CoRE Working Group M. Tiloca
Internet-Draft RISE SICS AB
Intended status: Standards Track G. Selander
Expires: August 16, 2018 F. Palombini
Ericsson AB
J. Park
Universitaet Duisburg-Essen
February 12, 2018
Secure group communication for CoAP
draft-ietf-core-oscore-groupcomm-00
Abstract
This document describes a method for protecting group communication
over the Constrained Application Protocol (CoAP). The proposed
approach relies on Object Security for Constrained RESTful
Environments (OSCORE) and the CBOR Object Signing and Encryption
(COSE) format. All security requirements fulfilled by OSCORE are
maintained for multicast OSCORE request messages and related OSCORE
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
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on August 16, 2018.
Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Assumptions and Security Objectives . . . . . . . . . . . . . 5
2.1. Assumptions . . . . . . . . . . . . . . . . . . . . . . . 5
2.2. Security Objectives . . . . . . . . . . . . . . . . . . . 7
3. OSCORE Security Context . . . . . . . . . . . . . . . . . . . 7
3.1. Management of Group Keying Material . . . . . . . . . . . 9
4. The COSE Object . . . . . . . . . . . . . . . . . . . . . . . 10
5. Message Processing . . . . . . . . . . . . . . . . . . . . . 12
5.1. Protecting the Request . . . . . . . . . . . . . . . . . 12
5.2. Verifying the Request . . . . . . . . . . . . . . . . . . 13
5.3. Protecting the Response . . . . . . . . . . . . . . . . . 13
5.4. Verifying the Response . . . . . . . . . . . . . . . . . 13
6. Synchronization of Sequence Numbers . . . . . . . . . . . . . 14
7. Security Considerations . . . . . . . . . . . . . . . . . . . 14
7.1. Group-level Security . . . . . . . . . . . . . . . . . . 15
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 15
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 15
10.1. Normative References . . . . . . . . . . . . . . . . . . 15
10.2. Informative References . . . . . . . . . . . . . . . . . 16
Appendix A. List of Use Cases . . . . . . . . . . . . . . . . . 18
Appendix B. Example of Group Identifier Format . . . . . . . . . 20
Appendix C. Set-up of New Endpoints . . . . . . . . . . . . . . 21
C.1. Join Process . . . . . . . . . . . . . . . . . . . . . . 21
C.2. Provisioning and Retrieval of Public Keys . . . . . . . . 23
C.3. Group Joining Based on the ACE Framework . . . . . . . . 24
Appendix D. Examples of Synchronization Approaches . . . . . . . 25
D.1. Best-Effort Synchronization . . . . . . . . . . . . . . . 25
D.2. Baseline Synchronization . . . . . . . . . . . . . . . . 25
D.3. Challenge-Response Synchronization . . . . . . . . . . . 26
Appendix E. No Verification of Signatures . . . . . . . . . . . 27
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 28
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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 A).
Furthermore, [RFC7390] recognizes the importance to introduce a
secure mode for CoAP group communication. This specification defines
such a mode.
Object Security for Constrained RESTful Environments
(OSCORE)[I-D.ietf-core-object-security] describes a security protocol
based on the exchange of protected CoAP messages. OSCORE builds on
CBOR Object Signing and Encryption (COSE) [RFC8152] 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 OSCORE, providing end-to-end
security of CoAP messages exchanged between members of a multicast
group. In particular, the described approach defines how OSCORE
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 CoAP responses sent as reply by multiple listener
nodes. Multicast OSCORE 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 OSCORE, 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", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
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14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
Readers are expected to be familiar with the terms and concepts
described in CoAP [RFC7252]; group communication for CoAP [RFC7390];
COSE and counter signatures [RFC8152].
Readers are also expected to be familiar with the terms and concepts
for protection and processing of CoAP messages through OSCORE, 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].
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. 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 handle multiple response messages
associated to a given multicast request message that it has
previously sent to the group.
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
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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 Endpoint ID: identifier assigned by the Group Manager to an
endpoint upon joining the group as a new member, unless configured
exclusively as pure listener. 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.
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. Assumptions and Security Objectives
This section presents a set of assumptions and security objectives
for the approach described in this document.
