Identifier Update for OSCORE
draft-ietf-core-oscore-id-update-00
| Document | Type | Active Internet-Draft (core WG) | |
|---|---|---|---|
| Authors | Rikard Höglund , Marco Tiloca | ||
| Last updated | 2024-03-05 | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Intended RFC status | (None) | ||
| Formats | |||
| Additional resources | Mailing list discussion | ||
| Stream | WG state | WG Document | |
| Document shepherd | (None) | ||
| IESG | IESG state | I-D Exists | |
| Consensus boilerplate | Unknown | ||
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | (None) |
draft-ietf-core-oscore-id-update-00
CoRE Working Group R. Höglund
Internet-Draft M. Tiloca
Intended status: Standards Track RISE AB
Expires: 5 September 2024 4 March 2024
Identifier Update for OSCORE
draft-ietf-core-oscore-id-update-00
Abstract
Two peers that communicate with the CoAP protocol can use the Object
Security for Constrained RESTful Environments (OSCORE) protocol to
protect their message exchanges end-to-end. To this end, the two
peers share an OSCORE Security Context and a number of related
identifiers. In particular, each of the two peers stores a Sender ID
that identifies its own Sender Context within the Security Context,
and a Recipient ID that identifies the Recipient Context associated
with the other peer within the same Security Context. These
identifiers are sent in plaintext within OSCORE-protected messages.
Hence, they can be used to correlate messages exchanged between peers
and track those peers, with consequent privacy implications. This
document defines an OSCORE ID update procedure that two peers can use
to update their OSCORE identifiers. This procedure can be run stand-
alone or seamlessly integrated in an execution of the Key Update for
OSCORE (KUDOS) procedure.
Discussion Venues
This note is to be removed before publishing as an RFC.
Discussion of this document takes place on the Constrained RESTful
Environments Working Group mailing list (core@ietf.org), which is
archived at https://mailarchive.ietf.org/arch/browse/core/.
Source for this draft and an issue tracker can be found at
https://github.com/core-wg/oscore-id-update.
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/.
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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 5 September 2024.
Copyright Notice
Copyright (c) 2024 IETF Trust and the persons identified as the
document authors. All rights reserved.
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
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provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Update of OSCORE Sender/Recipient IDs . . . . . . . . . . . . 4
2.1. The Recipient-ID Option . . . . . . . . . . . . . . . . . 6
2.1.1. Forward Message Flow . . . . . . . . . . . . . . . . 7
2.1.2. Reverse Message Flow . . . . . . . . . . . . . . . . 11
2.1.3. Additional Actions for Execution . . . . . . . . . . 15
2.2. Preserving Observations Across ID Updates . . . . . . . . 16
3. Security Considerations . . . . . . . . . . . . . . . . . . . 17
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
4.1. CoAP Option Numbers Registry . . . . . . . . . . . . . . 17
5. References . . . . . . . . . . . . . . . . . . . . . . . . . 17
5.1. Normative References . . . . . . . . . . . . . . . . . . 17
5.2. Informative References . . . . . . . . . . . . . . . . . 18
Appendix A. Examples of OSCORE ID update procedure Integrated in
KUDOS . . . . . . . . . . . . . . . . . . . . . . . . . . 18
A.1. Forward Message Flow . . . . . . . . . . . . . . . . . . 19
A.2. Reverse Message Flow . . . . . . . . . . . . . . . . . . 21
Appendix B. Document Updates . . . . . . . . . . . . . . . . . . 23
B.1. Version -00 . . . . . . . . . . . . . . . . . . . . . . . 23
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23
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1. Introduction
When using the CoAP protocol [RFC7252], two peers can use Object
Security for Constrained RESTful Environments (OSCORE) protocol to
protect their message exchanges end-to-end. To this end, the two
peers share an OSCORE Security Context and a number of related
identifiers.
As part of the shared Security Context, each peer stores one Sender
Context identified by a Sender ID and used to protect its outgoing
messages. Also, it stores a Recipient Context identified by a
Recipient ID and used to unprotect the incoming messages from the
other peer. That is, one's peer Sender ID (Recipient ID) is equal to
the other peer's Recipient ID (Sender ID).
When receiving an OSCORE-protected message, the recipient peer uses
its Recipient ID conveyed within the message or otherwise implied, in
order to retrieve the correct Security Context and unprotect the
message.
These identifiers are sent in plaintext within OSCORE-protected
messages and are immutable throughout the lifetime of a Security
Context, even in case the two peers migrate to a different network or
simply change their addressing information. Therefore, the
identifiers can be used to correlate messages that the two peers
exchange at different points in time or through different paths,
hence allowing for track them with consequent privacy implications.
