Group OSCORE - Secure Group Communication for CoAP
draft-ietf-core-oscore-groupcomm-06
CoRE Working Group M. Tiloca
Internet-Draft RISE AB
Intended status: Standards Track G. Selander
Expires: May 7, 2020 F. Palombini
Ericsson AB
J. Park
Universitaet Duisburg-Essen
November 04, 2019
Group OSCORE - Secure Group Communication for CoAP
draft-ietf-core-oscore-groupcomm-06
Abstract
This document describes a mode for protecting group communication
over the Constrained Application Protocol (CoAP). The proposed mode
relies on Object Security for Constrained RESTful Environments
(OSCORE) and the CBOR Object Signing and Encryption (COSE) format.
In particular, it defines how OSCORE is used in a group communication
setting, while fulfilling the same security requirements for group
requests and responses. Source authentication of all messages
exchanged within the group is provided by means of digital signatures
produced by the sender and embedded in the protected CoAP messages.
Status of This Memo
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Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2. OSCORE Security Context . . . . . . . . . . . . . . . . . . . 5
2.1. Management of Group Keying Material . . . . . . . . . . . 9
2.2. Wrap-Around of Partial IVs . . . . . . . . . . . . . . . 9
3. The COSE Object . . . . . . . . . . . . . . . . . . . . . . . 10
3.1. Updated external_aad . . . . . . . . . . . . . . . . . . 10
3.1.1. Updated external_aad for Encryption . . . . . . . . . 10
3.1.2. Updated external_aad for Signing . . . . . . . . . . 11
3.2. Use of the 'kid' Parameter . . . . . . . . . . . . . . . 12
3.3. Updated 'unprotected' Field . . . . . . . . . . . . . . . 12
4. OSCORE Header Compression . . . . . . . . . . . . . . . . . . 12
4.1. Encoding of the OSCORE Option Value . . . . . . . . . . . 12
4.2. Encoding of the OSCORE Payload . . . . . . . . . . . . . 13
4.3. Examples of Compressed COSE Objects . . . . . . . . . . . 14
5. Message Binding, Sequence Numbers, Freshness and Replay
Protection . . . . . . . . . . . . . . . . . . . . . . . . . 15
5.1. Synchronization of Sender Sequence Numbers . . . . . . . 15
6. Message Processing . . . . . . . . . . . . . . . . . . . . . 15
6.1. Protecting the Request . . . . . . . . . . . . . . . . . 16
6.2. Verifying the Request . . . . . . . . . . . . . . . . . . 16
6.3. Protecting the Response . . . . . . . . . . . . . . . . . 17
6.4. Verifying the Response . . . . . . . . . . . . . . . . . 17
7. Responsibilities of the Group Manager . . . . . . . . . . . . 18
8. Security Considerations . . . . . . . . . . . . . . . . . . . 19
8.1. Group-level Security . . . . . . . . . . . . . . . . . . 20
8.2. Uniqueness of (key, nonce) . . . . . . . . . . . . . . . 20
8.3. Management of Group Keying Material . . . . . . . . . . . 21
8.4. Update of Security Context and Key Rotation . . . . . . . 21
8.5. Collision of Group Identifiers . . . . . . . . . . . . . 22
8.6. Cross-group Message Injection . . . . . . . . . . . . . . 22
8.7. End-to-end Protection . . . . . . . . . . . . . . . . . . 24
8.8. Security Context Establishment . . . . . . . . . . . . . 24
8.9. Master Secret . . . . . . . . . . . . . . . . . . . . . . 24
8.10. Replay Protection . . . . . . . . . . . . . . . . . . . . 25
8.11. Client Aliveness . . . . . . . . . . . . . . . . . . . . 25
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8.12. Cryptographic Considerations . . . . . . . . . . . . . . 25
8.13. Message Segmentation . . . . . . . . . . . . . . . . . . 26
8.14. Privacy Considerations . . . . . . . . . . . . . . . . . 26
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 27
9.1. Counter Signature Parameters Registry . . . . . . . . . . 27
9.2. Counter Signature Key Parameters Registry . . . . . . . . 29
9.3. Expert Review Instructions . . . . . . . . . . . . . . . 31
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 32
10.1. Normative References . . . . . . . . . . . . . . . . . . 32
10.2. Informative References . . . . . . . . . . . . . . . . . 33
Appendix A. Assumptions and Security Objectives . . . . . . . . 34
A.1. Assumptions . . . . . . . . . . . . . . . . . . . . . . . 34
A.2. Security Objectives . . . . . . . . . . . . . . . . . . . 36
Appendix B. List of Use Cases . . . . . . . . . . . . . . . . . 36
Appendix C. Example of Group Identifier Format . . . . . . . . . 39