2.1. Assumptions
The following assumptions are assumed to be already addressed and are
out of the scope of 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
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.
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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 Establishment and management of Security Contexts: 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
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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.
2.2. Security Objectives
The approach described in this document aims at fulfilling the
following security objectives:
o Data replay protection: replayed group request messages or
response messages MUST be detected.
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
OSCORE [I-D.ietf-core-object-security], this results in providing
relative freshness of group requests and absolute freshness of
responses. It is not required to determine ordering of messages
from different sender endpoints.
3. OSCORE Security Context
To support multicast communication secured with OSCORE, 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:
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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. In addition to what is defined in Section 3 of
[I-D.ietf-core-object-security], the Common Context includes the
following information.
* Group Identifier (Gid). Variable length byte string
identifying the Security Context and used as Master Salt
parameter in the derivation of keying material. The Gid is
used together with the multicast IP address of the group to
retrieve the Security Context, upon receiving a secure
multicast request message (see Section 5.2). The Gid
associated to a multicast group is determined by the
responsible Group Manager. The choice of the Gid for a given
group's Security Context is application specific. However, a
Gid MUST be random as well as long enough, in order to achieve
a negligible probability of collisions between Group
Identifiers from different Group Managers. It is the role of
the application to specify how to handle possible collisions.
An example of specific formatting of the Group Identifier that
would follow this specification is given in Appendix B.
* Counter signature algorithm. Value identifying the algorithm
used for source authenticating messages sent within the group,
by means of a counter signature (see Section 4.5 of
[RFC8152]). Its value is immutable once the Security Context
is established. All the endpoints in the group agree on the
same counter signature algorithm. The Group Manager MUST
define a list of supported signature algorithms as part of the
group communication policy. Such a list MUST include 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 OSCORE
messages. The Sender ID in the Sender Context coincides with the
Endpoint ID received upon joining the group. 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.
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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.
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.
It is RECOMMENDED that the Group Manager acts as trusted key
repository, and hence is configured to store public keys of group
members and provide them to other members of the same group upon
request. Possible approaches to provision public keys upon joining
the group and to retrieve public keys of group members are discussed
in Appendix C.2.
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].
3.1. Management of Group Keying Material
The approach described in this specification should take into account
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. Nevertheless, the adoption of key
management schemes for secure revocation and renewal of Security
Contexts and group keying material should be considered.
Consistently with the security assumptions in Section 2, it is
RECOMMENDED to adopt a group key management scheme, and securely
distribute a new value for the Master Secret parameter of the group's
Security Context, before a new joining endpoint is added to the group
or after a currently present endpoint leaves the group. This is
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necessary in order to preserve backward security and forward security
in the multicast group. The Group Manager responsible for the group
is entrusted with such a task.
In particular, the Group Manager MUST distribute also a new Group
Identifier (Gid) for that group, together with a new value for the
Master Secret parameter. An example of how this can be done is
provided in Appendix B. Then, each group member re-derives the
keying material stored in its own Sender Context and Recipient
Contexts as described in Section 3, using the updated Group
Identifier.
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.
4. The COSE Object
When creating a protected CoAP message, an endpoint in the group
computes the COSE object using the untagged COSE_Encrypt0 structure
[RFC8152] as defined in Section 5 of [I-D.ietf-core-object-security],
with the following modifications.
o The value of the "kid" parameter in the "unprotected" field of
responses SHALL be set to the Sender ID of the endpoint
transmitting the group message.
o The "unprotected" field of the "Headers" field SHALL additionally
include the following parameters:
* gid : its value is set to the Group Identifier (Gid) 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 [RFC8152]), computed by the
endpoint by means of its own private key as described in
Section 4.5 of [RFC8152].
In particular, "gid" is included as COSE header parameter as defined
in Figure 1.
