CoRE Working Group R. Höglund
Internet-Draft M. Tiloca
Updates: 8613 (if approved) RISE AB
Intended status: Standards Track 3 March 2025
Expires: 4 September 2025
Key Update for OSCORE (KUDOS)
draft-ietf-core-oscore-key-update-10
Abstract
This document defines Key Update for OSCORE (KUDOS), a lightweight
procedure that two CoAP endpoints can use to update their keying
material by establishing a new OSCORE Security Context. Accordingly,
it updates the use of the OSCORE flag bits in the CoAP OSCORE Option
as well as the protection of CoAP response messages with OSCORE, and
it deprecates the key update procedure specified in Appendix B.2 of
RFC 8613. Thus, this document updates RFC 8613.
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-key-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
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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
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on 4 September 2025.
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Copyright Notice
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document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2. Current Methods for Rekeying OSCORE . . . . . . . . . . . . . 4
3. Updated Protection of Responses with OSCORE . . . . . . . . . 7
4. Key Update for OSCORE (KUDOS) . . . . . . . . . . . . . . . . 7
4.1. Extensions to the OSCORE Option . . . . . . . . . . . . . 8
4.2. Function for Security Context Update . . . . . . . . . . 10
4.3. Key Update . . . . . . . . . . . . . . . . . . . . . . . 14
4.3.1. Nonces and X Bytes . . . . . . . . . . . . . . . . . 15
4.3.2. KUDOS States . . . . . . . . . . . . . . . . . . . . 16
4.3.3. KUDOS State Machine . . . . . . . . . . . . . . . . . 17
4.3.4. Handling of OSCORE Security Contexts . . . . . . . . 20
4.3.5. KUDOS Messages as CoAP Requests or Responses . . . . 20
4.3.6. Avoiding In-Transit Requests During a Key Update . . 21
4.4. Key Update Admitting no Forward Secrecy . . . . . . . . . 21
4.4.1. Handling and Use of Keying Material . . . . . . . . . 22
4.4.2. Selection of KUDOS Mode . . . . . . . . . . . . . . . 24
4.4.3. Non-CAPABLE Devices Operating in FS Mode . . . . . . 26
4.5. Preserving Observations Across Key Updates . . . . . . . 27
4.5.1. Management of Observations . . . . . . . . . . . . . 28
4.6. Retention Policies . . . . . . . . . . . . . . . . . . . 30
4.7. Discussion . . . . . . . . . . . . . . . . . . . . . . . 30
4.7.1. Communication Overhead . . . . . . . . . . . . . . . 31
4.7.2. Resource Type core.kudos . . . . . . . . . . . . . . 32
4.7.3. Well-Known KUDOS Resource . . . . . . . . . . . . . . 33
4.7.4. Rekeying when Using SCHC with OSCORE . . . . . . . . 33
4.8. Signaling KUDOS support in EDHOC . . . . . . . . . . . . 33
5. Security Considerations . . . . . . . . . . . . . . . . . . . 36
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 37
6.1. OSCORE Flag Bits Registry . . . . . . . . . . . . . . . . 37
6.2. CoAP Option Numbers Registry . . . . . . . . . . . . . . 39
6.3. EDHOC External Authorization Data Registry . . . . . . . 39
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6.4. The Well-Known URI Registry . . . . . . . . . . . . . . . 39
6.5. Resource Type (rt=) Link Target Attribute Values
Registry . . . . . . . . . . . . . . . . . . . . . . . . 40
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 40
7.1. Normative References . . . . . . . . . . . . . . . . . . 40
7.2. Informative References . . . . . . . . . . . . . . . . . 41
Appendix A. Examples . . . . . . . . . . . . . . . . . . . . . . 43
A.1. Successful KUDOS Initiated with a Request . . . . . . . . 43
A.2. Successful KUDOS Initiated with a Response . . . . . . . 45
Appendix B. Document Updates . . . . . . . . . . . . . . . . . . 47
B.1. Version -09 to -10 . . . . . . . . . . . . . . . . . . . 47
B.2. Version -08 to -09 . . . . . . . . . . . . . . . . . . . 47
B.3. Version -07 to -08 . . . . . . . . . . . . . . . . . . . 48
B.4. Version -06 to -07 . . . . . . . . . . . . . . . . . . . 49
B.5. Version -05 to -06 . . . . . . . . . . . . . . . . . . . 49
B.6. Version -04 to -05 . . . . . . . . . . . . . . . . . . . 49
B.7. Version -03 to -04 . . . . . . . . . . . . . . . . . . . 50
B.8. Version -02 to -03 . . . . . . . . . . . . . . . . . . . 50
B.9. Version -01 to -02 . . . . . . . . . . . . . . . . . . . 51
B.10. Version -00 to -01 . . . . . . . . . . . . . . . . . . . 51
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 52
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 52
1. Introduction
Object Security for Constrained RESTful Environments (OSCORE)
[RFC8613] provides end-to-end protection of CoAP [RFC7252] messages
at the application-layer, ensuring message confidentiality and
integrity, replay protection, as well as binding of response to
request between a sender and a recipient.
To ensure secure communication when using OSCORE, peers may need to
update their shared keying material. Among other reasons,
approaching key usage limits
[I-D.irtf-cfrg-aead-limits][I-D.ietf-core-oscore-key-limits] requires
updating the OSCORE keying material before communications can
securely continue.
This document updates [RFC8613] as follows.
* It specifies KUDOS, a lightweight key update procedure that the
two peers can use in order to update their current keying material
and establish a new OSCORE Security Context. This deprecates and
replaces the procedure specified in Appendix B.2 of [RFC8613].
* With reference to the "OSCORE Flag Bits" registry defined in
Section 13.7 of [RFC8613] as part of the "Constrained RESTful
Environments (CoRE) Parameters" registry group, it updates the
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entries with Bit Position 0 and 1 (see Section 6), both originally
marked as "Reserved". That is, it defines and registers the usage
of the OSCORE flag bit with Bit Position 0, as the one intended to
expand the space for the OSCORE flag bits in the OSCORE Option
(see Section 4.1). Also, it marks the bit with Bit Position of 1
as "Unassigned".
* It updates the protection of CoAP responses with OSCORE originally
specified in Section 8.3 of [RFC8613], as defined in Section 3 of
this document.
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
related to CoAP [RFC7252], Observe [RFC7641], CBOR [RFC8949], OSCORE
[RFC8613], and EDHOC [RFC9528].
This document additionally defines the following terminology.
* FS mode: the KUDOS execution mode that achieves forward secrecy
(see Section 4.3).
* No-FS mode: the KUDOS execution mode that does not achieve forward
secrecy (see Section 4.4).
* Equilibrium: The situation where a peer has no execution of KUDOS
currently ongoing with the other KUDOS peer for updating one of
their OSCORE Security Contexts.
2. Current Methods for Rekeying OSCORE
Two peers communicating using OSCORE may choose to renew their shared
keying information by establishing a new OSCORE Security Context for
a variety of reasons. A particular reason is approaching limits set
for safe key usage [I-D.ietf-core-oscore-key-limits]. Practically,
when the relevant limits have been reached for an OSCORE Security
Context, the two peers have to establish a new OSCORE Security
Context, in order to continue using OSCORE for secure communication.
That is, the two peers have to establish new Sender and Recipient
Keys, as the keys actually used by the AEAD algorithm.