Appendix D. Set-up of New Endpoints . . . . . . . . . . . . . . 40
Appendix E. Examples of Synchronization Approaches . . . . . . . 40
E.1. Best-Effort Synchronization . . . . . . . . . . . . . . . 40
E.2. Baseline Synchronization . . . . . . . . . . . . . . . . 41
E.3. Challenge-Response Synchronization . . . . . . . . . . . 41
Appendix F. No Verification of Signatures . . . . . . . . . . . 43
Appendix G. Document Updates . . . . . . . . . . . . . . . . . . 43
G.1. Version -05 to -06 . . . . . . . . . . . . . . . . . . . 43
G.2. Version -04 to -05 . . . . . . . . . . . . . . . . . . . 44
G.3. Version -03 to -04 . . . . . . . . . . . . . . . . . . . 44
G.4. Version -02 to -03 . . . . . . . . . . . . . . . . . . . 45
G.5. Version -01 to -02 . . . . . . . . . . . . . . . . . . . 46
G.6. Version -00 to -01 . . . . . . . . . . . . . . . . . . . 47
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 47
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 48
1. Introduction
The Constrained Application Protocol (CoAP) [RFC7252] is a web
transfer protocol specifically designed for constrained devices and
networks [RFC7228].
Group communication for CoAP [RFC7390][I-D.dijk-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 utilisation. Use cases include
lighting control, integrated building control, software and firmware
updates, parameter and configuration updates, commissioning of
constrained networks, and emergency multicast (see Appendix B).
Furthermore, [RFC7390] recognizes the importance to introduce a
secure mode for CoAP group communication. This specification defines
such a mode.
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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 receipient, 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 end-to-end security of
CoAP messages exchanged between members of a group, and preserving
independence of transport layer. In particular, the described
approach defines how OSCORE should be used in a group communication
setting, so that end-to-end security is assured in the same way as
OSCORE for unicast communication. That is, end-to-end security is
provided for CoAP multicast requests sent by a client to the group,
and for related CoAP responses sent by multiple servers. Group
OSCORE provides source authentication of all CoAP messages exchanged
within the group, by means of digital signatures produced through
private keys of sender devices and embedded in the protected CoAP
messages.
As defined in the latest [I-D.dijk-core-groupcomm-bis], Group OSCORE
is the security protocol to use for applications that rely on CoAP
group communication. As in OSCORE, it is still possible to
simultaneously rely on DTLS [RFC6347] to protect hop-by-hop
communication between a sender and a proxy (and vice versa), and
between a proxy and a recipient (and vice versa). Note that DTLS
cannot be used to secure messages sent over multicast.
1.1. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
Readers are expected to be familiar with the terms and concepts
described in CoAP [RFC7252] including "endpoint", "client", "server",
"sender" and "recipient"; group communication for CoAP
[RFC7390][I-D.dijk-core-groupcomm-bis]; COSE and counter signatures
[RFC8152].
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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). The term
group used in this specification refers thus to a "security
group", not to be confused with 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 7.
o Silent server: member of a group that never responds to requests.
Note that a silent server can act as a client, the two roles are
independent.
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.
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. OSCORE Security Context
To support group communication secured with OSCORE, each endpoint
registered as member of a group maintains a Security Context as
defined in Section 3 of [RFC8613], extended as defined below. Each
endpoint in a group makes use of:
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1. one Common Context, shared by all the endpoints in a given group.
In particular:
* The ID Context parameter contains the Gid of the group, which
is used to retrieve the Security Context for processing
messages intended to the endpoints of the group (see
Section 6). The choice of the Gid is application specific.
