Group OSCORE - Secure Group Communication for CoAP
draft-ietf-core-oscore-groupcomm-08
CoRE Working Group M. Tiloca
Internet-Draft RISE AB
Intended status: Standards Track G. Selander
Expires: October 8, 2020 F. Palombini
Ericsson AB
J. Park
Universitaet Duisburg-Essen
April 06, 2020
Group OSCORE - Secure Group Communication for CoAP
draft-ietf-core-oscore-groupcomm-08
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. using IP
multicast. In particular, the described approach defines how OSCORE
should be used in a group communication setting to provide source
authentication for CoAP group requests, sent by a client to multiple
servers, and the corresponding CoAP responses.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5
2. Security Context . . . . . . . . . . . . . . . . . . . . . . 7
2.1. Common Context . . . . . . . . . . . . . . . . . . . . . 7
2.2. Sender Context and Recipient Context . . . . . . . . . . 8
2.3. The Group Manager . . . . . . . . . . . . . . . . . . . . 9
2.4. Management of Group Keying Material . . . . . . . . . . . 10
2.5. Exhaustion of Partial IV Values . . . . . . . . . . . . . 11
3. Pairwise Keys . . . . . . . . . . . . . . . . . . . . . . . . 12
3.1. Key Derivation . . . . . . . . . . . . . . . . . . . . . 12
3.2. Usage of Sequence Numbers . . . . . . . . . . . . . . . . 13
3.3. Note on Implementation . . . . . . . . . . . . . . . . . 13
4. The COSE Object . . . . . . . . . . . . . . . . . . . . . . . 14
4.1. Counter Signature . . . . . . . . . . . . . . . . . . . . 14
4.2. The 'kid' and 'kid context' parameters . . . . . . . . . 14
4.3. external_aad . . . . . . . . . . . . . . . . . . . . . . 15
4.3.1. external_aad for Encryption . . . . . . . . . . . . . 15
4.3.2. external_aad for Signing . . . . . . . . . . . . . . 16
5. OSCORE Header Compression . . . . . . . . . . . . . . . . . . 17
5.1. Examples of Compressed COSE Objects . . . . . . . . . . . 17
6. Message Binding, Sequence Numbers, Freshness and Replay
Protection . . . . . . . . . . . . . . . . . . . . . . . . . 19
6.1. Synchronization of Sender Sequence Numbers . . . . . . . 19
7. Message Processing . . . . . . . . . . . . . . . . . . . . . 19
7.1. Protecting the Request . . . . . . . . . . . . . . . . . 20
7.1.1. Supporting Observe . . . . . . . . . . . . . . . . . 21
7.2. Verifying the Request . . . . . . . . . . . . . . . . . . 21
7.2.1. Supporting Observe . . . . . . . . . . . . . . . . . 22
7.3. Protecting the Response . . . . . . . . . . . . . . . . . 22
7.3.1. Supporting Observe . . . . . . . . . . . . . . . . . 23
7.4. Verifying the Response . . . . . . . . . . . . . . . . . 23
7.4.1. Supporting Observe . . . . . . . . . . . . . . . . . 24
8. Responsibilities of the Group Manager . . . . . . . . . . . . 24
9. Optimized Mode . . . . . . . . . . . . . . . . . . . . . . . 25
9.1. Optimized Request . . . . . . . . . . . . . . . . . . . . 25
9.1.1. Optimized Compressed Request . . . . . . . . . . . . 25
9.2. Optimized Response . . . . . . . . . . . . . . . . . . . 26
9.2.1. Optimized Compressed Response . . . . . . . . . . . . 26
10. Security Considerations . . . . . . . . . . . . . . . . . . . 26
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10.1. Group-level Security . . . . . . . . . . . . . . . . . . 27
10.2. Uniqueness of (key, nonce) . . . . . . . . . . . . . . . 28
10.3. Management of Group Keying Material . . . . . . . . . . 28
10.4. Update of Security Context and Key Rotation . . . . . . 29
10.4.1. Late Update on the Sender . . . . . . . . . . . . . 29
10.4.2. Late Update on the Recipient . . . . . . . . . . . . 30
10.5. Collision of Group Identifiers . . . . . . . . . . . . . 30
10.6. Cross-group Message Injection . . . . . . . . . . . . . 30
10.7. Group OSCORE for Unicast Requests . . . . . . . . . . . 32
10.8. End-to-end Protection . . . . . . . . . . . . . . . . . 33
10.9. Security Context Establishment . . . . . . . . . . . . . 33
10.10. Master Secret . . . . . . . . . . . . . . . . . . . . . 34
10.11. Replay Protection . . . . . . . . . . . . . . . . . . . 34
10.12. Client Aliveness . . . . . . . . . . . . . . . . . . . . 35
10.13. Cryptographic Considerations . . . . . . . . . . . . . . 35
10.14. Message Segmentation . . . . . . . . . . . . . . . . . . 36
10.15. Privacy Considerations . . . . . . . . . . . . . . . . . 36
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 37
11.1. Counter Signature Parameters Registry . . . . . . . . . 37
11.2. Counter Signature Key Parameters Registry . . . . . . . 40
11.3. OSCORE Flag Bits Registry . . . . . . . . . . . . . . . 42
11.4. Expert Review Instructions . . . . . . . . . . . . . . . 42
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 43
12.1. Normative References . . . . . . . . . . . . . . . . . . 43
12.2. Informative References . . . . . . . . . . . . . . . . . 44
Appendix A. Assumptions and Security Objectives . . . . . . . . 46
A.1. Assumptions . . . . . . . . . . . . . . . . . . . . . . . 47
A.2. Security Objectives . . . . . . . . . . . . . . . . . . . 48
Appendix B. List of Use Cases . . . . . . . . . . . . . . . . . 49
Appendix C. Example of Group Identifier Format . . . . . . . . . 51
Appendix D. Set-up of New Endpoints . . . . . . . . . . . . . . 52
Appendix E. Examples of Synchronization Approaches . . . . . . . 53
E.1. Best-Effort Synchronization . . . . . . . . . . . . . . . 53
E.2. Baseline Synchronization . . . . . . . . . . . . . . . . 53
E.3. Challenge-Response Synchronization . . . . . . . . . . . 54
Appendix F. No Verification of Signatures . . . . . . . . . . . 56
Appendix G. Pairwise Mode . . . . . . . . . . . . . . . . . . . 57
G.1. Pre-Requirements . . . . . . . . . . . . . . . . . . . . 57
G.2. Pairwise Protected Request . . . . . . . . . . . . . . . 57
G.3. Pairwise Protected Response . . . . . . . . . . . . . . . 58
Appendix H. Document Updates . . . . . . . . . . . . . . . . . . 58
H.1. Version -07 to -08 . . . . . . . . . . . . . . . . . . . 58
H.2. Version -06 to -07 . . . . . . . . . . . . . . . . . . . 60
H.3. Version -05 to -06 . . . . . . . . . . . . . . . . . . . 60
H.4. Version -04 to -05 . . . . . . . . . . . . . . . . . . . 61
H.5. Version -03 to -04 . . . . . . . . . . . . . . . . . . . 61
H.6. Version -02 to -03 . . . . . . . . . . . . . . . . . . . 62
H.7. Version -01 to -02 . . . . . . . . . . . . . . . . . . . 63
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H.8. Version -00 to -01 . . . . . . . . . . . . . . . . . . . 64
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 64
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 65
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) [RFC8152] 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 should be used in a group communication setting to provide
source authentication for CoAP group requests, sent by a client to
multiple servers, and the corresponding CoAP responses.
