CoRE Working Group M. Tiloca
Internet-Draft RISE AB
Intended status: Standards Track G. Selander
Expires: April 25, 2019 F. Palombini
Ericsson AB
J. Park
Universitaet Duisburg-Essen
October 22, 2018
Group OSCORE - Secure Group Communication for CoAP
draft-ietf-core-oscore-groupcomm-03
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
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 25, 2019.
Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved.
Tiloca, et al. Expires April 25, 2019 [Page 1]
Internet-Draft Group OSCORE October 2018
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2. OSCORE Security Context . . . . . . . . . . . . . . . . . . . 5
2.1. Management of Group Keying Material . . . . . . . . . . . 7
2.2. Wrap-Around of Partial IVs . . . . . . . . . . . . . . . 8
3. The COSE Object . . . . . . . . . . . . . . . . . . . . . . . 8
4. OSCORE Header Compression . . . . . . . . . . . . . . . . . . 9
4.1. Encoding of the OSCORE Option Value . . . . . . . . . . . 9
4.2. Encoding of the OSCORE Payload . . . . . . . . . . . . . 10
4.3. Examples of Compressed COSE Objects . . . . . . . . . . . 10
5. Message Binding, Sequence Numbers, Freshness and Replay
Protection . . . . . . . . . . . . . . . . . . . . . . . . . 11
5.1. Synchronization of Sender Sequence Numbers . . . . . . . 12
6. Message Processing . . . . . . . . . . . . . . . . . . . . . 12
6.1. Protecting the Request . . . . . . . . . . . . . . . . . 13
6.2. Verifying the Request . . . . . . . . . . . . . . . . . . 13
6.3. Protecting the Response . . . . . . . . . . . . . . . . . 13
6.4. Verifying the Response . . . . . . . . . . . . . . . . . 14
7. Responsibilities of the Group Manager . . . . . . . . . . . . 14
8. Security Considerations . . . . . . . . . . . . . . . . . . . 15
8.1. Group-level Security . . . . . . . . . . . . . . . . . . 15
8.2. Uniqueness of (key, nonce) . . . . . . . . . . . . . . . 16
8.3. Management of Group Keying Material . . . . . . . . . . . 16
8.4. Update of Security Context and Key Rotation . . . . . . . 17
8.5. Collision of Group Identifiers . . . . . . . . . . . . . 17
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
9.1. OSCORE Flag Bits Registry . . . . . . . . . . . . . . . . 18
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 18
10.1. Normative References . . . . . . . . . . . . . . . . . . 18
10.2. Informative References . . . . . . . . . . . . . . . . . 19
Appendix A. Assumptions and Security Objectives . . . . . . . . 20
A.1. Assumptions . . . . . . . . . . . . . . . . . . . . . . . 20
A.2. Security Objectives . . . . . . . . . . . . . . . . . . . 21
Appendix B. List of Use Cases . . . . . . . . . . . . . . . . . 22
Appendix C. Example of Group Identifier Format . . . . . . . . . 24
Appendix D. Set-up of New Endpoints . . . . . . . . . . . . . . 25
Tiloca, et al. Expires April 25, 2019 [Page 2]
Internet-Draft Group OSCORE October 2018
Appendix E. Examples of Synchronization Approaches . . . . . . . 26
E.1. Best-Effort Synchronization . . . . . . . . . . . . . . . 26
E.2. Baseline Synchronization . . . . . . . . . . . . . . . . 26
E.3. Challenge-Response Synchronization . . . . . . . . . . . 27
Appendix F. No Verification of Signatures . . . . . . . . . . . 28
Appendix G. Document Updates . . . . . . . . . . . . . . . . . . 29
G.1. Version -02 to -03 . . . . . . . . . . . . . . . . . . . 29
G.2. Version -01 to -02 . . . . . . . . . . . . . . . . . . . 30
G.3. Version -00 to -01 . . . . . . . . . . . . . . . . . . . 31
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 31
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 32
1. Introduction
The Constrained Application Protocol (CoAP) [RFC7252] is a web
transfer protocol specifically designed for constrained devices and
networks [RFC7228].