An example of specific formatting of the Gid is given in
Appendix C. The application needs to specify how to handle
possible collisions between Gids, see Section 8.5.
* A new parameter Counter Signature Algorithm is included. Its
value identifies the digital signature algorithm used to
compute a counter signature on the COSE object (see
Section 4.5 of [RFC8152]) which provides source authentication
within the group. 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].
* A new parameter Counter Signature Parameters is included.
This parameter 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 9.1), where each entry indicates a
specified structure of the Counter Signature Parameters.
* A new parameter Counter Signature Key Parameters is included.
This parameter 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 9.2), where
each entry indicates a specified structure of the Counter
Signature Key Parameters.
2. one Sender Context, unless the endpoint is configured exclusively
as silent server. The Sender Context is used to secure outgoing
messages and is initialized according to Section 3 of [RFC8613],
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once the endpoint has joined the group. The Sender Context of a
given endpoint matches the corresponding Recipient Context in all
the endpoints receiving a protected message from that endpoint.
Besides, in addition to what is defined in [RFC8613], the Sender
Context stores also the endpoint's private key.
3. one Recipient Context for each distinct endpoint from which
messages are received, used to process incoming messages. The
recipient may generate a Recipient Context whenever in possession
of all the required pieces of information on the corresponding
endpoint, e.g. they may be provided to the recipient upon joining
the group. Alternatively, the recipient may generate a Recipient
Context upon receiving an incoming message from another endpoint
in the group for the first time (see Section 6.2 and
Section 6.4). Each Recipient Context matches the Sender Context
of the endpoint from which protected messages are received.
Besides, in addition to what is defined in [RFC8613], each
Recipient Context stores also the public key of the associated
other endpoint from which messages are received. Note that each
Recipient Context includes a Replay Window, unless the recipient
acts only as client and hence processes only responses as
incoming messages.
The table in Figure 1 overviews the new information included in the
OSCORE Security Context, with respect to what defined in Section 3 of
[RFC8613].
+---------------------------+------------------------------+
| Context portion | New information |
+---------------------------+------------------------------+
| | |
| Common Context | Counter signature algorithm |
| | |
| Common Context | Counter signature parameters |
| | |
| Sender Context | Endpoint's own private key |
| | |
| Each Recipient Context | Public key of the |
| | associated other endpoint |
| | |
+---------------------------+------------------------------+
Figure 1: Additions to the OSCORE Security Context
Upon receiving a secure CoAP message, a recipient uses the sender's
public key, in order to verify the counter signature of the COSE
Object (see Section 3).
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If not already stored in the Recipient Context associated to the
sender, the recipient retrieves the sender's public key from the
Group Manager, which collects public keys upon endpoints' joining the
group, acts as trusted key repository and ensures the correct
association between the public key and the identifier of the sender,
for instance by means of public key certificates.
For very constrained devices, it may be not feasible to
simultaneously handle the ongoing processing of a just received
message and the parallel retrieval of the sender's public key. Such
devices can be configured to drop that received message altogether,
switch to the retrieval of the sender's public key, and thus have it
available to verify following messages from that sender.
Note that a group member can retrieve public keys from the Group
Manager and generate the Recipient Context associated to another
group member at any point in time, as long as this is done before
verifying a received secure CoAP message. The exact configuration is
application dependent. For example, an application can configure a
group member to retrieve all the required information and to create
the Recipient Context exactly upon receiving a message from another
group member for the first time. As an alternative, the application
can configure a group member to asynchronously retrieve the required
information and update its list of Recipient Contexts well before
receiving any message, e.g. by Observing [RFC7641] the Group Manager
to get updates on the group membership.
It is RECOMMENDED that the Group Manager collects public keys and
provides them to group members upon request 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.
An endpoint receives 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. An endpoint uses its own Sender ID
(together with other data) to generate unique AEAD nonces for
outgoing messages, as in [RFC8613]. Endpoints which are configured
only as silent servers do not have a Sender ID.