Group OSCORE provides source authentication of CoAP messages, by
means of two possible methods. The first method relies on a digital
signature produced with the private key of the sender and embedded in
the protected CoAP message. The second method relies on a symmetric
key, which is derived from a pairwise shared secred computed from the
asymmetric keys of the message sender and recipient.
The second method is intended for one-to-one messages sent in the
group. These include all responses, as individually sent by servers,
as well as requests addressed to an individual server. For instance,
such requests are sent as part of an exchange using the CoAP Echo
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option [I-D.ietf-core-echo-request-tag], or as part of a block-wise
transfer [RFC7959] in the group, 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]).
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 cannot be used to secure
messages sent over IP multicast.
Group OSCORE defines different modes of operation:
o In the signature mode, Group OSCORE requests and responses are
digitally signed. The signature mode supports all COSE algorithms
as well as signature verification by intermediaries.
o The pairwise mode allows two group members to exchange unicast
OSCORE requests and responses protected with symmetric keys.
These symmetric keys are derived from Diffie-Hellman shared
secrets, calculated with the asymmetric keys of the two group
members. This allows for shorter integrity tags and therefore
lower message overhead.
o In the (hybrid) optimized mode, the CoAP requests are digitally
signed as in the signature mode, and the CoAP responses are
integrity protected with the symmetric key of the pairwise mode.
The signature and optimized modes are detailed in the body of this
document. The pairwise mode is detailed in Appendix G. Unless
otherwise stated, this specification refers to the signature mode.
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.
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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 [RFC8152].
Readers are also expected to be familiar with the terms and concepts
for protection and processing of CoAP messages through OSCORE, such
as "Security Context" and "Master Secret", defined in [RFC8613].
Terminology for constrained environments, such as "constrained
device", "constrained-node network", is defined in [RFC7228].
This document refers also to the following terminology.
o Keying material: data that is necessary to establish and maintain
secure communication among 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", not to be confused with
CoAP/network/multicast 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 8.
o Silent server: member of a group that never responds to requests.
Given that, for CoAP group communications, messages are normally
sent without requesting a confirmation, the idea of a server
silently acting on a message is not unreasonable. Note that an
endpoint can implement both a silent server and a client, the two
roles are independent. An endpoint implementing only a silent
server processes only incoming requests, and, in case it supports
only the signature mode, it maintains less keying material and
especially does not have a Sender Context for the group.
o Group Identifier (Gid): identifier assigned to the group. Group
Identifiers must be 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.
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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
Each endpoint registered as 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.
o One Sender Context, extended with the endpoint's private key. 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. No Recipient Contexts are maintained as 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.
+---------------------------+----------------------------------+
| Context Component | New Information Elements |
+---------------------------+----------------------------------+
| | Counter Signature Algorithm |
| Common Context | Counter Signature Parameters |
| | Counter Signature Key Parameters |
+---------------------------+----------------------------------+
| Sender Context | Endpoint's own private key |
+---------------------------+----------------------------------+
| Each Recipient Context | Public key of the other endpoint |
+---------------------------+----------------------------------+
Figure 1: Additions to the OSCORE Security Context
2.1. Common 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 is application specific. An example of
specific formatting of the Gid is given in Appendix C. The
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application needs to specify how to handle possible collisions
between Gids, see Section 10.5.
The Counter Signature Algorithm identifies the digital signature
algorithm used to compute a counter signature on the COSE object (see
Section 4.5 of [RFC8152]). Its value is immutable once the Common
Context is established. The used Counter Signature Algorithm MUST be
selected among the signing ones defined in the COSE Algorithms
Registry (see section 16.4 of [RFC8152]). The EdDSA signature
algorithm Ed25519 [RFC8032] is mandatory to implement. If Elliptic
Curve Digital Signature Algorithm (ECDSA) is used, it is RECOMMENDED
that implementations implement "deterministic ECDSA" as specified in
[RFC6979].
The Counter Signature Parameters identifies the parameters associated
to the digital signature algorithm specified in the Counter Signature
Algorithm. This parameter MAY be empty and is immutable once the
Common Context is established. The exact structure of this parameter
depends on the value of Counter Signature Algorithm, and is defined
in the Counter Signature Parameters Registry (see Section 11.1),
where each entry indicates a specified structure of the Counter
Signature Parameters.
The Counter Signature Key Parameters identifies the parameters
associated to the keys used with the digital signature algorithm
specified in the Counter Signature Algorithm. This parameter MAY be
empty and is immutable once the Common Context is established. The
exact structure of this parameter depends on the value of Counter
Signature Algorithm, and is defined in the Counter Signature Key
Parameters Registry (see Section 11.2), where each entry indicates a
specified structure of the Counter Signature Key Parameters.
2.2. Sender Context and Recipient Context
OSCORE specifies the derivation of Sender Context and Recipient
Context, specifically 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]. However, for Group
OSCORE the Sender Context and Recipient Context additionally contain
asymmetric keys.
The Sender Context needs to be configured with the private key of the
endpoint. The private key is used to generate a signature (see
Section 4) included in the sent OSCORE message. How the private key
is established is out of scope for this specification.
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Each Recipient Context needs to be configured with the public key of
the associated endpoint. The public key is used to verify a
signature (see Section 4) included in the received OSCORE message.
The input parameters for deriving the Recipient Context parameters
and the public key of the associated endpoint may be provided to the
recipient endpoint upon joining the group. These parameters may
alternatively be acquired at a later time, for example the first time
a message is received from this particular endpoint in the group (see
Section 7.2 and Section 7.4). The received message together with the
Common Context contains the necessary information to derive a
security context for verifying a message, except for the public key
of the associated endpoint.
