarios based on group communication can display
particularly strict requirements, for instance low message latency in
non-emergency lighting applications [I-D.somaraju-ace-multicast]. In
such and similar non-critical applications with performance
constraints and more relaxed security requirements, it can be
inconvenient for some endpoints to verify digital signatures in order
to assert source authenticity of received group messages.
Although it is NOT RECOMMENDED by this specification, such endpoints
may optionally not verify the counter signature of received group
messages. As a consequence, they assert only group-authenticity of
received group messages, when decrypting them by means of the AEAD
algorithm and the Sender Key/IV used by the sender endpoint. That
is, such endpoints have evidence that a received message has been
originated by a group member, although not specifically identifiable
in a secure way.
Authors' Addresses
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Marco Tiloca
RISE SICS AB
Isafjordsgatan 22
Kista SE-16440 Stockholm
Sweden
Email: marco.tiloca@ri.se
Goeran Selander
Ericsson AB
Farogatan 6
Kista SE-16480 Stockholm
Sweden
Email: goran.selander@ericsson.com
Francesca Palombini
Ericsson AB
Farogatan 6
Kista SE-16480 Stockholm
Sweden
Email: francesca.palombini@ericsson.com
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