The Sender/Recipient Keys and the Common IV are derived according to
the same scheme defined in Section 3.2 of [RFC8613]. The mandatory-
to-implement HKDF and AEAD algorithms for Group OSCORE are the same
as in [RFC8613].
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2.1. 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. When doing so, a new value for the
Master Salt parameter MAY also be distributed, and the Group Manager
SHOULD preserve the current value of the Sender ID of each group
member. An example of Gid format supporting this operation is
provided in Appendix C. 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.
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 verifying countersignatures of received messages.
As a consequence, group members may end up retaining stale Recipient
Contexts, that are no longer useful to verify incoming secure
messages. Applications may define policies to delete (long-)unused
Recipient Contexts and reduce the impact on storage space.
If the application requires so (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].
2.2. Wrap-Around of Partial IVs
An endpoint can eventually experience a wrap-around of 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.
When a wrap-around happens, the endpoint MUST NOT transmit further
messages including a Partial IV 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 OSCORE Security Context in the group
(see Section 2.1).
o The Group Manager provides a new Sender ID value to the endpoint
that has experienced the wrap-around. Then, the endpoint derives
a new Sender Context using the new Sender ID, as described in
Section 3.2 of [RFC8613].
Either case, same considerations from Section 2.1 hold about possible
retaining of stale Recipient Contexts.
3. 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
Group OSCORE, the following modifications apply.
3.1. Updated external_aad
The external_aad of the Additional Authenticated Data (AAD) is
extended as follows. In particular, it has one structure used for
the encryption process producing the ciphertext, and one structure
used for the signing process producing the counter signature.
3.1.1. Updated 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 9.1) for this counter signature
algorithm.
The 'algorithms' array in the aad_array MAY also include:
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
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"Parameters" field of the Counter Signature Parameters Registry
(see Section 9.1), 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 9.2), 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]).
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
]
3.1.2. Updated 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 3.1.1;
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.
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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,
OSCORE_option: bstr
]
Note for implementation: this requires the value of the OSCORE option
to be fully ready, before starting the signing process.
3.2. Use of the 'kid' Parameter
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.
3.3. Updated 'unprotected' Field
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 using its own private key
as described in Appendix A.2 of [RFC8152]. In particular, the
Sig_structure contains the external_aad as defined in
Section 3.1.2 and the ciphertext of the COSE_Encrypt0 object as
payload.
4. OSCORE Header Compression
The OSCORE compression defined in Section 6 of [RFC8613] is used,
with the following additions for the encoding of the OSCORE Option
and the OSCORE Payload.
4.1. Encoding of the OSCORE Option Value
Analogously to [RFC8613], the value of the OSCORE option SHALL
contain the OSCORE flag bits, the Partial IV parameter, the kid
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context parameter (length and value), and the kid parameter, with the
following modifications:
o The first byte, containing the OSCORE flag bits, has the following
encoding modifications:
* The fourth least significant bit MUST be set to 1 in every
message, to indicate the presence of the 'kid' parameter for
all group requests and responses. That is, unlike in
[RFC8613], the 'kid' parameter is always present in all
messages.
* The fifth least significant bit MUST be set to 1 for group
requests, to indicate the presence of the 'kid context'
parameter in the compressed COSE object. The 'kid context' MAY
be present in responses if the application requires it. In
such a case, the kid context flag MUST be set to 1.
The flag bits are registered in the OSCORE Flag Bits registry
specified in Section 13.7 of [RFC8613].
o The 'kid context' value encodes the Group Identifier value (Gid)
of the group's Security Context.
o The remaining bytes in the OSCORE Option value encode the value of
the 'kid' parameter, which is always present both in group
requests and in responses.
0 1 2 3 4 5 6 7 <------------ n bytes ------------>
+-+-+-+-+-+-+-+-+-----------------------------------+
|0 0|0|h|1| n | Partial IV (if any) |
+-+-+-+-+-+-+-+-+-----------------------------------+
<-- 1 byte ---> <------ s bytes ------>
+---------------+-----------------------+-----------+
| s (if any) | kid context = Gid | kid |
+---------------+-----------------------+-----------+
Figure 2: OSCORE Option Value
4.2. Encoding of the OSCORE Payload
The payload of the OSCORE message SHALL encode the ciphertext of the
COSE object concatenated with the value of the CounterSignature0 of
the COSE object, computed as in Appendix A.2 of [RFC8152] according
to the Counter Signature Algorithm and Counter Signature Parameters
in the Security Context.