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 Recipient Context, and retrieve the
public key in order to have it available to verify subsequent
messages from that endpoint.
2.3. The Group Manager
Endpoints communicating with Group OSCORE need, in addition to the
OSCORE input parameters, also to be provisioned with information
about the group(s) and other endpoints in the group(s).
The Group Manager is an entity responsible for the group, including
the Group Identifier (Gid) used as ID Context, as well as the Sender
ID and Recipient ID of the group members (see Section 8).
The Group Manager is exclusively in control of the Gid values
uniquely assigned to the different groups under its control, as well
as of the Sender ID and Recipient ID values uniquely assigned to the
members of each 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 records the
public keys of endpoints joining a group, and provides information
about the group and its members to other members.
An endpoint receives the Group Identifier and OSCORE input
parameters, including its own Sender ID, from the Group Manager upon
joining the group. That Sender ID is valid only within that group,
and is unique within the group. Endpoints which are configured only
as silent servers do not have a Sender ID.
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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.
According to this specification, 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].
Further details about how public keys can be handled and retrieved in
the group is out of the scope of this document.
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 is required to be authorized for
retrieving public keys of members in a specific group from the Group
Manager. To this end, the same method mentioned above and based on
the ACE framework can be used.
2.4. Management of Group Keying Material
In order to establish a new Security Context in a group, a new Group
Identifier (Gid) for that group and a new value for the Master Secret
parameter MUST be distributed. 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
value for the Master Salt parameter, and SHOULD preserve the current
value of the Sender ID of each group member.
Then, each group member re-derives the keying material stored in its
own Sender Context and Recipient Contexts as described in Section 2,
using the updated Gid and Master Secret parameter. 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.
From then on, each group member MUST use its latest installed Sender
Context to protect outgoing messages.
After a new Gid has been distributed, a same Recipient ID ('kid')
should not be considered as a persistent and reliable indicator of
the same group member. Such an indication can be actually achieved
only by using that members's public key. This occurs when verifying
countersignatures of received messages (in signature mode), or when
verifying messages integrity-protected with pairwise keying material
derived from asymmetric keys (in pairwise mode). As a consequence,
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group members may end up retaining stale Recipient Contexts, that are
no longer useful to verify incoming secure messages.
In order to alleviate this issue, the Group Manager SHOULD NOT
recycle 'kid' values within a same group, especially in the short
term. 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 an owned
public key is currently the one associated to a 'kid' value, after a
number of consecutive failed verifications.
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 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 is provided in Section 10.4.
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 the new Gid and Master
Secret parameter to the group 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].
2.5. Exhaustion of Partial IV Values
An endpoint can eventually exhaust its own Sender Sequence Number,
which is incremented after sending each new message including a
Partial IV. This is the case for all group requests, all Observe
notifications [RFC7641] and, optionally, any other response.
If an implementation's integers only support wrapping addition, the
implementation MUST detect a wrap-around of the Sender Sequence
Number value and treat that as exhausted instead.
Upon exhausting its own Sender Sequence Number, the endpoint MUST NOT
transmit further messages for that group until it has derived a new
Sender Context, in order to avoid reusing nonces with the same keys.
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Furthermore, the endpoint SHOULD inform the Group Manager, that can
take one of the following actions:
o The Group Manager renews the Security Context in the group (see
Section 2.4).
o The Group Manager provides a new Sender ID value to the endpoint
that has experienced the exhaustion. Then, the endpoint derives a
new Sender Context using the new Sender ID, as described in
Section 3.2 of [RFC8613].
In either case, same considerations from Section 2.4 hold about
possible retaining of stale Recipient Contexts.
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 encryption keys for use in the pairwise and
optimized modes of Group OSCORE.
3.1. Key Derivation
Two group members can derive a symmetric pairwise key, from their
Sender/Recipient Key and a static-static Diffe-Hellman shared secret
[NIST-800-56A]. The key derivation is as follows, and uses the same
construction used in Section 3.2.1 of [RFC8613].
Pairwise key = HKDF(Sender/Recipient Key, Shared Secret, info, L)
where:
o The Sender/Recipient key is the Sender Key of the sender, i.e. the
Recipient Key that the recipient stores in its own Recipient
Context corresponding to the sender.
o The Shared Secret is computed as a static-static Diffie-Hellman
shared secret [NIST-800-56A], where the sender uses its own
private key and the recipient's public key, while the recipient
uses its own private key and the senders's public key. The Shared
Secret may be stored in memory, rather than recomputed each time
it is needed.
o info and L are defined as in Section 3.2.1 of [RFC8613].
The security of using the same key pair for Diffie-Hellman and for
signing is proven in [Degabriele]. The derivation of pairwise keys
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defined above is compatible with ECDSA and EdDSA asymmetric keys, but
is not compatible with RSA asymmetric keys.
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 completing the establishment of a new Security Context (see
Section 2.4), every group member MUST delete all its pairwise keys.
Since new Sender/Recipient keys are derived from the new group keying
material (see Section 2.2), every group member MUST use such new
Sender/Recipient keys when possibly deriving new pairwise keys.
As long as any two group members preserve the same asymmetric keys,
the Diffie-Hellman shared secret does not change across updates of
the group keying material.
3.2. Usage of Sequence Numbers
When using any of its pairwise keys, a sender endpoint MUST use the
current fresh value of its own Sender Sequence Number, from its own
Sender Context (see Section 2.2). That is, the same Sender Sequence
Number space is used for all outgoing messages sent to the group and
protected with Group OSCORE, thus limiting both storage and
complexity.
On the other hand, when combining one-to-many and one-to-one
communication in the group, this may result in the Partial IV values
moving forward more often. Fundamentally, this is due to the fact
that not all the recipients receive all messages from a given sender.
For instance, requests sent over multicast (in signature mode) are
addressed to the whole group, while requests sent over unicast (in
signature mode or pairwise mode) are not.
As a consequence, replay checks may be invoked more often on the
recipient side, where larger replay windows should be considered.
3.3. Note on Implementation
In order to optimize performance, an endpoint A may derive a pairwise
key used with an endpoint B in the OSCORE group only once, and then
store it in its own Security Context for future retrieval. This can
work as follows.
Endpoint A can have a Pairwise Sender Context associated to B, within
its own Sender Context. This Pairwise Sender Context includes:
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o The Recipient ID of B for A, i.e. the Sender ID of B.
o The Pairwise Key derived as defined in Section 3, with A acting as
sender and B acting as recipient.