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+------+-------+------------+----------------+-------------------+
| name | label | value type | value registry | description |
+------+-------+------------+----------------+-------------------+
| gid | TBD | bstr | | Identifies the |
| | | | | OSCORE group |
| | | | | Security Context |
+------+-------+------------+----------------+-------------------+
Figure 1: Additional common header parameter for the COSE object
o The Additional Authenticated Data (AAD) considered to compute the
COSE object is extended, in order to include also the Group
Identifier (Gid) of the Security Context used to protect the
request message. In particular, the "external_aad" in Section 5.3
of [I-D.ietf-core-object-security] SHALL include also gid as
follows:
external_aad = [
version : uint,
alg : int,
request_kid : bstr,
request_piv : bstr,
gid : bstr,
options : bstr
]
o The OSCORE compression 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 fourth least significant bit of the first byte of the
object-security option value SHALL be set to 1, to indicate the
presence of the "kid" parameter for both multicast requests and
responses.
* The fifth least significant bit of the first byte MUST be set
to 1 for multicast requests, to indicate the presence of the
Context Hint in the OSCORE payload. The Context Hint flag MAY
be set to 1 for responses.
* 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 Context Hint value encodes the Group Identifier value (Gid)
of the group's Security Context.
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* 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 remaining bytes in the Object-Security value encode the
value of the "kid" parameter, which is always present both in
multicast requests and in responses.
0 1 2 3 4 5 6 7 <----------- n bytes -----------> <-- 1 byte -->
+-+-+-+-+-+-+-+-+---------------------------------+--------------+
|0 0|1|h|1| n | Partial IV (if any) | s (if any) |
+-+-+-+-+-+-+-+-+---------------------------------+--------------+
<------ s bytes ------> <--------- q bytes --------->
-----------------------+-----------------------------+-----------+
Gid (if any) | countersign | kid |
-----------------------+-----------------------------+-----------+
Figure 2: Object-Security Value
5. Message Processing
Each multicast request message and response message is protected and
processed as specified in [I-D.ietf-core-object-security], with the
modifications described in the following sections.
Furthermore, endpoints in the multicast group locally perform error
handling and processing of invalid messages according to the same
principles adopted in [I-D.ietf-core-object-security]. However, a
receiver endpoint MUST stop processing and silently reject any
message which is malformed and does not follow the format specified
in Section 4, without sending back any error message. This prevents
listener endpoints from sending multiple error messages to a
multicaster endpoint, so avoiding the risk of flooding the multicast
group.
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 - Group
Identifier. That is, it SHALL be able to find the correct
Security Context used to protect the multicast request and verify
the response(s) by using the CoAP Token used in the message
exchange.
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2. The multicaster computes the COSE object as defined in Section 4
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 Group Identifier from the
"gid" parameter of the received COSE object. Then, it uses the
Group Identifier together with the destination IP address of the
multicast request message to identify the correct group's
Security Context.
2. The listener endpoint retrieves the Sender ID from the "kid"
parameter of the received 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 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 computes the COSE object as defined in
Section 4 of this specification.
5.4. Verifying the Response
Upon receiving a secure 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 by using
the Token of the received response message.
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2. The multicaster endpoint retrieves the Sender ID from the "kid"
parameter of the received 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 response messages from listener endpoints and the
associated multicast request message from a multicaster endpoint
relies on the 3-tuple (Group ID, Sender ID, Partial IV) associated to
the secure multicast request message. This is used by listener
endpoints as part of the Additional Authenticated Data when
protecting their own response message, as described in Section 4.
6. 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
for the first time, they are not able to verify if that request is
fresh and has not been replayed. The same applies when a listener
endpoint loses synchronization with sequence numbers of multicasters,
for instance after a device reboot.
The exact way to address this issue depends on the specific use case
and its synchronization requirements. The Group Manager should
define also how to handle synchronization of sequence numbers, as
part of the policies enforced in the multicast group. In particular,
the Group Manager can suggest to single specific listener endpoints
how they can exceptionally behave in order to synchronize with
sequence numbers of multicasters. Appendix D describes three
possible approaches that can be considered.
7. Security Considerations
The same security considerations from OSCORE (Section 11 of
[I-D.ietf-core-object-security]) apply to this specification.
Additional security aspects to be taken into account are discussed
below.
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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.
8. IANA Considerations
TBD. Header parameter 'gid'.
9. Acknowledgments
The authors sincerely thank Stefan Beck, Rolf Blom, Carsten Bormann,
Klaus Hartke, Richard Kelsey, John Mattsson, Jim Schaad and Ludwig
Seitz for their feedback and comments.