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4.3. Examples of Compressed COSE Objects
This section covers a list of OSCORE Header Compression examples for
group requests and responses. 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 4 and divided into two 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. COUNTERSIGN is the CounterSignature0 byte
string as described in Section 3 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 = 60b035059d9ef5667c5a0710823b, 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)
5. 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.
5.1. 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.
6. Message Processing
Each request message and response message is protected and processed
as specified in [RFC8613], with the modifications described in the
following sections. 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][RFC7390][I-D.dijk-core-groupcomm-bis], group
requests sent over multicast MUST be Non-Confirmable. Thus, senders
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should store their outgoing messages for an amount of time defined by
the application and sufficient to correctly handle possible
retransmissions. 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 3, or which is not cryptographically
validated in a successful way. 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.
6.1. 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 3 of this specification.
o In step 4, the encryption of the COSE object is modified as
described in Section 3 of this specification. The encoding of the
compressed COSE object is modified as described in Section 4 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 4.2 of this
specification. In particular, the payload of the OSCORE message
includes also the counter signature.
6.2. 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 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 server MAY create a new
Recipient Context and initializes it according to Section 3 of
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[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 3 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
prevent attackers from overloading the server's storage and
creating processing overhead on the server.
6.3. 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 3 of this specification.
o In step 4, the encryption of the COSE object is modified as
described in Section 3 of this specification. The encoding of the
compressed COSE object is modified as described in Section 4 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 4.2 of this
specification. In particular, the payload of the OSCORE message
includes also the counter signature.
6.4. 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 3 of this specification. If the received
Recipient ID ('kid') does not match with any Recipient Context for
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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 3 of this specification.
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.
7. Responsibilities of the Group Manager
The Group Manager is responsible for performing the following tasks:
1. Creating and managing OSCORE groups. This includes the
assignment of a Gid to every newly created group, as well as
ensuring uniqueness of Gids within the set of its OSCORE groups.
2. Defining policies for authorizing the joining of its OSCORE
groups.
3. Handling the join process to add new endpoints as group members.
4. Establishing the Common Context part of the Security Context,
and providing it to authorized group members during the join
process, together with the corresponding Sender Context.
5. Generating and managing Sender IDs within its OSCORE groups, as
well as assigning and providing them to new endpoints during the
join process. This includes ensuring uniqueness of Sender IDs
within each of its OSCORE groups.
6. Defining 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).
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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.
8. 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
further discussed in Section 8.1.
The same considerations on supporting Proxy operations discussed for
OSCORE in Appendix D.2 of [RFC8613] hold for Group OSCORE.
The same considerations on protected message fields for OSCORE
discussed in Appendix D.3 of [RFC8613] hold for Group OSCORE.
The same considerations on uniqueness of (key, nonce) pairs for
OSCORE discussed in Appendix D.4 of [RFC8613] hold for Group OSCORE.
This is further discussed in Section 8.2.
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 8.6.
As discussed in Section 6.2.3 of [I-D.dijk-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
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covered in the security considerations of OSCORE (Section 12 of
[RFC8613]), and discusses how they hold when Group OSCORE is used.
8.1. Group-level Security
The approach described in this document 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.
Note that, even if an endpoint is authorized to be a group member and
to take part in group communications, there is a risk that it behaves
inappropriately. For instance, it can forward the content of
messages in the group to unauthorized entities. However, in many use
cases, the devices in the group belong to a common authority and are
configured by a commissioner (see Appendix B), which results in a
practically limited risk and enables a prompt detection/reaction in
case of misbehaving.
8.2. 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.
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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.
8.3. 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.
8.4. 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. In the following two cases, this
may result in misaligned Security Contexts between the sender and the
recipient.
In the first 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 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.
