Group OSCORE - Secure Group Communication for CoAP
draft-ietf-core-oscore-groupcomm-09
CoRE Working Group M. Tiloca
Internet-Draft RISE AB
Intended status: Standards Track G. Selander
Expires: December 25, 2020 F. Palombini
Ericsson AB
J. Park
Universitaet Duisburg-Essen
June 23, 2020
Group OSCORE - Secure Group Communication for CoAP
draft-ietf-core-oscore-groupcomm-09
Abstract
This document defines Group Object Security for Constrained RESTful
Environments (Group OSCORE), providing end-to-end security of CoAP
messages exchanged between members of a group, e.g. sent over IP
multicast. In particular, the described approach defines how OSCORE
is used in a group communication setting to provide source
authentication for CoAP group requests, sent by a client to multiple
servers, and for protection of the corresponding CoAP responses.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 6
2. Security Context . . . . . . . . . . . . . . . . . . . . . . 7
2.1. Common Context . . . . . . . . . . . . . . . . . . . . . 8
2.1.1. ID Context . . . . . . . . . . . . . . . . . . . . . 8
2.1.2. Counter Signature Algorithm . . . . . . . . . . . . . 9
2.1.3. Counter Signature Parameters . . . . . . . . . . . . 9
2.1.4. Counter Signature Key Parameters . . . . . . . . . . 10
2.2. Sender Context and Recipient Context . . . . . . . . . . 10
2.3. Pairwise Keys . . . . . . . . . . . . . . . . . . . . . . 11
2.3.1. Derivation of Pairwise Keys . . . . . . . . . . . . . 11
2.3.2. Usage of Sequence Numbers . . . . . . . . . . . . . . 12
2.3.3. Security Context for Pairwise Mode . . . . . . . . . 12
2.4. Update of Security Context . . . . . . . . . . . . . . . 13
2.4.1. Loss of Mutable Security Context . . . . . . . . . . 13
2.4.2. Exhaustion of Sender Sequence Numbers . . . . . . . . 13
2.4.3. Retrieving New Security Context Parameters . . . . . 14
3. The Group Manager . . . . . . . . . . . . . . . . . . . . . . 15
3.1. Management of Group Keying Material . . . . . . . . . . . 16
3.2. Responsibilities of the Group Manager . . . . . . . . . . 17
4. The COSE Object . . . . . . . . . . . . . . . . . . . . . . . 18
4.1. Counter Signature . . . . . . . . . . . . . . . . . . . . 18
4.2. The 'kid' and 'kid context' parameters . . . . . . . . . 19
4.3. external_aad . . . . . . . . . . . . . . . . . . . . . . 19
4.3.1. external_aad for Encryption . . . . . . . . . . . . . 19
4.3.2. external_aad for Signing . . . . . . . . . . . . . . 20
5. OSCORE Header Compression . . . . . . . . . . . . . . . . . . 21
5.1. Examples of Compressed COSE Objects . . . . . . . . . . . 21
5.1.1. Examples in Group Mode . . . . . . . . . . . . . . . 22
5.1.2. Examples in Pairwise Mode . . . . . . . . . . . . . . 23
6. Message Binding, Sequence Numbers, Freshness and Replay
Protection . . . . . . . . . . . . . . . . . . . . . . . . . 24
6.1. Update of Replay Window . . . . . . . . . . . . . . . . . 24
7. Message Reception . . . . . . . . . . . . . . . . . . . . . . 24
8. Message Processing in Group Mode . . . . . . . . . . . . . . 25
8.1. Protecting the Request . . . . . . . . . . . . . . . . . 26
8.1.1. Supporting Observe . . . . . . . . . . . . . . . . . 26
8.2. Verifying the Request . . . . . . . . . . . . . . . . . . 26
8.2.1. Supporting Observe . . . . . . . . . . . . . . . . . 27
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8.3. Protecting the Response . . . . . . . . . . . . . . . . . 28
8.3.1. Supporting Observe . . . . . . . . . . . . . . . . . 28
8.4. Verifying the Response . . . . . . . . . . . . . . . . . 29
8.4.1. Supporting Observe . . . . . . . . . . . . . . . . . 30
9. Message Processing in Pairwise Mode . . . . . . . . . . . . . 30
9.1. Pre-Conditions . . . . . . . . . . . . . . . . . . . . . 31
9.2. Protecting the Request . . . . . . . . . . . . . . . . . 31
9.3. Verifying the Request . . . . . . . . . . . . . . . . . . 32
9.4. Protecting the Response . . . . . . . . . . . . . . . . . 32
9.5. Verifying the Response . . . . . . . . . . . . . . . . . 33
10. Security Considerations . . . . . . . . . . . . . . . . . . . 33
10.1. Group-level Security . . . . . . . . . . . . . . . . . . 34
10.2. Uniqueness of (key, nonce) . . . . . . . . . . . . . . . 35
10.3. Management of Group Keying Material . . . . . . . . . . 35
10.4. Update of Security Context and Key Rotation . . . . . . 36
10.4.1. Late Update on the Sender . . . . . . . . . . . . . 36
10.4.2. Late Update on the Recipient . . . . . . . . . . . . 37
10.5. Collision of Group Identifiers . . . . . . . . . . . . . 37
10.6. Cross-group Message Injection . . . . . . . . . . . . . 38
10.6.1. Attack Description . . . . . . . . . . . . . . . . . 38
10.6.2. Attack Prevention in Group Mode . . . . . . . . . . 39
10.7. Group OSCORE for Unicast Requests . . . . . . . . . . . 40
10.8. End-to-end Protection . . . . . . . . . . . . . . . . . 41
10.9. Master Secret . . . . . . . . . . . . . . . . . . . . . 41
10.10. Replay Protection . . . . . . . . . . . . . . . . . . . 42
10.11. Client Aliveness . . . . . . . . . . . . . . . . . . . . 43
10.12. Cryptographic Considerations . . . . . . . . . . . . . . 43
10.13. Message Segmentation . . . . . . . . . . . . . . . . . . 44
10.14. Privacy Considerations . . . . . . . . . . . . . . . . . 44
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 45
11.1. OSCORE Flag Bits Registry . . . . . . . . . . . . . . . 45
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 45
12.1. Normative References . . . . . . . . . . . . . . . . . . 45
12.2. Informative References . . . . . . . . . . . . . . . . . 47
Appendix A. Assumptions and Security Objectives . . . . . . . . 49
A.1. Assumptions . . . . . . . . . . . . . . . . . . . . . . . 49
A.2. Security Objectives . . . . . . . . . . . . . . . . . . . 51
Appendix B. List of Use Cases . . . . . . . . . . . . . . . . . 52
Appendix C. Example of Group Identifier Format . . . . . . . . . 54
Appendix D. Set-up of New Endpoints . . . . . . . . . . . . . . 55
Appendix E. Examples of Synchronization Approaches . . . . . . . 56
E.1. Best-Effort Synchronization . . . . . . . . . . . . . . . 56
E.2. Baseline Synchronization . . . . . . . . . . . . . . . . 56
E.3. Challenge-Response Synchronization . . . . . . . . . . . 57
Appendix F. No Verification of Signatures in Group Mode . . . . 60
Appendix G. Optimized Request . . . . . . . . . . . . . . . . . 60
Appendix H. Example Values of Parameters for Countersignatures . 61
Appendix I. Document Updates . . . . . . . . . . . . . . . . . . 61
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I.1. Version -08 to -09 . . . . . . . . . . . . . . . . . . . 62
I.2. Version -07 to -08 . . . . . . . . . . . . . . . . . . . 62
I.3. Version -06 to -07 . . . . . . . . . . . . . . . . . . . 64
I.4. Version -05 to -06 . . . . . . . . . . . . . . . . . . . 64
I.5. Version -04 to -05 . . . . . . . . . . . . . . . . . . . 65
I.6. Version -03 to -04 . . . . . . . . . . . . . . . . . . . 65
I.7. Version -02 to -03 . . . . . . . . . . . . . . . . . . . 66
I.8. Version -01 to -02 . . . . . . . . . . . . . . . . . . . 67
I.9. Version -00 to -01 . . . . . . . . . . . . . . . . . . . 68
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 69
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 69
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
[I-D.ietf-core-groupcomm-bis] addresses use cases where deployed
devices benefit from a group communication model, for example to
reduce latencies, improve performance and reduce bandwidth
utilization. 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). This specification defines the security
protocol for Group communication for CoAP
[I-D.ietf-core-groupcomm-bis].
Object Security for Constrained RESTful Environments (OSCORE)
[RFC8613] describes a security protocol based on the exchange of
protected CoAP messages. OSCORE builds on CBOR Object Signing and
Encryption (COSE)
[I-D.ietf-cose-rfc8152bis-struct][I-D.ietf-cose-rfc8152bis-algs] and
provides end-to-end encryption, integrity, replay protection and
binding of response to request between a sender and a recipient,
independent of transport also in the presence of intermediaries. To
this end, a CoAP message is protected by including its payload (if
any), certain options, and header fields in a COSE object, which
replaces the authenticated and encrypted fields in the protected
message.
This document defines Group OSCORE, providing the same end-to-end
security properties as OSCORE in the case where CoAP requests have
multiple recipients. In particular, the described approach defines
how OSCORE is used in a group communication setting to provide source
authentication for CoAP group requests, sent by a client to multiple
servers, and for protection of the corresponding CoAP responses.
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Just like OSCORE, Group OSCORE is independent of transport layer and
works wherever CoAP does. Group communication for CoAP
[I-D.ietf-core-groupcomm-bis] uses UDP/IP multicast as the underlying
data transport.
As with OSCORE, it is possible to combine Group OSCORE with
communication security on other layers. One example is the use of
transport layer security, such as DTLS [RFC6347], between one client
and one proxy (and vice versa), or between one proxy and one server
(and vice versa), in order to protect the routing information of
packets from observers. Note that DTLS [RFC6347] does not define how
to secure messages sent over IP multicast.
Group OSCORE defines two modes of operation:
o In the group mode, Group OSCORE requests and responses are
digitally signed with the private key of the sender and the
signature is embedded in the protected CoAP message. The group
mode supports all COSE algorithms as well as signature
verification by intermediaries. This mode is defined in Section 8
and MUST be supported.
o In the pairwise mode, two group members exchange Group OSCORE
requests and responses over unicast, and the messages are
protected with symmetric keys. These symmetric keys are derived
from Diffie-Hellman shared secrets, calculated with the asymmetric
keys of the sender and recipient, allowing for shorter integrity
tags and therefore lower message overhead. This mode is OPTIONAL
to support as defined in Section 9.
Both modes provide source authentication of CoAP messages. The
application decides what mode to use, potentially on a per-message
basis. Such decision can be based, for instance, on pre-configured
policies or dynamic assessing of the target recipient and/or
resource, among other things. One important case is when requests
are protected with the group mode, and responses with the pairwise
mode, since this significantly reduces the overhead in case of many
responses to one request.
A special deployment of Group OSCORE is to use pairwise mode only.
For example, consider the case of a constrained-node network
[RFC7228] with a large number of CoAP endpoints and the objective to
establish secure communication between any pair of endpoints with a
small provisioning effort and message overhead. Since the total
number of security associations that needs to be established grows
with the square of the number of nodes, it is desirable to restrict
the provisioned keying material. Moreover, a key establishment
protocol would need to be executed for each security association.
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One solution to this is to deploy Group OSCORE with the endpoints
being part of a group and use the pairwise mode. This solution
assumes a trusted third party called Group Manager (see Section 3),
but has the benefit of restricting the symmetric keying material
while distributing only the public key of each group member. After
that, a CoAP endpoint can locally derive the OSCORE Security Context
for the other endpoint and protect the CoAP communication with very
low overhead [I-D.ietf-lwig-security-protocol-comparison].
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
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] including "endpoint", "client", "server",
"sender" and "recipient"; group communication for CoAP
[I-D.ietf-core-groupcomm-bis]; COSE and counter signatures
[I-D.ietf-cose-rfc8152bis-struct][I-D.ietf-cose-rfc8152bis-algs].
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" and "Master Secret", defined in [RFC8613].
Terminology for constrained environments, such as "constrained
device" and "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 endpoints. This includes, for
instance, keys and IVs [RFC4949].
o Group: a set of endpoints that share group keying material and
security parameters (Common Context, see Section 2). Unless
specified otherwise, the term group used in this specification
refers thus to a "security group" (see Section 2.1 of
[I-D.ietf-core-groupcomm-bis]), not to be confused with "CoAP
group" or "application group".
o Group Manager: entity responsible for a group. Each endpoint in a
group communicates securely with the respective Group Manager,
which is neither required to be an actual group member nor to take
part in the group communication. The full list of
responsibilities of the Group Manager is provided in Section 3.2.
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o Silent server: member of a group that never sends protected
responses in reply to requests. For CoAP group communications,
requests are normally sent without necessarily expecting a
response. A silent server may send unprotected responses, as
error responses reporting an OSCORE error. Note that an endpoint
can implement both a silent server and a client, i.e. the two
roles are independent. An endpoint acting only as a silent server
performs only Group OSCORE processing on incoming requests.
Silent servers maintain less keying material and in particular do
not have a Sender Context for the group. Since silent servers do
not have a Sender ID they cannot support pairwise mode.
o Group Identifier (Gid): identifier assigned to the group, unique
within the set of groups of a given Group Manager.
o Group request: CoAP request message sent by a client in the group
to all servers in that group.
o Source authentication: evidence that a received message in the
group originated from a specific identified group member. This
also provides assurance that the message was not tampered with by
anyone, be it a different legitimate group member or an endpoint
which is not a group member.
2. Security Context
This specification refers to a group as a set of endpoints sharing
keying material and security parameters for executing the Group
OSCORE protocol (see Section 1.1). Each endpoint which is member of
a group maintains a Security Context as defined in Section 3 of
[RFC8613], extended as follows (see Figure 1):
o One Common Context, shared by all the endpoints in the group.
Three new parameters are included in the Common Context: Counter
Signature Algorithm, Counter Signature Parameters and Counter
Signature Key Parameters, which all relate to the signature of the
message included in group mode (see Section 8).
o One Sender Context, extended with the endpoint's private key. The
private key is used to sign the message in group mode, and for
calculating the pairwise keys in pairwise mode (see Section 2.3).