More generally, A has one of such Paiwise Sender Contexts within its
own Sender Context, for each different intended recipient.
Furthermore, A can additionally store in its own Recipient Context
associated to B the Pairwise Key to use for incoming traffic from B.
That is, this Pairwise Key is derived as defined in Section 3, with A
acting as recipient and B acting as sender.
4. The COSE Object
Building on Section 5 of [RFC8613], this section defines how to use
COSE [RFC8152] 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. For
the signature mode of Group OSCORE the following modifications apply.
4.1. Counter Signature
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
Appendix A.2 of [RFC8152], by using its own 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.
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, i.e. 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's Security Context. That is,
unlike in [RFC8613], the 'kid context' parameter is always present in
requests.
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4.3. external_aad
The external_aad of the Additional Authenticated Data (AAD) is built
differently. In particular, it has one structure used for the
encryption process producing the ciphertext, and a second structure
used for the signing process producing the counter signature.
4.3.1. external_aad for Encryption
The first external_aad structure used for the encryption process
producing the ciphertext (see Section 5.3 of [RFC8152]) includes also
the counter signature algorithm and related parameters used to sign
messages. In particular, compared with Section 5.4 of [RFC8613], the
'algorithms' array in the aad_array MUST also include:
o 'alg_countersign', which contains the Counter Signature Algorithm
from the Common Context (see Section 2). This parameter has the
value specified in the "Value" field of the Counter Signature
Parameters Registry (see Section 11.1) for this counter signature
algorithm.
o 'par_countersign', which contains the Counter Signature Parameters
from the Common Context (see Section 2). This parameter contains
the counter signature parameters encoded as specified in the
"Parameters" field of the Counter Signature Parameters Registry
(see Section 11.1), for the used counter signature algorithm. If
the Counter Signature Parameters in the Common Context is empty,
'par_countersign' MUST be encoding the CBOR simple value Null.
o 'par_countersign_key', which contains the Counter Signature Key
Parameters from the Common Context (see Section 2). This
parameter contains the counter signature key parameters encoded as
specified in the "Parameters" field of the Counter Signature Key
Parameters Registry (see Section 11.2), for the used counter
signature algorithm. If the Counter Signature Key Parameters in
the Common Context is empty, 'par_countersign_key' MUST be
encoding the CBOR simple value Null.
Thus, the following external_aad structure is used for the encryption
process producing the ciphertext (see Section 5.3 of [RFC8152]).
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external_aad = bstr .cbor aad_array
aad_array = [
oscore_version : uint,
algorithms : [alg_aead : int / tstr,
alg_countersign : int / tstr,
par_countersign : any / nil,
par_countersign_key : any / nil],
request_kid : bstr,
request_piv : bstr,
options : bstr
]
4.3.2. external_aad for Signing
The second external_aad structure used for the signing process
producing the counter signature as defined below includes also:
o the counter signature algorithm and related parameters used to
sign messages, encoded as in the external_aad structure defined in
Section 4.3.1;
o the value of the OSCORE Option included in the OSCORE message,
encoded as a binary string.
Thus, the following external_aad structure is used for the signing
process producing the counter signature, as defined below.
external_aad = bstr .cbor aad_array
aad_array = [
oscore_version : uint,
algorithms : [alg_aead : int / tstr,
alg_countersign : int / tstr,
par_countersign : any / nil,
par_countersign_key : any / nil],
request_kid : bstr,
request_piv : bstr,
options : bstr,
OSCORE_option: bstr
]
Note for implementation: this requires the value of the OSCORE option
to be fully ready, before starting the signing process. Also, this
requires that the aad_array is long enough to contain the longest
possible OSCORE option.
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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 signature mode as well as in the
optimized compressed requests of the optimized mode (see
Section 9.1.1), 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, namely the Pairwise Flag bit, in the first byte
of the OSCORE option containing the OSCORE flag bits. This flag
bit is registered in Section 11.3 of this specification.
o The Pairwise Flag bit is set to 1 if the OSCORE message is
protected using pairwise keying material, which is shared with a
single group member as single intended recipient and derived as
defined in Section 3. This is used, for instance, to send
responses with the optimized mode defined in Section 9. In any
other case, especially when the OSCORE message is protected as per
Section 7.1 and Section 7.3, this bit MUST be set to 0.
If any of the following conditions holds, a recipient MUST discard
an incoming OSCORE message with the Pairwise Flag bit set to 1:
* The recipient does not support this feature, i.e. it is not
capable or willing to process OSCORE messages protected using
pairwise keying material.
* The recipient can not retrieve a Security Context which is both
valid to process the message and also associated to an OSCORE
group.
5.1. Examples of Compressed COSE Objects
This section covers a list of OSCORE Header Compression examples for
group requests and responses, with Group OSCORE used in signature
mode.
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 label for the new kid context defined in
[RFC8613] has value 10. The examples also 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. Finally, COUNTERSIGN is the CounterSignature0 byte string as
described in Section 4 and is 64 bytes long.
1. 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: 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 COUNTERSIGN
(14 bytes + size of COUNTERSIGN)
1. 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: 0b00001000 = 0x08
Option Value: 08 52 (2 bytes)
Payload: 60 b0 35 05 9d 9e f5 66 7c 5a 07 10 82 3b COUNTERSIGN
(14 bytes + size of COUNTERSIGN)
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
provides relative freshness of responses, and replay protection of
requests.
6.1. Synchronization of Sender Sequence Numbers
Upon joining the group, new servers are not aware of the Sender
Sequence Number values currently used by different clients to
transmit group requests. This means that, when such servers receive
a secure group request from a given client for the first time, they
are not able to verify if that request is fresh and has not been
replayed or (purposely) delayed. The same holds when a server loses
synchronization with Sender Sequence Numbers of clients, 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 synchronization
requirements. The list of methods to handle synchronization of
Sender Sequence Numbers is part of the group communication policy,
and different servers can use different methods.
Appendix E describes three possible approaches that can be considered
for synchronization of sequence numbers.
7. Message Processing
Each request message and response message is protected and processed
as specified in [RFC8613], with the modifications described in the
following sections. In particular, the following sections refer to
the signature mode of Group OSCORE, while the optimized mode and the
pairwise mode are described in Section 9 and Appendix G,
respectively.
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The following security objectives are fulfilled, as further discussed
in Appendix A.2: data replay protection, group-level data
confidentiality, source authentication and message integrity.