In the second case, the sender protects a message using the new
Security Context, but the recipient receives that request before
having installed the new Security Context. Therefore, the recipient
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would not be able to correctly process the request and hence discards
it. If the recipient receives 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.
8.5. 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.
However, 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.
8.6. 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
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 3.1.2). 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
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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.
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.
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8.7. 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.
8.8. 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
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].
8.9. Master Secret
Group OSCORE derives the Security Context using the same construction
as OSCORE, and by using the Group Identifier of a group as the
related ID Context. Hence, the same required properties of the
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
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generating and distributing a new Master Secret. Randomness
requirements for security are described in [RFC4086].
8.10. Replay Protection
As in OSCORE, also Group OSCORE relies on sender sequence numbers
included in the COSE message field 'Partial IV' and used to build
AEAD nonces.
As discussed in Section 5.1, 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 6.4 also
ensures that a client accepts a single valid response to a given
request from each replying server, unless CoAP observation is used.
8.11. 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).
8.12. Cryptographic Considerations
The same considerations from Section 12.6 of [RFC8613] about the
maximum Sender Sequence Number hold for Group OSCORE.
As discussed in Section 2.2, an endpoint that experiences a wrap-
around 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
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by means of group rekeying, or provide only that endpoint with a new
Sender ID value. 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.
8.13. Message Segmentation
The same considerations from Section 12.7 of [RFC8613] hold for Group
OSCORE.
8.14. Privacy Considerations
Group OSCORE ensures end-to-end integrity protection and encryption
of the message payload and all options that are not used for proxy
operations. In particular, options are processed according to the
same class U/I/E that they have for OSCORE. Therefore, the same
privacy considerations from Section 12.8 of [RFC8613] hold for Group
OSCORE.
Furthermore, the following privacy considerations hold, about the
OSCORE option that may reveal information on the communicating
endpoints.
o The 'kid' parameter, which is intended to help a recipient
endpoint to find the right Recipient Context, may reveal
information about the Sender Endpoint. Since both requests and
responses always include the 'kid' parameter, this may reveal
information about both a client sending a group request and all
the possibly replying servers sending their own individual
response.
o The 'kid context' parameter, which is intended to help a recipient
endpoint to find the right Recipient Context, reveals information
about the sender endpoint. In particular, it reveals that the
sender endpoint is a member of a particular OSCORE group, whose
current Group ID is indicated in the 'kid context' parameter.
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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9. 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.
9.1. 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 9.3.
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.
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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9.2. 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 9.3.
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 | |
| | | | | |
+-------------+-------+--------------+-------------------+-----------+
9.3. 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).
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
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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.
10. References
10.1. Normative References
[I-D.dijk-core-groupcomm-bis]
Dijk, E., Wang, C., and M. Tiloca, "Group Communication
for the Constrained Application Protocol (CoAP)", draft-
dijk-core-groupcomm-bis-01 (work in progress), July 2019.
[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>.
[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>.
[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>.
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[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>.
10.2. Informative References
[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-03 (work in progress), November 2019.
[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-25
(work in progress), October 2019.
[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-08 (work in progress), November 2019.
[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>.
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[RFC6282] Hui, J., Ed. and P. Thubert, "Compression Format for IPv6
Datagrams over IEEE 802.15.4-Based Networks", RFC 6282,
DOI 10.17487/RFC6282, September 2011,
<https://www.rfc-editor.org/info/rfc6282>.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
January 2012, <https://www.rfc-editor.org/info/rfc6347>.
[RFC7228] Bormann, C., Ersue, M., and A. Keranen, "Terminology for
Constrained-Node Networks", RFC 7228,
DOI 10.17487/RFC7228, May 2014,
<https://www.rfc-editor.org/info/rfc7228>.
[RFC7390] Rahman, A., Ed. and E. Dijk, Ed., "Group Communication for
the Constrained Application Protocol (CoAP)", RFC 7390,
DOI 10.17487/RFC7390, October 2014,
<https://www.rfc-editor.org/info/rfc7390>.
[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>.
Appendix A. Assumptions and Security Objectives
This section presents a set of assumptions and security objectives
for the approach described in this document.