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In addition to approaching the key usage limits, there may be other
reasons for a peer to initiate a key update procedure. These
include: the OSCORE Security Context approaching its expiration time;
application policies prescribing a regular key rollover; approaching
the exhaustion of the Sender Sequence Number space in the OSCORE
Sender Context.
It is RECOMMENDED that the peer initiating the key update procedure
starts it with some margin, i.e., well before actually experiencing
the trigger event forcing to perform a key update, e.g., the OSCORE
Security Context expiration or the exhaustion of the Sender Sequence
Number space. If the rekeying is not initiated ahead of these
events, it may become practically impossible to perform a key update
with certain methods, and/or without aborting ongoing message
exchanges.
Other specifications define a number of ways for rekeying OSCORE, as
summarized below.
* The two peers can run the procedure defined in Appendix B.2 of
[RFC8613]. That is, the two peers exchange three or four
messages, protected with temporary Security Contexts adding
randomness to the ID Context.
As a result, the two peers establish a new OSCORE Security Context
with new ID Context, Sender Key, and Recipient Key, while keeping
the same OSCORE Master Secret and OSCORE Master Salt from the old
OSCORE Security Context.
This procedure does not require any additional components to what
OSCORE already provides, and it does not provide forward secrecy.
The procedure defined in Appendix B.2 of [RFC8613] is used in
6TiSCH networks [RFC7554][RFC8180] when handling failure events.
That is, a node acting as Join Registrar/Coordinator (JRC) assists
new devices, namely "pledges", to securely join the network as per
the Constrained Join Protocol [RFC9031]. In particular, a pledge
exchanges OSCORE-protected messages with the JRC, from which it
obtains a short identifier, link-layer keying material and other
configuration parameters. As per Section 8.3.3 of [RFC9031], a
JRC that experiences a failure event may likely lose information
about joined nodes, including their assigned identifiers. Then,
the reinitialized JRC can establish a new OSCORE Security Context
with each pledge, through the procedure defined in Appendix B.2 of
[RFC8613].
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* The two peers can run the OSCORE profile [RFC9203] of the
Authentication and Authorization for Constrained Environments
(ACE) Framework [RFC9200].
When a CoAP client uploads an Access Token to a CoAP server as an
access credential, the two peers also exchange two nonces. Then,
the two peers use the two nonces together with information
provided by the ACE Authorization Server that issued the Access
Token, in order to derive an OSCORE Security Context.
This procedure does not provide forward secrecy.
* The two peers can run the EDHOC key exchange protocol based on
Diffie-Hellman and defined in [RFC9528], in order to establish a
pseudo-random key in a mutually authenticated way.
Then, the two peers can use the established pseudo-random key to
derive external application keys. This allows the two peers to
securely derive an OSCORE Master Secret and an OSCORE Master Salt,
from which an OSCORE Security Context can be established.
This procedure additionally provides forward secrecy.
EDHOC also specifies an optional function, EDHOC_KeyUpdate, to
perform a key update in a more efficient way than re-running
EDHOC. The two communicating peers call EDHOC_KeyUpdate with
equivalent input, which results in derivation of a new shared
pseudo-random key. Usage of EDHOC_KeyUpdate preserves forward
secrecy.
Note that EDHOC may be run standalone or as part of other
workflows, such as when using the EDHOC and OSCORE profile of ACE
[I-D.ietf-ace-edhoc-oscore-profile].
* If one peer is acting as LwM2M Client and the other peer as LwM2M
Server, according to the OMA Lightweight Machine to Machine Core
specification [LwM2M], then the LwM2M Client peer may take the
initiative to bootstrap again with the LwM2M Bootstrap Server, and
receive again an OSCORE Security Context. Alternatively, the
LwM2M Server can instruct the LwM2M Client to initiate this
procedure.
If the OSCORE Security Context information on the LwM2M Bootstrap
Server has been updated, the LwM2M Client will thus receive a
fresh OSCORE Security Context to use with the LwM2M Server.
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In addition to that, the LwM2M Client, the LwM2M Server as well as
the LwM2M Bootstrap server are required to use the procedure
defined in Appendix B.2 of [RFC8613] and overviewed above, when
they use a certain OSCORE Security Context for the first time
[LwM2M-Transport].
Manually updating the OSCORE Security Context at the two peers should
be a last resort option, and it might often be not practical or
feasible.
Even when any of the alternatives mentioned above is available, it is
RECOMMENDED that two OSCORE peers update their Security Context by
using the KUDOS procedure as defined in Section 4 of this document.
3. Updated Protection of Responses with OSCORE
The protection of CoAP responses with OSCORE is updated, by adding
the following text at the end of Step 3 of Section 8.3 of [RFC8613].
| If the server is using a different Security Context for the
| response compared to what was used to verify the request (e.g.,
| due to an occurred key update), then the server MUST take the
| second alternative. That is, the server MUST include its Sender
| Sequence Number as Partial IV in the response and use it to build
| the AEAD nonce to protect the response.
|
| This prevents the server from using the same AEAD (key, nonce)
| pair for two responses, protected with different OSCORE Security
| Contexts.
|
| An exception is the procedure in Appendix B.2 of [RFC8613], which
| is secure although not complying with the above. The reason is
| that, in that procedure, the server uses the new OSCORE Security
| Context only and solely to protect the outgoing response (response
| #1), and no other message is protected with that OSCORE Security
| Context. Other procedures where that holds would also remain
| secure.
4. Key Update for OSCORE (KUDOS)
This section defines KUDOS, a lightweight procedure that two OSCORE
peers can use to update their keying material and establish a new
OSCORE Security Context. Hereafter, the document refers to two
specific peers that run KUDOS to update specifically one of their
OSCORE Security Contexts.
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KUDOS relies on the OSCORE Option defined in [RFC8613] and extended
as defined in Section 4.1, as well as on the support function
updateCtx() defined in Section 4.2.
In order to run KUDOS, two peers exchange OSCORE-protected CoAP
messages. The key update procedure is defined in Section 4.3, with
particular reference to the stateful FS mode providing forward
secrecy. The possible use of the stateless no-FS mode is defined in
Section 4.4, as intended to peers that are not able to write in non-
volatile memory. Two peers MUST run KUDOS in FS mode if they are
both capable to.
The key update procedure has the following properties.
* KUDOS can be initiated by either peer.
* The new OSCORE Security Context enjoys forward secrecy, unless
KUDOS is run in no-FS mode (see Section 4.4).
* The same ID Context value used in the old OSCORE Security Context
(if set) is preserved in the new OSCORE Security Context.
* The same Sender and Recipient IDs used in the old OSCORE Security
Context are preserved in the new OSCORE Security Context.
* KUDOS is robust against a peer rebooting and loss of state, and it
especially avoids the reuse of AEAD (nonce, key) pairs.
* KUDOS typically completes in one round trip by exchanging two
OSCORE-protected CoAP messages. The two peers achieve mutual key
confirmation in a following exchange, which is protected with the
newly established OSCORE Security Context.
4.1. Extensions to the OSCORE Option
This document extends the use of the OSCORE Option originally defined
in [RFC8613] as follows.
* This document defines the usage of the eight least significant
bit, called "Extension-1 Flag", in the first byte of the OSCORE
Option containing the OSCORE flag bits. The registration of this
flag bit in the "OSCORE Flag Bits" registry is specified in
Section 6.1.
When the Extension-1 Flag is set to 1, the second byte of the
OSCORE Option MUST include the OSCORE flag bits 8-15.