In order to address this issue, this document defines an OSCORE ID
update procedure that two peers can use to update their OSCORE Sender
and Recipient IDs. For instance, two peers may want to use this
procedure before switching to a different network, in order to make
it more difficult to understand that their communication is
continuing in the new network.
The OSCORE ID update procedure can be run stand-alone or seamlessly
integrated in an execution of the Key Update for OSCORE (KUDOS)
procedure [I-D.ietf-core-oscore-key-update].
1.1. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
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Readers are expected to be familiar with the terms and concepts
related to CoAP [RFC7252], Observe [RFC7641], CBOR [RFC8949], OSCORE
[RFC8613], and KUDOS [I-D.ietf-core-oscore-key-update].
This document additionally uses the following terminology.
* Initiator: the peer starting the OSCORE ID update procedure, by
sending the first message.
* Responder: the peer that receives the first message in an
execution of the OSCORE ID update procedure.
* Forward message flow: the execution workflow where the initiator
acts as CoAP client (see Section 2.1.1).
* Reverse message flow: the execution workflow where the initiator
acts as CoAP server (see Section 2.1.2).
2. Update of OSCORE Sender/Recipient IDs
This section defines the procedure that two peers can perform, in
order to update the OSCORE Sender/Recipient IDs that they use in
their shared OSCORE Security Context.
When performing an update of OSCORE Sender/Recipient IDs, a peer
provides its new intended OSCORE Recipient ID to the other peer, by
means of the Recipient-ID Option defined in Section 2.1. Hereafter,
this document refers to a message including the Recipient-ID Option
as an "ID update (request/response) message".
This procedure can be initiated by either peer, i.e., the CoAP client
or the CoAP server may start it by sending the first OSCORE IDs
update message. The former case is denoted as the "forward message
flow" and the latter as the "reverse message flow".
Furthermore, this procedure can be executed stand-alone, or instead
seamlessly integrated in an execution of the KUDOS procedure for
updating OSCORE keying material (see Section 4 of
[I-D.ietf-core-oscore-key-update]) used in its FS mode or no-FS mode
(see Section 4.5 of [I-D.ietf-core-oscore-key-update]).
* In the former stand-alone case, updating the OSCORE Sender/
Recipient IDs effectively results in updating part of the current
OSCORE Security Context.
That is, both peers derive a new Sender Key, Recipient Key, and
Common IV, as defined in Section 3.2 of [RFC8613]. Also, both
peers re-initialize the Sender Sequence Number and the Replay
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Window accordingly, as defined in Section 3.2.2 of [RFC8613].
Since the same Master Secret is preserved, forward secrecy is not
achieved.
As defined in Section 2.1.3, the two peers must take additional
actions to ensure a safe execution of the OSCORE ID update
procedure.
A peer can safely discard the old OSCORE Security Context
including the old OSCORE Sender/Recipient IDs after the following
two events have occurred, in this order: first, the peer has sent
to the other peer a message protected with the new OSCORE Security
Context including the new OSCORE Sender/Recipient IDs; then, the
peer has received from the other peer and successfully verified a
message protected with that new OSCORE Security Context.
* In the latter integrated case, the KUDOS initiator (responder)
also acts as initiator (responder) for the OSCORE ID update
procedure. That is, both KUDOS and the OSCORE ID update procedure
MUST be run either in their forward message flow or in their
reverse message flow.
The new OSCORE Sender/Recipient IDs MUST NOT be used with the
OSCORE Security Context CTX_OLD, and MUST NOT be used with the
temporary OSCORE Security Context CTX_1 used to protect the first
KUDOS message of a KUDOS execution.
The first use of the new OSCORE Sender/Recipient IDs with the new
OSCORE Security Context CTX_NEW occurs: for the KUDOS initiator,
after having received from the KUDOS responder and successfully
verified the second KUDOS message of the KUDOS execution in
question; for the KUDOS responder, after having sent to the KUDOS
initiator the second KUDOS message of the KUDOS execution in
question.
An initiator terminates an ongoing OSCORE ID update procedure with
another peer as failed, in case, after having sent the first ID
update message for the procedure in question, a pre-defined amount of
time has elapsed without receiving and successfully verifying the
second ID update message from the other peer. It is RECOMMENDED that
such an amount of time is equal to MAX_TRANSMIT_WAIT (see
Section 4.8.2 of [RFC7252]).
A peer terminates an ongoing OSCORE ID update procedure with another
peer as successful, in any of the following two cases.
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* The peer is acting as initiator, and it has received and
successfully verified the second ID update message from the other
peer.