10. References
10.1. Normative References
[I-D.ietf-core-object-security]
Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
"Object Security for Constrained RESTful Environments
(OSCORE)", draft-ietf-core-object-security-08 (work in
progress), January 2018.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252,
DOI 10.17487/RFC7252, June 2014,
<https://www.rfc-editor.org/info/rfc7252>.
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[RFC8032] Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital
Signature Algorithm (EdDSA)", RFC 8032,
DOI 10.17487/RFC8032, January 2017,
<https://www.rfc-editor.org/info/rfc8032>.
[RFC8152] Schaad, J., "CBOR Object Signing and Encryption (COSE)",
RFC 8152, DOI 10.17487/RFC8152, July 2017,
<https://www.rfc-editor.org/info/rfc8152>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
10.2. Informative 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.aragon-ace-ipsec-profile]
Aragon, S., Tiloca, M., and S. Raza, "IPsec profile of
ACE", draft-aragon-ace-ipsec-profile-01 (work in
progress), October 2017.
[I-D.ietf-ace-dtls-authorize]
Gerdes, S., Bergmann, O., Bormann, C., Selander, G., and
L. Seitz, "Datagram Transport Layer Security (DTLS)
Profiles for Authentication and Authorization for
Constrained Environments (ACE)", draft-ietf-ace-dtls-
authorize-02 (work in progress), October 2017.
[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-09 (work in progress), November 2017.
[I-D.ietf-ace-oscore-profile]
Seitz, L., Palombini, F., and M. Gunnarsson, "OSCORE
profile of the Authentication and Authorization for
Constrained Environments Framework", draft-ietf-ace-
oscore-profile-00 (work in progress), December 2017.
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[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.
[I-D.tiloca-ace-oscoap-joining]
Tiloca, M. and J. Park, "Joining of OSCORE multicast
groups in ACE", draft-tiloca-ace-oscoap-joining-02 (work
in progress), October 2017.
[RFC2093] Harney, H. and C. Muckenhirn, "Group Key Management
Protocol (GKMP) Specification", RFC 2093,
DOI 10.17487/RFC2093, July 1997,
<https://www.rfc-editor.org/info/rfc2093>.
[RFC2094] Harney, H. and C. Muckenhirn, "Group Key Management
Protocol (GKMP) Architecture", RFC 2094,
DOI 10.17487/RFC2094, July 1997,
<https://www.rfc-editor.org/info/rfc2094>.
[RFC2627] Wallner, D., Harder, E., and R. Agee, "Key Management for
Multicast: Issues and Architectures", RFC 2627,
DOI 10.17487/RFC2627, June 1999,
<https://www.rfc-editor.org/info/rfc2627>.
[RFC3376] Cain, B., Deering, S., Kouvelas, I., Fenner, B., and A.
Thyagarajan, "Internet Group Management Protocol, Version
3", RFC 3376, DOI 10.17487/RFC3376, October 2002,
<https://www.rfc-editor.org/info/rfc3376>.
[RFC3740] Hardjono, T. and B. Weis, "The Multicast Group Security
Architecture", RFC 3740, DOI 10.17487/RFC3740, March 2004,
<https://www.rfc-editor.org/info/rfc3740>.
[RFC3810] Vida, R., Ed. and L. Costa, Ed., "Multicast Listener
Discovery Version 2 (MLDv2) for IPv6", RFC 3810,
DOI 10.17487/RFC3810, June 2004,
<https://www.rfc-editor.org/info/rfc3810>.
[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,
<https://www.rfc-editor.org/info/rfc4046>.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, DOI 10.17487/RFC4301,
December 2005, <https://www.rfc-editor.org/info/rfc4301>.
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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,
<https://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,
<https://www.rfc-editor.org/info/rfc4944>.
[RFC4949] Shirey, R., "Internet Security Glossary, Version 2",
FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,
<https://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,
<https://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, <https://www.rfc-editor.org/info/rfc6347>.
[RFC6749] Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
RFC 6749, DOI 10.17487/RFC6749, October 2012,
<https://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,
<https://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,
<https://www.rfc-editor.org/info/rfc7390>.