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* This document defines the usage of the least significant bit
"Nonce Flag", 'd', in the second byte of the OSCORE Option
containing the OSCORE flag bits 8-15. This flag bit is specified
in Section 6.1.
When it is set to 1, the compressed COSE object contains a field
'x' and a field 'nonce', to be used for the steps defined in
Section 4.3. In particular, the 1 byte 'x' following 'kid
context' (if any) encodes the size of the following field 'nonce',
together with signaling bits that indicate the specific behavior
to adopt during the KUDOS execution.
Hereafter, a message is referred to as a "KUDOS message", if and
only if the second byte of flags is present and the 'd' bit is set
to 1. If that is not the case, the message is referred to as a
"non KUDOS message".
The encoding of 'x' is as follows:
- The four least significant bits encode the 'nonce' size in
bytes minus 1, namely 'm'.
- The fifth least significant bit is the "No Forward Secrecy" 'p'
bit. The sender peer indicates its wish to run KUDOS in FS
mode or in no-FS mode, by setting the 'p' bit to 0 or 1,
respectively. This makes KUDOS possible to run also for peers
that cannot support the FS mode. At the same time, two peers
MUST run KUDOS in FS mode if they are both capable to, as per
Section 4.3. The execution of KUDOS in no-FS mode is defined
in Section 4.4.
- The sixth least significant bit is the "Preserve Observations"
'b' bit. The sender peer indicates its wish to preserve
ongoing observations beyond the KUDOS execution or not, by
setting the 'b' bit to 1 or 0, respectively. The related
processing is defined in Section 4.5.
- The seventh least significant bit is the 'z' bit. The meaning
of 'z' is as follows:
o z = 0: This is a "divergent" message, that is a KUDOS
message protected with a temporary OSCORE Security Context
and indicating that the sender peer is moving away from
"equilibrium". The sender peer is offering its own nonce in
the 'nonce' field of this message and is waiting to receive
the other peer's nonce.
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o z = 1: This is a "convergent" message, that is a KUDOS
message protected with the newly established OSCORE Security
Context and indicating that the sender peer is offering its
own nonce in the 'nonce' field of this message, has received
the other peer's nonce, and is going to wait for key
confirmation (to return to equilibrium).
- The eight least significant bit is reserved for future use.
This bit SHALL be set to zero when not in use. According to
this specification, if this bit is set to 1: i) if the message
is a request, it is considered to be malformed and
decompression fails as specified in item 2 of Section 8.2 of
[RFC8613]; ii) if the message is a response, it is considered
to be malformed and decompression fails as specified in item 2
of Section 8.4 of [RFC8613] and the client SHALL discard the
response as specified in item 8 of Section 8.4 of [RFC8613].
Figure 1 shows extended OSCORE Option value, with the presence of
'nonce'.
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 <----- n bytes ----->
+-+-+-+-+-+-+-+-+---+---+---+---+---+---+---+---+---------------------+
|1|0|0|h|k| n | 0 | 0 | 0 | 0 | 0 | 0 | 0 | d | Partial IV (if any) |
+-+-+-+-+-+-+-+-+---+---+---+---+---+---+---+---+---------------------+
<- 1 byte -> <----- s bytes ------> <- 1 byte -> <--- m + 1 bytes --->
+------------+----------------------+------------+--------------------+
| s (if any) | kid context (if any) | x (if any) | nonce (if any) |
+------------+----------------------+------------+--------------------+
/ \____
/ |
/ 0 1 2 3 4 5 6 7 |
| +-+-+-+-+-+-+-+-+ |
| |0|z|b|p| m | |
| +-+-+-+-+-+-+-+-+ |
Figure 1: The extended OSCORE Option value
4.2. Function for Security Context Update
The updateCtx() function shown in Figure 2 takes as input the three
parameters input1, input2, and CTX_IN. In particular, input1 and
input2 are built from the 'x' and 'nonce' fields transported in the
OSCORE Option value of the exchanged KUDOS messages (see
Section 4.1), while CTX_IN is the OSCORE Security Context to update.
The function returns a new OSCORE Security Context CTX_OUT.
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As a first step, the updateCtx() function builds the two CBOR byte
strings input1_cbor and input2_cbor, with value the input parameter
input1 and input2, respectively. Then, it builds X_N, as the byte
concatenation of input1_cbor and input2_cbor. In order for
updateCtx() to be agnostic of the order the nonce values were
exchanged, the input1_cbor and input2_cbor values are first sorted in
lexicographical order before they are concatenated. E.g., if
input1_cbor comes before input2_cbor in lexicographical ordering,
then X_N takes the value input1_cbor | input2_cbor and vice-versa.
After that, the updateCtx() function derives the new values of the
Master Secret and Master Salt for CTX_OUT. In particular, the new
Master Secret is derived through a KUDOS-Expand step, which takes as
input the Master Secret value from the Security Context CTX_IN, the
literal string "key update", X_N, and the length of the Master
Secret. Instead, the new Master Salt takes X_N as value.
The definition of KUDOS-Expand depends on the key derivation function
used for OSCORE by the two peers, as specified in CTX_IN. If the key
derivation function is an HKDF Algorithm (see Section 3.1 of
[RFC8613]), then KUDOS-Expand is mapped to HKDF-Expand [RFC5869], as
shown below. Also, the hash algorithm is the same one used by the
HKDF Algorithm specified in CTX_IN.
KUDOS-Expand(CTX_IN.MasterSecret, ExpandLabel, key_length) =
HKDF-Expand(CTX_IN.MasterSecret, ExpandLabel, key_length)
If a future specification updates [RFC8613] by admitting different
key derivation functions than HKDF Algorithms (e.g., KMAC as based on
the SHAKE128 or SHAKE256 hash functions), that specification has to
update also the present document in order to define the mapping
between such key derivation functions and KUDOS-Expand.
When an HKDF Algorithm is used, the derivation of new values follows
the same approach used in TLS 1.3, which is also based on HKDF-Expand
(see Section 7.1 of [RFC8446]) and used for computing new keying
material in case of key update (see Section 4.6.3 of [RFC8446]).
After that, the new Master Secret and Master Salt parameters are used
to derive a new Security Context CTX_OUT as per Section 3.2 of
[RFC8613]. Any other parameter required for the derivation takes the
same value as in the Security Context CTX_IN.
Note that the following holds for the newly derived CTX_OUT:
* In its Sender Context, the Sender Sequence Number is initialized
to 0 as per Section 3.2.2 of [RFC8613].
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* If the peer that has derived CTX_OUT supports CoAP Observe
[RFC7641], the Notification Number used for the replay protection
of Observe notifications (see Section 7.4.1 of [RFC8613]) is left
as not initialized.
Finally, the updateCtx() function returns the newly derived Security
Context CTX_OUT.
Since the updateCtx() function also takes X as input, the derivation
of CTX_OUT also considers as input the information from the 'x' field
transported in the OSCORE Option value of the exchanged KUDOS
messages. In turn, this ensures that, if successfully completed, a
KUDOS execution occurs as intended by the two peers.
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function updateCtx(input1, input2, CTX_IN):
// Output values
CTX_OUT // The new Security Context
MSECRET_NEW // The new Master Secret
MSALT_NEW // The new Master Salt
// Define the label for the key update
Label = "key update"
// Create CBOR byte strings from input1 and input2
input1_cbor = create_cbor_bstr(input1)
input2_cbor = create_cbor_bstr(input2)
// Concatenate the CBOR-encoded input1 and input2
// according to their lexicographic order
X_N = is_lexicographic_shorter(input1_cbor, input2_cbor) ?