* The peer is acting as responder, and it has sent the second ID
update message to the other peer.
A peer MUST NOT initiate an OSCORE ID update procedure with another
peer, if it has another such procedure ongoing with that other peer.
Upon receiving a valid ID update message, a responder that supports
the ID update update procedure MUST send the second ID update
message, except in the following case.
* The received ID update message is not a KUDOS message (i.e., the
OSCORE ID update procedure is being performed stand-alone) and the
responder has no eligible Recipient ID to offer to the initiator
(see Section 2.1.3).
If the responder is a server, the responder MUST also reply to the
received ID update request message with a protected 5.03 (Service
Unavailable) error response. The error response MUST NOT include
the Recipient-ID Option, and its diagnostic payload MAY provide
additional information.
When receiving the error response, the initiator terminates the
OSCORE IDs procedure as failed.
2.1. The Recipient-ID Option
The Recipient ID-Option defined in this section has the properties
summarized in Figure 1, which extends Table 4 of [RFC7252]. That is,
the option is elective, safe to forward, part of the cache key, and
non repeatable.
+-------+---+---+---+---+--------------+--------+--------+---------+
| No. | C | U | N | R | Name | Format | Length | Default |
+-------+---+---+---+---+--------------+--------+--------+---------+
| | | | | | | | | |
| TBD24 | | | | | Recipient-ID | opaque | any | (none) |
| | | | | | | | | |
+-------+---+---+---+---+--------------+--------+--------+---------+
C=Critical, U=Unsafe, N=NoCacheKey, R=Repeatable
Figure 1: The Recipient-ID Option.
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Note to RFC Editor: Following the registration of the CoAP Option
Number 24, please replace "TBD24" with "24" in the figure above.
Then, please delete this paragraph.
The option value can have an arbitrary length. Implementations can
limit its length to that of the longest supported Recipient ID.
This document particularly defines how this option is used in
messages protected with OSCORE. That is, when the option is included
in an outgoing message, the option value specifies the new OSCORE
Recipient ID that the sender endpoint intends to use with the other
endpoint sharing the OSCORE Security Context.
Therefore, the maximum length of the option value is equal to the
maximum length of OSCORE Sender/Recipient IDs. As defined in
Section 3.3 of [RFC8613], this is determined by the size of the AEAD
nonce of the used AEAD Algorithm in the OSCORE Security Context.
The Recipient-ID Option is of class E in terms of OSCORE processing
(see Section 4.1 of [RFC8613]).
2.1.1. Forward Message Flow
Figure 2 shows an example of the OSCORE ID update procedure, run
stand-alone and in the forward message flow, with the client acting
as initiator. On each peer, SID and RID denote the OSCORE Sender ID
and Recipient ID of that peer, respectively.
Appendix A.1 provides a different example of the OSCORE ID update
procedure, as run integrated in an execution of KUDOS and in the
forward message flow (see Section 4.3.1 of
[I-D.ietf-core-oscore-key-update]).
Client Server
(initiator) (responder)
| |
CTX_A { | | CTX_A {
SID = 0x01 | | SID = 0x00
RID = 0x00 | | RID = 0x01
} | | }
| |
| Request #1 |
Protect |---------------------------------->| /temp
with CTX_A | OSCORE { |
| ... |
| kid: 0x01 | Verify
| } | with CTX_A
| Encrypted Payload { |
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| ... |
| Recipient-ID: 0x42 |
| ... |
| Application Payload |
| } |
| |
| Response #1 |
|<----------------------------------| Protect
| OSCORE { | with CTX_A
| ... |
Verify | } |
with CTX_A | Encrypted Payload { |
| ... |
| Recipient-ID: 0x78 |
| ... |
| Application Payload |
| } |
| |
CTX_B { | | CTX_B {
SID = 0x78 | | SID = 0x42
RID = 0x42 | | RID = 0x78
} | | }
| |
| Request #2 |
Protect |---------------------------------->| /temp
with CTX_B | OSCORE { |
| ... |
| kid: 0x78 | Verify
| } | with CTX_B
| Encrypted Payload { |
| ... |
| Application Payload |
| } |
| |
| Response #2 |
|<----------------------------------| Protect
| OSCORE { | with CTX_B
| ... |
Verify | } |
with CTX_B | Encrypted Payload { |
| ... |
| Application Payload |
| } |
| |
Discard | |
CTX_A | |
| |
| Request #3 |
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Protect |---------------------------------->| /temp
with CTX_B | OSCORE { |
| ... |
| kid: 0x78 | Verify
| } | with CTX_B
| Encrypted Payload { |
| ... |
| Application Payload |
| } |
| |
| | Discard
| | CTX_A
| |
| Response #3 |
|<----------------------------------| Protect
| OSCORE { | with CTX_B
| ... |
Verify | } |
with CTX_B | Encrypted Payload { |
| ... |
| Application Payload |
| } |
| |
Figure 2: Example of the OSCORE ID update procedure with Forward
Message Flow
Before the OSCORE ID update procedure starts, the client (the server)
shares with the server (the client) an OSCORE Security Context CTX_A
with Sender ID 0x01 (0x00) and Recipient ID 0x00 (0x01).