Group communication for CoAP [RFC7390] addresses use cases where
deployed devices benefit from a group communication model, for
example to reduce latencies, 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.
Object Security for Constrained RESTful Environments
(OSCORE)[I-D.ietf-core-object-security] describes a security protocol
based on the exchange of protected CoAP messages. OSCORE builds on
CBOR Object Signing and Encryption (COSE) [RFC8152] and provides end-
to-end encryption, integrity, 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
Tiloca, et al. Expires April 25, 2019 [Page 3]
Internet-Draft Group OSCORE October 2018
within the group, by means of digital signatures produced through
private keys of sender devices and embedded in the protected CoAP
messages.
As in OSCORE, it is still possible to simultaneously rely on DTLS
[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];
COSE and counter signatures [RFC8152].
Readers are also expected to be familiar with the terms and concepts
for protection and processing of CoAP messages through OSCORE, such
as "Security Context" and "Master Secret", defined in
[I-D.ietf-core-object-security].
Terminology for constrained environments, such as "constrained
device", "constrained-node network", is defined in [RFC7228].
This document refers also to the following terminology.
o Keying material: data that is necessary to establish and maintain
secure communication among 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 (GM): entity responsible for a group. Each endpoint
in a group communicates securely with the respective GM, 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.
Tiloca, et al. Expires April 25, 2019 [Page 4]
Internet-Draft Group OSCORE October 2018
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 should be unique within the set of groups of a given
Group Manager, in order to avoid collisions. In case they are
not, the considerations in Section 8.5 apply.
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 [I-D.ietf-core-object-security], extended as
defined below. Each endpoint in a group makes use of:
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 EdDSA signature algorithm ed25519
[RFC8032] is mandatory to implement.
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
[I-D.ietf-core-object-security], once the endpoint has joined the
Tiloca, et al. Expires April 25, 2019 [Page 5]
Internet-Draft Group OSCORE October 2018
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 [I-D.ietf-core-object-security], 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 the 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 [I-D.ietf-core-object-security], each Recipient Context stores
also the public key of the associated other endpoint from which
messages are received.
The table in Figure 1 overviews the new information included in the
OSCORE Security Context, with respect to what defined in Section 3 of
[I-D.ietf-core-object-security].
+---------------------------+-----------------------------+
| Context portion | New information |
+---------------------------+-----------------------------+
| | |
| Common Context | Counter signature algorithm |
| | |
| 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).
If not already stored in the Recipient Context associated to the
sender, the recipient retrieves the public key from the Group
Manager, which collects public keys upon endpoints' joining, 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.
Tiloca, et al. Expires April 25, 2019 [Page 6]
Internet-Draft Group OSCORE October 2018
It is RECOMMENDED that the Group Manager collects public keys and
provides them to group members upon request as described in
[I-D.tiloca-ace-oscoap-joining], 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 [I-D.ietf-core-object-security]. 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 Sections 3.2 and 5.2 of
[I-D.ietf-core-object-security]. The mandatory-to-implement HKDF and
AEAD algorithms for Group OSCORE are the same as in
[I-D.ietf-core-object-security].
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. 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.
Consistently with the security assumptions in 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.tiloca-ace-oscoap-joining].
Tiloca, et al. Expires April 25, 2019 [Page 7]
Internet-Draft Group OSCORE October 2018
2.2. Wrap-Around of Partial IVs
A client can eventually experience a wrap-around of its own Sender
Sequence Number, which is used as Partial IV in outgoing requests and
incremented after each request. When this happens, the OSCORE
Security Context MUST be renewed in the group, in order to avoid
reusing nonces with the same keys.
Therefore, when experiencing a wrap-around of its own Sender Sequence
Number, a group member MUST NOT transmit further group requests until
a new OSCORE Security Context has been derived. In particular, the
endpoint SHOULD inform the Group Manager of the occurred wrap-around,
in order to trigger a prompt renewal of the OSCORE Security Context.