(input1_cbor | input2_cbor) : (input2_cbor | input1_cbor)
// Determine the length in bytes of the current Master Secret
oscore_key_length = length(CTX_IN.MasterSecret)
// Create the new Master Secret using KUDOS-Expand-Label
MSECRET_NEW = KUDOS_Expand_Label(CTX_IN.MasterSecret, Label,
X_N, oscore_key_length)
// Set the new Master Salt to X_N
MSALT_NEW = X_N
// Derive the new Security Context CTX_OUT, using
// the new Master Secret, the new Master Salt,
// and other parameters from CTX_IN
CTX_OUT = derive_security_context(MSECRET_NEW, MSALT_NEW, CTX_IN)
// Return the new Security Context
return CTX_OUT
function KUDOS_Expand_Label(master_secret, Label, X_N, key_length):
struct {
uint16 length = key_length;
opaque label<7..255> = "oscore " + Label;
opaque context<0..255> = X_N;
} ExpandLabel;
return KUDOS_Expand(master_secret, ExpandLabel, key_length)
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Figure 2: Functions for deriving a new OSCORE Security Context
4.3. Key Update
This section defines KUDOS, a lightweight procedure that two OSCORE
peers can use to update their keying material and establish a new
OSCORE Security Context.
Using KUDOS as described in this section will achieve forward secrecy
for the new keying material produced by the execution of KUDOS, as
long as the original OSCORE keying material was also established with
forward secrecy. For peers unable to store information to persistent
memory, Section 4.4 provides an alternative approach to perform key
update without achieving forward secrecy. This alternative ensures
that also very constrained peers are able to use KUDOS, although
without achieving forward secrecy.
A peer can run KUDOS for active rekeying at any time, or for a
variety of more compelling reasons. These include the (approaching)
expiration of the OSCORE Security Context, approaching limits for the
key usage [I-D.ietf-core-oscore-key-limits], application policies,
and imminent exhaustion of the OSCORE Sender Sequence Number space.
The expiration time of an OSCORE Security Context and the key usage
limits are hard limits. Once reached them, a peer MUST stop using
the keying material in the OSCORE Security Context for exchanging
application data with the other peer, and has to perform a rekeying
before resuming secure communication.
Before starting KUDOS, the two peers share the OSCORE Security
Context CTX_OLD. In particular, CTX_OLD is the most recent OSCORE
Security Context that a peer has with the other peer, before
initiating the KUDOS procedure or upon having received and
successfully verified a divergent KUDOS message. During an execution
of KUDOS, a temporary OSCORE Security Context CTX_TEMP is derived.
Once successfully completed the KUDOS execution, the two peers agree
on a newly established OSCORE Security Context CTX_NEW that replaces
CTX_OLD. In particular, CTX_NEW is the most recent OSCORE Security
Context that a peer has, before sending a convergent KUDOS message or
upon having received and successfully verified a convergent KUDOS
message. CTX_OLD can be safely deleted upon receiving key
confirmation from the other peer, i.e., that the other peer also has
CTX_NEW.
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The following specifically defines how KUDOS is run in its stateful
FS mode achieving forward secrecy. That is, in the OSCORE Option
value of all the exchanged KUDOS messages, the "No Forward Secrecy"
'p' bit is set to 0.
In order to run KUDOS in FS mode, both peers have to be able to write
in non-volatile memory. From the newly derived Security Context
CTX_NEW, the peers MUST store to non-volatile memory the immutable
parts of the OSCORE Security Context as specified in Section 3.1 of
[RFC8613], with the possible exception of the Common IV, Sender Key,
and Recipient Key that can be derived again when needed, as specified
in Section 3.2.1 of [RFC8613]. If the peer is unable to write in
non-volatile memory, the two peers have to run KUDOS in its stateless
no-FS mode (see Section 4.4).
4.3.1. Nonces and X Bytes
When running KUDOS, each peer contributes by generating a nonce value
N1 or N2, and providing it to the other peer. The size of the nonces
N1 and N2 is application specific, and the use of 8 byte nonce values
is RECOMMENDED. The nonces N1 and N2 MUST be random values, with the
possible exception described later in Section 4.4.1. Note that a
good amount of randomness is important for the nonce generation.
[RFC4086] provides guidance on the generation of random values.
Furthermore, X1 and X2 are the value of the 'x' field specified in
the OSCORE Option of a KUDOS message. From one peer's point of view,
X1 and N1 are generated by that peer, while X2 and N2 are obtained
from the other peer.
In a KUDOS message, the sender peer sets the X1 value based on: the
length of N1 in bytes, the specific settings the peer wishes to run
KUDOS with, and in accordance with the message being divergent or
convergent. During the same KUDOS execution, all the KUDOS messages
sent by a peer MUST have the same value of the bit 'b' in the 'x'
field conveying X1.
N1, N2, X1, and X2 are used by the peers to build the 'input1' and
'input2' values provided to the updateCtx() function, in order to
derive an OSCORE Security Context. As for any newly derived OSCORE
Security Context, the Sender Sequence Number and the Replay Window
are re-initialized accordingly (see Section 3.2.2 of [RFC8613]).
Specifically, the input to updateCtx() is built as follows, where |
denotes byte concatenation:
* When deriving CTX_TEMP to protect a divergent outgoing message,
input1 is X1 | N1 and input2 is 0x.
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* When deriving CTX_TEMP to unprotect a divergent incoming message,
input1 is X2 | N2 and input2 is 0x.
* When deriving CTX_NEW to protect or unprotect a convergent
message, input1 is X1 | N1 and input2 is X2 | N2.
For any divergent KUDOS message, the sender peer's (X, nonce) are
included in the message. Also, both peers use those as input to
updateCtx() for deriving CTX_TEMP, which is used to protect or
unprotect that KUDOS message.
For any convergent KUDOS message, the sender peer's (X, nonce) are
included in the message. Both the sender peer's (X, nonce) and the
recipient peer's (X, nonce) are used as input to updateCtx() for
deriving CTX_NEW, which is used to protect or unprotect that KUDOS
message.
A pair (X, nonce) offered by a peer is bound to CTX_OLD, and is
reused as much as possible during the same KUDOS execution. A peer
generates its (X, nonce) pair before invoking updateCtx(), in case
the peer does not have one such pair already associated with the
CTX_OLD to use as input for updateCtx(). The peer associates the
newly generated pair with CTX_OLD before entering updateCtx().
4.3.2. KUDOS States
A peer performs a KUDOS execution according to the state machine
specified in Section 4.3.3, where the following states are
considered.
The peer can be in three possible states: IDLE, BUSY, and PENDING.
Normally, the peer is in the IDLE state, i.e., in "equilibrium". The
peer starts a KUDOS execution upon entering the BUSY state from a
state different than BUSY. The peer succesfully completes a KUDOS
execution by entering the IDLE state, at which point the peer has the
OSCORE Security Context CTX_NEW and has achieved key confirmation.
The sending of a KUDOS message is per the KUDOS state machine, and is
based on the perception that the sender peer has about what the other
peer has done.
The processing of a received KUDOS message is per the KUDOS state
machine, and is based on the local status of the recipient peer.
Moving to a state due to a received KUDOS message occurs only in case
of successful decryption and verification of the message with OSCORE.
In its local status, a peer tracks its current KUDOS state by means
of the bits (c_tx, c_rx) as follows:
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* (00) IDLE - The peer is not running KUDOS.