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. However, this does not
prevent the acknowledgment of Confirmable group requests in non-
multicast environments. Besides, according to Section 5.2.3 of
[RFC7252], responses to Non-Confirmable group requests SHOULD be also
Non-Confirmable. However, endpoints MUST be prepared to receive
Confirmable responses in reply to a Non-Confirmable group request.
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 group.
7.1. 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 'algorithms' array in the Additional Authenticated
Data is modified as described in Section 4 of this specification.
o In step 4, the encryption of the COSE object is modified as
described in Section 4 of this specification. The encoding of the
compressed COSE object is modified as described in Section 5 of
this specification.
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
of this specification. In particular, the payload of the OSCORE
message includes also the counter signature.
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7.1.1. 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 2.5).
7.2. Verifying the Request
Upon receiving a secure group request, a server proceeds as described
in Section 8.2 of [RFC8613], with the following modifications.
o In step 2, the decoding of the compressed COSE object follows
Section 5 of this specification. In particular:
* If the Pairwise Flag bit is set to 1, and the server discards
the request due to not supporting this feature or 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 Recipient ID ('kid') does not match with any
Recipient Context for the retrieved Gid ('kid context'), then
the server MAY create a new Recipient Context and initializes
it according to Section 3 of [RFC8613], also retrieving the
client's 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 'algorithms' array in the Additional Authenticated
Data is modified as described in Section 4 of this specification.
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 MAY
reply with a 4.00 (Bad Request) response.
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, would
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prevent attackers from overloading the server's storage and
creating processing overhead on the server.
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.
7.2.1. 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.
The server MUST NOT update the stored value, even in case the
observer client is individually rekeyed and starts using a new Sender
ID received from the Group Manager (see Section 2.5).
7.3. Protecting the Response
A server that has received a secure group request may reply with a
secure response, which is protected as described in Section 8.3 of
[RFC8613], with the following modifications.
o In step 2, the 'algorithms' array in the Additional Authenticated
Data is modified as described in Section 4 of this specification.
o In step 4, the encryption of the COSE object is modified as
described in Section 4 of this specification. The encoding of the
compressed COSE object is modified as described in Section 5 of
this specification.
o In step 5, the counter signature is computed and the format of the
OSCORE mesage is modified as described in Section 5 of this
specification. In particular, the payload of the OSCORE message
includes also the counter signature.
Note that the server MUST always protect a response by using its own
Sender Context from the latest owned Security Context.
Consistently, upon the establishment of a new Security Context, the
server may end up protecting a response by using a Security Context
different from the one used to protect the group request (see
Section 10.4). In such a case:
o The server MUST encode the Partial IV (Sender Sequence Number in
network byte order), which is set to the Sender Sequence Number of
the server; increment the Sender Sequence Number by one; compute
the AEAD nonce from the Sender ID, Common IV, and Partial IV.
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o The server MUST include in the response the 'Partial IV'
parameter, which is set to the encoded Partial IV value above.
o The server SHOULD include in the response the 'kid context'
parameter, which is set to the ID Context of the new Security
Context, i.e. the new Group Identifier (Gid).
7.3.1. 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 its own Sender
Context from the new Security Context.
For each ongoing observation, the server SHOULD include in the first
notification protected with the new Security Context also the 'kid
context' parameter, which is set to the ID Context of the new
Security Context, i.e. the new Group Identifier (Gid). It is
OPTIONAL for the server to include the 'kid context' parameter, as
set to the new Gid, 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.
7.4. Verifying the Response
Upon receiving a secure response message, the client proceeds as
described in Section 8.4 of [RFC8613], with the following
modifications.
o In step 2, the decoding of the compressed COSE object is modified
as described in Section 4 of this specification. If the received
Recipient ID ('kid') does not match with any Recipient Context for
the retrieved Gid ('kid context'), then the client MAY create a
new Recipient Context and initializes it according to Section 3 of
[RFC8613], also retrieving the server's 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 'algorithms' array in the Additional Authenticated
Data is modified as described in Section 4 of this specification.
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o In step 5, the client also verifies the counter signature using
the public key of the server from the associated Recipient
Context.
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
a configuration, which is specified by the application, would
prevent attackers from overloading the client's storage and
creating processing overhead on the client.
Note that, as discussed in Section 10.4, a client may receive a
response protected with a Security Context different from the one
used to protect the corresponding group request.
7.4.1. 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 2.5).
8. 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.
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6. Defining a communication policy for each of its OSCORE groups,
and signalling it to new endpoints during the join process.
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.
9. Optimized Mode
For use cases that do not require an intermediary performing
signature verification and that use a compatible signature algorithm,
the optimized mode defined in this section provides significant
smaller message sizes and increases the security by making responses
confidential to other group members than the intended recipient.
9.1. Optimized Request
No changes.
9.1.1. Optimized Compressed Request
The OSCORE header compression defined in Section 5 is used, 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 compressed request is compatible with all AEAD
algorithms defined in [RFC8152], but would not be compatible with
AEAD algorithms that do not have a well-defined tag.
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9.2. Optimized Response
An optimized response is protected as defined in Section 7.3, with
the following differences.
o The server MUST set to 1 the sixth least significant bit of the
OSCORE flag bits in the OSCORE option, i.e. the Pairwise Flag.
o The COSE_Encrypt0 object included in the optimized response is
encrypted using a symmetric pairwise key K, that the server
derives as defined in Section 3. In particular, the Sender/
Recipient Key is the Sender Key of the server from its own Sender
Context, i.e. the Recipient Key that the client stores in its own
Recipient Context corresponding to the server.
o The Counter Signature is not computed. That is, unlike defined in
Section 5, the payload of the OSCORE message terminates with the
encoded ciphertext of the COSE object.
Note that no changes are made to the AEAD nonce and AAD.
Upon receiving a response with the Pairwise Flag set to 1, the client
MUST process it as defined in Section 7.4, with the following
differences.
o No countersignature to verify is included.
o The COSE_Encrypt0 object included in the optimized response is
decrypted and verified using the same symmetric pairwise key K,
that the client derives as described above for the server side and
as defined in Section 3.
9.2.1. Optimized Compressed Response
No changes.
10. Security Considerations
The same threat model discussed for OSCORE in Appendix D.1 of
[RFC8613] holds for Group OSCORE. In addition, 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.
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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 countersignature included in a Group
OSCORE message 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 mounted over IP multicast.
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 signature mode described in Section 7 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 Appendix G 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 3). Therefore, in the
parwise mode, the AEAD algorithm provides both pairwise data-
confidentiality and source authentication of messages, without using
counter signatures.
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
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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
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.