3. The COSE Object
Building on Section 5 of [I-D.ietf-core-object-security], 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 Additional Data
(AEAD) algorithm. For Group OSCORE, the following modifications
apply:
o The external_aad in the Additional Authenticated Data (AAD) is
extended with the counter signature algorithm used to sign
messages. In particular, compared with Section 5.4 of
[I-D.ietf-core-object-security], the 'algorithms' array in the
aad_array MUST also include 'alg_countersign', which contains the
Counter Signature Algorithm from the Common Context (see
Section 2). This external_aad structure is used both for the
encryption process producing the ciphertext (see Section 5.3 of
[RFC8152]) and for the signing process producing the counter
signature, as defined below.
external_aad = bstr .cbor aad_array
aad_array = [
oscore_version : uint,
algorithms : [alg_aead : int / tstr , alg_countersign : int / tstr],
request_kid : bstr,
request_piv : bstr,
options : bstr
]
o 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
Tiloca, et al. Expires April 25, 2019 [Page 8]
Internet-Draft Group OSCORE October 2018
[I-D.ietf-core-object-security], the 'kid' parameter is always
present in all messages, i.e. both requests and responses.
o The 'unprotected' field MUST additionally include the following
parameter:
* 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 above and the ciphertext of the COSE_Encrypt0 object as
payload.
4. OSCORE Header Compression
The OSCORE compression defined in Section 6 of
[I-D.ietf-core-object-security] 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 [I-D.ietf-core-object-security], the value of the
OSCORE option SHALL contain the OSCORE flag bits, the Partial IV
parameter, the kid 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
[I-D.ietf-core-object-security], 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 sixth least significant bit is set to 1 if the
'CounterSignature0' parameter is present, or to 0 otherwise.
In order to ensure source authentication of messages as
described in this specification, this bit MUST be set to 1.
Tiloca, et al. Expires April 25, 2019 [Page 9]
Internet-Draft Group OSCORE October 2018
The flag bits are registered in the OSCORE Flag Bits registry
specified in Section 13.7 of [I-D.ietf-core-object-security] and in
Section 9.1 of this Specification.
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|1|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 (if
present) of the COSE object, computed as in Appendix A.2 of
[RFC8152].
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
[I-D.ietf-core-object-security] has value 10. COUNTERSIGN is the
CounterSignature0 byte string as described in Section 3 and is 64
bytes long.
Tiloca, et al. Expires April 25, 2019 [Page 10]
Internet-Draft Group OSCORE October 2018
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: 0b00111001 = 0x39
Option Value: 39 05 03 44 61 6c 25 (7 bytes)
Payload: ae a0 15 56 67 92 4d ff 8a 24 e4 cb 35 b9 COUNTERSIGN
(14 bytes + size of COUNTERSIGN)
1. Response with ciphertext = 60b035059d9ef5667c5a0710823b, kid =
0x52 and no Partial IV.
Before compression (88 bytes):
[
h'',
{ 4:h'52', 9:COUNTERSIGN },
h'60b035059d9ef5667c5a0710823b'
]
After compression (80 bytes):
Flag byte: 0b00101000 = 0x28
Option Value: 28 52 (2 bytes)
Payload: 60 b0 35 05 9d 9e f5 66 7c 5a 07 10 82 3b COUNTERSIGN
(14 bytes + size of COUNTERSIGN)
5. Message Binding, Sequence Numbers, Freshness and Replay Protection
The requirements and properties described in Section 7 of
[I-D.ietf-core-object-security] 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. More details about
error processing for replay detection in group OSCORE are specified
Tiloca, et al. Expires April 25, 2019 [Page 11]
Internet-Draft Group OSCORE October 2018
in Section 6 of this specification. The mechanisms describing replay
protection and freshness of Observe notifications do not apply to
group OSCORE, as Observe is not defined for group settings.
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.
Requests sent over Multicast must be Non-Confirmable (Section 8.1 of
[RFC7252]), as overall most of the messages sent within a group.
Thus, senders should store their outgoing messages for an amount of
time defined by the application and sufficient to correctly handle
possible retransmissions.
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 [I-D.ietf-core-object-security], 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, message integrity, and
message ordering.