Appendix A. 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.
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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
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
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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
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 B. Example of Group Identifier Format
This section provides an example of how the Group Identifier (Gid)
can be specifically formatted. That is, the Gid can be composed of
two parts, namely a Group Prefix and a Group Epoch.
The Group Prefix is uniquely defined in the set of all the multicast
groups associated to the same Group Manager. The choice of the Group
Prefix for a given group's Security Context is application specific.
Group Prefixes are random as well as long enough, in order to achieve
a negligible probability of collisions between Group Identifiers from
different Group Managers.
The Group Epoch is set to 0 upon the group's initialization, and is
incremented by 1 upon completing each renewal of the Security Context
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and keying material in the group (see Section 3.1). In particular,
once a new Master Secret has been distributed to the group, all the
group members increment by 1 the Group Epoch in the Group Identifier
of that group (see Section 3).
Appendix C. Set-up of New Endpoints
An endpoint joins a multicast group by explicitly interacting with
the responsible Group Manager. All communications between a joining
endpoint and the Group Manager rely on the CoAP protocol and MUST be
secured. Specific details on how to secure communications between
joining endpoints and a Group Manager are out of the scope of this
specification.
In order to receive multicast messages sent to the group, a joining
endpoint has to register with a network router device
[RFC3376][RFC3810], signaling its intent to receive packets sent to
the multicast IP address of that group. As a particular case, the
Group Manager can also act as such a network router device. Upon
joining the group, endpoints are not required to know how many and
what endpoints are active in the same group.
Furthermore, 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 (see Section 3). The
following Appendix C.1 describes which of this information is
provided to an endpoint upon joining a multicast group through the
responsible Group Manager.
C.1. Join Process
An endpoint requests to join a multicast group by sending a
confirmable CoAP POST request to the Group Manager responsible for
that group. The join request is addressed to a CoAP resource
associated to that group and carries the following information.
o Role: the exact role of the joining endpoint in the multicast
group. Possible values are: "multicaster", "listener", "pure
listener", "multicaster and listener", or "multicaster and pure
listener".
o Identity credentials: information elements to enforce source
authentication of group messages from the joining endpoint, such
as its public key. The exact content depends on whether the Group
Manager is configured to store the public keys of group members.
If this is the case, this information is omitted if it has been
provided to the same Group Manager upon previously joining the
same or a different multicast group under its control. This
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information is also omitted if the joining endpoint is configured
exclusively as pure listener for the joined group. Appendix C.2
discusses additional details on provisioning of public keys and
other information to enforce source authentication of joining
node's messages.
o Retrieval flag: indication of interest to receive the public keys
of the endpoints currently in the multicast group, as included in
the following join response. This flag MUST be set to false if
the Group Manager is not configured to store the public keys of
group members, or if the joining endpoint is configured
exclusively as pure listener for the joined group.
The Group Manager MUST be able to verify that the joining enpoint is
authorized to become a member of the multicast group. To this end,
the Group Manager can directly authorize the joining endpoint, or
expect it to provide authorization evidence previously obtained from
a trusted entity. Appendix C.3 describes how this can be achieved by
leveraging the ACE framework for Authentication and Authorization in
constrained environments [I-D.ietf-ace-oauth-authz].
In case of successful authorization check, the Group Manager
generates an Endpoint ID assigned to the joining node, before
proceeding with the rest of the join process. Instead, in case the
authorization check fails, the Group Manager MUST abort the join
process. Further details about the authorization of joining endpoint
are out of the scope of this specification.
As discussed in Section 3.1, it is then RECOMMENDED that the Security
Context is renewed before the joining endpoint becomes a new active
member of the multicast group. This is achieved by securely
distributing a new Master Secret and a new Group Identifier to the
endpoints currently present in the same group.