If the pairwise mode is supported, the Sender Context is also
extended with the Pairwise Sender Keys associated to the other
endpoints (see Section 2.3). The Sender Context is omitted if the
endpoint is configured exclusively as silent server.
o One Recipient Context for each endpoint from which messages are
received. It is not necessary to maintain Recipient Contexts
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associated to endpoints from which messages are not (expected to
be) received. The Recipient Context is extended with the public
key of the associated endpoint, used to verify the signature in
group mode and for calculating the pairwise keys in pairwise mode
(see Section 2.3). If the pairwise mode is supported, then the
Recipient Context is also extended with the Pairwise Recipient Key
associated to the other endpoint (see Section 2.3).
+-------------------+-----------------------------------------------+
| Context Component | New Information Elements |
+-------------------+-----------------------------------------------+
| | Counter Signature Algorithm |
| Common Context | Counter Signature Parameters |
| | Counter Signature Key Parameters |
+-------------------+-----------------------------------------------+
| Sender Context | Endpoint's own private key |
| | *Pairwise Sender Keys for the other endpoints |
+-------------------+-----------------------------------------------+
| Each | Public key of the other endpoint |
| Recipient Context | *Pairwise Recipient Key of the other endpoint |
+-------------------+-----------------------------------------------+
Figure 1: Additions to the OSCORE Security Context. Optional
additions are labeled with an asterisk.
Further details about the Security Context of Group OSCORE are
provided in the remainder of this section. How the Security Context
is established by the group members is out of scope for this
specification, but if there is more than one Security Context
applicable to a message, then the endpoints MUST be able to tell
which Security Context was latest established.
The default setting for how to manage information about the group is
described in terms of a Group Manager (see Section 3).
2.1. Common Context
The Common Context may be acquired from the Group Manager (see
Section 3). The following sections define how the Common Context is
extended, compared to [RFC8613].
2.1.1. ID Context
The ID Context parameter (see Sections 3.3 and 5.1 of [RFC8613]) in
the Common Context SHALL contain the Group Identifier (Gid) of the
group. The choice of the Gid format is application specific. An
example of specific formatting of the Gid is given in Appendix C.
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The application needs to specify how to handle potential collisions
between Gids (see Section 10.5).
2.1.2. Counter Signature Algorithm
Counter Signature Algorithm identifies the digital signature
algorithm used to compute a counter signature on the COSE object (see
Section 5.2 of [I-D.ietf-cose-rfc8152bis-struct]).
This parameter is immutable once the Common Context is established.
Counter Signature Algorithm MUST take value from the "Value" column
of the "COSE Algorithms" Registry [COSE.Algorithms]. The value is
associated to a COSE key type, specified in the "Capabilities" column
of the Registry. COSE capabilities for algorithms are defined in
Section 8 of [I-D.ietf-cose-rfc8152bis-algs].
The EdDSA signature algorithm Ed25519 [RFC8032] is mandatory to
implement. For endpoints that support the pairwise mode of Group
OSCORE, the X25519 function [RFC7748] is also mandatory to implement.
If elliptic curve signatures are used, it is RECOMMENDED to implement
deterministic signatures with additional randomness as specified in
[I-D.mattsson-cfrg-det-sigs-with-noise].
2.1.3. Counter Signature Parameters
Counter Signature Parameters identifies the parameters associated to
the digital signature algorithm specified in Counter Signature
Algorithm. This parameter is immutable once the Common Context is
established.
This parameter is a CBOR array including the following two elements,
whose exact structure and value depend on the value of Counter
Signature Algorithm:
o The first element is the array of COSE capabilities for Counter
Signature Algorithm, as specified for that algorithm in the
"Capabilities" column of the "COSE Algorithms" Registry
[COSE.Algorithms] (see Section 8.2 of
[I-D.ietf-cose-rfc8152bis-algs]).
o The second element is the array of COSE capabilities for the COSE
key type associated to Counter Signature Algorithm, as specified
for that key type in the "Capabilities" column of the "COSE Key
Types" Registry [COSE.Key.Types] (see Section 8.1 of
[I-D.ietf-cose-rfc8152bis-algs]).
Examples of Counter Signature Parameters are in Appendix H.
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2.1.4. Counter Signature Key Parameters
Counter Signature Key Parameters identifies the parameters associated
to the keys used with the digital signature algorithm specified in
Counter Signature Algorithm. This parameter is immutable once the
Common Context is established.
The exact structure and value of this parameter depends on the value
of Counter Signature Algorithm. In particular, this parameter takes
the same structure and value of the array of COSE capabilities for
the COSE key type associated to Counter Signature Algorithm, as
specified for that key type in the "Capabilities" column of the "COSE
Key Types" Registry [COSE.Key.Types] (see Section 8.1 of
[I-D.ietf-cose-rfc8152bis-algs]).
Examples of Counter Signature Key Parameters are in Appendix H.
2.2. Sender Context and Recipient Context
OSCORE specifies the derivation of Sender Context and Recipient
Context, specifically of Sender/Recipient Keys and Common IV, from a
set of input parameters (see Section 3.2 of [RFC8613]). This
derivation applies also to Group OSCORE, and the mandatory-to-
implement HKDF and AEAD algorithms are the same as in [RFC8613]. The
Sender ID SHALL be unique for each endpoint in a group with a fixed
Master Secret, Master Salt and Group Identifier (see Section 3.3 of
[RFC8613]).
For Group OSCORE the Sender Context and Recipient Context
additionally contain asymmetric keys, as described previously in
Section 2. The private/public key pair of the sender can, for
example, be generated by the endpoint or provisioned during
manufacturing.
With the exception of the public key of the sending endpoint, a
receiving endpoint can derive a complete security context from a
received Group OSCORE message and the Common Context. The public
keys in the Recipient Contexts can be accessed from the Group Manager
(see Section 3) upon joining the group. A public key can
alternatively be acquired from the Group Manager at a later time, for
example the first time a message is received from a particular
endpoint in the group (see Section 8.2 and Section 8.4).
For severely constrained devices, it may be not feasible to
simultaneously handle the ongoing processing of a recently received
message in parallel with the retrieval of the associated endpoint's
public key. Such devices can be configured to drop a received
message for which there is no (complete) Recipient Context, and
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retrieve the public key in order to have it available to verify
subsequent messages from that endpoint.
2.3. Pairwise Keys
Certain signature schemes, such as EdDSA and ECDSA, support a secure
combined signature and encryption scheme. This section specifies the
derivation of "pairwise keys", for use in the pairwise mode of Group
OSCORE defined in Section 9.
2.3.1. Derivation of Pairwise Keys
Using the Group OSCORE Security Context (see Section 2), a group
member can derive AEAD keys to protect point-to-point communication
between itself and any other endpoint in the group. The same AEAD
algorithm as in the group mode is used. The key derivation of these
so-called pairwise keys follows the same construction as in
Section 3.2.1 of [RFC8613]:
Pairwise Recipient Key = HKDF(Recipient Key, Shared Secret, info, L)
Pairwise Sender Key = HKDF(Sender Key, Shared Secret, info, L)
where:
o The Pairwise Recipient Key is the AEAD key for receiving from
endpoint X.
o The Pairwise Sender Key is the AEAD key for sending to endpoint X.
o The Shared Secret is computed as a static-static Diffie-Hellman
shared secret [NIST-800-56A], where the endpoint uses its private
key and the public key of the other endpoint X.
o The Recipient Key and the public key are from the Recipient
Context associated to endpoint X.
o The Sender Key and private key are from the Sender Context.
o info and L are defined as in Section 3.2.1 of [RFC8613].
If EdDSA asymmetric keys are used, the Edward coordinates are mapped
to Montgomery coordinates using the maps defined in Sections 4.1 and
4.2 of [RFC7748], before using the X25519 and X448 functions defined
in Section 5 of [RFC7748].
After establishing a partially or completely new Security Context
(see Section 3.1 and Section 2.4), the old pairwise keys MUST be
deleted. Since new Sender/Recipient Keys are derived from the new
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group keying material (see Section 2.2), every group member MUST use
the new Sender/Recipient Keys when deriving new pairwise keys.
As long as any two group members preserve the same asymmetric keys,
their Diffie-Hellman shared secret does not change across updates of
the group keying material.
2.3.2. Usage of Sequence Numbers
When using any of its Pairwise Sender Keys, a sender endpoint
including the 'Partial IV' parameter in the protected message MUST
use the current fresh value of the Sender Sequence Number from its
Sender Context (see Section 2.2). That is, the same Sender Sequence
Number space is used for all outgoing messages protected with Group
OSCORE, thus limiting both storage and complexity.
On the other hand, when combining group and pairwise communication
modes, this may result in the Partial IV values moving forward more
often. This can happen when a client engages in frequent or long
sequences of one-to-one exchanges with servers in the group, by
sending requests over unicast.
As a consequence, replay checks may be invoked more often on the
recipient side, where larger replay windows should be considered.
2.3.3. Security Context for Pairwise Mode
If the pairwise mode is supported, the pairwise keys are added to the
Security Context, as described at the beginning of Section 2.
The pairwise keys as well as the shared secrets used in their
derivation (see Section 2.3.1) may be stored in memory or recomputed
each time they are needed. The shared secret changes only when a
public/private key pair used for its derivation changes, which
results in the pairwise keys also changing. Additionally, the
pairwise keys change if the Sender ID changes or if a new Security
Context is established for the group (see Section 2.4.3). In order
to optimize protocol performance, an endpoint may store the derived
pairwise keys for easy retrieval.
In the pairwise mode, the Sender Context includes the Pairwise Sender
Keys for the other endpoints (see Figure 1). In order to identify
the right key to use, the Pairwise Sender Key for endpoint X may be
associated to the Recipient ID of endpoint X, as defined in the
Recipient Context (i.e. the Sender ID from the point of view of
endpoint X). In this way, the Recipient ID can be used to lookup for
the right Pairwise Sender Key. This association may be implemented in
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different ways, e.g. by storing the pair (Recipient ID, Pairwise
Sender Key), or linking a Pairwise Sender Key to a Recipient Context.
2.4. Update of Security Context
The mutable parts of the Security Context are updated by the endpoint
when executing the security protocol, but may nevertheless become
outdated, e.g. due to loss of the mutable Security Context (see
Section 2.4.1) or exhaustion of Sender Sequence Numbers (see
Section 2.4.2). The endpoint MUST be able to detect a loss of
mutable security context (see Section 2.4.1). If an endpoint detects
a loss of mutable Sender Security Context, it MUST NOT protect
further messages using this Security Context to avoid reusing a nonce
with the same AEAD key.
It is RECOMMENDED that the immutable part of the Security Context is
stored in non-volatile memory, or that it can otherwise be reliably
accessed throughout the operation of the group, e.g. after device
reboot. However, also immutable parts of the Security Context may
need to be updated, for example due to scheduled key renewal, new or
re-joining members in the group, or the fact that the endpoint
changes Sender ID (see Section 2.4.3).
2.4.1. Loss of Mutable Security Context
An endpoint losing its mutable Security Context, e.g., due to reboot,
needs to prevent the re-use of Sender Sequence Numbers, and to handle
incoming replayed messages. Appendix B.1 of [RFC8613] describes
secure procedures for handling the loss of Sender Sequence Number and
the update of Replay Window. The procedure in Appendix B.1.1 of
[RFC8613] applies also to servers in Group OSCORE and is RECOMMENDED
to use. A variant of Appendix B.1.2 of [RFC8613] applicable to Group
OSCORE is specified in Appendix E.3 of this specification.
If an endpoint is not able to establish an updated Sender Security
Context, e.g. because of lack of connectivity with the Group Manager,
it MUST NOT protect further messages using this Security Context.
The endpoint SHOULD inform the Group Manager and retrieve new
Security Context parameters from the Group Manager (see
Section 2.4.3).
2.4.2. Exhaustion of Sender Sequence Numbers
An endpoint can eventually exhaust the Sender Sequence Numbers, which
are incremented for each new outgoing message including a Partial IV.
This is the case for group requests, Observe notifications [RFC7641]
and, optionally, any other response.
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If an implementation's integers support wrapping addition, when a
wrap-around is detected the implementation MUST treat Sender Sequence
Numbers as exhausted.
Upon exhausting the Sender Sequence Numbers, the endpoint MUST NOT
protect further messages using this Security Context. The endpoint
SHOULD inform the Group Manager and retrieve new Security Context
parameters from the Group Manager (see Section 2.4.3).
2.4.3. Retrieving New Security Context Parameters
The Group Manager can assist an endpoint with an incomplete Sender
Security Context to retrieve missing data of the Security Context and
thereby become fully operational in the group again. The two main
options are described in this section: i) assignment of a new Sender
ID (see Section 2.4.3.1); and ii) establishment of a new Security
Context for the group (see Section 2.4.3.2). Update of Replay Window
in Recipient Contexts is discussed in Section 6.1.
As group membership changes, or as group members get new Sender IDs
(see Section 2.4.3.1) so do the relevant Recipient IDs that the other
endpoints need to keep track of. As a consequence, group members may
end up retaining stale Recipient Contexts, that are no longer useful
to verify incoming secure messages.
The Recipient ID ('kid') SHOULD NOT be considered as a persistent and
reliable indicator of a group member. Such an indication can be
achieved only by using that member's public key, when verifying
countersignatures of received messages (in group mode), or when
verifying messages integrity-protected with pairwise keying material
derived from asymmetric keys (in pairwise mode).
Furthermore, applications MAY define policies to: i) delete
(long-)unused Recipient Contexts and reduce the impact on storage
space; as well as ii) check with the Group Manager that a public key
is currently the one associated to a 'kid' value, after a number of
consecutive failed verifications.
2.4.3.1. New Sender ID for the Endpoint
The Group Manager may assign the endpoint a new Sender ID, leaving
the Gid, Master Secret and Master Salt unchanged. In this case the
Group Manager MUST assign an unused Sender ID. Having retrieved the
new Sender ID, and potentially other missing data of the immutable
Security Context, the endpoint can derive a new Sender Context (see
Section 2.2). The Sender Sequence Number is initialized to 0.
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The assignment of a new Sender ID may be the result of different
processes. The endpoint may request a new Sender ID, e.g. because of
exhaustion of Sender Sequence Numbers (see Section 2.4.2). An
endpoint may request to re-join the group, e.g. because of losing its
mutable Security Context (see Section 2.4.1), and receive as response
a new Sender ID together with the latest immutable Security Context.
The Recipient Context of the other group members corresponding to the
old Sender ID becomes stale (see Section 3.1).