Furthermore, endpoints in the group locally perform error handling
and processing of invalid messages according to the same principles
adopted in [I-D.ietf-core-object-security]. However, a 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, the recipient MUST NOT send back any error message. This
prevents servers from replying with multiple error messages to a
Tiloca, et al. Expires April 25, 2019 [Page 12]
Internet-Draft Group OSCORE October 2018
client sending a group request, so avoiding the risk of flooding and
possibly congesting the group.
As per [RFC7252][RFC7390], group requests sent over multicast must be
Non-confirmable. However, this does not prevent the acknowledgment
of Confirmable group requests in non-multicast environments.
6.1. Protecting the Request
A client transmits a secure group request as described in Section 8.1
of [I-D.ietf-core-object-security], with the following modifications.
o In step 2, the 'algorithms' array in the Additional Authenticated
Data is modified as described in Section 3.
o In step 4, the encryption of the COSE object is modified as
described in Section 3. The encoding of the compressed COSE
object is modified as described in Section 4.
6.2. Verifying the Request
Upon receiving a secure group request, a server proceeds as described
in Section 8.2 of [I-D.ietf-core-object-security], with the following
modifications.
o In step 2, the decoding of the compressed COSE object follows
Section 4. If the received Recipient ID ('kid') does not match
with any Recipient Context for the retrieved Gid ('kid context'),
then the server creates a new Recipient Context, initializes it
according to Section 3 of [I-D.ietf-core-object-security], also
retrieving the client's public key.
o In step 4, the 'algorithms' array in the Additional Authenticated
Data is modified as described in Section 3.
o In step 6, the server also verifies the counter signature using
the public key of the client from the associated Recipient
Context.
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
[I-D.ietf-core-object-security], with the following modifications.
o In step 2, the 'algorithms' array in the Additional Authenticated
Data is modified as described in Section 3.
Tiloca, et al. Expires April 25, 2019 [Page 13]
Internet-Draft Group OSCORE October 2018
o In step 4, the encryption of the COSE object is modified as
described in Section 3. The encoding of the compressed COSE
object is modified as described in Section 4.
6.4. Verifying the Response
Upon receiving a secure response message, the client proceeds as
described in Section 8.4 of [I-D.ietf-core-object-security], with the
following modifications.
o In step 2, the decoding of the compressed COSE object is modified
as described in Section 3. If the received Recipient ID ('kid')
does not match with any Recipient Context for the retrieved Gid
('kid context'), then the client creates a new Recipient Context,
initializes it according to Section 3 of
[I-D.ietf-core-object-security], also retrieving the server's
public key.
o In step 3, the 'algorithms' array in the Additional Authenticated
Data is modified as described in Section 3.
o In step 5, the client also verifies the counter signature using
the public key of the server from the associated Recipient
Context.
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. Such policies can be enforced locally by the Group
Manager, or by a third party in a trust relation with the Group
Manager and entrusted to enforce join policies on behalf of the
Group Manager.
3. Driving the join process to add new endpoints as group members.
4. Establishing Security Common Contexts and providing them to
authorized group members during the join process, together with
a corresponding Security Sender Context.
5. Generating and managing Sender IDs within its OSCORE groups, as
well as assigning and providing them to new endpoints during the
Tiloca, et al. Expires April 25, 2019 [Page 14]
Internet-Draft Group OSCORE October 2018
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).
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.
8. Security Considerations
The same security considerations from OSCORE (Section 11 of
[I-D.ietf-core-object-security]) apply to this specification.
Additional security aspects to be taken into account are discussed
below.
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.
Tiloca, et al. Expires April 25, 2019 [Page 15]
Internet-Draft Group OSCORE October 2018
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.3 of
[I-D.ietf-core-object-security] 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.3 of [I-D.ietf-core-object-security] for
messages including a Partial IV concerns only group requests, and
same considerations from [I-D.ietf-core-object-security] apply
here as well.
o The case B in Appendix D.3 of [I-D.ietf-core-object-security] for
messages not including a Partial IV concerns all group responses,
and same considerations from [I-D.ietf-core-object-security] apply
here as well.
As a consequence, each message encrypted/decrypted with the same
Sender Key is processed by using a different (ID_PIV, PIV) pair.
This means that nonces used by any fixed encrypting endpoint are
unique. Thus, each message is processed with a different (key,
nonce) pair.