When starting the OSCORE ID update procedure, the client determines
its new intended OSCORE Recipient ID 0x42. Then, the client prepares
a CoAP request targeting an application resource at the server. The
request includes the Recipient-ID Option, with value the client's new
Recipient ID 0x42.
The client protects the request with CTX_A, i.e., by using the keying
material derived from the client's current Sender ID 0x01. The
protected request specifies the client's current Sender ID 0x01 in
the 'kid' field of the OSCORE Option. After that, the client sends
the request to the server as Request #1.
Upon receiving, decrypting, and successfully verifying the OSCORE
message Request #1, the server retrieves the value 0x42 from the
Recipient-ID Option, and determines its new intended OSCORE Recipient
ID 0x78. Then, the server prepares a CoAP response including the
Recipient-ID Option, with value the server's new Recipient ID 0x78.
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The server protects the response with CTX_A, i.e., by using the
keying material derived from the server's current Sender ID 0x00.
After that, the server sends the response to the client.
Then, the server considers 0x42 and 0x78 as its new Sender ID and
Recipient ID to use with the client, respectively. As shown in the
example, the server practically installs a new OSCORE Security
Context CTX_B where: i) its Sender ID and Recipient ID are 0x42 and
0x78, respectively; ii) the Sender Sequence Number and the Replay
Window are re-initialized (see Section 3.2.2 of [RFC8613]); iii)
anything else is like in the OSCORE Security Context used to encrypt
the OSCORE message Response #1.
Upon receiving, decrypting, and successfully verifying the OSCORE
message Response #1, the client considers 0x78 and 0x42 as the new
Sender ID and Recipient ID to use with the server, respectively. As
shown in the example, the client practically installs a new OSCORE
Security Context CTX_B where: i) its Sender ID and Recipient ID are
0x78 and 0x42, respectively; ii) the Sender Sequence Number and the
Replay Window are re-initialized (see Section 3.2.2 of [RFC8613]);
iii) anything else is like in the OSCORE Security Context used to
decrypt the OSCORE message Response #1.
From then on, the client and the server can exchange messages
protected with the OSCORE Security Context CTX_B, i.e., according to
the new OSCORE Sender/Recipient IDs and using new keying material
derived from those.
That is, the client sends the OSCORE message Request #2, which is
protected with CTX_B and specifies the new client's Sender ID 0x78 in
the 'kid' field of the OSCORE Option.
Upon receiving the OSCORE message Request #2, the server retrieves
the OSCORE Security Context CTX_B, according to its new Recipient ID
0x78 specified in the 'kid' field of the OSCORE Option. Then, the
server decrypts and verifies the response by using CTX_B. Finally,
the server prepares a CoAP response Response #2, protects it with
CTX_B, and sends it to the client.
Upon receiving the OSCORE message Response #2, the client decrypts
and verifies it with the OSCORE Security Context CTX_B. In case of
successful verification, the client confirms that the server is
aligned with the new OSCORE Sender/Recipient IDs, and thus discards
the OSCORE Security Context CTX_A.
After that, one further exchange occurs, where both the CoAP request
and the CoAP response are protected with the OSCORE Security Context
CTX_B. In particular, upon receiving, decrypting, and successfully
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verifying the OSCORE message Request #3, the server confirms that the
client is aligned with the new OSCORE Sender/Recipient IDs, and thus
discards the OSCORE Security Context CTX_A.
2.1.2. Reverse Message Flow
Figure 3 shows an example of the OSCORE ID update procedure, run
stand-alone and in the reverse message flow, with the server acting
as initiator. On each peer, SID and RID denote the OSCORE Sender ID
and Recipient ID of that peer, respectively.
Appendix A.2 provides a different example of the OSCORE ID update
procedure, as run integrated in an execution of KUDOS and in the
reverse message flow (see Section 4.3.2 of
[I-D.ietf-core-oscore-key-update]).