2.4.3.2. New Security Context for the Group
The Group Manager may establish a new Security Context for the group
(see Section 3.1). The Group Manager does not necessarily establish
a new Security Context for the group if one member has an outdated
Security Context (see Section 2.4.3.1), unless that was already
planned or required for other reasons. All endpoints in the group
need to acquire new Security Context parameters from the Group
Manager.
Having acquired new Security Context parameters, each member can re-
derive the keying material stored in its Sender Context and Recipient
Contexts (see Section 2.2). The Master Salt used for the re-
derivations is the updated Master Salt parameter if provided by the
Group Manager, or the empty byte string otherwise. Unless otherwise
specified by the application, a group member does not reset the
Sender Sequence Number in its Sender Context, and does not reset the
Replay Windows in its Recipient Contexts. From then on, each group
member MUST use its latest installed Sender Context to protect
outgoing messages.
The distribution of a new Gid and Master Secret may result in
temporarily misaligned Security Contexts among group members. In
particular, this may result in a group member not being able to
process messages received right after a new Gid and Master Secret
have been distributed. A discussion on practical consequences and
possible ways to address them, as well as on how to handle the old
Security Context, is provided in Section 10.4.
3. The Group Manager
As with OSCORE, endpoints communicating with Group OSCORE need to
establish the relevant Security Context. Group OSCORE endpoints need
to acquire OSCORE input parameters, information about the group(s)
and about other endpoints in the group(s). This specification is
based on the existence of an entity called Group Manager which is
responsible for the group, but does not mandate how the Group Manager
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interacts with the group members. The responsibilities of the Group
Manager are compiled in Section 3.2.
It is RECOMMENDED to use a Group Manager as described in
[I-D.ietf-ace-key-groupcomm-oscore], where the join process is based
on the ACE framework for authentication and authorization in
constrained environments [I-D.ietf-ace-oauth-authz].
The Group Manager assigns unique Group Identifiers (Gids) to
different groups under its control, as well as unique Sender IDs (and
thereby Recipient IDs) to the members of those groups. According to
a hierarchical approach, the Gid value assigned to a group is
associated to a dedicated space for the values of Sender ID and
Recipient ID of the members of that group. In addition, the Group
Manager maintains records of the public keys of endpoints in a group,
and provides information about the group and its members to other
members and selected roles. Upon nodes' joining, the Group Manager
collects such public keys and MUST verify proof-of-possession of the
respective private key.
An endpoint acquires group data such as the Gid and OSCORE input
parameters including its own Sender ID from the Group Manager, and
provides information about its public key to the Group Manager, for
example upon joining the group.
A group member can retrieve from the Group Manager the public key and
other information associated to another group member, with which it
can generate the corresponding Recipient Context. An application can
configure a group member to asynchronously retrieve information about
Recipient Contexts, e.g. by Observing [RFC7641] the Group Manager to
get updates on the group membership.
The Group Manager MAY serve additional entities acting as signature
checkers, e.g. intermediary gateways. These entities do not join a
group as members, but can retrieve public keys of group members from
the Group Manager, in order to verify counter signatures of group
messages. A signature checker MUST be authorized for retrieving
public keys of members in a specific group from the Group Manager.
To this end, the same method mentioned above based on the ACE
framework [I-D.ietf-ace-oauth-authz] can be used.
3.1. Management of Group Keying Material
In order to establish a new Security Context for a group, a new Group
Identifier (Gid) for that group and a new value for the Master Secret
parameter MUST be generated. An example of Gid format supporting
this operation is provided in Appendix C. When distributing the new
Gid and Master Secret, the Group Manager MAY distribute also a new
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value for the Master Salt parameter, and SHOULD preserve the current
value of the Sender ID of each group member.
The Group Manager MUST NOT reassign a previously used Sender ID
('kid') with the same Gid, Master Secret and Master Salt. Even if
Gid and Master Secret are renewed as described in this section, the
Group Manager SHOULD NOT reassign an endpoint's Sender ID ('kid')
within a same group, especially in the short term.
If required by the application (see Appendix A.1), it is RECOMMENDED
to adopt a group key management scheme, and securely distribute a new
value for the Gid and 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
necessary to preserve backward security and forward security in the
group, if the application requires it.
The specific approach used to distribute new group data is out of the
scope of this document. However, it is RECOMMENDED that the Group
Manager supports the distribution of the new Gid and Master Secret
parameter to the group according to the Group Rekeying Process
described in [I-D.ietf-ace-key-groupcomm-oscore].
3.2. Responsibilities of the Group Manager
The Group Manager is responsible for performing the following tasks:
1. Creating and managing OSCORE groups. This includes the
assignment of a Gid to every newly created group, as well as
ensuring uniqueness of Gids within the set of its OSCORE groups.
2. Defining policies for authorizing the joining of its OSCORE
groups.
3. Handling the join process to add new endpoints as group members.
4. Establishing the Common Context part of the Security Context,
and providing it to authorized group members during the join
process, together with the corresponding Sender Context.
5. Generating and managing Sender IDs within its OSCORE groups, as
well as assigning and providing them to new endpoints during the
join process. This includes ensuring uniqueness of Sender IDs
within each of its OSCORE groups.
6. Defining communication policies for each of its OSCORE groups,
and signalling them to new endpoints during the join process.
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7. Renewing the Security Context of an OSCORE group upon membership
change, by revoking and renewing common security parameters and
keying material (rekeying).
8. Providing the management keying material that a new endpoint
requires to participate in the rekeying process, consistent with
the key management scheme used in the group joined by the new
endpoint.
9. Updating the Gid of its OSCORE groups, upon renewing the
respective Security Context.
10. Acting as key repository, in order to handle the public keys of
the members of its OSCORE groups, and providing such public keys
to other members of the same group upon request. The actual
storage of public keys may be entrusted to a separate secure
storage device.
11. Validating that the format and parameters of public keys of
group members are consistent with the countersignature algorithm
and related parameters used in the respective OSCORE group.
The Group Manager described in [I-D.ietf-ace-key-groupcomm-oscore]
provides these functionalities.
4. The COSE Object
Building on Section 5 of [RFC8613], this section defines how to use
COSE [I-D.ietf-cose-rfc8152bis-struct] to wrap and protect data in
the original message. OSCORE uses the untagged COSE_Encrypt0
structure with an Authenticated Encryption with Associated Data
(AEAD) algorithm. Unless otherwise specified, the following
modifications apply for both the group mode and the pairwise mode of
Group OSCORE.
4.1. Counter Signature
For the group mode only, the 'unprotected' field MUST additionally
include the following parameter:
o CounterSignature0: its value is set to the counter signature of
the COSE object, computed by the sender as described in
Section 5.2 of [I-D.ietf-cose-rfc8152bis-struct], by using the
private key and according to the Counter Signature Algorithm and
Counter Signature Parameters in the Security Context. In
particular, the Sig_structure contains the external_aad as defined
in Section 4.3.2 and the ciphertext of the COSE_Encrypt0 object as
payload.
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4.2. The 'kid' and 'kid context' parameters
The value of the 'kid' parameter in the 'unprotected' field of
response messages MUST be set to the Sender ID of the endpoint
transmitting the message. That is, unlike in [RFC8613], the 'kid'
parameter is always present in all messages, both requests and
responses.
The value of the 'kid context' parameter in the 'unprotected' field
of requests messages MUST be set to the ID Context, i.e. the Group
Identifier value (Gid) of the group. That is, unlike in [RFC8613],
the 'kid context' parameter is always present in requests.
4.3. external_aad
The external_aad of the Additional Authenticated Data (AAD) is
different compared to OSCORE. In particular, there is one
external_aad used for encryption (both in group mode and pairwise
mode), and another external_aad used for signing (only in group
mode).
4.3.1. external_aad for Encryption
The external_aad for encryption (see Section 6.3 of
[I-D.ietf-cose-rfc8152bis-struct]), used both in group mode and
pairwise mode, includes also the counter signature algorithm and
related signature parameters, see Figure 2.
external_aad = bstr .cbor aad_array
aad_array = [
oscore_version : uint,
algorithms : [alg_aead : int / tstr,
alg_countersign : int / tstr,
par_countersign : [countersign_alg_capab,
countersign_key_type_capab],
par_countersign_key : countersign_key_type_capab],
request_kid : bstr,
request_piv : bstr,
options : bstr
]
Figure 2: external_aad for Encryption
Compared with Section 5.4 of [RFC8613], the 'algorithms' array in the
aad_array additionally includes:
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o 'alg_countersign', which specifies Counter Signature Algorithm
from the Common Context (see Section 2.1.2). This parameter MUST
encode the value of Counter Signature Algorithm as a CBOR integer
or text string, consistently with the "Value" field in the "COSE
Algorithms" Registry for this counter signature algorithm.
o 'par_countersign', which specifies the CBOR array Counter
Signature Parameters from the Common Context (see Section 2.1.3).
In particular:
* 'countersign_alg_capab' is the array of COSE capabilities for
the countersignature algorithm indicated in 'alg_countersign'.
* 'countersign_key_type_capab' is the array of COSE capabilities
for the COSE key type used by the countersignature algorithm
indicated in 'alg_countersign'.
o 'par_countersign_key', which specifies Counter Signature Key
Parameters from the Common Context (see Section 2.1.4). In
particular, 'countersign_key_type_capab' is the array of COSE
capabilities for the COSE key type used by the countersignature
algorithm indicated in 'alg_countersign'.
4.3.2. external_aad for Signing
The external_aad for signing (see Section 4.4 of
[I-D.ietf-cose-rfc8152bis-struct]) used in group mode is identical to
the external_aad for encryption (see Section 4.3.1) with the addition
of the OSCORE option, see Figure 3.
external_aad = bstr .cbor aad_array
aad_array = [
oscore_version : uint,
algorithms : [alg_aead : int / tstr,
alg_countersign : int / tstr,
par_countersign : [countersign_alg_capab,
countersign_key_type_capab],
par_countersign_key : countersign_key_type_capab],
request_kid : bstr,
request_piv : bstr,
options : bstr,
OSCORE_option: bstr
]
Figure 3: external_aad for Signing
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Compared with Section 5.4 of [RFC8613] the aad_array additionally
includes:
o the 'algorithms' array as defined in the external_aad for
encryption, see Section 4.3.1;
o the value of the OSCORE Option encoded as a binary string.
Note for implementation: this construction requires the OSCORE option
of the message to be generated before calculating the signature.
Also, the aad_array needs to be large enough to contain the largest
possible OSCORE option.
5. OSCORE Header Compression
The OSCORE header compression defined in Section 6 of [RFC8613] is
used, with the following differences.
o The payload of the OSCORE message SHALL encode the ciphertext of
the COSE object. In the group mode, the ciphertext above is
concatenated with the value of the CounterSignature0 of the COSE
object, computed as described in Section 4.1.
o This specification defines the usage of the sixth least
significant bit, called the "Group Flag", in the first byte of the
OSCORE option containing the OSCORE flag bits. This flag bit is
specified in Section 11.1.
o The Group Flag MUST be set to 1 if the OSCORE message is protected
using the group mode (see Section 8).
o The Group Flag MUST be set to 0 if the OSCORE message is protected
using the pairwise mode (see Section 9). The Group Flag MUST also
be set to 0 for ordinary OSCORE messages processed according to
[RFC8613].
5.1. Examples of Compressed COSE Objects
This section covers a list of OSCORE Header Compression examples of
Group OSCORE used in group mode (see Section 5.1.1) or in pairwise
mode (see Section 5.1.2).
The examples assume that the COSE_Encrypt0 object is set (which means
the CoAP message and cryptographic material is known). Note that the
examples do not include the full CoAP unprotected message or the full
Security Context, but only the input necessary to the compression
mechanism, i.e. the COSE_Encrypt0 object. The output is the
compressed COSE object as defined in Section 5 and divided into two
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parts, since the object is transported in two CoAP fields: OSCORE
option and payload.
The examples assume that the plaintext (see Section 5.3 of [RFC8613])
is 6 bytes long, and that the AEAD tag is 8 bytes long, hence
resulting in a ciphertext which is 14 bytes long. When using the
group mode, COUNTERSIGN denotes the CounterSignature0 byte string as
described in Section 4, and is 64 bytes long.
5.1.1. Examples in Group Mode
o Request with ciphertext = 0xaea0155667924dff8a24e4cb35b9, kid =
0x25, Partial IV = 5 and kid context = 0x44616c
Before compression (96 bytes):
[
h'',
{ 4:h'25', 6:h'05', 10:h'44616c', 9:COUNTERSIGN },
h'aea0155667924dff8a24e4cb35b9'
]
After compression (85 bytes):
Flag byte: 0b00111001 = 0x39
Option Value: 39 05 03 44 61 6c 25 (7 bytes)
Payload: ae a0 15 56 67 92 4d ff 8a 24 e4 cb 35 b9 COUNTERSIGN
(14 bytes + size of COUNTERSIGN)
o Response with ciphertext = 0x60b035059d9ef5667c5a0710823b, kid =
0x52 and no Partial IV.
Before compression (88 bytes):
[
h'',
{ 4:h'52', 9:COUNTERSIGN },
h'60b035059d9ef5667c5a0710823b'
]
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After compression (80 bytes):
Flag byte: 0b00101000 = 0x28
Option Value: 28 52 (2 bytes)
Payload: 60 b0 35 05 9d 9e f5 66 7c 5a 07 10 82 3b COUNTERSIGN
(14 bytes + size of COUNTERSIGN)
5.1.2. Examples in Pairwise Mode
o Request with ciphertext = 0xaea0155667924dff8a24e4cb35b9, kid =
0x25, Partial IV = 5 and kid context = 0x44616c
Before compression (32 bytes):
[
h'',
{ 4:h'25', 6:h'05', 10:h'44616c' },
h'aea0155667924dff8a24e4cb35b9'
]
After compression (21 bytes):
Flag byte: 0b00011001 = 0x19
Option Value: 19 05 03 44 61 6c 25 (7 bytes)
Payload: ae a0 15 56 67 92 4d ff 8a 24 e4 cb 35 b9 (14 bytes)
o Response with ciphertext = 0x60b035059d9ef5667c5a0710823b, kid =
0x52 and no Partial IV.
Before compression (24 bytes):
[
h'',
{ 4:h'52'},
h'60b035059d9ef5667c5a0710823b'
]
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After compression (16 bytes):
Flag byte: 0b00001000 = 0x08
Option Value: 08 52 (2 bytes)
Payload: 60 b0 35 05 9d 9e f5 66 7c 5a 07 10 82 3b (14 bytes)
6. Message Binding, Sequence Numbers, Freshness and Replay Protection
The requirements and properties described in Section 7 of [RFC8613]
also apply to OSCORE used in group communication. In particular,
Group OSCORE provides message binding of responses to requests, which
enables absolute freshness of responses that are not notifications,
relative freshness of requests and notification responses, and replay
protection of requests.