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.
Tiloca, et al. Expires April 25, 2019 [Page 16]
Internet-Draft Group OSCORE October 2018
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. Note that a former (compromised) group
member can take advantage of this by sending messages protected with
the old retained Security Context. Therefore, a conservative
application policy should not admit the storage 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
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. That can also happen if
the application can not guarantee unique Group Identifiers within a
given Group Manager. 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, and as long as the Sender
IDs within a group are unique, AEAD keys are different among
different groups.
Tiloca, et al. Expires April 25, 2019 [Page 17]
Internet-Draft Group OSCORE October 2018
9. IANA Considerations
Note to RFC Editor: Please replace all occurrences of "[[this
document]]" with the RFC number of this specification.
9.1. OSCORE Flag Bits Registry
The entry with Bit Position TBD is added to the "OSCORE Flag Bits"
registry.
+--------------+-------------+---------------------+-------------------+
| Bit Position | Name | Description | Specification |
+--------------+-------------+---------------------+-------------------+
| TBD | Counter | Set to 1 if counter | [[this document]] |
| | Signature | signature present | |
| | | in the compressed | |
| | | COSE object | |
+--------------+-------------+---------------------+-------------------+
10. References
10.1. Normative References
[I-D.ietf-core-object-security]
Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
"Object Security for Constrained RESTful Environments
(OSCORE)", draft-ietf-core-object-security-15 (work in
progress), August 2018.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252,
DOI 10.17487/RFC7252, June 2014,
<https://www.rfc-editor.org/info/rfc7252>.
[RFC8032] Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital
Signature Algorithm (EdDSA)", RFC 8032,
DOI 10.17487/RFC8032, January 2017,
<https://www.rfc-editor.org/info/rfc8032>.
[RFC8152] Schaad, J., "CBOR Object Signing and Encryption (COSE)",
RFC 8152, DOI 10.17487/RFC8152, July 2017,
<https://www.rfc-editor.org/info/rfc8152>.
Tiloca, et al. Expires April 25, 2019 [Page 18]
Internet-Draft Group OSCORE October 2018
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
10.2. Informative References
[I-D.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-16
(work in progress), October 2018.
[I-D.ietf-core-echo-request-tag]
Amsuess, C., Mattsson, J., and G. Selander, "Echo and
Request-Tag", draft-ietf-core-echo-request-tag-02 (work in
progress), June 2018.
[I-D.somaraju-ace-multicast]
Somaraju, A., Kumar, S., Tschofenig, H., and W. Werner,
"Security for Low-Latency Group Communication", draft-
somaraju-ace-multicast-02 (work in progress), October
2016.
[I-D.tiloca-ace-oscoap-joining]
Tiloca, M., Park, J., and F. Palombini, "Key Management
for OSCORE Groups in ACE", draft-tiloca-ace-oscoap-
joining-05 (work in progress), October 2018.
[RFC4944] Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
"Transmission of IPv6 Packets over IEEE 802.15.4
Networks", RFC 4944, DOI 10.17487/RFC4944, September 2007,
<https://www.rfc-editor.org/info/rfc4944>.
[RFC4949] Shirey, R., "Internet Security Glossary, Version 2",
FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,
<https://www.rfc-editor.org/info/rfc4949>.
[RFC6282] Hui, J., Ed. and P. Thubert, "Compression Format for IPv6
Datagrams over IEEE 802.15.4-Based Networks", RFC 6282,
DOI 10.17487/RFC6282, September 2011,
<https://www.rfc-editor.org/info/rfc6282>.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
January 2012, <https://www.rfc-editor.org/info/rfc6347>.
Tiloca, et al. Expires April 25, 2019 [Page 19]
Internet-Draft Group OSCORE October 2018
[RFC7228] Bormann, C., Ersue, M., and A. Keranen, "Terminology for
Constrained-Node Networks", RFC 7228,
DOI 10.17487/RFC7228, May 2014,
<https://www.rfc-editor.org/info/rfc7228>.
[RFC7390] Rahman, A., Ed. and E. Dijk, Ed., "Group Communication for
the Constrained Application Protocol (CoAP)", RFC 7390,
DOI 10.17487/RFC7390, October 2014,
<https://www.rfc-editor.org/info/rfc7390>.