Especially in dynamic, large-scale, groups where endpoints can join
and leave at any time, it is important that the considered group key
management scheme is efficient and highly scalable with the group
size, in order to limit the impact on performance due to the Security
Context and keying material update.
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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. That is, 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. Since
a sender always protects an outgoing message using the latest owned
Security Context, the server discussed above protects the possible
response using the new Security Context. Then, 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
simply discard an incoming group request which is not successfully
processed with the new Security Context.
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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.
10.6. Cross-group Message Injection
A same endpoint is allowed to and would likely use the same signature
key in multiple OSCORE groups, possibly administered by different
Group Managers. Also, the same endpoint can register several times
in the same group, getting multiple unique Sender IDs. This requires
that, when a sender endpoint sends a message to an OSCORE group using
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a Sender ID, the countersignature included in the message is
explicitly bound also to that group and to the used Sender ID.
To this end, the countersignature of each message protected with
Group OSCORE 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
below, where a malicious group member injects forged messages to a
different OSCORE group than the originally intended one. 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 victim endpoint V which is member of both G1 and G2, and uses
the same signature key in both groups. The endpoint V 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 V 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 V in G1 and G2.
If countersignatures were not computed also over the value of the
OSCORE option as discussed above, Z can intercept a group message M1
sent by V to G1, and forge a valid signed message M2 to be injected
in G2, making it appear as sent by V and valid to be accepted.
More in detail, Z first intercepts a message M1 sent by V in G1, and
tries to forge 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.
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If M2 is used as a request message, there is a probability equal to
2^-64 that the same unchanged MAC is successfully verified by using
Sid2 as 'request_kid' and the symmetric key associated to V in G2.
In such a case, the same unchanged signature would be also valid.
Note that Z can check offline if a performed forgery is actually
valid before sending the forged message to G2. That is, this attack
has a complexity of 2^64 offline calculations.
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 V in G2.
In such a case, the same unchanged signature would be also valid.
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 is 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 7.1 for the
signature mode and in Section 9.1.1 for the optimized mode, it is NOT
RECOMMENDED for a client to use Group OSCORE for securing a request
sent to a single group member over unicast.
If the client uses 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 to 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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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 an 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.
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 may instead use the pairwise mode defined in Appendix G.2,
in order to protect a request sent to a single group member by using
pairwise keying material (see Section 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 signature mode.
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. Instead, it is not
possible to combine DTLS and Group OSCORE for protecting message
exchanges where messages are (also) sent over multicast.
10.9. Security Context Establishment
The use of COSE_Encrypt0 and AEAD to protect messages as specified in
this document requires an endpoint to be a member of an OSCORE group.
That is, upon joining the group, the endpoint securely receives from
the Group Manager the necessary input parameters, which are used to
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derive the Common Context and the Sender Context (see Section 2).
The Group Manager ensures uniqueness of Sender IDs in the same group.
Each different Recipient Context for decrypting messages from a
particular sender can be derived at runtime, at the latest upon
receiving a message from that sender for the first time.
Countersignatures of group messages are verified by means of the
public key of the respective sender endpoint. Upon nodes' joining,
the Group Manager collects such public keys and MUST verify proof-of-
possession of the respective private key. Later on, a group member
can request from the Group Manager the public keys of other group
members.
The joining process can occur, for instance, as defined in
[I-D.ietf-ace-key-groupcomm-oscore].
10.10. 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
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].
10.11. 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 2.5, 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.
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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 7.4 also
ensures that a client accepts a single valid response to a given
request from each replying server, unless CoAP observation is used.
10.12. Client Aliveness
As discussed in Section 12.5 of [RFC8613], a server may use the 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).
10.13. 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 2.5, an endpoint that experiences a
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.
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The EdDSA signature algorithm Ed25519 [RFC8032] is mandatory to
implement. For many constrained IoT devices, it is problematic to
support more than one signature algorithm or multiple whole cipher
suites. This means that some deployments using, for instance, ECDSA
with NIST P-256 may not support the mandatory signature algorithm.
However, this is not a problem for local deployments.
10.14. Message Segmentation
The same considerations from Section 12.7 of [RFC8613] hold for Group
OSCORE.
10.15. 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.
Moreover, this parameter explicitly relates two or more
communicating endpoints, as members of the same OSCORE group.
Also, 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 Idenfier changes when the corresponding group is
rekeyed. Thus, changes in the shared list of Group Identifiers may
be used to infer about 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 such privacy
concerns, 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. Counter Signature Parameters Registry
This specification establishes the IANA "Counter Signature
Parameters" Registry. The Registry has been created to use the
"Expert Review Required" registration procedure [RFC8126]. Expert
review guidelines are provided in Section 11.4.
This registry specifies the parameters of each admitted
countersignature algorithm, as well as the possible structure they
are organized into. This information is used to populate the
parameter Counter Signature Parameters of the Common Context (see
Section 2).
The columns of this table are:
o Name: A value that can be used to identify an algorithm in
documents for easier comprehension. Its value is taken from the
'Name' column of the "COSE Algorithms" Registry.
o Value: The value to be used to identify this algorithm. Its
content is taken from the 'Value' column of the "COSE Algorithms"
Registry. The value MUST be the same one used in the "COSE
Algorithms" Registry for the entry with the same 'Name' field.
o Parameters: This indicates the CBOR encoding of the parameters (if
any) for the counter signature algorithm indicated by the 'Value'
field.
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o Description: A short description of the parameters encoded in the
'Parameters' field (if any).
o Reference: This contains a pointer to the public specification for
the field, if one exists.
Initial entries in the registry are as follows.
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+-------------+-------+--------------+-----------------+-----------+
| Name | Value | Parameters | Description | Reference |
+-------------+-------+--------------+-----------------+-----------+
| | | | | |
| EdDSA | -8 | crv : int | crv value taken | [This |
| | | | from the COSE | Document] |
| | | | Elliptic Curve | |
| | | | Registry | |
| | | | | |
+-------------+-------+--------------+-----------------+-----------+
| | | | | |
| ES256 | -7 | crv : int | crv value taken | [This |
| | | | from the COSE | Document] |
| | | | Elliptic Curve | |
| | | | Registry | |
| | | | | |
+-------------+-------+--------------+-----------------+-----------+
| | | | | |
| ES384 | -35 | crv : int | crv value taken | [This |
| | | | from the COSE | Document] |
| | | | Elliptic Curve | |
| | | | Registry | |
| | | | | |
+-------------+-------+--------------+-----------------+-----------+
| | | | | |
| ES512 | -36 | crv : int | crv value taken | [This |
| | | | from the COSE | Document] |
| | | | Elliptic Curve | |
| | | | Registry | |
| | | | | |
+-------------+-------+--------------+-----------------+-----------+
| | | | | |
| PS256 | -37 | | Parameters not | [This |
| | | | present | Document] |
| | | | | |
+-------------+-------+--------------+-----------------+-----------+
| | | | | |
| PS384 | -38 | | Parameters not | [This |
| | | | present | Document] |
| | | | | |
+-------------+-------+--------------+-----------------+-----------+
| | | | | |
| PS512 | -39 | | Parameters not | [This |
| | | | present | Document] |
| | | | | |
+-------------+-------+--------------+-----------------+-----------+
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11.2. Counter Signature Key Parameters Registry
This specification establishes the IANA "Counter Signature Key
Parameters" Registry. The Registry has been created to use the
"Expert Review Required" registration procedure [RFC8126]. Expert
review guidelines are provided in Section 11.4.