6.1. Update of Replay Window
A new server joining a group may not be aware of the current Partial
IVs (Sender Sequence Numbers of the clients). The first time the new
server receives a request from a particular client, it is not able to
verify if that request is a replay. The same holds when a server
loses its mutable Security Context (see Section 2.4.1), for instance
after a device reboot.
The exact way to address this issue is application specific, and
depends on the particular use case and its replay requirements. The
list of methods to handle the update of a Replay Window is part of
the group communication policy, and different servers can use
different methods.
Appendix E describes three possible approaches that can be considered
to update a Replay Window.
7. Message Reception
Upon receiving a protected message, a recipient endpoint retrieves a
Security Context as in [RFC8613]. An endpoint MUST be able to
distinguish between a Security Context to process OSCORE messages as
in [RFC8613] and a Security Context to process Group OSCORE messages
as defined in this specification.
To this end, an endpoint can take into account the different
structure of the Security Context defined in Section 2, for example
based on the presence of Counter Signature Algorithm in the Common
Context. Alternatively implementations can use an additional
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parameter in the Security Context, to explicitly signal that it is
intended for processing Group OSCORE messages.
If any of the following two conditions holds, a recipient endpoint
MUST discard the incoming protected message:
o The Group Flag is set to 1, and the recipient endpoint can not
retrieve a Security Context which is both valid to process the
message and also associated to an OSCORE group.
o The Group Flag is set to 0, and the recipient endpoint retrieves a
Security Context which is both valid to process the message and
also associated to an OSCORE group, but the endpoint does not
support the pairwise mode.
Otherwise, if a Security Context associated to an OSCORE group and
valid to process the message is retrieved, the recipient endpoint
processes the message with Group OSCORE, using the group mode (see
Section 8) if the Group Flag is set to 1, or the pairwise mode (see
Section 9) if the Group Flag is set to 0.
Note that, if the Group Flag is set to 0, and the recipient endpoint
retrieves a Security Context which is valid to process the message
but is not associated to an OSCORE group, then the message is
processed according to [RFC8613].
8. Message Processing in Group Mode
When using the group mode, messages are protected and processed as
specified in [RFC8613], with the modifications described in this
section. The security objectives of the group mode are discussed in
Appendix A.2. The group mode MUST be supported.
The group mode MUST be used to protect group requests intended for
multiple recipients or for the whole group. This includes both
requests directly addressed to multiple recipients, e.g. sent by the
client over multicast, as well as requests sent by the client over
unicast to a proxy, that forwards them to the intended recipients
over multicast [I-D.ietf-core-groupcomm-bis].
As per [RFC7252][I-D.ietf-core-groupcomm-bis], group requests sent
over multicast MUST be Non-Confirmable, and thus cannot be
retransmitted by the CoAP messaging layer. Instead, applications
should store such outgoing messages for a pre-defined, sufficient
amount of time, in order to correctly perform possible
retransmissions at the application layer. According to Section 5.2.3
of [RFC7252], responses to Non-Confirmable group requests SHOULD also
be Non-Confirmable, but endpoints MUST be prepared to receive
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Confirmable responses in reply to a Non-Confirmable group request.
Confirmable group requests are acknowledged in non-multicast
environments, as specified in [RFC7252].
Furthermore, endpoints in the group locally perform error handling
and processing of invalid messages according to the same principles
adopted in [RFC8613]. However, a recipient MUST stop processing and
silently reject any message which is malformed and does not follow
the format specified in Section 4, or which is not cryptographically
validated in a successful way. In either case, it is RECOMMENDED
that the recipient does not send back any error message. This
prevents servers from replying with multiple error messages to a
client sending a group request, so avoiding the risk of flooding and
possibly congesting the network.
8.1. Protecting the Request
A client transmits a secure group request as described in Section 8.1
of [RFC8613], with the following modifications.
o In step 2, the Additional Authenticated Data is modified as
described in Section 4.
o In step 4, the encryption of the COSE object is modified as
described in Section 4. The encoding of the compressed COSE
object is modified as described in Section 5. In particular, the
Group Flag MUST be set to 1.
o In step 5, the counter signature is computed and the format of the
OSCORE message is modified as described in Section 4 and
Section 5. In particular, the payload of the OSCORE message
includes also the counter signature.
8.1.1. Supporting Observe
If Observe [RFC7641] is supported, for each newly started
observation, the client MUST store the value of the 'kid' parameter
from the original Observe request.
The client MUST NOT update the stored value, even in case it is
individually rekeyed and receives a new Sender ID from the Group
Manager (see Section 2.4.3.1).
8.2. Verifying the Request
Upon receiving a secure group request with the Group Flag set to 1,
following the procedure in Section 7, a server proceeds as described
in Section 8.2 of [RFC8613], with the following modifications.
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o In step 2, the decoding of the compressed COSE object follows
Section 5. In particular:
* If the server discards the request due to not retrieving a
Security Context associated to the OSCORE group, the server MAY
respond with a 4.02 (Bad Option) error. When doing so, the
server MAY set an Outer Max-Age option with value zero, and MAY
include a descriptive string as diagnostic payload.
* If the received 'kid context' matches an existing ID Context
(Gid) but the received 'kid' does not match any Recipient ID in
this Security Context, then the server MAY create a new
Recipient Context for this Recipient ID and initialize it
according to Section 3 of [RFC8613], and also retrieve the
associated public key. Such a configuration is application
specific. If the application does not specify dynamic
derivation of new Recipient Contexts, then the server SHALL
stop processing the request.
o In step 4, the Additional Authenticated Data is modified as
described in Section 4.
o In step 6, the server also verifies the counter signature using
the public key of the client from the associated Recipient
Context. If the signature verification fails, the server SHALL
stop processing the request and MAY respond with a 4.00 (Bad
Request) response. If the verification fails, the same steps are
taken as if the decryption had failed. In particular, the Replay
Window is only updated if both the signature verification and the
decryption succeed.
o Additionally, if the used Recipient Context was created upon
receiving this group request and the message is not verified
successfully, the server MAY delete that Recipient Context. Such
a configuration, which is specified by the application, mitigates
attacks that aim at overloading the server's storage.
A server SHOULD NOT process a request if the received Recipient ID
('kid') is equal to its own Sender ID in its own Sender Context. For
an example where this is not fulfilled, see Section 5.2.1 in
[I-D.tiloca-core-observe-multicast-notifications].
8.2.1. Supporting Observe
If Observe [RFC7641] is supported, for each newly started
observation, the server MUST store the value of the 'kid' parameter
from the original Observe request.
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The server MUST NOT update the stored value of a 'kid' parameter
associated to a particular Observe request, even in case the observer
client is individually rekeyed and starts using a new Sender ID
received from the Group Manager (see Section 2.4.3.1).
8.3. Protecting the Response
If a server generates a CoAP message in response to a Group OSCORE
request, then the server SHALL follow the description in Section 8.3
of [RFC8613], with the modifications described in this section.
Note that the server always protects a response with the Sender
Context from its latest Security Context, and that a new Security
Context does not reset the Sender Sequence Number unless otherwise
specified by the application (see Section 3.1).
o In step 2, the Additional Authenticated Data is modified as
described in Section 4.
o In step 3, if the server is using a different Security Context for
the response compared to what was used to verify the request (see
Section 3.1), then the AEAD nonce from the request MUST NOT be
used.
o In step 4, the encryption of the COSE object is modified as
described in Section 4. The encoding of the compressed COSE
object is modified as described in Section 5. In particular, the
Group Flag MUST be set to 1. If the server is using a different
ID Context (Gid) for the response compared to what was used to
verify the request (see Section 3.1), then the new ID Context MUST
be included in the 'kid context' parameter of the response.
o In step 5, the counter signature is computed and the format of the
OSCORE message is modified as described in Section 5. In
particular, the payload of the OSCORE message includes also the
counter signature.
8.3.1. Supporting Observe
If Observe [RFC7641] is supported, the server may have ongoing
observations, started by Observe requests protected with an old
Security Context.
After completing the establishment of a new Security Context, the
server MUST protect the following notifications with the Sender
Context of the new Security Context.
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For each ongoing observation, the server MUST include in the first
notification protected with the new Security Context also the 'kid
context' parameter, which is set to the ID Context (Gid) of the new
Security Context. It is OPTIONAL for the server to include the ID
Context (Gid) in the 'kid context' parameter also in further
following notifications for those observations.
Furthermore, for each ongoing observation, the server MUST use the
stored value of the 'kid' parameter from the original Observe
request, as value for the 'request_kid' parameter in the two
external_aad structures (see Section 4.3.1 and Section 4.3.2), when
protecting notifications for that observation.
8.4. Verifying the Response
Upon receiving a secure response message with the Group Flag set to
1, following the procedure in Section 7, the client proceeds as
described in Section 8.4 of [RFC8613], with the following
modifications.
Note that a client may receive a response protected with a Security
Context different from the one used to protect the corresponding
group request, and that, upon the establishment of a new Security
Context, the client does not reset its own replay windows in its
Recipient Contexts, unless otherwise specified by the application
(see Section 3.1).
o In step 2, the decoding of the compressed COSE object is modified
as described in Section 5. If the received 'kid context' matches
an existing ID Context (Gid) but the received 'kid' does not match
any Recipient ID in this Security Context, then the client MAY
create a new Recipient Context for this Recipient ID and
initialize it according to Section 3 of [RFC8613], and also
retrieve the associated public key. If the application does not
specify dynamic derivation of new Recipient Contexts, then the
client SHALL stop processing the response.
o In step 3, the Additional Authenticated Data is modified as
described in Section 4.
o In step 5, the client also verifies the counter signature using
the public key of the server from the associated Recipient
Context. If the verification fails, the same steps are taken as
if the decryption had failed.
o Additionally, if the used Recipient Context was created upon
receiving this response and the message is not verified
successfully, the client MAY delete that Recipient Context. Such
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a configuration, which is specified by the application, mitigates
attacks that aim at overloading the client's storage.
8.4.1. Supporting Observe
If Observe [RFC7641] is supported, for each ongoing observation, the
client MUST use the stored value of the 'kid' parameter from the
original Observe request, as value for the 'request_kid' parameter in
the two external_aad structures (see Section 4.3.1 and
Section 4.3.2), when verifying notifications for that observation.
This ensures that the client can correctly verify notifications, even
in case it is individually rekeyed and starts using a new Sender ID
received from the Group Manager (see Section 2.4.3.1).
9. Message Processing in Pairwise Mode
When using the pairwise mode of Group OSCORE, messages are protected
and processed as in Section 8, with the modifications described in
this section. The security objectives of the pairwise mode are
discussed in Appendix A.2.
The pairwise mode takes advantage of an existing Security Context for
the group mode to establish a Security Context shared exclusively
with any other member. In order to use the pairwise mode, the
signature scheme of the group mode MUST support a combined signature
and encryption scheme. This can be, for example, signature using
ECDSA, and encryption using AES-CCM with a key derived with ECDH.
The pairwise mode does not support intermediary verification of
source authentication or integrity.
The pairwise mode MAY be supported. The pairwise mode MUST be
supported by endpoints that use the CoAP Echo Option
[I-D.ietf-core-echo-request-tag] and/or block-wise transfers
[RFC7959], for instance for responses after the first block-wise
request, possibly targeting all servers in the group and including
the CoAP Block2 option (see Section 2.3.6 of
[I-D.ietf-core-groupcomm-bis]). An endpoint implementing only a
silent server does not support the pairwise mode.
The pairwise mode protects messages between two members of a group,
essentially following [RFC8613], but with the following notable
differences:
o The 'kid' and 'kid context' parameters of the COSE object are used
as defined in Section 4.2.
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o The external_aad defined in Section 4.3.1 is used for the
encryption process.
o The Sender/Recipient Keys used in the pairwise mode are derived as
defined in Section 2.3.
Senders MUST NOT use the pairwise mode to protect a message intended
for multiple recipients. The pairwise mode is defined only between
two endpoints and the keying material is thus only available to one
recipient.
The Group Manager MAY indicate that the group uses also the pairwise
mode, as part of the group communication policies signalled to
candidate group members when joining the group.
9.1. Pre-Conditions
In order to protect an outgoing message in pairwise mode, the sender
needs to know the public key and the Recipient ID for the recipient
endpoint, as stored in the Recipient Context associated to that
endpoint (see Pairwise Sender Context of Section 2.3.3).
Furthermore, the sender needs to know the individual address of the
recipient endpoint. This information may not be known at any given
point in time. For instance, right after having joined the group, a
client may know the public key and Recipient ID for a given server,
but not the addressing information required to reach it with an
individual, one-to-one request.
To make addressing information of individual endpoints available,
servers in the group MAY expose a resource to which a client can send
a group request targeting a server or a set of servers, identified by
their 'kid' value(s). The specified set may be empty, hence
identifying all the servers in the group. Further details of such an
interface are out of scope for this document.
9.2. Protecting the Request
When using the pairwise mode, the request is protected as defined in
Section 8.1, with the following differences.
o The Group Flag MUST be set to 0.
o The Sender Key used is the Pairwise Sender Key (see Section 2.3).
o The counter signature is not computed and therefore not included
in the message. The payload of the protected request thus
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terminates with the encoded ciphertext of the COSE object, just as
in [RFC8613].
Note that, just as in the group mode, the external_aad for encryption
is generated as in Section 4.3.1, and the Partial IV is the current
fresh value of the Sender Sequence Number (see Section 2.3.2).
9.3. Verifying the Request
Upon receiving a request with the Group Flag set to 0, following the
procedure in Section 7, the server MUST process it as defined in
Section 8.2, with the following differences.
o If the server discards the request due to not retrieving a
Security Context associated to the OSCORE group or to not
supporting the pairwise mode, the server MAY respond with a 4.02
(Bad Option) error. When doing so, the server MAY set an Outer
Max-Age option with value zero, and MAY include a descriptive
string as diagnostic payload.
o If a new Recipient Context is created for this Recipient ID, new
Pairwise Sender/Recipient Keys are also derived (see
Section 2.3.1). The new Pairwise Sender/Recipient Keys are
deleted if the Recipient Context is deleted as a result of the
message not being successfully verified.
o The Recipient Key used is the Pairwise Recipient Key (see
Section 2.3).
o No verification of counter signature occurs, as there is none
included in the message.