* BUSY - The peer is running KUDOS and:
- (01) has not offered a nonce, but has received the nonce from
the other peer; or
- (10) has offered a nonce, but has not received the nonce from
the other peer.
* (11) PENDING: the peer is running KUDOS, has offered its nonce,
has received the nonce from the other peer, and is waiting for key
confirmation.
While in the *BUSY* or the *PENDING* state, the peer MUST NOT send
non KUDOS messages.
4.3.3. KUDOS State Machine
From a peer's point of view, the change of KUDOS states works as
follows.
4.3.3.1. Startup
At startup, the peer enter a *Pre-IDLE* stage.
4.3.3.2. Pre-IDLE Stage
1. If the peer has any CTX_TEMP Security Contexts, it deletes them.
2. If the peer has both an old and a new OSCORE Security Context: a.
Delete the (X, nonce) pair associated with the old OSCORE
Security Context. b. Delete the old OSCORE Security Context, or
retain it only for processing late incoming messages as allowed
by retention policies (see Section 4.6).
3. Move to *IDLE*.
4.3.3.3. IDLE
* Upon receiving a divergent message while in *IDLE*
1. Move to *BUSY*.
* Upon sending a divergent message while in *IDLE*
1. Move to *BUSY*.
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* Upon receiving a convergent message while in *IDLE*
1. Ignore the message for the sake of KUDOS processing, but
process it as a CoAP message.
2. Stay in *IDLE*.
4.3.3.4. BUSY
* Upon entering *BUSY* due to receiving a divergent message
1. Send a convergent message.
2. Move to *PENDING*.
* Upon receiving a divergent message while in *BUSY*
1. Send a convergent message.
2. Move to *PENDING*.
* Upon receiving a convergent message while in *BUSY*
1. Achieve key confirmation.
2. Move to the *Pre-IDLE* stage.
* Upon sending a divergent message while in *BUSY*
- If, as in most cases, CTX_TEMP is usable to protect the
intended divergent message, send the message and then stay in
*BUSY*.
- Otherwise, perform the following steps. (E.g., this happens
upon eventually exhausting the Sender Sequence Number values of
CTX_TEMP)
1. Delete CTX_TEMP.
2. Delete the (X, nonce) pair associated with the Security
Context CTX_IN that was used to generate the CTX_TEMP
deleted at the previous step.
3. Generate a new (X, nonce) pair and associate it with
CTX_IN.
4. Generate a new CTX_TEMP from CTX_IN.
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5. Send the intended divergent message protected with the
CTX_TEMP generated at the previous step.
6. Stay in *BUSY*.
4.3.3.5. Pending
* Upon receiving a convergent message while in *PENDING*
1. Achieve key confirmation.
2. Move to the *Pre-IDLE* stage.
* Upon receiving a non KUDOS message protected with the latest
CTX_NEW while in *PENDING*
1. Achieve key confirmation.
2. Move to the *Pre-IDLE* stage.
* Upon needing to send a message (e.g., the application wants to
send a request) while in *PENDING*
1. Send the message as a convergent message.
2. Stay in *PENDING*.
* Upon receiving a divergent message while in *PENDING*
- In case of successful decryption and verification of the
divergent message using a CTX_TEMP derived from CTX_OLD:
1. Delete CTX_NEW.
2. Delete the pair (X, nonce) associated with the Security
Context CTX_IN used to generate the CTX_NEW deleted at the
previous step.
3. Abort the ongoing KUDOS execution.
4. Move to *BUSY*.
- In case of successful decryption and verification of the
divergent message using a CTX_TEMP derived from CTX_NEW:
1. Delete the oldest CTX_TEMP.
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2. Delete the Security Context that was used as CTX_IN to
generate the CTX_TEMP deleted at the previous step.
3. CTX_NEW becomes the oldest Security Context. From this
point on, that Security Context is what this KUDOS
execution refers to as CTX_OLD.
4. Abort the ongoing KUDOS execution.
5. Move to *BUSY*.
4.3.4. Handling of OSCORE Security Contexts
A peer completes the key update procedure when it has derived the new
OSCORE Security Context CTX_NEW and achieved key confirmation, and
thus has moved back to the IDLE state.
Once the peer has successfully derived CTX_NEW, the peer MUST use
CTX_NEW to protect outgoing non KUDOS messages, and MUST NOT use the
originally shared OSCORE Security Context CTX_OLD for protecting
outgoing messages.
The peer MUST terminate all the ongoing observations [RFC7641] that
it has with the other peer as protected with the old Security Context
CTX_OLD, unless the two peers have explicitly agreed otherwise as
defined in Section 4.5.
More specifically, if either or both peers indicate the wish to
cancel their observations, those will be all cancelled following a
successful KUDOS execution.
Note that, even though a peer had no real reason to update its OSCORE
keying material, running KUDOS can be intentionally exploited as a
more efficient way to terminate all the ongoing observations with the
other peer, compared to sending one cancellation request per
observation (see Section 3.6 of [RFC7641]).
4.3.5. KUDOS Messages as CoAP Requests or Responses
If a KUDOS message is a CoAP request, then it can target two
different types of resources at the recipient CoAP server:
* The well-known KUDOS resource at /.well-known/kudos, or an
alternative KUDOS resource with resource type "core.kudos" (see
Section 4.7.3 and Section 6.5). In such a case, no application
processing is expected at the CoAP server, and the plain CoAP
request composed before OSCORE protection should not include an
application payload.
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* A non-KUDOS resource, i.e., an actual application resource that a
CoAP request can target in order to trigger application processing
at the CoAP server. In such a case, the plain CoAP request
composed before OSCORE protection can include an application
payload, if admitted by the request method.
In either case, the link to the target resource can have the "osc"
target attribute to indicate that the resource is only accessible
using OSCORE (see Section 9 of [RFC8613]).
Similarly, any CoAP response can also be a KUDOS message. If the
corresponding CoAP request has targeted a KUDOS resource, then the
plain CoAP response composed before OSCORE encryption SHOULD NOT
include an application payload. Otherwise, an application payload
MAY be included.
4.3.6. Avoiding In-Transit Requests During a Key Update
Before sending a KUDOS message, the peer MUST ensure that it has no
outstanding interactions with the other peer (see Section 4.7 of
[RFC7252]), with the exception of ongoing observations [RFC7641].
If any such outstanding interactions are found, the peer MUST NOT
initiate or continue the KUDOS execution, before either: i) having
all those outstanding interactions cleared; or ii) freeing up the
Token values used with those outstanding interactions, with the
exception of ongoing observations with the other peer.
Later on, this prevents a non KUDOS response protected with the new
Security Context CTX_NEW from cryptographically matching with both
the corresponding request also protected with CTX_NEW and with an
older request protected with CTX_OLD, in case the two requests were
protected using the same OSCORE Partial IV.
During an ongoing KUDOS execution, a peer MUST NOT send a non KUDOS
message to the other peer, until having aborted or successfully
completed the key update procedure on its side. This could otherwise
be possible, if the client is using a value of NSTART greater than 1
(see Section 4.7 of [RFC7252]).
4.4. Key Update Admitting no Forward Secrecy
The FS mode of the KUDOS procedure defined in Section 4.3 ensures
forward secrecy of the OSCORE keying material. However, it requires
peers executing KUDOS to preserve their state (e.g., across a device
reboot), by writing information such as data from the newly derived
OSCORE Security Context CTX_NEW in non-volatile memory.