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 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 group members are clients. According to [RFC7390],
any possible proxy entity is supposed to know about the clients in
the group and to not perform aggregation of response messages from
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multiple servers. Also, every client expects and is able to
handle multiple response messages associated to a same request
sent to the group.
o Group size: security solutions for group communication should be
able to adequately support different and possibly large groups.
The group size is the current number of members in a 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. 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 group member.
o Provisioning and management of Security Contexts: an OSCORE
Security Context must be established among the group members. A
secure mechanism must be used to generate, revoke and
(re-)distribute keying material, multicast security policies and
security parameters in the group. The actual provisioning and
management of the Security Context is out of the scope of this
document.
o Multicast data security ciphersuite: all group members 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 group should not have
access to any old Security Contexts used before its joining. This
ensures that a new group member is not able to decrypt
confidential data sent before it has joined the group. The
adopted key management scheme should ensure that the Security
Context is updated to ensure backward confidentiality. The actual
mechanism to update the Security Context and renew the group
keying material upon a group member's joining has to be defined as
part of the group key management scheme.
o Forward security: entities that leave the group should not have
access to any future Security Contexts or message exchanged within
the group after their leaving. This ensures that a former group
member is not able to decrypt confidential data sent within the
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group anymore. Also, it ensures that a former member is not able
to send encrypted and/or integrity protected messages to the group
anymore. The actual mechanism to update the Security Context and
renew the group keying material upon a group member's leaving has
to be defined as part of the group key management scheme.
A.2. Security Objectives
The approach described in this document aims at fulfilling the
following security objectives:
o Data replay protection: replayed group request messages or
response messages must be detected.
o Group-level data confidentiality: messages sent within the group
shall be encrypted if privacy sensitive data is exchanged within
the 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
group, but not by an external adversary or other external
entities.
o Source authentication: messages sent within the group shall be
authenticated. That is, it is essential to ensure that a message
is originated by a member of the group in the first place, and in
particular by a specific member of the group.
o Message integrity: messages sent within the group shall be
integrity protected. That is, it is essential to ensure that a
message has not been tampered with by an external adversary or
other external entities which are not group members.
o Message ordering: it must be possible to determine the ordering of
messages coming from a single sender. 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 [RFC7390][I-D.dijk-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
[RFC7390][I-D.dijk-core-groupcomm-bis] to understand the non-security
related details. This section discusses a number of use cases that
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benefit from secure group communication. 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 devices are
organized into groups according to their physical location in the
building. For instance, lighting devices and switches in a room
or corridor can be configured as members of a single group.
Switches are then used to control the lighting devices by sending
on/off/dimming commands to all lighting devices in a group, while
border routers connected to an IP network backbone (which is also
multicast-enabled) can be used to interconnect routers in the
building. Consequently, this would also enable logical groups to
be formed even if devices in the lighting group may be physically
in different subnets (e.g. on wired and wireless networks).
Connectivity between 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 group of connected lights, ensuring that the light
preset (e.g. dimming level or color) of a large group of
luminaires are changed at the same perceived time. This is
especially useful for providing a visual synchronicity of light
effects to the user. 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 group of lighting devices.
In more advanced lighting control use cases, a M-to-N
communication topology would be required, for instance in case
multiple sensors (presence or day-light) are responsible to
trigger events to a group of lighting devices. 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 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 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
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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 group of devices at once displays a number of
benefits. For instance, it can significantly reduce the network
load and decrease the overall time latency for propagating this
data to all devices. Even if the complete whole update process
itself is secured, securing the individual messages is important,
in case updates consist of relatively large amounts of data. In
fact, checking individual received data piecemeal for tampering
avoids that devices store large amounts of partially corrupted
data and that they detect tampering hereof only after all data has
been received. Devices receiving software and firmware updates
are expected to possibly reply back, in order to provide a
feedback about the execution of the update operation (e.g. OK,
failure, error) and their current operational status.
o Parameter and configuration update: by means of multicast
communication, it is possible to update the settings of a group of
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 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
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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
and keying material in the group (see Section 2.1). In particular,
once a new Master Secret has been distributed to the group, all the
group members increment by 1 the Group Epoch in the Group Identifier
of that group.