9.4. Protecting the Response
When using the pairwise mode, a response is protected as defined in
Section 8.3, with the following differences.
o The Group Flag MUST be set to 0.
o The Sender Key used is the Pairwise Sender Key (see Section 2.3).
o The counter signature is not computed and therefore not included
in the message. The payload of the protected response thus
terminates with the encoded ciphertext of the COSE object, just as
in [RFC8613].
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9.5. Verifying the Response
Upon receiving a response with the Group Flag set to 0, following the
procedure in Section 7, the client MUST process it as defined in
Section 8.4, with the following differences.
o If a new Recipient Context is created for this Recipient ID, new
Pairwise Sender/Recipient Keys are also derived (see
Section 2.3.1). The new Pairwise Sender/Recipient Keys are
deleted if the Recipient Context is deleted as a result of the
message not being successfully verified.
o The Recipient Key used is the Pairwise Recipient Key (see
Section 2.3).
o No verification of counter signature occurs, as there is none
included in the message.
10. Security Considerations
The same threat model discussed for OSCORE in Appendix D.1 of
[RFC8613] holds for Group OSCORE. In addition, when using the group
mode, source authentication of messages is explicitly ensured by
means of counter signatures, as discussed in Section 10.1.
The same considerations on supporting Proxy operations discussed for
OSCORE in Appendix D.2 of [RFC8613] hold for Group OSCORE.
The same considerations on protected message fields for OSCORE
discussed in Appendix D.3 of [RFC8613] hold for Group OSCORE.
The same considerations on uniqueness of (key, nonce) pairs for
OSCORE discussed in Appendix D.4 of [RFC8613] hold for Group OSCORE.
This is further discussed in Section 10.2.
The same considerations on unprotected message fields for OSCORE
discussed in Appendix D.5 of [RFC8613] hold for Group OSCORE, with
the following difference. The counter signature included in a Group
OSCORE message protected in group mode is computed also over the
value of the OSCORE option, which is part of the Additional
Authenticated Data used in the signing process. This is further
discussed in Section 10.6.
As discussed in Section 6.2.3 of [I-D.ietf-core-groupcomm-bis], Group
OSCORE addresses security attacks against CoAP listed in Sections
11.2-11.6 of [RFC7252], especially when run over IP multicast.
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The rest of this section first discusses security aspects to be taken
into account when using Group OSCORE. Then it goes through aspects
covered in the security considerations of OSCORE (Section 12 of
[RFC8613]), and discusses how they hold when Group OSCORE is used.
10.1. Group-level Security
The group mode described in Section 8 relies on commonly shared group
keying material to protect communication within a group. This has
the following implications.
o Messages are encrypted at a group level (group-level data
confidentiality), i.e. they can be decrypted by any member of the
group, but not by an external adversary or other external
entities.
o The AEAD algorithm provides only group authentication, i.e. it
ensures that a message sent to a group has been sent by a member
of that group, but not by the alleged sender. This is why source
authentication of messages sent to a group is ensured through a
counter signature, which is computed by the sender using its own
private key and then appended to the message payload.
Instead, the pairwise mode described in Section 9 protects messages
by using pairwise symmetric keys, derived from the static-static
Diffie-Hellman shared secret computed from the asymmetric keys of the
sender and recipient endpoint (see Section 2.3). Therefore, in the
parwise mode, the AEAD algorithm provides both pairwise data-
confidentiality and source authentication of messages, without using
counter signatures.
The long-term storing of the Diffie-Hellman shared secret is a
potential security issue. In fact, if the shared secret of two group
members is leaked, a third group member can exploit it to impersonate
any of those two group members, by deriving and using their pairwise
key. The possibility of such leakage should be contemplated, as more
likely to happen than the leakage of a private key, which could be
rather protected at a significantly higher level than generic memory,
e.g. by using a Trusted Platform Module. Therefore, there is a
trade-off between the maximum amount of time a same shared secret is
stored and the frequency of its re-computing.
Note that, even if an endpoint is authorized to be a group member and
to take part in group communications, there is a risk that it behaves
inappropriately. For instance, it can forward the content of
messages in the group to unauthorized entities. However, in many use
cases, the devices in the group belong to a common authority and are
configured by a commissioner (see Appendix B), which results in a
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practically limited risk and enables a prompt detection/reaction in
case of misbehaving.
10.2. Uniqueness of (key, nonce)
The proof for uniqueness of (key, nonce) pairs in Appendix D.4 of
[RFC8613] is also valid in group communication scenarios. That is,
given an OSCORE group:
o Uniqueness of Sender IDs within the group is enforced by the Group
Manager.
o The case A in Appendix D.4 of [RFC8613] concerns all group
requests and responses including a Partial IV (e.g. Observe
notifications). In this case, same considerations from [RFC8613]
apply here as well.
o The case B in Appendix D.4 of [RFC8613] concerns responses not
including a Partial IV (e.g. single response to a group request).
In this case, same considerations from [RFC8613] apply here as
well.
As a consequence, each message encrypted/decrypted with the same
Sender Key is processed by using a different (ID_PIV, PIV) pair.
This means that nonces used by any fixed encrypting endpoint are
unique. Thus, each message is processed with a different (key,
nonce) pair.
10.3. Management of Group Keying Material
The approach described in this specification should take into account
the risk of compromise of group members. In particular, this
document specifies that a key management scheme for secure revocation
and renewal of Security Contexts and group keying material should be
adopted.
[I-D.ietf-ace-key-groupcomm-oscore] provides a simple rekeying scheme
for renewing the Security Context in a group.
Alternative rekeying schemes which are more scalable with the group
size may be needed in dynamic, large-scale, groups where endpoints
can join and leave at any time, in order to limit the impact on
performance due to the Security Context and keying material update.
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10.4. Update of Security Context and Key Rotation
A group member can receive a message shortly after the group has been
rekeyed, and new security parameters and keying material have been
distributed by the Group Manager.
This may result in a client using an old Security Context to protect
a group request, and a server using a different new Security Context
to protect a corresponding response. As a consequence, clients may
receive a response protected with a Security Context different from
the one used to protect the corresponding group request.
In particular, a server may first get a group request protected with
the old Security Context, then install the new Security Context, and
only after that produce a response to send back to the client. In
such a case, as specified in Section 8.3, the server MUST protect the
potential response using the new Security Context. Specifically, the
server MUST use its own Sender Sequence Number as Partial IV to
protect that response, and not the Partial IV from the request, in
order to prevent reuse of AEAD nonces in the new Security Context.
The client will process that response using the new Security Context,
provided that it has installed the new security parameters and keying
material before the message reception.
In case block-wise transfer [RFC7959] is used, the same
considerations from Section 7.2 of [I-D.ietf-ace-key-groupcomm] hold.
Furthermore, as described below, a group rekeying may temporarily
result in misaligned Security Contexts between the sender and
recipient of a same message.
10.4.1. Late Update on the Sender
In this case, the sender protects a message using the old Security
Context, i.e. before having installed the new Security Context.
However, the recipient receives the message after having installed
the new Security Context, hence not being able to correctly process
it.
A possible way to ameliorate this issue is to preserve the old,
recent, Security Context for a maximum amount of time defined by the
application. By doing so, the recipient can still try to process the
received message using the old retained Security Context as second
attempt. This makes particular sense when the recipient is a client,
that would hence be able to process incoming responses protected with
the old, recent, Security Context used to protect the associated
group request. Instead, a recipient server would better and more
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simply discard an incoming group request which is not successfully
processed with the new Security Context.
This tolerance preserves the processing of secure messages throughout
a long-lasting key rotation, as group rekeying processes may likely
take a long time to complete, especially in large scale groups. On
the other hand, a former (compromised) group member can abusively
take advantage of this, and send messages protected with the old
retained Security Context. Therefore, a conservative application
policy should not admit the retention of old Security Contexts.
10.4.2. Late Update on the Recipient
In this case, the sender protects a message using the new Security
Context, but the recipient receives that message before having
installed the new Security Context. Therefore, the recipient would
not be able to correctly process the message and hence discards it.
If the recipient installs the new Security Context shortly after that
and the sender endpoint uses CoAP retransmissions, the former will
still be able to receive and correctly process the message.
In any case, the recipient should actively ask the Group Manager for
an updated Security Context according to an application-defined
policy, for instance after a given number of unsuccessfully decrypted
incoming messages.
10.5. Collision of Group Identifiers
In case endpoints are deployed in multiple groups managed by
different non-synchronized Group Managers, it is possible for Group
Identifiers of different groups to coincide.
This does not impair the security of the AEAD algorithm. In fact, as
long as the Master Secret is different for different groups and this
condition holds over time, AEAD keys are different among different
groups.
The entity assigning an IP multicast address may help limiting the
chances to experience such collisions of Group Identifiers. In
particular, it may allow the Group Managers of groups using the same
IP multicast address to share their respective list of assigned Group
Identifiers currently in use.
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10.6. Cross-group Message Injection
A same endpoint is allowed to and would likely use the same public/
private key pair in multiple OSCORE groups, possibly administered by
different Group Managers.
When a sender endpoint sends a message protected in pairwise mode to
a recipient endpoint in an OSCORE group, a malicious group member may
attempt to inject the message to a different OSCORE group also
including the same endpoints (see Section 10.6.1).
This practically relies on altering the content of the OSCORE option,
and having the same MAC in the ciphertext still correctly validating,
which has a success probability depending on the size of the MAC.
As discussed in Section 10.6.2, the attack is practically infeasible
if the message is protected in group mode, since the counter
signature is bound also to the OSCORE option, through the Additional
Authenticated Data used in the signing process (see Section 4.3.2).
10.6.1. Attack Description
Let us consider:
o Two OSCORE groups G1 and G2, with ID Context (Group ID) Gid1 and
Gid2, respectively. Both G1 and G2 use the AEAD cipher AES-CCM-
16-64-128, i.e. the MAC of the ciphertext is 8 bytes in size.
o A sender endpoint X which is member of both G1 and G2, and uses
the same public/private key pair in both groups. The endpoint X
has Sender ID Sid1 in G1 and Sender ID Sid2 in G2. The pairs
(Sid1, Gid1) and (Sid2, Gid2) identify the same public key of X in
G1 and G2, respectively.
o A recipient endpoint Y which is member of both G1 and G2, and uses
the same public/private key pair in both groups. The endpoint Y
has Sender ID Sid3 in G1 and Sender ID Sid4 in G2. The pairs
(Sid3, Gid1) and (Sid4, Gid2) identify the same public key of Y in
G1 and G2, respectively.
o A malicious endpoint Z is also member of both G1 and G2. Hence, Z
is able to derive the symmetric keys associated to X in G1 and G2.
When X sends a message M1 addressed to Y in G1 and protected in
pairwise mode, Z can intercept M1, and forge a valid message M2 to be
injected in G2, making it appear as still sent by X to Y and valid to
be accepted.
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More in detail, Z intercepts and stops message M1, and forges a
message M2 by changing the value of the OSCORE option from M1 as
follows: the 'kid context' is changed from G1 to G2; and the 'kid' is
changed from Sid1 to Sid2. Then, Z injects message M2 as addressed
to Y in G2.
Upon receiving M2, there is a probability equal to 2^-64 that Y
successfully verifies the same unchanged MAC by using Sid2 as
'request_kid' and using the Pairwise Recipient Key associated to X in
G2.
Note that Z does not know the pairwise keys of X and Y, since it does
not know and is not able to compute their shared Diffie-Hellman
secret. Therefore, Z is not able to check offline if a performed
forgery is actually valid, before sending the forged message to G2.
10.6.2. Attack Prevention in Group Mode
When a Group OSCORE message is protected with the group mode, the
counter signature is computed also over the value of the OSCORE
option, which is part of the Additional Authenticated Data used in
the signing process (see Section 4.3.2).
That is, the countersignature is computed also over: the ID Context
(Group ID) and the Partial IV, which are always present in group
requests; as well as the Sender ID of the message originator, which
is always present in all group requests and responses.
Since the signing process takes as input also the ciphertext of the
COSE_Encrypt0 object, the countersignature is bound not only to the
intended OSCORE group, hence to the triplet (Master Secret, Master
Salt, ID Context), but also to a specific Sender ID in that group and
to its specific symmetric key used for AEAD encryption, hence to the
quartet (Master Secret, Master Salt, ID Context, Sender ID).
This makes it practically infeasible to perform the attack described
in Section 10.6.1, since it would require the adversary to
additionally forge a valid countersignature that replaces the
original one in the forged message M2.
If the countersignature did not cover the OSCORE option, the attack
would be possible also in group mode, since the same unchanged
countersignature from messsage M1 would be also valid in message M2.
Also, the following attack simplifications would hold, since Z is
able to derive the Sender/Recipient Keys of X and Y in G1 and G2.
o If M2 is used as a request, Z can check offline if a performed
forgery is actually valid before sending the forged message to G2.
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That is, this attack would have a complexity of 2^64 offline
calculations.
o If M2 is used as a response, Z can also change the response
Partial IV, until the same unchanged MAC is successfully verified
by using Sid2 as 'request_kid' and the symmetric key associated to
X in G2. Since the Partial IV is 5 bytes in size, this requires
2^40 operations to test all the Partial IVs, which can be done in
real-time. Also, the probability that a single given message M1
can be used to forge a response M2 for a given request would be
equal to 2^-24, since there are more MAC values (8 bytes in size)
than Partial IV values (5 bytes in size).
Note that, by changing the Partial IV as discussed above, any
member of G1 would also be able to forge a valid signed response
message M2 to be injected in G1.
10.7. Group OSCORE for Unicast Requests
With reference to the processing defined in Section 8.1 for the group
mode and in Appendix G for the optimized request, it is NOT
RECOMMENDED for a client to use the group mode for securing a request
intended for a single group member and sent over unicast.
This does not include the case where the client sends a request over
unicast to a proxy, to be forwarded to multiple intended recipients
over multicast [I-D.ietf-core-groupcomm-bis]. In this case, the
client MUST protect the request with the group mode, even though it
is sent to the proxy over unicast (see Section 8).
If the client uses the group mode with its own Sender Key to protect
a unicast request to a group member, an on-path adversary can, right
then or later on, redirect that request to one/many different group
member(s) over unicast, or to the whole OSCORE group over multicast.