This registry specifies the parameters of countersignature keys for
each admitted countersignature algorithm, as well as the possible
structure they are organized into. This information is used to
populate the parameter Counter Signature Key Parameters of the Common
Context (see Section 2).
The columns of this table are:
o Name: A value that can be used to identify an algorithm in
documents for easier comprehension. Its value is taken from the
'Name' column of the "COSE Algorithms" Registry.
o Value: The value to be used to identify this algorithm. Its
content is taken from the 'Value' column of the "COSE Algorithms"
Registry. The value MUST be the same one used in the "COSE
Algorithms" Registry for the entry with the same 'Name' field.
o Parameters: This indicates the CBOR encoding of the key parameters
(if any) for the counter signature algorithm indicated by the
'Value' field.
o Description: A short description of the parameters encoded in the
'Parameters' field (if any).
o Reference: This contains a pointer to the public specification for
the field, if one exists.
Initial entries in the registry are as follows.
+-------------+-------+--------------+-------------------+-----------+
| Name | Value | Parameters | Description | Reference |
+-------------+-------+--------------+-------------------+-----------+
| | | | | |
| EdDSA | -8 | [kty : int , | kty value is 1, | [This |
| | | | as Key Type "OKP" | Document] |
| | | | from the COSE Key | |
| | | | Types Registry | |
| | | | | |
| | | | | |
| | | crv : int] | crv value taken | |
| | | | from the COSE | |
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| | | | Elliptic Curve | |
| | | | Registry | |
| | | | | |
+-------------+-------+--------------+-------------------+-----------+
| | | | | |
| ES256 | -7 | [kty : int , | kty value is 2, | [This |
| | | | as Key Type "EC2" | Document] |
| | | | from the COSE Key | |
| | | | Types Registry | |
| | | | | |
| | | | | |
| | | crv : int] | crv value taken | |
| | | | from the COSE | |
| | | | Elliptic Curve | |
| | | | Registry | |
| | | | | |
+-------------+-------+--------------+-------------------+-----------+
| | | | | |
| ES384 | -35 | [kty : int , | kty value is 2, | [This |
| | | | as Key Type "EC2" | Document] |
| | | | from the COSE Key | |
| | | | Types Registry | |
| | | | | |
| | | crv : int] | crv value taken | |
| | | | from the COSE | |
| | | | Elliptic Curve | |
| | | | Registry | |
| | | | | |
+-------------+-------+--------------+-------------------+-----------+
| | | | | |
| ES512 | -36 | [kty : int , | kty value is 2, | [This |
| | | | as Key Type "EC2" | Document] |
| | | | from the COSE Key | |
| | | | Types Registry | |
| | | | | |
| | | crv : int] | crv value taken | |
| | | | from the COSE | |
| | | | Elliptic Curve | |
| | | | Registry | |
| | | | | |
+-------------+-------+--------------+-------------------+-----------+
| | | | | |
| PS256 | -37 | kty : int | kty value is 3, | [This |
| | | | as Key Type "RSA" | Document] |
| | | | from the COSE Key | |
| | | | Types Registry | |
| | | | | |
+-------------+-------+--------------+-------------------+-----------+
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| | | | | |
| PS384 | -38 | kty : int | kty value is 3, | [This |
| | | | as Key Type "RSA" | Document] |
| | | | from the COSE Key | |
| | | | Types Registry | |
| | | | | |
+-------------+-------+--------------+-------------------+-----------+
| | | | | |
| PS512 | -39 | kty : int | kty value is 3, | [This |
| | | | as Key Type "RSA" | Document] |
| | | | from the COSE Key | |
| | | | Types Registry | |
| | | | | |
+-------------+-------+--------------+-------------------+-----------+
11.3. 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 | Pairwise | Set to 1 if the message | [This |
| | Protection | is protected with | Document] |
| | Flag | pairwise keying material | |
+--------------+-------------+--------------------------+-----------+
11.4. Expert Review Instructions
The IANA Registries established in this document are defined as
"Expert Review". This section gives some general guidelines for what
the experts should be looking for, but they are being designated as
experts for a reason so they should be given substantial latitude.
Expert reviewers should take into consideration the following points:
o Clarity and correctness of registrations. Experts are expected to
check the clarity of purpose and use of the requested entries.
Experts should inspect the entry for the algorithm considered, to
verify the conformity of the encoding proposed against the
theoretical algorithm, including completeness of the 'Parameters'
column. Expert needs to make sure values are taken from the right
registry, when that's required. Expert should consider requesting
an opinion on the correctness of registered parameters from the
CBOR Object Signing and Encryption Working Group (COSE).
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Encodings that do not meet these objective of clarity and
completeness should not be registered.
o Duplicated registration and point squatting should be discouraged.
Reviewers are encouraged to get sufficient information for
registration requests to ensure that the usage is not going to
duplicate one that is already registered and that the point is
likely to be used in deployments.
o Experts should take into account the expected usage of fields when
approving point assignment. The length of the 'Parameters'
encoding should be weighed against the usage of the entry,
considering the size of device it will be used on. Additionally,
the length of the encoded value should be weighed against how many
code points of that length are left, the size of device it will be
used on, and the number of code points left that encode to that
size.
o Specifications are recommended. When specifications are not
provided, the description provided needs to have sufficient
information to verify the points above.
12. References
12.1. Normative References
[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.
[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>.
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[RFC6979] Pornin, T., "Deterministic Usage of the Digital Signature
Algorithm (DSA) and Elliptic Curve Digital Signature
Algorithm (ECDSA)", RFC 6979, DOI 10.17487/RFC6979, August
2013, <https://www.rfc-editor.org/info/rfc6979>.
[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>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.