Client Server
(responder) (initiator)
| |
CTX_A { | | CTX_A {
SID = 0x01 | | SID = 0x00
RID = 0x00 | | RID = 0x01
} | | }
| |
| Request #1 |
Protect |---------------------------------->| /temp
with CTX_A | OSCORE { |
| ... |
| kid: 0x01 | Verify
| } | with CTX_A
| Encrypted Payload { |
| ... |
| Application Payload |
| } |
| |
| Response #1 |
|<----------------------------------| Protect
| OSCORE { | with CTX_A
| ... |
Verify | } |
with CTX_A | Encrypted Payload { |
| ... |
| Recipient-ID: 0x78 |
| ... |
| Application Payload |
| } |
| |
| Request #2 |
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Protect |---------------------------------->| /temp
with CTX_A | OSCORE { |
| ... |
| kid: 0x01 | Verify
| } | with CTX_A
| Encrypted Payload { |
| ... |
| Recipient-ID: 0x42 |
| ... |
| Application Payload |
| } |
| |
| | CTX_B {
| | SID = 0x42
| | RID = 0x78
| | }
| |
| Response #2 |
|<----------------------------------| Protect
| OSCORE { | with CTX_A
| ... |
Verify | } |
with CTX_A | Encrypted Payload { |
| ... |
| Application Payload |
| } |
| |
CTX_B { | |
SID = 0x78 | |
RID = 0x42 | |
} | |
| |
| Request #3 |
Protect |---------------------------------->| /temp
with CTX_B | OSCORE { |
| ... |
| kid: 0x78 | Verify
| } | with CTX_B
| Encrypted Payload { |
| ... |
| Application Payload |
| } |
| |
| Response #3 |
|<----------------------------------| Protect
| OSCORE { | with CTX_B
| ... |
Verify | } |
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with CTX_B | Encrypted Payload { |
| ... |
| Application Payload |
| } |
| |
Discard | |
CTX_A | |
| |
| Request #4 |
Protect |---------------------------------->| /temp
with CTX_B | OSCORE { |
| ... |
| kid: 0x78 | Verify
| } | with CTX_B
| Encrypted Payload { |
| ... |
| Application Payload |
| } |
| |
| | Discard
| | CTX_A
| |
| Response #4 |
|<----------------------------------| Protect
| OSCORE { | with CTX_B
| ... |
Verify | } |
with CTX_B | Encrypted Payload { |
| ... |
| Application Payload |
| } |
| |
Figure 3: Example of the OSCORE ID update procedure with Reverse
Message Flow
Before the OSCORE ID update procedure starts, the client (the server)
shares with the server (the client) an OSCORE Security Context CTX_A
with Sender ID 0x01 (0x00) and Recipient ID 0x00 (0x01).
At first, the client prepares a CoAP Request #1 targeting an
application resource at the server. The client protects the request
with CTX_A, i.e., by using the keying material derived from the
client's current Sender ID 0x01. The protected request specifies the
client's current Sender ID 0x01 in the 'kid' field of the OSCORE
Option. After that, the client sends the request to the server as
Request #1.
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Upon receiving, decrypting, and successfully verifying the OSCORE
message Request #1, the server decides to start an OSCORE ID update
procedure. To this end, the server determines its new intended
OSCORE Recipient ID 0x78. Then, the server prepares a CoAP response
as a reply to the just received request and including the Recipient-
ID Option, with value the server's new Recipient ID 0x78.
The server protects the response with CTX_A, i.e., by using the
keying material derived from the server's current Sender ID 0x00.
After that, the server sends the response to the client as Response
#1.
Upon receiving, decrypting, and successfully verifying the OSCORE
message Response #1, the client retrieves the value 0x78 from the
Recipient-ID Option, and determines its new intended OSCORE Recipient
ID 0x42. Then, the client prepares a CoAP request targeting an
application resource at the server. The request includes the
Recipient-ID Option, with value the client's new Recipient ID 0x42.
The client protects the request with CTX_A, i.e., by using the keying
material derived from the client's current Sender ID 0x01. The
protected request specifies the client's current Sender ID 0x01 in
the 'kid' field of the OSCORE Option. After that, the client sends
the request to the server as Request #2.
Upon receiving, decrypting, and successfully verifying the OSCORE
message Request #2, the server retrieves the value 0x42 from the
Recipient-ID Option. Then the server considers 0x42 and 0x78 as the
new Sender ID and Recipient ID to use with the client, respectively.
As shown in the example, the server practically installs a new OSCORE
Security Context CTX_B where: i) its Sender ID and Recipient ID are
0x42 and 0x78, respectively; ii) the Sender Sequence Number and the
Replay Window are re-initialized (see Section 3.2.2 of [RFC8613]);
iii) anything else is like in the OSCORE Security Context used to
encrypt the OSCORE message Request #2.