Once renewed the Security Context in the multicast group, the Group
Manager replies to the joining endpoint with a CoAP response carrying
the following information.
o Security Common Context: the OSCORE Security Common Context
associated to the joined multicast group (see Section 3).
o Endpoint ID: the Endpoint ID associated to the joining node. This
information is not included in case "Role" in the join request is
equal to "pure listener".
o Management keying material: the set of administrative keying
material used to participate in the group rekeying process run by
the Group Manager (see Section 3.1). The specific elements of
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this management keying material depend on the group rekeying
protocol used in the group. For instance, this can simply consist
in a group key encryption key and a pairwise symmetric key shared
between the joining node and the Group Manager, in case GKMP
[RFC2093][RFC2094] is used. Instead, if key-tree based rekeying
protocols like LKH [RFC2627] are used, it can consist in the set
of symmetric keys associated to the key-tree leaf representing the
group member up to the key-tree root representing the group key
encryption key.
o Member public keys: the public keys of the endpoints currently
present in the multicast group. This includes: the public keys of
the non-pure listeners currently in the group, if the joining
endpoint is configured (also) as multicaster; and the public keys
of the multicasters currently in the group, if the joining
endpoint is configured (also) as listener or pure listener. This
information is omitted in case the Group Manager is not configured
to store the public keys of group members or if the "Retrieval
flag" was set to false in the join request. Appendix C.2
discusses additional details on provisioning public keys upon
joining the group and on retrieving public keys of group members.
C.2. Provisioning and Retrieval of Public Keys
As mentioned in Section 3, it is RECOMMENDED that the Group Manager
acts as trusted key repository, stores public keys of group members
and provide them to other members of the same group upon request. In
such a case, a joining endpoint provides its own public key to the
Group Manager, as "Identity credentials" of the join request, when
joining the multicast group (see Appendix C.1).
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-response. In case of success, the
Group Manager stores the received public key as associated to the
joining endpoint and its Endpoint ID, before sending the join
response and continuing with the rest of the join process. From then
on, that public key will be available for secure and trusted delivery
to other endpoints in the multicast group.
The joining node does not have to provide its own public key if that
already occurred upon previously joining the same or a different
multicast group under the same Group Manager. However, separately
for each multicast group under its control, the Group Manager
maintains an updated list of active Endpoint IDs associated to a same
endpoint's public key.
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Instead, in case the Group Manager does not act as trusted key
repository, the following information is exchanged with the Group
Manager during the join process.
1. The joining endpoint signs its own certificate by using its own
private key. There is no restriction on the Certificate Subject
included in the joining node's certificate.
2. The joining endpoint includes the following information as
"Identity credentials" in the join request (Appendix C.1): the
signed certificate; and the identifier of the Certification
Authority that issued the certificate. The joining endpoint can
optionally specify also a list of public key repositories storing
its own certificate.
3. When processing the join request, the Group Manager first
validates the certificate by verifying the signature of the
issuer CA, and then verifies the signature of the joining node.
4. The Group Manager stores the association between the Certificate
Subject of the joining node's certificate and the pair {Group ID,
Endpoint ID of the joining node}. If received from the joining
endpoint, the Group Manager also stores the list of public key
repositories storing the certificate of the joining endpoint.
When a group member X wants to retrieve the public key of another
group member Y in the same multicast group, the endpoint X proceeds
as follows.
1. The endpoint X contacts the Group Manager, specifying the pair
{Group ID, Endpoint ID of the endpoint Y}.
2. The Group Manager provides the endpoint X with the Certificate
Subject CS from the certificate of endpoint Y. If available, the
Group Manager provides the endpoint X also with the list of
public key repositories storing the certificate of the endpoint
Y.
3. The endpoint X retrieves the certificate of the endpoint X from a
key repository storing it, by using the Certificate Subject CS.
C.3. 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 for Authentication
and Authorization in constrained environments
[I-D.ietf-ace-oauth-authz], built on re-use of OAuth 2.0 [RFC6749].
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In particular, the approach described in
[I-D.tiloca-ace-oscoap-joining] uses the ACE framework to delegate
the authentication and authorization of joining endpoints to an
Authorization Server in a trust relation with the Group Manager. At
the same time, it allows a joining endpoint to establish a secure
channel with the Group Manager, by leveraging protocol-specific
profiles of ACE [I-D.ietf-ace-oscore-profile]
[I-D.ietf-ace-dtls-authorize] [I-D.aragon-ace-ipsec-profile] to
achieve communication security, proof-of-possession and server
authentication.