[RFC8152] Schaad, J., "CBOR Object Signing and Encryption (COSE)",
RFC 8152, DOI 10.17487/RFC8152, July 2017,
<https://www.rfc-editor.org/info/rfc8152>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[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>.
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[I-D.ietf-ace-key-groupcomm]
Palombini, F. and M. Tiloca, "Key Provisioning for Group
Communication using ACE", draft-ietf-ace-key-groupcomm-05
(work in progress), March 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-05 (work in progress), March 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-33
(work in progress), February 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.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.
[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>.
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[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>.
[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. 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 CoAP groups and application 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.
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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.
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, multicast security 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.
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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
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
encrypted and/or integrity 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 Group-level data confidentiality: messages sent within the
security group shall be encrypted if privacy sensitive data is
exchanged within the security group. This document considers
group-level data confidentiality since messages are encrypted at a
group level, i.e. in such a way that they can be decrypted by any
member of the security group, but not by an external adversary or
other external entities.
o Source 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
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first place, and in particular by a specific 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 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 relative freshness of group
requests and absolute freshness of 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
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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
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
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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
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 upon completing each renewal of the Security Context
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and keying material in the group (see Section 2.4). In particular,
once a new Master Secret has been distributed to the group, all the
group members increment by 1 the Group Epoch in the Group Identifier
of that group.
As an example, a 3-byte Group Identifier 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 65535. Then, after having established the Common Context 61532
times in the group, its Group Identifier 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.
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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.
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.
E.1. Best-Effort Synchronization
Upon receiving a group request from a client, a server does not take
any action to synchonize 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 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.
Since it provides no assurance as to freshness of requests, it is
thus RECOMMENDED that a server using this synchronization approach
always uses its own sender sequence number as 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. This 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.
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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.
In case the baseline synchronization state related to a client is
lost, it is RECOMMENDED that the server uses its own sender sequence
number as Partial IV when protecting a response to that client, until
a new baseline synchronization state for that client is established.
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. The server stores the
option value included therein.
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. 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 verifies that the option value equals the stored and
previously sent value. If not, 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. It is the role of the application to
define under what circumstances sender sequence numbers lose
synchronization. This can include a minimum gap between the sender
sequence number of the latest accepted group request from a client
and the sender sequence number of a group request just received from
the same client. A client has to be always ready to perform the
challenge-response based on the Echo Option in case a server starts
it.
This approach provides an assurance of absolute message freshness.
However, it can result in an impact on performance which is
undesirable or unbearable, especially in large 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 signature mode of Group OSCORE,
as described in Section 7.1.
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Instead, protecting requests with the Echo Option by using the
pairwise mode of Group OSCORE as described in Appendix G.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,
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.
Appendix F. No Verification of Signatures
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. In other cases, the signature verification can be deferred
or only checked for specific actions. For instance, a command to
turn a bulb on where the bulb is already on does not need the
signature to be checked. In such situations, the counter signature
needs to be included anyway as part of the message, so that an
endpoint that needs to validate the signature for any reason has the
ability to do so.
In this specification, it is NOT RECOMMENDED that endpoints do not
verify the counter signature of received messages. However, it is
recognized that there may be situations where it is not always
required. The consequence of not doing the signature validation is
that security in the group is based only on the group-authenticity of
the shared keying material used for encryption. That is, endpoints
in the group have evidence that a received message has been
originated by a group member, although not specifically identifiable
in a secure way. This can violate a number of security requirements,
as the compromise of any element in the group means that the attacker
has the ability to control the entire group. Even worse, the group
may not be limited in scope, and hence the same keying material might
be used not only for light bulbs but for locks as well. Therefore,
extreme care must be taken in situations where the security
requirements are relaxed, so that deployment of the system will
always be done safely.
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Appendix G. Pairwise Mode
For use cases that do not require an intermediary performing
signature verification and that use a compatible signature algorithm,
the pairwise mode defined in this section can be used for unicast
communication.
This mode uses the derivation process defined in Section 3, and
allows two group members to protect requests and responses exchanged
with each other using pairwise keying material.
Senders MUST NOT use the pairwise mode to protect a message addressed
to multiple recipients or to the whole group. This prevents a client
that wants to address one specific server from protecting a request
with the pairwise key associated to that server, and then send the
request over multicast.
The pairwise mode results in the same performance and security
improvements displayed by optimized responses (see Section 9.2).
G.1. Pre-Requirements
In order to protect an outgoing message in pairwise mode, a sender
needs to know the public key and the Recipient ID for the message
recipient, as stored in its own Recipient Context associated to that
recipient.
Furthermore, the sender needs to know the individual address of the
message recipient. 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 this information available, servers MAY provide a resource to
which a client can send a request for a server identified by its
'kid' value, or a set thereof. 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.
G.2. Pairwise Protected Request
A request in pairwise mode is protected as defined in Section 7.1,
with the following differences.
o The client MUST set to 1 the sixth least significant bit of the
OSCORE flag bits in the OSCORE option, i.e. the Pairwise Flag.
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o The COSE_Encrypt0 object included in the request is encrypted
using a symmetric pairwise key K, that the client derives as
defined in Section 3. In particular, the Sender/Recipient Key is
the Sender Key of the client from its own Sender Context, i.e. the
Recipient Key that the server stores in its own Recipient Context
corresponding to the client.
o The Counter Signature is not computed. That is, unlike defined in
Section 5, the payload of the OSCORE message terminates with the
encoded ciphertext of the COSE object.
Note that no changes are made to the AEAD nonce and AAD.
Upon receiving a request with the Pairwise Flag set to 1, the server
MUST process it as defined in Section 7.2, with the following
differences.
o No countersignature to verify is included.
o The COSE_Encrypt0 object included in the request is decrypted and
verified using the same symmetric pairwise key K, that the server
derives as described above for the client side and as defined in
Section 3.
G.3. Pairwise Protected Response
When using the pairwise mode, the processing of a response occurs as
described in Section 9.2 for an optimized response.
Appendix H. Document Updates
RFC EDITOR: PLEASE REMOVE THIS SECTION.
H.1. 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).
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).
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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).
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.
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H.2. 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.
H.3. 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.
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.
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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.
H.4. 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).
H.5. 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).
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).
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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).
H.6. 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).
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".
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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.
H.7. 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.
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.
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H.8. 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.
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; and by the EIT-Digital High Impact
Initiative ACTIVE.
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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
Jiye Park
Universitaet Duisburg-Essen
Schuetzenbahn 70
Essen 45127
Germany
Email: ji-ye.park@uni-due.de
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