Appendix A. 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
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
Tiloca, et al. Expires April 25, 2019 [Page 20]
Internet-Draft Group OSCORE October 2018
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
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.
Tiloca, et al. Expires April 25, 2019 [Page 21]
Internet-Draft Group OSCORE October 2018
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
[I-D.ietf-core-object-security], this results in providing
relative freshness of group requests and absolute freshness of
responses. It is not required to determine ordering of messages
from different senders.
Appendix B. List of Use Cases
Group Communication for CoAP [RFC7390] provides the necessary
background for multicast-based CoAP communication, with particular
reference to low-power and lossy networks (LLNs) and resource
constrained environments. The interested reader is encouraged to
first read [RFC7390] to understand the non-security related details.
This section discusses a number of use cases that benefit from secure
group communication. Specific security requirements for these use
cases are discussed in 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).
Tiloca, et al. Expires April 25, 2019 [Page 22]
Internet-Draft Group OSCORE October 2018
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
execution of the requested operation (e.g. OK, failure, error)
and their current operational status.
o Software and firmware updates: software and firmware updates often
comprise quite a large amount of data. This can overload a LLN
that is otherwise typically used to deal with only small amounts
of data, on an infrequent base. Rather than sending software and
firmware updates as unicast messages to each individual device,
multicasting such updated data to a larger group of devices at
once displays a number of benefits. For instance, it can
significantly reduce the network load and decrease the overall
time latency for propagating this data to all devices. Even if
the complete whole update process itself is secured, securing the
individual messages is important, in case updates consist of
relatively large amounts of data. In fact, checking individual
received data piecemeal for tampering avoids that devices store
Tiloca, et al. Expires April 25, 2019 [Page 23]
Internet-Draft Group OSCORE October 2018
large amounts of partially corrupted data and that they detect
tampering hereof only after all data has been received. Devices
receiving software and firmware updates are expected to possibly
reply back, in order to provide a feedback about the execution of
the update operation (e.g. OK, failure, error) and their current
operational status.
o Parameter and configuration update: by means of multicast
communication, it is possible to update the settings of a group of
similar devices, both simultaneously and efficiently. Possible
parameters are related, for instance, to network load management
or network access controls. Devices receiving parameter and
configuration updates are expected to possibly reply back, to
provide a feedback about the execution of the update operation
(e.g. OK, failure, error) and their current operational status.
o Commissioning of LLNs systems: a commissioning device is
responsible for querying all devices in the local network or a
selected subset of them, in order to discover their presence, and
be aware of their capabilities, default configuration, and
operating conditions. Queried devices displaying similarities in
their capabilities and features, or sharing a common physical
location can be configured as members of a single group. Queried
devices are expected to reply back to the commissioning device, in
order to notify their presence, and provide the requested
information and their current operational status.
o Emergency multicast: a particular emergency related information
(e.g. natural disaster) is generated and multicast by an emergency
notifier, and relayed to multiple devices. The latters may reply
back to the emergency notifier, in order to provide their feedback
and local information related to the ongoing emergency. This kind
of setups should additionally rely on a fault tolerance multicast
algorithm, such as 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.
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.
Tiloca, et al. Expires April 25, 2019 [Page 24]
Internet-Draft Group OSCORE October 2018
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 Security 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 OSCORE Security
Contexts to process an incoming message, until the right one is found
and the message is correctly verified. Therefore, it is favourable
that Group Identifiers from different Group Managers have a size that
result in a small probability of collision. How small this
probability should be is up to system designers.
Appendix D. Set-up of New Endpoints
An endpoint joins a group by explicitly interacting with the
responsible Group Manager. When becoming members of a group,
endpoints are not required to know how many and what endpoints are in
the same group.
Communications between a joining endpoint and the Group Manager rely
on the CoAP protocol and must be secured. Specific details on how to
secure communications between joining endpoints and a Group Manager
are out of the scope of this document.