By doing so, the adversary can induce the target group member(s) to
perform actions intended for one group member only. Note that the
adversary can be external, i.e. (s)he does not need to also be a
member of the OSCORE group.
This is due to the fact that the client is not able to indicate the
single intended recipient in a way which is secure and possible to
process for Group OSCORE on the server side. In particular, Group
OSCORE does not protect network addressing information such as the IP
address of the intended recipient server. It follows that the
server(s) receiving the redirected request cannot assert whether that
was the original intention of the client, and would thus simply
assume so.
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The impact of such an attack depends especially on the REST method of
the request, i.e. the Inner CoAP Code of the OSCORE request message.
In particular, safe methods such as GET and FETCH would trigger
(several) unintended responses from the targeted server(s), while not
resulting in destructive behavior. On the other hand, non safe
methods such as PUT, POST and PATCH/iPATCH would result in the target
server(s) taking active actions on their resources and possible
cyber-physical environment, with the risk of destructive consequences
and possible implications for safety.
A client can instead use the pairwise mode defined in Section 9.2, in
order to protect a request sent to a single group member by using
pairwise keying material (see Section 2.3). This prevents the attack
discussed above by construction, as only the intended server is able
to derive the pairwise keying material used by the client to protect
the request. A client supporting the pairwise mode SHOULD use it to
protect requests sent to a single group member over unicast, instead
of using the group mode. For an example where this is not fulfilled,
see Section 5.2.1 in
[I-D.tiloca-core-observe-multicast-notifications].
With particular reference to block-wise transfers [RFC7959],
Section 2.3.6 of [I-D.ietf-core-groupcomm-bis] points out that, while
an initial request including the CoAP Block2 option can be sent over
multicast, any other request in a transfer has to occur over unicast,
individually addressing the servers in the group.
Additional considerations are discussed in Appendix E.3, with respect
to requests including a CoAP Echo Option
[I-D.ietf-core-echo-request-tag] that has to be sent over unicast, as
a challenge-response method for servers to achieve synchronization of
client Sender Sequence Numbers.
10.8. End-to-end Protection
The same considerations from Section 12.1 of [RFC8613] hold for Group
OSCORE.
Additionally, (D)TLS and Group OSCORE can be combined for protecting
message exchanges occurring over unicast. However, it is not
possible to combine DTLS and Group OSCORE for protecting message
exchanges where messages are (also) sent over multicast.
10.9. Master Secret
Group OSCORE derives the Security Context using the same construction
as OSCORE, and by using the Group Identifier of a group as the
related ID Context. Hence, the same required properties of the
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Security Context parameters discussed in Section 3.3 of [RFC8613]
hold for this document.
With particular reference to the OSCORE Master Secret, it has to be
kept secret among the members of the respective OSCORE group and the
Group Manager responsible for that group. Also, the Master Secret
must have a good amount of randomness, and the Group Manager can
generate it offline using a good random number generator. This
includes the case where the Group Manager rekeys the group by
generating and distributing a new Master Secret. Randomness
requirements for security are described in [RFC4086].
10.10. Replay Protection
As in OSCORE, also Group OSCORE relies on sender sequence numbers
included in the COSE message field 'Partial IV' and used to build
AEAD nonces.
Note that the Partial IV of an endpoint does not necessarily grow
monotonically. For instance, upon exhaustion of the endpoint Sender
Sequence Number, the Partial IV also gets exhausted. As discussed in
Section 2.4.3, this results either in the endpoint being individually
rekeyed and getting a new Sender ID, or in the establishment of a new
Security Context in the group. Therefore, uniqueness of (key, nonce)
pairs (see Section 10.2) is preserved also when a new Security
Context is established.
As discussed in Section 6.1, an endpoint that has just joined a group
is exposed to replay attack, as it is not aware of the sender
sequence numbers currently used by other group members. Appendix E
describes how endpoints can synchronize with senders' sequence
numbers.
Unless exchanges in a group rely only on unicast messages, Group
OSCORE cannot be used with reliable transport. Thus, unless only
unicast messages are sent in the group, it cannot be defined that
only messages with sequence numbers that are equal to the previous
sequence number + 1 are accepted.
The processing of response messages described in Section 2.3.1 of
[I-D.ietf-core-groupcomm-bis] also ensures that a client accepts a
single valid response to a given request from each replying server,
unless CoAP observation is used.
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10.11. Client Aliveness
As discussed in Section 12.5 of [RFC8613], a server may use the CoAP
Echo Option [I-D.ietf-core-echo-request-tag] to verify the aliveness
of the client that originated a received request. This would also
allow the server to (re-)synchronize with the client's sequence
number, as well as to ensure that the request is fresh and has not
been replayed or (purposely) delayed, if it is the first one received
from that client after having joined the group or rebooted (see
Appendix E.3).
10.12. Cryptographic Considerations
The same considerations from Section 12.6 of [RFC8613] about the
maximum Sender Sequence Number hold for Group OSCORE.
As discussed in Section 2.4.2, an endpoint that experiences an
exhaustion of its own Sender Sequence Number MUST NOT transmit
further messages including a Partial IV, until it has derived a new
Sender Context. This prevents the endpoint to reuse the same AEAD
nonces with the same Sender Key.
In order to renew its own Sender Context, the endpoint SHOULD inform
the Group Manager, which can either renew the whole Security Context
by means of group rekeying, or provide only that endpoint with a new
Sender ID value. In either case, the endpoint derives a new Sender
Context, and in particular a new Sender Key.
Additionally, the same considerations from Section 12.6 of [RFC8613]
hold for Group OSCORE, about building the AEAD nonce and the secrecy
of the Security Context parameters.
The EdDSA signature algorithm Ed25519 [RFC8032] is mandatory to
implement. For endpoints that support the pairwise mode of Group
OSCORE, the X25519 function [RFC7748] is also mandatory to implement.
Montgomery curves and (twisted) Edwards curves [RFC7748] can be
alternatively represented in short-Weierstrass form as described in
[I-D.ietf-lwig-curve-representations].
For many constrained IoT devices, it is problematic to support more
than one signature algorithm or multiple whole cipher suites. As a
consequence, some deployments using, for instance, ECDSA with NIST
P-256 may not support the mandatory signature algorithm but that
should not be an issue for local deployments.
The derivation of pairwise keys defined in Section 2.3.1 is
compatible with ECDSA and EdDSA asymmetric keys, but is not
compatible with RSA asymmetric keys. The security of using the same
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key pair for Diffie-Hellman and for signing is demonstrated in
[Degabriele].
10.13. Message Segmentation
The same considerations from Section 12.7 of [RFC8613] hold for Group
OSCORE.
10.14. Privacy Considerations
Group OSCORE ensures end-to-end integrity protection and encryption
of the message payload and all options that are not used for proxy
operations. In particular, options are processed according to the
same class U/I/E that they have for OSCORE. Therefore, the same
privacy considerations from Section 12.8 of [RFC8613] hold for Group
OSCORE.
Furthermore, the following privacy considerations hold, about the
OSCORE option that may reveal information on the communicating
endpoints.
o The 'kid' parameter, which is intended to help a recipient
endpoint to find the right Recipient Context, may reveal
information about the Sender Endpoint. Since both requests and
responses always include the 'kid' parameter, this may reveal
information about both a client sending a group request and all
the possibly replying servers sending their own individual
response.
o The 'kid context' parameter, which is intended to help a recipient
endpoint to find the right Recipient Context, reveals information
about the sender endpoint. In particular, it reveals that the
sender endpoint is a member of a particular OSCORE group, whose
current Group ID is indicated in the 'kid context' parameter.
When receiving a group request, each of the recipient endpoints can
reply with a response that includes its Sender ID as 'kid' parameter.
All these responses will be matchable with the request through the
Token. Thus, even if these responses do not include a 'kid context'
parameter, it becomes possible to understand that the responder
endpoints are in the same group of the requester endpoint.
Furthermore, using the mechanisms described in Appendix E.3 to
achieve sequence number synchronization with a client may reveal when
a server device goes through a reboot. This can be mitigated by the
server device storing the precise state of the replay window of each
known client on a clean shutdown.
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Finally, the mechanism described in Section 10.5 to prevent
collisions of Group Identifiers from different Group Managers may
reveal information about events in the respective OSCORE groups. In
particular, a Group Identifier changes when the corresponding group
is rekeyed. Thus, Group Managers might use the shared list of Group
Identifiers to infer the rate and patterns of group membership
changes triggering a group rekeying, e.g. due to newly joined members
or evicted (compromised) members. In order to alleviate this privacy
concern, it should be hidden from the Group Managers which exact
Group Manager has currently assigned which Group Identifiers in its
OSCORE groups.
11. IANA Considerations
Note to RFC Editor: Please replace all occurrences of "[This
Document]" with the RFC number of this specification and delete this
paragraph.
This document has the following actions for IANA.
11.1. OSCORE Flag Bits Registry
IANA is asked to add the following value entry to the "OSCORE Flag
Bits" subregistry defined in Section 13.7 of [RFC8613] as part of the
"CoRE Parameters" registry.
+--------------+------------+-------------------------------+-----------+
| Bit Position | Name | Description | Reference |
+--------------+------------+-------------------------------+-----------+
| 2 | Group Flag | Set to 1 if the message is | [This |
| | | protected with the group mode | Document] |
| | | of Group OSCORE | |
+--------------+------------+-------------------------------+-----------+
12. References
12.1. Normative References
[COSE.Algorithms]
IANA, "COSE Algorithms",
<https://www.iana.org/assignments/cose/
cose.xhtml#algorithms>.
[COSE.Key.Types]
IANA, "COSE Key Types",
<https://www.iana.org/assignments/cose/
cose.xhtml#key-type>.
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[I-D.ietf-core-groupcomm-bis]
Dijk, E., Wang, C., and M. Tiloca, "Group Communication
for the Constrained Application Protocol (CoAP)", draft-
ietf-core-groupcomm-bis-00 (work in progress), March 2020.
[I-D.ietf-cose-rfc8152bis-algs]
Schaad, J., "CBOR Object Signing and Encryption (COSE):
Initial Algorithms", draft-ietf-cose-rfc8152bis-algs-09
(work in progress), June 2020.
[I-D.ietf-cose-rfc8152bis-struct]
Schaad, J., "CBOR Object Signing and Encryption (COSE):
Structures and Process", draft-ietf-cose-rfc8152bis-
struct-10 (work in progress), June 2020.
[NIST-800-56A]
Barker, E., Chen, L., Roginsky, A., Vassilev, A., and R.
Davis, "Recommendation for Pair-Wise Key-Establishment
Schemes Using Discrete Logarithm Cryptography - NIST
Special Publication 800-56A, Revision 3", April 2018,
<https://nvlpubs.nist.gov/nistpubs/SpecialPublications/
NIST.SP.800-56Ar3.pdf>.
[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>.
[RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106, RFC 4086,
DOI 10.17487/RFC4086, June 2005,
<https://www.rfc-editor.org/info/rfc4086>.
[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>.
[RFC7748] Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves
for Security", RFC 7748, DOI 10.17487/RFC7748, January
2016, <https://www.rfc-editor.org/info/rfc7748>.
[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>.
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[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>.
[RFC8613] Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
"Object Security for Constrained RESTful Environments
(OSCORE)", RFC 8613, DOI 10.17487/RFC8613, July 2019,
<https://www.rfc-editor.org/info/rfc8613>.
12.2. Informative References
[Degabriele]
Degabriele, J., Lehmann, A., Paterson, K., Smart, N., and
M. Strefler, "On the Joint Security of Encryption and
Signature in EMV", December 2011,
<https://eprint.iacr.org/2011/615>.
[I-D.ietf-ace-key-groupcomm]
Palombini, F. and M. Tiloca, "Key Provisioning for Group
Communication using ACE", draft-ietf-ace-key-groupcomm-07
(work in progress), June 2020.
[I-D.ietf-ace-key-groupcomm-oscore]
Tiloca, M., Park, J., and F. Palombini, "Key Management
for OSCORE Groups in ACE", draft-ietf-ace-key-groupcomm-
oscore-07 (work in progress), June 2020.
[I-D.ietf-ace-oauth-authz]
Seitz, L., Selander, G., Wahlstroem, E., Erdtman, S., and
H. Tschofenig, "Authentication and Authorization for
Constrained Environments (ACE) using the OAuth 2.0
Framework (ACE-OAuth)", draft-ietf-ace-oauth-authz-34
(work in progress), June 2020.
[I-D.ietf-core-echo-request-tag]
Amsuess, C., Mattsson, J., and G. Selander, "CoAP: Echo,
Request-Tag, and Token Processing", draft-ietf-core-echo-
request-tag-09 (work in progress), March 2020.
[I-D.ietf-lwig-curve-representations]
Struik, R., "Alternative Elliptic Curve Representations",
draft-ietf-lwig-curve-representations-10 (work in
progress), April 2020.
[I-D.ietf-lwig-security-protocol-comparison]
Mattsson, J., Palombini, F., and M. Vucinic, "Comparison
of CoAP Security Protocols", draft-ietf-lwig-security-
protocol-comparison-04 (work in progress), March 2020.
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[I-D.mattsson-cfrg-det-sigs-with-noise]
Mattsson, J., Thormarker, E., and S. Ruohomaa,
"Deterministic ECDSA and EdDSA Signatures with Additional
Randomness", draft-mattsson-cfrg-det-sigs-with-noise-02
(work in progress), March 2020.
[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-core-observe-multicast-notifications]
Tiloca, M., Hoeglund, R., Amsuess, C., and F. Palombini,
"Observe Notifications as CoAP Multicast Responses",
draft-tiloca-core-observe-multicast-notifications-02 (work
in progress), March 2020.
[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>.
[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>.
[RFC7641] Hartke, K., "Observing Resources in the Constrained
Application Protocol (CoAP)", RFC 7641,
DOI 10.17487/RFC7641, September 2015,
<https://www.rfc-editor.org/info/rfc7641>.
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[RFC7959] Bormann, C. and Z. Shelby, Ed., "Block-Wise Transfers in
the Constrained Application Protocol (CoAP)", RFC 7959,
DOI 10.17487/RFC7959, August 2016,
<https://www.rfc-editor.org/info/rfc7959>.
Appendix A. Assumptions and Security Objectives
This section presents a set of assumptions and security objectives
for the approach described in this document. The rest of this
section refers to three types of groups:
o Application group, i.e. a set of CoAP endpoints that share a
common pool of resources.
o Security group, as defined in Section 1.1 of this specification.