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This can be problematic for devices that cannot dynamically write
information to non-volatile memory. For example, some devices may
support only a single writing in persistent memory when initial
keying material is provided (e.g., at manufacturing or commissioning
time), but no further writing after that. Therefore, these devices
cannot perform a stateful key update procedure, and thus are not
capable to run KUDOS in FS mode to achieve forward secrecy.
In order to address these limitations, KUDOS can be run in its
stateless no-FS mode, as defined in the following. This allows two
peers to achieve the same results as when running KUDOS in FS mode
(see Section 4.3), with the difference that forward secrecy is not
achieved and no state information is required to be dynamically
written in non-volatile memory.
From a practical point of view, the two modes differ as to what exact
OSCORE Master Secret and Master Salt are used as part of the OSCORE
Security Context CTX_IN that is provided as input to the updateCtx()
function (see Section 4.2).
If either or both peers are not able to write in non-volatile memory,
then the two peers have to run KUDOS in no-FS mode.
4.4.1. Handling and Use of Keying Material
In the following, a device is denoted as "CAPABLE" if it is able to
store information in non-volatile memory (e.g., on disk), beyond a
one-time-only writing occurring at manufacturing or
(re-)commissioning time. If that is not the case, the device is
denoted as "non-CAPABLE".
The following terms are used to refer to OSCORE keying material.
* Bootstrap Master Secret and Bootstrap Master Salt. If pre-
provisioned during manufacturing or (re-)commissioning, these
OSCORE Master Secret and Master Salt are initially stored on disk
and are never going to be overwritten by the device.
* Latest Master Secret and Latest Master Salt. These OSCORE Master
Secret and Master Salt can be dynamically updated by the device.
In case of reboot, they are lost unless they have been stored on
disk.
Note that:
* A peer running KUDOS can have none of the pairs above associated
with another peer, only one, or both.
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* A peer that has neither of the pairs above associated with another
peer, cannot run KUDOS in any mode with that other peer.
* A peer that has only one of the pairs above associated with
another peer can attempt to run KUDOS with that other peer, but
the procedure might fail depending on the other peer's
capabilities. In particular:
- In order to run KUDOS in FS mode, a peer must be a CAPABLE
device. It follows that two peers have to both be CAPABLE
devices in order to be able to run KUDOS in FS mode with one
another.
- In order to run KUDOS in no-FS mode, a peer must have Bootstrap
Master Secret and Bootstrap Master Salt available as stored on
disk.
* A peer that is a non-CAPABLE device MUST support the no-FS mode.
Note that an exception described in Section 4.4.3 exists for non-
CAPABLE devices that lack Bootstrap Master Secret and Bootstrap
Master Salt.
* A peer that is a CAPABLE device MUST support the FS mode and the
no-FS mode.
* As an exception to the nonces being generated as random values
(see Section 4.3.1), a peer that is a CAPABLE device MAY use a
value obtained from a monotonically incremented counter as nonce.
This has privacy implications, which are described in Section 5.
In such a case, the peer MUST enforce measures to ensure freshness
of the nonce values. For example, the peer can use the same
procedure described in Appendix B.1.1 of [RFC8613] for handling
the OSCORE Sender Sequence Number values. These measures require
to regularly store the used counter values in non-volatile memory,
which makes non-CAPABLE devices unable to safely use counter
values as nonce values.
As a general rule, once successfully generated a new OSCORE Security
Context CTX (e.g., CTX is the CTX_NEW resulting from a KUDOS
execution, or it has been established through the EDHOC protocol
[RFC9528]), a peer considers the Master Secret and Master Salt of CTX
as Latest Master Secret and Latest Master Salt. After that:
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* If the peer is a CAPABLE device, it MUST store Latest Master
Secret and Latest Master Salt on disk, with the exception of
possible temporary OSCORE Security Contexts used during a key
update procedure, such as CTX_TEMP used during the KUDOS
execution. That is, the OSCORE Master Secret and Master Salt from
such temporary Security Contexts are not stored on disk.
* The peer MUST store Latest Master Secret and Latest Master Salt in
volatile memory, thus making them available to OSCORE message
processing and possible key update procedures.
Following a state loss (e.g., due to a reboot), a device MUST
complete a successful KUDOS execution before performing an exchange
of OSCORE-protected application data with another peer, unless:
* The device is CAPABLE and implements a functionality for safely
reusing old keying material, such as that described in
Appendix B.1 of [RFC8613]; or
* The device is exchanging OSCORE-protected data within a KUDOS
messages, as described in Section 4.3.5. In such case, the plain
CoAP message composed before OSCORE protection of the KUDOS
message can include an application payload, if admitted.
4.4.2. Selection of KUDOS Mode
The following refers to CTX_BOOTSTRAP as to the OSCORE Security
Context where the OSCORE Master Secret is the Bootstrap Master Secret
and the Master Salt is the Bootstrap Master Salt Section 4.4.1.
During a KUDOS execution, the two peers agree on whether to perform
the key update procedure in FS mode or no-FS mode, by leveraging the
"No Forward Secrecy" bit, 'p', in the 'x' byte of the OSCORE Option
value of the KUDOS messages (see Section 4.1). The 'p' bit
practically determines what OSCORE Security Context CTX_IN to use as
input to updateCtx() during the KUDOS execution, consistently with
the indicated mode.
* If the 'p' bit is set to 0 (FS mode), the updateCtx() function
used to derive CTX_TEMP or CTX_NEW considers CTX_IN to be CTX_OLD,
i.e., the current OSCORE Security Context shared with the other
peer as is. In particular, CTX_OLD includes Latest Master Secret
as OSCORE Master Secret and Latest Master Salt as OSCORE Master
Salt.
* If the 'p' bit is set to 1 (no-FS mode), the updateCtx() function
used to derive CTX_TEMP or CTX_NEW considers CTX_IN to be
CTX_BOOTSTRAP. Thus, every execution of KUDOS in no-FS mode
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between these two peers considers the same CTX_BOOTSTRAP, i.e.,
the same CTX_IN, as input to the updateCtx() function, hence the
impossibility to achieve forward secrecy.
Note that, in the state machine described in section {#ssec-state-
machine}, updateCtx() will take CTX_BOOTSTRAP as input when
creating every OSCORE Security Context for protecting or
unprotecting a KUDOS message where the 'p' bit is set to 1.
If at least one KUDOS message in a successful KUDOS execution had
the 'p' bit set to 1, that KUDOS execution is considered to have
been run in the no-FS mode.
When a peer moves to the *Pre-IDLE* stage after having succesfully
completed a KUDOS execution in the no-FS mode, then the peer MUST
additionally perform the following as first step of the *Pre-IDLE*
stage:
1. Delete CTX_BOOTSTRAP
A peer determines to run KUDOS either in FS or no-FS mode with
another peer as follows.
* If a peer A is a non-CAPABLE device, it MUST run KUDOS only in no-
FS mode. That is, when sending a KUDOS message, it MUST set to 1
the 'p' bit of the 'x' byte in the OSCORE Option value. Note
that, if peer A lacks a Bootstrap Master Secret and Bootstrap
Master Salt to use with the other peer B, it can still run KUDOS
in FS mode according to what is defined in Section 4.4.3.
* If a peer A is a CAPABLE device, it SHOULD run KUDOS only in FS
mode. That is, when sending a KUDOS message, it SHOULD set to 0
the 'p' bit of the 'x' byte in the OSCORE Option value. An
exception applies in the following cases.