Then, the server prepares a CoAP response, as a reply to the just
received request, and protects it with CTX_A, i.e., by using the
keying material derived from the server's current Sender ID 0x00.
After that, the server sends the response to the client as Response
#2.
Upon receiving, decrypting, and successfully verifying the OSCORE
message Response #2, the client considers 0x78 and 0x42 as the new
Sender ID and Recipient ID to use with the server, respectively. As
shown in the example, the client practically installs a new OSCORE
Security Context CTX_B where: i) its Sender ID and Recipient ID are
0x78 and 0x42, respectively; ii) the Sender Sequence Number and the
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Replay Window are re-initialized (see Section 3.2.2 of [RFC8613]);
iii) anything else is like in the OSCORE Security Context used to
decrypt the OSCORE response.
From then on, the client and the server can exchange messages
protected with the OSCORE Security Context CTX_B, i.e., according to
the new OSCORE Sender/Recipient IDs and using new keying material
derived from those.
That is, the client sends the OSCORE message Request #3, which is
protected with CTX_B and specifies the new client's Sender ID 0x78 in
the 'kid' field of the OSCORE Option.
Upon receiving the OSCORE message Request #3, the server retrieves
the OSCORE Security Context CTX_B, according to its new Recipient ID
0x78 specified in the 'kid' field of the OSCORE Option. Then, the
server decrypts and verifies the response by using CTX_B. Finally,
the server prepares a CoAP response, protects it with CTX_B, and
sends it to the client as Response #3.
Upon receiving the OSCORE message Response #3, the client decrypts
and verifies it with the OSCORE Security Context CTX_B. In case of
successful verification, the client confirms that the server is
aligned with the new OSCORE Sender/Recipient IDs, and thus discards
the OSCORE Security Context CTX_A.
After that, one further exchange occurs, where both the CoAP request
and the CoAP response are protected with the OSCORE Security Context
CTX_B. In particular, upon receiving, decrypting, and successfully
verifying the OSCORE message Request #4, the server confirms that the
client is aligned with the new OSCORE Sender/Recipient IDs, and thus
discards the OSCORE Security Context CTX_A.
2.1.3. Additional Actions for Execution
After having experienced a loss of state, a peer MUST NOT participate
in a stand-alone OSCORE ID update procedure with another peer, until
having performed a full-fledged establishment/renewal of an OSCORE
Security Context with the other peer (e.g., by running KUDOS
[I-D.ietf-core-oscore-key-update] or the authenticated key
establishment protocol EDHOC [I-D.ietf-lake-edhoc]).
More precisely, a peer has experienced a loss of state if it cannot
access the latest snapshot of the latest OSCORE Security Context
CTX_OLD or the whole set of OSCORE Sender/Recipient IDs that have
been used with the triplet (Master Secret, Master Salt, ID Context)
of CTX_OLD. This can happen, for instance, after a device reboots.
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Furthermore, when participating in a stand-alone OSCORE ID update
procedure, a peer performs the following additional steps.
* When a peer sends an ID update message, the value of the
Recipient-ID Option that the peer specifies as its new intended
OSCORE Recipient ID MUST fulfill both the following conditions: it
is currently available as Recipient ID to use for the peer (see
Section 3.3 of [RFC8613]); and the peer has never used it as
Recipient ID with the current triplet (Master Secret, Master Salt,
ID Context).
* When receiving an ID update message, the peer MUST abort the
procedure if it has already used the identifier specified in the
Recipient-ID Option as its own Sender ID with the current triplet
(Master Secret, Master Salt, ID Context).
In order to fulfill the conditions above, a peer has to keep track of
the OSCORE Sender/Recipient IDs that it has used with the current
triplet (Master Secret, Master Salt, ID Context) since the latest
update of the OSCORE Master Secret (e.g., performed by running
KUDOS).
2.2. Preserving Observations Across ID Updates
When running the OSCORE ID update procedure stand-alone or integrated
in an execution of KUDOS, the following holds if Observe [RFC7641] is
supported, in order to preserve ongoing observations beyond a change
of OSCORE identifiers.
* If a peer intends to keep active beyond an update of its Sender ID
the observations where it is acting as CoAP client, then the peer:
- MUST store the value of the 'kid' parameter from the original
Observe requests, and retain it for the whole duration of the
observations, throughout which the client MUST NOT update the
stored value associated with the corresponding Observe
registration request; and
- MUST use the stored value of the 'kid' parameter from the
original Observe registration request as value for the
'request_kid' parameter in the external_aad structure (see
Section 5.4 of [RFC8613]), when verifying notifications for
that observation as per Section 8.4.2 of [RFC8613].