The Group Manager must verify that the joining endpoint is authorized
to join the group. To this end, the Group Manager can directly
authorize the joining endpoint, or expect it to provide authorization
Tiloca, et al. Expires April 25, 2019 [Page 25]
Internet-Draft Group OSCORE October 2018
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 OSCORE 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.tiloca-ace-oscoap-joining] 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.
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.
Tiloca, et al. Expires April 25, 2019 [Page 26]
Internet-Draft Group OSCORE October 2018
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 Section 7.5.2 of
[I-D.ietf-core-object-security].
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 a 4.03 Forbidden response message including an Echo
Option, and stores the option value included therein.
Upon receiving a 4.03 Forbidden response that includes an Echo Option
and originates from a verified group member, a client sends a request
as a unicast message addressed to the same server, echoing the Echo
Option value. In particular, the client does not necessarily resend
the same group request, but can instead send a more recent one, if
the application permits it. This makes it possible for the client to
not retain previously sent group requests for full retransmission,
unless the application explicitly requires otherwise. In either
case, the client uses the sender sequence number value currently
stored in its own Sender Context. If the client stores group
requests for possible retransmission with the Echo Option, it should
not store a given request for longer than a pre-configured time
interval. Note that the unicast request echoing the Echo Option is
correctly treated and processed as a message, since the 'kid context'
field including the Group Identifier of the OSCORE group is still
present in the OSCORE Option as part of the COSE object (see
Section 3).
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.
Tiloca, et al. Expires April 25, 2019 [Page 27]
Internet-Draft Group OSCORE October 2018
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 4.03 Forbidden 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.
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
Tiloca, et al. Expires April 25, 2019 [Page 28]
Internet-Draft Group OSCORE October 2018
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 -02 to -03
o Revised structure and phrasing for improved readability and better
alignment with draft-ietf-core-object-security.
o Added discussion on wrap-Around of Partial IVs (see Section 2.2).
o Separate sections for the COSE Object (Section 3) and the OSCORE
Header Compression (Section 4).
o The countersignature is now appended to the encrypted payload of
the OSCORE message, rather than included in the OSCORE Option (see
Section 4).
o Extended scope of Section 5, now titled " Message Binding,
Sequence Numbers, Freshness and Replay Protection".
o Clarifications about Non-Confirmable messages in Section 5.1
"Synchronization of Sender Sequence Numbers".
o Clarifications about error handling in Section 6 "Message
Processing".
Tiloca, et al. Expires April 25, 2019 [Page 29]
Internet-Draft Group OSCORE October 2018
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.2. Version -01 to -02
o Terminology has been made more aligned with RFC7252 and draft-
ietf-core-object-security: i) "client" and "server" replace the
old "multicaster" and "listener", respectively; ii) "silent
server" replaces the old "pure listener".
o Section 2 has been updated to have the Group Identifier stored in
the 'ID Context' parameter defined in draft-ietf-core-object-
security.
o Section 3 has been updated with the new format of the Additional
Authenticated Data.
o Major rewriting of Section 4 to better highlight the differences
with the message processing in draft-ietf-core-object-security.
o Added Sections 7.2 and 7.3 discussing security considerations
about uniqueness of (key, nonce) and collision of group
identifiers, respectively.
o Minor updates to Appendix A.1 about assumptions on multicast
communication topology and group size.
o Updated Appendix C on format of group identifiers, with practical
implications of possible collisions of group identifiers.
o Updated Appendix D.2, adding a pointer to draft-palombini-ace-key-
groupcomm about retrieval of nodes' public keys through the Group
Manager.
o Minor updates to Appendix E.3 about Challenge-Response
synchronization of sequence numbers based on the Echo option from
draft-ietf-core-echo-request-tag.
Tiloca, et al. Expires April 25, 2019 [Page 30]
Internet-Draft Group OSCORE October 2018
G.3. 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, Jim Schaad, Ludwig Seitz and Peter van der Stok for their
feedback and comments.
The work on this document has been partly supported by the EIT-
Digital High Impact Initiative ACTIVE.
Tiloca, et al. Expires April 25, 2019 [Page 31]
Internet-Draft Group OSCORE October 2018
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
Tiloca, et al. Expires April 25, 2019 [Page 32]