More specifically and with reference to the terminology defined in
OAuth 2.0:
o The joining endpoint acts as Client;
o The Group Manager acts as Resource Server, with different CoAP
resources for different multicast groups it is responsible for;
o An Authorization Server enables and enforces authorized access of
the joining endpoint to the Group Manager and its CoAP resources
paired with multicast groups to join.
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, such as
OSCORE, DTLS [RFC6347] or IPsec [RFC4301].
Appendix D. Examples of Synchronization Approaches
This section describes three possible approaches that can be
considered by listener endpoints to synchronize with sequence numbers
of multicasters.
D.1. Best-Effort Synchronization
Upon receiving a multicast request from a multicaster, a listener
endpoint does not take any action to synchonize with the sequence
number of that multicaster. This provides no assurance at all as to
message freshness, which can be acceptable in non-critical use cases.
D.2. Baseline Synchronization
Upon receiving a multicast request from a given multicaster for the
first time, a listener endpoint initializes its last-seen sequence
number in its Recipient Context associated to that multicaster.
However, the listener drops the multicast request without delivering
it to the application layer. This provides a reference point to
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identify if future multicast requests from the same multicaster are
fresher than the last one received.
A replay time interval exists, between when a possibly replayed
message is originally transmitted by a given multicaster and the
first authentic fresh message from that same multicaster is received.
This can be acceptable for use cases where listener endpoints admit
such a trade-off between performance and assurance of message
freshness.
D.3. Challenge-Response Synchronization
A listener endpoint performs 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 Group
Identifier of the OSCORE group is still present in the Object-
Security Option as part of the COSE object (see Section 4).
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
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is further processed and verified; otherwise, it is silently
discarded. Finally, the listener updates the Recipient Context
associated to that multicaster, by setting the Replay Window
according to the Sequence Number from 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.
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.
This approach provides an assurance of absolute message freshness.
However, it can result in an impact on performance which is
undesirable or unbearable, especially in large multicast groups where
many nodes at the same time might join as new members or lose
synchronization.
Appendix E. No Verification of Signatures
There are some application scenarios using group communications that
have particularly strict requirements. One example of this is the
requirement of low message latency in non-emergency lighting
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applications [I-D.somaraju-ace-multicast]. For those applications
which have tight performance constraints and relaxed security
requirements, it can be inconvenient for some endpoints to verify
digital signatures in order to assert source authenticity of received
group messages. In other cases, the signature verification can be
deferred or only checked for specific actions. For instance, a
command to turn a bulb on where the bulb is already on does not need
the signature to be checked. In such situations, the counter
signature needs to be included anyway as part of the group message,
so that an endpoint that needs to validate the signature for any
reason has the ability to do so.
In this specification, it is NOT RECOMMENDED that endpoints do not
verify the counter signature of received group messages. However, it
is recognized that there may be situations where it is not always
required. The consequence of not doing the signature validation is
that security in the group is based only on the group-authenticity of
the shared keying material used for encryption. That is, endpoints
in the multicast group have evidence that a received message has been
originated by a group member, although not specifically identifiable
in a secure way. This can violate a number of security requirements,
as the compromise of any element in the group means that the attacker
has the ability to control the entire group. Even worse, the group
may not be limited in scope, and hence the same keying material might
be used not only for light bulbs but for locks as well. Therefore,
extreme care must be taken in situations where the security
requirements are relaxed, so that deployment of the system will
always be done safely.
Authors' Addresses
Marco Tiloca
RISE SICS AB
Isafjordsgatan 22
Kista SE-16440 Stockholm
Sweden
Email: marco.tiloca@ri.se
Goeran Selander
Ericsson AB
Torshamnsgatan 23
Kista SE-16440 Stockholm
Sweden
Email: goran.selander@ericsson.com
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Francesca Palombini
Ericsson AB
Torshamnsgatan 23
Kista SE-16440 Stockholm
Sweden
Email: francesca.palombini@ericsson.com
Jiye Park
Universitaet Duisburg-Essen
Schuetzenbahn 70
Essen 45127
Germany
Email: ji-ye.park@uni-due.de
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