* If a peer is acting as CoAP server in an ongoing observation, then
the peer:
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- MUST store the value of the 'kid' parameter from the original
Observe registration request, and retain it for the whole
duration of the observation, throughout which the peer MUST NOT
update the stored value associated with the corresponding
Observe registration request; and
- MUST use the stored value of the 'kid' parameter from the
original Observe registration request as value for the
'request_kid' parameter in the external_aad structure (see
Section 5.4 of [RFC8613]), when protecting notifications for
that observation as per Section 8.3.1 of [RFC8613].
3. Security Considerations
TODO Security
4. IANA Considerations
This document has the following actions for IANA.
Note to RFC Editor: Please replace all occurrences of "[RFC-XXXX]"
with the RFC number of this specification and delete this paragraph.
4.1. CoAP Option Numbers Registry
IANA is asked to enter the following option number to the "CoAP
Option Numbers" registry within the "Constrained RESTful Environments
(CoRE) Parameters" registry group.
+========+==============+============+
| Number | Name | Reference |
+========+==============+============+
| TBD24 | Recipient-ID | [RFC-XXXX] |
+--------+--------------+------------+
Table 1: New CoAP Option Number
Note to RFC Editor: Following the registration of the CoAP Option
Number 24, please replace "TBD24" with "24" in the table above.
Then, please delete this paragraph.
5. References
5.1. Normative References
[I-D.ietf-core-oscore-key-update]
Höglund, R. and M. Tiloca, "Key Update for OSCORE
(KUDOS)", Work in Progress, Internet-Draft, draft-ietf-
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core-oscore-key-update-07, 4 March 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-core-
oscore-key-update-07>.
[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>.
[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>.
[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>.
[RFC8949] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", STD 94, RFC 8949,
DOI 10.17487/RFC8949, December 2020,
<https://www.rfc-editor.org/info/rfc8949>.
5.2. Informative References
[I-D.ietf-lake-edhoc]
Selander, G., Mattsson, J. P., and F. Palombini,
"Ephemeral Diffie-Hellman Over COSE (EDHOC)", Work in
Progress, Internet-Draft, draft-ietf-lake-edhoc-23, 22
January 2024, <https://datatracker.ietf.org/doc/html/
draft-ietf-lake-edhoc-23>.
Appendix A. Examples of OSCORE ID update procedure Integrated in KUDOS
The following section shows two examples where the OSCORE ID update
procedure is performed together with the KUDOS procedure for updating
OSCORE keying material.
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A.1. Forward Message Flow
Figure 4 provides an example of the OSCORE ID update procedure, as
run integrated in an execution of KUDOS and in the forward message
flow (see Section 4.3.1 of [I-D.ietf-core-oscore-key-update]). On
each peer, SID and RID denote the OSCORE Sender ID and Recipient ID
of that peer, respectively.
Client Server
(initiator) (responder)
| |
CTX_OLD { | | CTX_OLD {
SID = 0x01 | | SID = 0x00
RID = 0x00 | | RID = 0x01
} | | }
| |
Generate N1 | |
| |
CTX_1 = updateCtx( | |
X1, | |
N1, | |
CTX_OLD) | |
| |
| Request #1 |
Protect with CTX_1 |--------------------->| /.well-known/kudos
| OSCORE { |
| ... |
| d flag: 1 | CTX_1 = updateCtx(
| x: X1 | X1,
| nonce: N1 | N1,
| ... | CTX_OLD)
| kid: 0x01 |
| } | Verify with CTX_1
| Encrypted Payload { |
| ... | Generate N2
| Recipient-ID: 0x42 |
| ... | CTX_NEW = updateCtx(
| } | Comb(X1,X2),
| | Comb(N1,N2),
| | CTX_OLD)
| |
| Response #1 |
|<---------------------| Protect with CTX_NEW
| OSCORE { |
| ... |
CTX_NEW = updateCtx( | Partial IV: 0 |
Comb(X1,X2), | ... |
Comb(N1,N2), | |
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CTX_OLD) | d flag: 1 |
| x: X2 |
Verify with CTX_NEW | nonce: N2 |
| ... |
Discard CTX_OLD | } |
| Encrypted Payload { |
Update SID and | ... | Update SID and
RID in CTX_NEW | Recipient-ID: 0x78 | RID in CTX_NEW
| ... |
CTX_NEW { | } | CTX_NEW {
SID = 0x78 | | SID = 0x42
RID = 0x42 | | RID = 0x78
} | | }
| |
// The actual key update process ends here.