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 8.5, 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
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result in a small probability of collision. How small this
probability should be is up to system designers.
Appendix D. Set-up of New Endpoints
An endpoint joins a group by explicitly interacting with the
responsible Group Manager. When becoming members of a group,
endpoints are not required to know how many and what endpoints are in
the same group.
Communications between a joining endpoint and the Group Manager rely
on the CoAP protocol and must be secured. Specific details on how to
secure communications between joining endpoints and a Group Manager
are out of the scope of this document.
The Group Manager must verify that the joining endpoint is authorized
to join the group. To this end, the Group Manager can directly
authorize the joining endpoint, or expect it to provide authorization
evidence previously obtained from a trusted entity. Further details
about the authorization of joining endpoints are out of scope.
In case of successful authorization check, the Group Manager
generates a Sender ID assigned to the joining endpoint, before
proceeding with the rest of the join process. That is, the Group
Manager provides the joining endpoint with the keying material and
parameters to initialize the Security Context (see Section 2). The
actual provisioning of keying material and parameters to the joining
endpoint is out of the scope of this document.
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.
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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. However, the server
drops the group request without delivering it to the application
layer. 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.
This can be acceptable for use cases where servers admit such a
trade-off between performance and assurance of message freshness.
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
Section 6.2 of 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. 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 3).
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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; otherwise, the request is silently discarded.
Then, the server verifies that the unicast request has been received
within a pre-configured time interval, as described in
[I-D.ietf-core-echo-request-tag]. In such a case, the request is
further processed and verified; otherwise, it is silently discarded.
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.
In case it does not receive a valid unicast request including the
Echo Option within the configured time interval, the server endpoint
should perform the same challenge-response upon receiving the next
group request from that same client.
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.
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.
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.
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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.
Appendix G. Document Updates
RFC EDITOR: PLEASE REMOVE THIS SECTION.
G.1. 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.
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o Optional 4.00 response for failed signature verification on the
server.
o Removed client handling of duplicated responses to multicast
requests.
o Additional considerations on public key retrieval and group
rekeying.
o Added Group Manager responsibility on validating public keys.
o Updates IANA registries.
o Reference to RFC 8613.
o Editorial improvements.
G.2. 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).
G.3. 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 key material from the
Group Manager, in order to process incoming messages (see
Section 2).
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o Structured Section 3 into subsections.
o Added the new 'par_countersign' to the aad_array of the
external_aad (see Section 3.1).
o Clarified non reliability of 'kid' as identity indicator for a
group member (see Section 2.1).
o Described possible provisioning of new Sender ID in case of
Partial IV wrap-around (see Section 2.2).
o The former signature bit in the Flag Byte of the OSCORE option
value is reverted to reserved (see Section 4.1).
o Updated examples of compressed COSE object, now with the sixth
less significant bit in the Flag Byte of the OSCORE option value
set to 0 (see Section 4.3).
o Relaxed statements on sending error messages (see Section 6).
o Added explicit step on computing the counter signature for
outgoing messages (see Setions 6.1 and 6.3).
o Handling of just created Recipient Contexts in case of
unsuccessful message verification (see Sections 6.2 and 6.4).
o Handling of replied/repeated responses on the client (see
Section 6.4).
o New IANA Registry "Counter Signature Parameters" (see
Section 9.1).
G.4. 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".
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o Clarifications about Non-Confirmable messages in Section 5.1
"Synchronization of Sender Sequence Numbers".
o Clarifications about error handling in Section 6 "Message
Processing".
o Compacted list of responsibilities of the Group Manager in
Section 7.
o Revised and extended security considerations in Section 8.
o Added IANA considerations for the OSCORE Flag Bits Registry in
Section 9.
o Revised Appendix D, now giving a short high-level description of a
new endpoint set-up.
G.5. 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.
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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.
G.6. 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 Stefan Beck, Rolf Blom, Carsten Bormann,
Esko Dijk, Klaus Hartke, Rikard Hoeglund, Richard Kelsey, John
Mattsson, Dave Robin, Jim Schaad, Ludwig Seitz and Peter van der Stok
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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