There can be a one-to-one or a one-to-many relation between
security groups and application groups, and vice versa. Any two
application groups associated to the same security group do not
share any same resource.
o CoAP group, as defined in [I-D.ietf-core-groupcomm-bis] i.e. a set
of CoAP endpoints, where each endpoint is configured to receive
CoAP multicast requests that are sent to the group's associated IP
multicast address and UDP port. An endpoint may be a member of
multiple CoAP groups. There can be a one-to-one or a one-to-many
relation between application groups and CoAP groups. Note that a
device sending a CoAP request to a CoAP group is not necessarily
itself a member of that group: it is a member only if it also has
a CoAP server endpoint listening to requests for this CoAP group,
sent to the associated IP multicast address and port. In order to
provide secure group communication, all members of a CoAP group as
well as all further endpoints configured only as clients sending
CoAP (multicast) requests to the CoAP group have to be member of a
security group. There can be a one-to-one or a one-to-many
relation between security groups and CoAP groups, and vice versa.
A.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 sender and multiple recipients) and M-to-N (multiple
senders and multiple recipients) 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). Examples of use cases that benefit from
secure group communication are provided in Appendix B.
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In a 1-to-N communication model, only a single client transmits
data to the CoAP group, in the form of request messages; in an
M-to-N communication model (where M and N do not necessarily have
the same value), M clients transmit data to the CoAP group.
According to [I-D.ietf-core-groupcomm-bis], any possible proxy
entity is supposed to know about the clients and to not perform
aggregation of response messages from multiple servers. Also,
every client expects and is able to handle multiple response
messages associated to a same request sent to the CoAP group.
o Group size: security solutions for group communication should be
able to adequately support different and possibly large security
groups. The group size is the current number of members in a
security group. In the use cases mentioned in this document, the
number of clients (normally the controlling devices) is expected
to be much smaller than the number of servers (i.e. the controlled
devices). A security solution for group communication that
supports 1 to 50 clients would be able to properly cover the group
sizes required for most use cases that are relevant for this
document. The maximum group size is expected to be in the range
of 2 to 100 devices. Security groups larger than that should be
divided into smaller independent groups.
o Communication with the Group Manager: an endpoint must use a
secure dedicated channel when communicating with the Group
Manager, also when not registered as a member of the security
group.
o Provisioning and management of Security Contexts: a Security
Context must be established among the members of the security
group. A secure mechanism must be used to generate, revoke and
(re-)distribute keying material, communication policies and
security parameters in the security group. The actual
provisioning and management of the Security Context is out of the
scope of this document.
o Multicast data security ciphersuite: all members of a security
group must agree on a ciphersuite to provide authenticity,
integrity and confidentiality of messages in the group. The
ciphersuite is specified as part of the Security Context.
o Backward security: a new device joining the security group should
not have access to any old Security Contexts used before its
joining. This ensures that a new member of the security group is
not able to decrypt confidential data sent before it has joined
the security 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
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Context and renew the group keying material in the security group
upon a new member's joining has to be defined as part of the group
key management scheme.
o Forward security: entities that leave the security group should
not have access to any future Security Contexts or message
exchanged within the security group after their leaving. This
ensures that a former member of the security group is not able to
decrypt confidential data sent within the security group anymore.
Also, it ensures that a former member is not able to send
protected messages to the security group anymore. The actual
mechanism to update the Security Context and renew the group
keying material in the security group upon a member's leaving has
to be defined as part of the group key management scheme.
A.2. Security Objectives
The approach described in this document aims at fulfilling the
following security objectives:
o Data replay protection: group request messages or response
messages replayed within the security group must be detected.
o Data confidentiality: messages sent within the security group
shall be encrypted.
o Group-level data confidentiality: the group mode provides 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 security group, but not by an external adversary or other
external entities.
o Pairwise data confidentiality: the pairwise mode especially
provides pairwise data confidentiality, since messages are
encrypted using pairwise keying material shared between any two
group members, hence they can be decrypted only by the intended
single recipient.
o Source message authentication: messages sent within the security
group shall be authenticated. That is, it is essential to ensure
that a message is originated by a member of the security group in
the first place, and in particular by a specific, identifiable
member of the security group.
o Message integrity: messages sent within the security group shall
be integrity protected. That is, it is essential to ensure that a
message has not been tampered with, either by a group member, or
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by an external adversary or other external entities which are not
members of the security group.
o Message ordering: it must be possible to determine the ordering of
messages coming from a single sender. In accordance with OSCORE
[RFC8613], this results in providing absolute freshness of
responses that are not notifications, as well as relative
freshness of group requests and notification responses. It is not
required to determine ordering of messages from different senders.
Appendix B. List of Use Cases
Group Communication for CoAP [I-D.ietf-core-groupcomm-bis] 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 [I-D.ietf-core-groupcomm-bis] to understand
the non-security related details. This section discusses a number of
use cases that benefit from secure group communication, and refers to
the three types of groups from Appendix A. Specific security
requirements for these use cases are discussed in Appendix A.
o Lighting control: consider a building equipped with IP-connected
lighting devices, switches, and border routers. The lighting
devices acting as servers are organized into application groups
and CoAP groups, according to their physical location in the
building. For instance, lighting devices in a room or corridor
can be configured as members of a single application group and
corresponding CoAP group. Those ligthing devices together with
the switches acting as clients in the same room or corridor can be
configured as members of the corresponding security group.
Switches are then used to control the lighting devices by sending
on/off/dimming commands to all lighting devices in the CoAP 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 groups to
be formed even if devices with a role in the lighting application
may be physically in different subnets (e.g. on wired and wireless
networks). Connectivity between lighting devices may be realized,
for instance, by means of IPv6 and (border) routers supporting
6LoWPAN [RFC4944][RFC6282]. Group communication enables
synchronous operation of a set of connected lights, ensuring that
the light preset (e.g. dimming level or color) of a large set of
luminaires are changed at the same perceived time. This is
especially useful for providing a visual synchronicity of light
effects to the user. As a practical guideline, events within a
200 ms interval are perceived as simultaneous by humans, which is
necessary to ensure in many setups. Devices may reply back to the
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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. In a typical
lighting control scenario, a single switch is the only entity
responsible for sending commands to a set 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 set
of lighting devices. Especially in professional lighting
scenarios, the roles of client and server are configured by the
lighting commissioner, and devices strictly follow those roles.
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 application groups and CoAP 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
application group and corresponding CoAP group. As a practical
guideline, events within intervals of seconds are typically
acceptable. 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 Low-
power and Lossy Network (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 set 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 set of
similar devices, both simultaneously and efficiently. Possible
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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 Low-power and Lossy Network (LLN) 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 application group and corresponding CoAP 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 latter may reply
back to the emergency notifier, in order to provide their feedback
and local information related to the ongoing emergency. This kind
of setups should additionally rely on a fault tolerance multicast
algorithm, such as Multicast Protocol for Low-Power and Lossy
Networks (MPL).
Appendix C. 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.
For each group, the Group Prefix is constant over time and is
uniquely defined in the set of all the groups associated to the same
Group Manager. The choice of the Group Prefix for a given group's
Security Context is application specific. The size of the Group
Prefix directly impact on the maximum number of distinct groups under
the same Group Manager.
The Group Epoch is set to 0 upon the group's initialization, and is
incremented by 1 each time new keying material, including a new Gid,
is distributed to the group in order to establish a new Security
Context (see Section 3.1).
As an example, a 3-byte Gid can be composed of: i) a 1-byte Group
Prefix '0xb1' interpreted as a raw byte string; and ii) a 2-byte
Group Epoch interpreted as an unsigned integer ranging from 0 to
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65535. Then, after having established the Common Context 61532 times
in the group, its Gid will assume value '0xb1f05c'.
Using an immutable Group Prefix for a group assumes that enough time
elapses between two consecutive usages of the same Group Epoch value
in that group. This ensures that the Gid value is temporally unique
during the lifetime of a given message. Thus, the expected highest
rate for addition/removal of group members and consequent group
rekeying should be taken into account for a proper dimensioning of
the Group Epoch size.
As discussed in Section 10.5, if endpoints are deployed in multiple
groups managed by different non-synchronized Group Managers, it is
possible that Group Identifiers of different groups coincide at some
point in time. In this case, a recipient has to handle coinciding
Group Identifiers, and has to try using different Security Contexts
to process an incoming message, until the right one is found and the
message is correctly verified. Therefore, it is favourable that
Group Identifiers from different Group Managers have a size that
result in a small probability of collision. How small this
probability should be is up to system designers.
Appendix D. Set-up of New Endpoints
An endpoint joins a group by explicitly interacting with the
responsible Group Manager. When becoming members of a group,
endpoints are not required to know how many and what endpoints are in
the same group.
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 document.
The Group Manager must verify that the joining endpoint is authorized
to join the 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. Further details
about the authorization of joining endpoints are out of scope.
In case of successful authorization check, the Group Manager
generates a Sender ID assigned to the joining endpoint, before
proceeding with the rest of the join process. That is, the Group
Manager provides the joining endpoint with the keying material and
parameters to initialize the Security Context (see Section 2). The
actual provisioning of keying material and parameters to the joining
endpoint is out of the scope of this document.
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It is RECOMMENDED that the join process adopts the approach described
in [I-D.ietf-ace-key-groupcomm-oscore] and based on the ACE framework
for Authentication and Authorization in constrained environments
[I-D.ietf-ace-oauth-authz].
Appendix E. Examples of Synchronization Approaches
This section describes three possible approaches that can be
considered by server endpoints to synchronize with Sender Sequence
Numbers of client endpoints sending group requests.
The Group Manager MAY indicate which of such approaches are used in
the group, as part of the group communication policies signalled to
candidate group members upon their group joining.
E.1. Best-Effort Synchronization
Upon receiving a group request from a client, a server does not take
any action to synchronize with the sender sequence number of that
client. This provides no assurance at all as to message freshness,
which can be acceptable in non-critical use cases.
With the notable exception of Observe notifications and responses
following a group rekeying, it is optional for the server to use the
sender sequence number as Partial IV. Instead, for efficiency
reasons, the server may rather use the request's Partial IV when
protecting a response.
E.2. Baseline Synchronization
Upon receiving a group request from a given client for the first
time, a server initializes its last-seen Sender Sequence Number in
its Recipient Context associated to that client. The server may also
drop the group request without delivering it to the application.
This method provides a reference point to identify if future group
requests from the same client are fresher than the last one received.
A replay time interval exists, between when a possibly replayed or
delayed message is originally transmitted by a given client and the
first authentic fresh message from that same client is received.
This can be acceptable for use cases where servers admit such a
trade-off between performance and assurance of message freshness.
With the notable exception of Observe notifications and responses
following a group rekeying, it is optional for the server to use its
own Sender Sequence Number as Partial IV. Instead, for efficiency
reasons, the server may rather use the request's Partial IV when
protecting a response.
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E.3. Challenge-Response Synchronization
A server performs a challenge-response exchange with a client, by
using the Echo Option for CoAP described in Section 2 of
[I-D.ietf-core-echo-request-tag] and according to Appendix B.1.2 of
[RFC8613].
That is, upon receiving a group request from a particular client for
the first time, the server processes the message as described in this
specification, but, even if valid, does not deliver it to the
application. Instead, the server replies to the client with an
OSCORE protected 4.01 (Unauthorized) response message, including only
the Echo Option and no diagnostic payload. Since this response is
protected with the Security Context used in the group, the client
will consider the response valid upon successfully decrypting and
verifying it.
The server stores the Echo Option value included therein, together
with the pair (gid,kid), where 'gid' is the Group Identifier of the
OSCORE group and 'kid' is the Sender ID of the client in the group,
as specified in the 'kid context' and 'kid' fields of the OSCORE
Option of the group request, respectively. After a group rekeying
has been completed and a new Security Context has been established in
the group, which results also in a new Group Identifier (see
Section 3.1), the server MUST delete all the stored Echo values
associated to members of that group.
Upon receiving a 4.01 (Unauthorized) response that includes an Echo
Option and originates from a verified group member, a client sends a
request as a unicast message addressed to the same server, echoing
the Echo Option value. The client MUST NOT send the request
including the Echo Option over multicast.
In particular, the client 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 client to not retain
previously sent group requests for full retransmission, unless the
application explicitly requires otherwise. In either case, the
client uses the Sender Sequence Number value currently stored in its
own Sender Context. If the client stores group requests for possible
retransmission with the Echo Option, it should not store a given
request for longer than a pre-configured time interval. Note that
the unicast request echoing the Echo Option is correctly treated and
processed as a message, since the 'kid context' field including the
Group Identifier of the OSCORE group is still present in the OSCORE
Option as part of the COSE object (see Section 4).
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Upon receiving the unicast request including the Echo Option, the
server performs the following verifications.
o If the server does not store an Echo Option value for the pair
(gid,kid), it considers: i) the time t1 when it has established
the Security Context used to protect the received request; and ii)
the time t2 when the request has been received. Since a valid
request cannot be older than the Security Context used to protect
it, the server verifies that (t2 - t1) is less than the largest
amount of time acceptable to consider the request fresh.
o If the server stores an Echo Option value for the pair (gid,kid)
associated to that same client in the same group, the server
verifies that the option value equals that same stored value
previously sent by that client.
If the verifications above fail, the server MUST NOT process the
request further and MAY send a 4.01 (Unauthorized) response including
an Echo Option.
In case of positive verification, the request is further processed
and verified. Finally, the server updates the Recipient Context
associated to that client, by setting the Replay Window according to
the Sequence Number from the unicast request conveying the Echo
Option. The server either delivers the request to the application if
it is an actual retransmission of the original one, or discards 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.
A server should not deliver group requests from a given client to the
application until one valid request from that same client has been
verified as fresh, as conveying an echoed Echo Option
[I-D.ietf-core-echo-request-tag]. Also, a server may perform the
challenge-response described above at any time, if synchronization
with Sender Sequence Numbers of clients is (believed to be) lost, for
instance after a device reboot. A client has to be always ready to
perform the challenge-response based on the Echo Option in case a
server starts it.
It is the role of the server application to define under what
circumstances Sender Sequence Numbers lose synchronization. This can
include experiencing a "large enough" gap D = (SN2 - SN1), between
the Sender Sequence Number SN1 of the latest accepted group request
from a client and the Sender Sequence Number SN2 of a group request
just received from that client. However, a client may send several
unicast requests to different group members as protected with the
pairwise mode (see Section 9.2), which may consume the gap D at the
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server relatively fast. This would induce the server to perform more
challenge-response exchanges than actually needed.