// The two peers can use the new Security Context CTX_NEW.
| |
| Request #2 |
Protect with CTX_NEW |--------------------->| /temp
| OSCORE { |
| ... |
| kid: 0x78 | Verify with CTX_NEW
| } |
| Encrypted Payload { | Discard CTX_OLD
| ... |
| Application Payload |
| } |
| |
| Response #2 |
|<---------------------| Protect with CTX_NEW
| OSCORE { |
| ... |
Verify with CTX_NEW | } |
| Encrypted Payload { |
| ... |
| Application Payload |
| } |
| |
Figure 4: Example of the OSCORE ID update procedure with Forward
Message Flow and Integrated in a KUDOS Execution.
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A.2. Reverse Message Flow
Figure 5 provides an example of the OSCORE ID update procedure, as
run integrated in an execution of KUDOS and in the reverse message
flow (see Section 4.3.2 of [I-D.ietf-core-oscore-key-update]). On
each peer, SID and RID denote the OSCORE Sender ID and Recipient ID
of that peer, respectively.
Client Server
(responder) (initiator)
| |
CTX_OLD { | | CTX_OLD {
SID = 0x01 | | SID = 0x00
RID = 0x00 | | RID = 0x01
} | | }
| |
| Request #1 |
Protect with CTX_OLD |--------------------->| /temp
| OSCORE { |
| ... |
| kid: 0x01 |
| } | Verify with CTX_OLD
| Encrypted Payload { |
| ... | Generate N1
| Application Payload |
| } | CTX_1 = updateCtx(
| | X1,
| | N1,
| | CTX_OLD)
| |
| Response #1 |
|<---------------------| Protect with CTX_1
| OSCORE { |
| ... |
CTX_1 = updateCtx( | Partial IV: 0 |
X1, | ... |
N1, | d flag: 1 |
CTX_OLD) | x: X1 |
| nonce: N1 |
Verify with CTX_1 | ... |
| } |
Generate N2 | Encrypted Payload { |
| ... |
CTX_NEW = updateCtx( | Recipient-ID: 0x78 |
Comb(X1,X2), | ... |
Comb(N1,N2), | } |
CTX_OLD) | |
| |
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| Request #2 |
Protect with CTX_NEW |--------------------->| /.well-known/kudos
| OSCORE { |
| ... |
| d flag: 1 | CTX_NEW = updateCtx(
| x: X2 | Comb(X1,X2),
| nonce: N2 | Comb(N1,N2),
| y: w | CTX_OLD)
| old_nonce: N1 |
| kid: 0x01 |
| ... |
| } | Verify with CTX_NEW
| Encrypted Payload { |
| ... | Discard CTX_OLD
| Recipient-ID: 0x42 |
| ... |
| Application Payload |
| } |
| |
Update SID and | | Update SID and
RID in CTX_NEW | | RID in CTX_NEW
| |
CTX_NEW { | | CTX_NEW {
SID = 0x78 | | SID = 0x42
RID = 0x42 | | RID = 0x78
} | | }
| |
// The actual key update process ends here.
// The two peers can use the new Security Context CTX_NEW.
| |
| Response #2 |
|<---------------------| Protect with CTX_NEW
| OSCORE { |
| ... |
Verify with CTX_NEW | } |
| Encrypted Payload { |
Discard CTX_OLD | ... |
| Application Payload |
| } |
| |
| Request #3 |
Protect with CTX_NEW |--------------------->| /temp
| OSCORE { |
| ... |
| kid: 0x78 | Verify with CTX_NEW
| } |
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| Encrypted Payload { |
| ... |
| Application Payload |
| } |
| |
| Response #3 |
|<---------------------| Protect with CTX_NEW
| OSCORE { |
| ... |
Verify with CTX_NEW | } |
| Encrypted Payload { |
| ... |
| Application Payload |
| } |
| |
Figure 5: Example of the OSCORE ID update procedure with Reverse
Message Flow and Integrated in a KUDOS Execution.
Appendix B. Document Updates
This section is to be removed before publishing as an RFC.
B.1. Version -00
* Split out material from Key Update for OSCORE draft into this new
document.
* Extended terminology
* Editorial improvements
Acknowledgments
The authors sincerely thank Christian Amsüss, Carsten Bormann, John
Preuß Mattsson, and Göran Selander 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 H2020 projects SIFIS-
Home (Grant agreement 952652) and ARCADIAN-IoT (Grant agreement
101020259).
Authors' Addresses
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Rikard Höglund
RISE AB
Isafjordsgatan 22
SE-16440 Stockholm Kista
Sweden
Email: rikard.hoglund@ri.se
Marco Tiloca
RISE AB
Isafjordsgatan 22
SE-16440 Stockholm Kista
Sweden
Email: marco.tiloca@ri.se
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