- The peer A is running KUDOS with another peer B, which A has
learned to be a non-CAPABLE device (and hence not able to run
KUDOS in FS mode).
Note that, if the peer A is a CAPABLE device, it is able to
store such information about the other peer B on disk and it
MUST do so. From then on, the peer A will perform every
execution of KUDOS with the peer B in no-FS mode, including
after a possible reboot.
- The peer A is running KUDOS with another peer B without knowing
its capabilities, and A receives a KUDOS message where the 'p'
bit of the 'x' byte in the OSCORE Option value is set to 1.
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* If a peer A is a CAPABLE device and has learned that another peer
B is also a CAPABLE device (and hence able to run KUDOS in FS
mode), then the peer A MUST NOT run KUDOS with the peer B in no-FS
mode. If the peer A receives a KUDOS message from the peer B
where the 'p' bit of the 'x' byte in the OSCORE Option value is
set to 1, then the peer A MUST ignore the message for the sake of
KUDOS processing, but process it as a CoAP message.
Note that, if the peer A is a CAPABLE device, it is able to
remember that the peer B is also a CAPABLE device and thus to
store such information on disk, which it MUST do. This ensures
that the peer A will perform every execution of KUDOS with the
peer B in FS mode. In turn, this prevents a possible downgrading
attack, aimed at making A believe that B is a non-CAPABLE device,
and thus to run KUDOS in no-FS mode although the FS mode can
actually be used by both peers.
4.4.3. Non-CAPABLE Devices Operating in FS Mode
Devices may not be pre-provisioned with Bootstrap material, for
instance due to storage limitations of persistent memory or to fulfil
particular use cases. Bootstrap material means specifically the
Bootstrap Master Secret and Bootstrap Master Salt, and Latest
material means the Latest Master Secret and Latest Master Salt as
defined in Section 4.4.1.
Normally, a non-CAPABLE device always uses KUDOS in no-FS mode. An
exception is possible, if the Bootstrap material is dynamically
installed at that device through an in-band process between that
device and the peer device. In such a case, it is possible for that
device to run KUDOS in FS mode with the peer device.
Note that, under the assumption that peer A does not have any
Bootstrap material with another peer B, peer A cannot use the no-FS
mode with peer B, even though peer A is a non-CAPABLE device. Thus,
allowing peer A to use KUDOS in FS mode ensures that peer A can
perform a key update using KUDOS at all.
The following describes how a non-CAPABLE device in the situation
outlined above, namely peer A, runs KUDOS in FS mode with another
peer B:
* Peer A is not provisioned with Bootstrap material associated with
peer B at the time of manufacturing or commissioning.
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* Peer A establishes OSCORE keying material associated with peer B
through an in-band procedure run with peer B. Then, peer A
considers that keying material as the Latest material with peer B,
and stores it only in volatile memory.
An example of such an in-band procedure is the EDHOC and OSCORE
profile of ACE [I-D.ietf-ace-edhoc-oscore-profile], according to
which the two peers run the EDHOC protocol [RFC9528] for
establishing an OSCORE Security Context to associate with access
rights. This in-band procedure may occur multiple times over the
device's lifetime.
* Peer A runs KUDOS in FS mode with peer B, thereby achieving
forward secrecy for subsequent key update epochs, as long as the
OSCORE keying material was originally established with forward
secrecy. Peer A stores each newly derived Security Context in
volatile memory.
As long as peer A does not reboot, executions of KUDOS rely on the
Latest material stored in volatile memory. If peer A reboots, no
OSCORE keying material associated with the peer B will be retained,
as peer A is non-CAPABLE and therefore stores it only in volatile
memory. Consequently, peer A must first establish new OSCORE keying
material to use as Latest material with peer B, before running KUDOS
again with peer B. This can be accomplished by running again the in-
band procedure mentioned above.
4.5. Preserving Observations Across Key Updates
As defined in Section 4.3, once a peer has completed the KUDOS
execution and successfully derived the new OSCORE Security Context
CTX_NEW, that peer normally terminates all the ongoing observations
it has with the other peer [RFC7641], as protected with the old
OSCORE Security Context CTX_OLD.
This section describes a method that the two peers can use to safely
preserve the ongoing observations that they have with one another,
beyond the completion of a KUDOS execution. In particular, this
method ensures that an Observe notification can never successfully
cryptographically match against the Observe requests of two different
observations, e.g., against an Observe request protected with CTX_OLD
and an Observe request protected with CTX_NEW.
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The actual preservation of ongoing observations has to be agreed by
the two peers at each execution of KUDOS that they run with one
another, as defined in Section 4.5.1. If, at the end of a KUDOS
execution, the two peers have not agreed on that, they MUST terminate
the ongoing observations that they have with one another, just as
defined in Section 4.3.4.
4.5.1. Management of Observations
As per Section 3.1 of [RFC7641], a client can register its interest
in observing a resource at a server, by sending a registration
request including the Observe Option with value 0.
If the server registers the observation as ongoing, the server sends
back a successful response also including the Observe Option, hence
confirming that an entry has been successfully added for that client.
If the client receives back the successful response above from the
server, then the client also registers the observation as ongoing.
In case the client can ever consider to preserve ongoing observations
beyond a key update as defined below, then the client MUST NOT simply
forget about an ongoing observation if not interested in it anymore.
Instead, the client MUST send an explicit cancellation request to the
server, i.e., a request including the Observe Option with value 1
(see Section 3.6 of [RFC7641]). After sending this cancellation
request, if the client does not receive back a response confirming
that the observation has been terminated, the client MUST NOT
consider the observation terminated. The client MAY try again to
terminate the observation by sending a new cancellation request.
In case a peer A performs a KUDOS execution with another peer B, and
A has ongoing observations with B that it is interested to preserve
beyond the key update, then A can explicitly indicate its interest to
do so. To this end, the peer A sets to 1 the bit "Preserve
Observations", 'b', in the 'x' byte of the OSCORE Option value (see
Section 4.1), in the KUDOS message it sends to the other peer B.
If a peer receives a KUDOS message with the bit 'b' set to 0, then
the peer MUST set to 0 the bit 'b' in the KUDOS message it sends as
follow-up, regardless of its wish to preserve ongoing observations
with the other peer.
If a peer has sent a KUDOS message with the bit 'b' set to 0, the
peer MUST ignore the bit 'b' in the follow-up KUDOS message that it
receives from the other peer.
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Note that during the same KUDOS execution, all the KUDOS messages
sent by a peer MUST have the same value in the bit 'b' for preserving
ongoing observations.
After successfully completing the KUDOS execution (i.e., after having
successfully derived the new OSCORE Security Context CTX_NEW), both
peers have expressed their interest in preserving their common
ongoing observations if and only if the bit 'b' was set to 1 in all
the exchanged KUDOS messages. In such a case, each peer X performs
the following actions.
1. The peer X considers all the still ongoing observations that it
has with the other peer, such that X acts as client in those
observations. If there are no such observations, the peer X
takes no further actions. Otherwise, it moves to step 2.
2. The peer X considers all the OSCORE Partial IV values used in the
Observe registration request associated with any of the still
ongoing observations determined at step 1.
3. The peer X determines the value PIV* as the highest OSCORE
Partial IV value among those considered at step 2.
4. In the Sender Context of the OSCORE Security Context shared with
the other peer, the peer X sets its own Sender Sequence Number to
(PIV* + 1), rather than to 0.
As a result, each peer X will "jump" beyond the OSCORE Partial IV
(PIV) values that are occupied and in use for ongoing observations
with the other peer where X acts as client.