To ameliorate this, the server may rather rely on a trade-off between
the Sender Sequence Number gap D and a time gap T = (t2 - t1), where
t1 is the time when the latest group request from a client was
accepted and t2 is the time when the latest group request from that
client has been received, respectively. Then, the server can start a
challenge-response when experiencing a time gap T larger than a
given, pre-configured threshold. Also, the server can start a
challenge-response when experiencing a Sender Sequence Number gap D
greater than a different threshold, computed as a monotonically
increasing function of the currently experienced time gap T.
The challenge-response approach described in this appendix provides
an assurance of absolute message freshness. However, it can result
in an impact on performance which is undesirable or unbearable,
especially in large groups where many endpoints at the same time
might join as new members or lose synchronization.
Note that endpoints configured as silent servers are not able to
perform the challenge-response described above, as they do not store
a Sender Context to secure the 4.01 (Unauthorized) response to the
client. Therefore, silent servers should adopt alternative
approaches to achieve and maintain synchronization with sender
sequence numbers of clients.
Since requests including the Echo Option are sent over unicast, a
server can be a victim of the attack discussed in Section 10.7, when
such requests are protected with the group mode of Group OSCORE, as
described in Section 8.1.
Instead, protecting requests with the Echo Option by using the
pairwise mode of Group OSCORE as described in Section 9.2 prevents
the attack in Section 10.7. In fact, only the exact server involved
in the Echo exchange is able to derive the correct pairwise key used
by the client to protect the request including the Echo Option.
In either case, an internal on-path adversary would not be able to
mix up the Echo Option value of two different unicast requests, sent
by a same client to any two different servers in the group. In fact,
if the group mode was used, this would require the adversary to forge
the client's counter signature in both such requests. As a
consequence, each of the two servers remains able to selectively
accept a request with the Echo Option only if it is waiting for that
exact integrity-protected Echo Option value, and is thus the intended
recipient.
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Appendix F. No Verification of Signatures in Group Mode
There are some application scenarios using group communication that
have particularly strict requirements. One example of this is the
requirement of low message latency in non-emergency lighting
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
messages protected with the group mode. 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 a message protected with the group mode, 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 messages protected with the
group mode. However, it is recognized that there may be situations
where it is not always required. The consequence of not doing the
signature validation in messages protected with the group mode 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 group would have evidence that the 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.
Appendix G. Optimized Request
An optimized request is processed as a request in group mode
(Section 8.1) and uses the OSCORE header compression defined in
Section 5 for the group mode, with the following difference: the
payload of the OSCORE message SHALL encode the ciphertext without the
tag, concatenated with the value of the CounterSignature0 of the COSE
object computed as described in Section 4.1.
The optimized request is compatible with all AEAD algorithms defined
in [I-D.ietf-cose-rfc8152bis-algs], but would not be compatible with
AEAD algorithms that do not have a well-defined tag.
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Appendix H. Example Values of Parameters for Countersignatures
The table below provides examples of values for Counter Signature
Parameters in the Common Context (see Section 2.1.3), for different
values of Counter Signature Algorithm.
+-------------------+---------------------------------------------+
| Counter Signature | Example Values for Counter |
| Algorithm | Signature Parameters |
+-------------------+---------------------------------------------+
| (-8) // EdDSA | [1], [1, 6] // 1: OKP ; 1: OKP, 6: Ed25519 |
| (-7) // ES256 | [2], [2, 1] // 2: EC2 ; 2: EC2, 1: P-256 |
| (-35) // ES384 | [2], [2, 2] // 2: EC2 ; 2: EC2, 2: P-384 |
| (-36) // ES512 | [2], [2, 3] // 2: EC2 ; 2: EC2, 3: P-512 |
| (-37) // PS256 | [], [3] // empty ; 3: RSA |
| (-38) // PS384 | [], [3] // empty ; 3: RSA |
| (-39) // PS512 | [], [3] // empty ; 3: RSA |
+-------------------+---------------------------------------------+
Figure 4: Examples of Counter Signature Parameters
The table below provides examples of values for Counter Signature Key
Parameters in the Common Context (see Section 2.1.4), for different
values of Counter Signature Algorithm.
+-------------------+---------------------------------+
| Counter Signature | Example Values for Counter |
| Algorithm | Signature Key Parameters |
+-------------------+---------------------------------+
| (-8) // EdDSA | [1, 6] // 1: OKP , 6: Ed25519 |
| (-7) // ES256 | [2, 1] // 2: EC2 , 1: P-256 |
| (-35) // ES384 | [2, 2] // 2: EC2 , 2: P-384 |
| (-36) // ES512 | [2, 3] // 2: EC2 , 3: P-512 |
| (-37) // PS256 | [3] // 3: RSA |
| (-38) // PS384 | [3] // 3: RSA |
| (-39) // PS512 | [3] // 3: RSA |
+-------------------+---------------------------------+
Figure 5: Examples of Counter Signature Key Parameters
Appendix I. Document Updates
RFC EDITOR: PLEASE REMOVE THIS SECTION.
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I.1. Version -08 to -09
o Pairwise keys are discarded after group rekeying.
o Signature mode renamed to group mode.
o The parameters for countersignatures use the updated COSE
registries. Newly defined IANA registries have been removed.
o Pairwise Flag bit renamed as Group Flag bit, set to 1 in group
mode and set to 0 in pairwise mode.
o Dedicated section on updating the Security Context.
o By default, sender sequence numbers and replay windows are not
reset upon group rekeying.
o An endpoint implementing only a silent server does not support the
pairwise mode.
o Separate section on general message reception.
o Pairwise mode moved to the document body.
o Considerations on using the pairwise mode in non-multicast
settings.
o Optimized requests are moved as an appendix.
o Normative support for the signature and pairwise mode.
o Revised methods for synchronization with clients' sender sequence
number.
o Appendix with example values of parameters for countersignatures.
o Clarifications and editorial improvements.
I.2. Version -07 to -08
o Clarified relation between pairwise mode and group communication
(Section 1).
o Improved definition of "silent server" (Section 1.1).
o Clarified when a Recipient Context is needed (Section 2).
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o Signature checkers as entities supported by the Group Manager
(Section 2.3).
o Clarified that the Group Manager is under exclusive control of Gid
and Sender ID values in a group, with Sender ID values under each
Gid value (Section 2.3).
o Mitigation policies in case of recycled 'kid' values
(Section 2.4).
o More generic exhaustion (not necessarily wrap-around) of sender
sequence numbers (Sections 2.5 and 10.11).
o Pairwise key considerations, as to group rekeying and Sender
Sequence Numbers (Section 3).
o Added reference to static-static Diffie-Hellman shared secret
(Section 3).
o Note for implementation about the external_aad for signing
(Sectino 4.3.2).
o Retransmission by the application for group requests over
multicast as Non-Confirmable (Section 7).
o A server MUST use its own Partial IV in a response, if protecting
it with a different context than the one used for the request
(Section 7.3).
o Security considerations: encryption of pairwise mode as
alternative to group-level security (Section 10.1).
o Security considerations: added approach to reduce the chance of
global collisions of Gid values from different Group Managers
(Section 10.5).
o Security considerations: added implications for block-wise
transfers when using the signature mode for requests over unicast
(Section 10.7).
o Security considerations: (multiple) supported signature algorithms
(Section 10.13).
o Security considerations: added privacy considerations on the
approach for reducing global collisions of Gid values
(Section 10.15).
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o Updates to the methods for synchronizing with clients' sequence
number (Appendix E).
o Simplified text on discovery services supporting the pairwise mode
(Appendix G.1).
o Editorial improvements.
I.3. Version -06 to -07
o Updated abstract and introduction.
o Clarifications of what pertains a group rekeying.
o Derivation of pairwise keying material.
o Content re-organization for COSE Object and OSCORE header
compression.
o Defined the Pairwise Flag bit for the OSCORE option.
o Supporting CoAP Observe for group requests and responses.
o Considerations on message protection across switching to new
keying material.
o New optimized mode based on pairwise keying material.
o More considerations on replay protection and Security Contexts
upon key renewal.
o Security considerations on Group OSCORE for unicast requests, also
as affecting the usage of the Echo option.
o Clarification on different types of groups considered
(application/security/CoAP).
o New pairwise mode, using pairwise keying material for both
requests and responses.
I.4. Version -05 to -06
o Group IDs mandated to be unique under the same Group Manager.
o Clarifications on parameter update upon group rekeying.
o Updated external_aad structures.
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o Dynamic derivation of Recipient Contexts made optional and
application specific.
o Optional 4.00 response for failed signature verification on the
server.
o Removed client handling of duplicated responses to multicast
requests.
o Additional considerations on public key retrieval and group
rekeying.
o Added Group Manager responsibility on validating public keys.
o Updates IANA registries.
o Reference to RFC 8613.
o Editorial improvements.
I.5. Version -04 to -05
o Added references to draft-dijk-core-groupcomm-bis.
o New parameter Counter Signature Key Parameters (Section 2).
o Clarification about Recipient Contexts (Section 2).
o Two different external_aad for encrypting and signing
(Section 3.1).
o Updated response verification to handle Observe notifications
(Section 6.4).
o Extended Security Considerations (Section 8).
o New "Counter Signature Key Parameters" IANA Registry
(Section 9.2).
I.6. Version -03 to -04
o Added the new "Counter Signature Parameters" in the Common Context
(see Section 2).
o Added recommendation on using "deterministic ECDSA" if ECDSA is
used as counter signature algorithm (see Section 2).
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o Clarified possible asynchronous retrieval of keying material from
the Group Manager, in order to process incoming messages (see
Section 2).
o Structured Section 3 into subsections.
o Added the new 'par_countersign' to the aad_array of the
external_aad (see Section 3.1).
o Clarified non reliability of 'kid' as identity indicator for a
group member (see Section 2.1).
o Described possible provisioning of new Sender ID in case of
Partial IV wrap-around (see Section 2.2).
o The former signature bit in the Flag Byte of the OSCORE option
value is reverted to reserved (see Section 4.1).
o Updated examples of compressed COSE object, now with the sixth
less significant bit in the Flag Byte of the OSCORE option value
set to 0 (see Section 4.3).
o Relaxed statements on sending error messages (see Section 6).
o Added explicit step on computing the counter signature for
outgoing messages (see Setions 6.1 and 6.3).
o Handling of just created Recipient Contexts in case of
unsuccessful message verification (see Sections 6.2 and 6.4).
o Handling of replied/repeated responses on the client (see
Section 6.4).
o New IANA Registry "Counter Signature Parameters" (see
Section 9.1).
I.7. Version -02 to -03
o Revised structure and phrasing for improved readability and better
alignment with draft-ietf-core-object-security.
o Added discussion on wrap-Around of Partial IVs (see Section 2.2).
o Separate sections for the COSE Object (Section 3) and the OSCORE
Header Compression (Section 4).
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o The countersignature is now appended to the encrypted payload of
the OSCORE message, rather than included in the OSCORE Option (see
Section 4).
o Extended scope of Section 5, now titled " Message Binding,
Sequence Numbers, Freshness and Replay Protection".
o Clarifications about Non-Confirmable messages in Section 5.1
"Synchronization of Sender Sequence Numbers".
o Clarifications about error handling in Section 6 "Message
Processing".
o Compacted list of responsibilities of the Group Manager in
Section 7.
o Revised and extended security considerations in Section 8.
o Added IANA considerations for the OSCORE Flag Bits Registry in
Section 9.
o Revised Appendix D, now giving a short high-level description of a
new endpoint set-up.
I.8. Version -01 to -02
o Terminology has been made more aligned with RFC7252 and draft-
ietf-core-object-security: i) "client" and "server" replace the
old "multicaster" and "listener", respectively; ii) "silent
server" replaces the old "pure listener".
o Section 2 has been updated to have the Group Identifier stored in
the 'ID Context' parameter defined in draft-ietf-core-object-
security.
o Section 3 has been updated with the new format of the Additional
Authenticated Data.
o Major rewriting of Section 4 to better highlight the differences
with the message processing in draft-ietf-core-object-security.
o Added Sections 7.2 and 7.3 discussing security considerations
about uniqueness of (key, nonce) and collision of group
identifiers, respectively.
o Minor updates to Appendix A.1 about assumptions on multicast
communication topology and group size.
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o Updated Appendix C on format of group identifiers, with practical
implications of possible collisions of group identifiers.
o Updated Appendix D.2, adding a pointer to draft-palombini-ace-key-
groupcomm about retrieval of nodes' public keys through the Group
Manager.
o Minor updates to Appendix E.3 about Challenge-Response
synchronization of sequence numbers based on the Echo option from
draft-ietf-core-echo-request-tag.
I.9. Version -00 to -01
o Section 1.1 has been updated with the definition of group as
"security group".
o Section 2 has been updated with:
* Clarifications on etablishment/derivation of Security Contexts.
* A table summarizing the the additional context elements
compared to OSCORE.
o Section 3 has been updated with:
* Examples of request and response messages.
* Use of CounterSignature0 rather than CounterSignature.
* Additional Authenticated Data including also the signature
algorithm, while not including the Group Identifier any longer.
o Added Section 6, listing the responsibilities of the Group
Manager.
o Added Appendix A (former section), including assumptions and
security objectives.
o Appendix B has been updated with more details on the use cases.
o Added Appendix C, providing an example of Group Identifier format.
o Appendix D has been updated to be aligned with draft-palombini-
ace-key-groupcomm.
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Acknowledgments
The authors sincerely thank Christian Amsuess, Stefan Beck, Rolf
Blom, Carsten Bormann, Esko Dijk, Klaus Hartke, Rikard Hoeglund,
Richard Kelsey, John Mattsson, Dave Robin, Jim Schaad, Ludwig Seitz,
Peter van der Stok and Erik Thormarker for their feedback and
comments.
The work on this document has been partly supported by VINNOVA and
the Celtic-Next project CRITISEC; the SSF project SEC4Factory under
the grant RIT17-0032; and the EIT-Digital High Impact Initiative
ACTIVE.
Authors' Addresses
Marco Tiloca
RISE 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
Francesca Palombini
Ericsson AB
Torshamnsgatan 23
Kista SE-16440 Stockholm
Sweden
Email: francesca.palombini@ericsson.com
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Jiye Park
Universitaet Duisburg-Essen
Schuetzenbahn 70
Essen 45127
Germany
Email: ji-ye.park@uni-due.de
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