Note that, each time it runs KUDOS, a peer must determine if it
wishes to preserve ongoing observations with the other peer or not,
before sending a KUDOS message.
To this end, the peer should also assess the new value that PIV*
would take after a successful completion of KUDOS, in case ongoing
observations with the other peer are going to be preserved. If the
peer considers such a new value of PIV* to be too close to or equal
to the maximum possible value admitted for the OSCORE Partial IV,
then the peer may choose to run KUDOS with no intention to preserve
its ongoing observations with the other peer, in order to "start
over" from a fresh, entirely unused PIV space.
Application policies can further influence whether attempting to
preserve observations beyond a key update is appropriate or not.
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4.6. Retention Policies
Applications MAY define policies that allow a peer to temporarily
keep the old Security Context CTX_OLD beyond having established the
new Security Context CTX_NEW and having achieved key confirmation,
rather than simply deleting CTX_OLD. This allows the peer to decrypt
late, still on-the-fly incoming non KUDOS messages protected with
CTX_OLD.
When enforcing such policies, the following applies.
* Outgoing non KUDOS messages MUST be protected by using only
CTX_NEW.
* Incoming non KUDOS messages MUST first be attempted to decrypt by
using CTX_NEW. If decryption fails, a second attempt can use
CTX_OLD.
* KUDOS messages MUST NOT be protected or unprotected by using
CTX_OLD.
* When an amount of time defined by the policy has elapsed since the
establishment of CTX_NEW, the peer deletes CTX_OLD.
A peer MUST NOT retain CTX_OLD beyond the establishment of CTX_NEW
and the achievement of key confirmation, if any of the following
conditions holds:
* CTX_OLD is expired.
* Limits set for safe key usage have been reached
[I-D.ietf-core-oscore-key-limits], for the Recipient Key of the
Recipient Context of CTX_OLD.
4.7. Discussion
KUDOS is intended to deprecate and replace the procedure defined in
Appendix B.2 of [RFC8613], as fundamentally achieving the same goal,
while displaying a number of improvements and advantages.
In particular, it is especially convenient for the handling of
failure events concerning the JRC node in 6TiSCH networks (see
Section 2). That is, among its intrinsic advantages compared to the
procedure defined in Appendix B.2 of [RFC8613], KUDOS preserves the
same ID Context value, when establishing a new OSCORE Security
Context.
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Since the JRC uses ID Context values as identifiers of network nodes,
namely "pledge identifiers", the above implies that the JRC does not
have to perform anymore a mapping between a new, different ID Context
value and a certain pledge identifier (see Section 8.3.3 of
[RFC9031]). It follows that pledge identifiers can remain constant
once assigned, and thus ID Context values used as pledge identifiers
can be employed in the long-term as originally intended.
4.7.1. Communication Overhead
Each KUDOS messages results in communication overhead. This is
determined by the following, additional information conveyed in the
OSCORE Option (see Section 4.1).
* The second byte of the OSCORE Option value.
* The byte 'x' of the OSCORE Option value.
* The nonce conveyed in the 'nonce' field of the OSCORE Option. Its
size ranges from 1 to 16 bytes as indicated in the 'x' byte, and
is typically of 8 bytes.
* The 'Partial IV' parameter of the OSCORE Option value, in a CoAP
response message that is a KUDOS message.
This takes into account the fact that OSCORE-protected CoAP
response messages normally do not include the 'Partial IV'
parameter, but they have to when they are KUDOS messages (see
Section 3).
* The first byte of the OSCORE Option value (i.e., the first OSCORE
flag byte), in a CoAP response message that is a KUDOS message.
This takes into account the fact that OSCORE-protected CoAP
response messages normally convey an OSCORE Option that only
consists of the all zero (0x00) flag byte. In turn, this results
in the OSCORE Option being encoded as with empty value (see
Section 2 of [RFC8613].
* The possible presence of the 1-byte Option Length (extended) field
in the OSCORE Option (see Section 3.1 of [RFC7252]). This is the
case where the length of the OSCORE Option value is between 13 and
255 bytes (see Section 2 of [RFC8613]).
The results shown in figure {table-overhead-forward} are the minimum,
typical, and maximum communication overhead in bytes introduced by
KUDOS, when considering a nonce with size 1, 8, and 16 bytes. All
the indicated values are in bytes.
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The overhead is calculated based on a scenario of a CoAP request
serving as the divergent message, followed by a CoAP response serving
as the convergent message.
In particular, the results build on the following assumptions.
* Both messages of the same KUDOS execution use nonces of the same
size, and do not include the 'kid context' parameter in the OSCORE
Option value.
* When included in the OSCORE Option value, the 'Partial IV'
parameter has size 1 byte.
* CoAP request messages include the 'kid' parameter with size 1 byte
in the OSCORE Option value.
* CoAP response messages do not include the 'kid' parameter in the
OSCORE Option value.
+============+=======================+===============+=======+
| Nonce size | Request KUDOS message | Response | Total |
| | | KUDOS message | |
+============+=======================+===============+=======+
| 1 | 3 | 5 | 8 |
+------------+-----------------------+---------------+-------+
| 8 | 11 | 12 | 23 |
+------------+-----------------------+---------------+-------+
| 16 | 19 | 21 | 40 |
+------------+-----------------------+---------------+-------+
Table 1: Communication overhead (bytes)
4.7.2. Resource Type core.kudos
The "core.kudos" resource type registered in Section 6.5 is defined
to ensure a means for clients to send KUDOS requests without
incurring any side effects. Specifically, a resource of this type
does not pertain to any real application, which ensures that no
application-level actions are triggered as a result of the KUDOS
request. This allows clients to issue KUDOS requests when they do
not include any actionable application payload in the plain CoAP
request composed before OSCORE protection, or when no application-
layer processing is intended to occur on the server.
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4.7.3. Well-Known KUDOS Resource
According to this specification, KUDOS is transferred in POST
requests and 2.04 (Changed) responses. If a client wishes to execute
the KUDOS procedure without triggering any application processing on
the server, then a request sent as a KUDOS message can target a KUDOS
resource with resource type "core.kudos" (see Section 4.7.2), e.g.,
at the Uri-Path "/.well-known/kudos" (see Section 6.4). An
alternative KUDOS resource can be discovered, e.g., by using a
resource directory [RFC9176], by using the resource type "core.kudos"
as filter criterion.
4.7.4. Rekeying when Using SCHC with OSCORE
In the interest of rekeying, the following points must be taken into
account when using the Static Context Header Compression and
fragmentation (SCHC) framework [RFC8724] for compressing CoAP
messages protected with OSCORE, as defined in [RFC8824].
Compression of the OSCORE Partial IV has implications for the
frequency of rekeying. That is, if the Partial IV is compressed, the
communicating peers must perform rekeying more often, as the
available Partial IV space becomes smaller due to the compression.
For instance, if only 3 bits of the Partial IV are sent, then the
maximum PIV before having to rekey is only 2^3 - 1 = 7.
Furthermore, any time the SCHC context Rules are updated on an OSCORE
endpoint, that endpoint must perform a rekeying (see Section 9 of
[RFC8824]).
That is, the use of SCHC plays a role in triggering KUDOS executions
and in affecting their cadence. Hence, the used SCHC Rules and their
update policies should ensure that the KUDOS executions occurring as
their side effect do not significantly impair the gain from message
compression.
4.8. Signaling KUDOS support in EDHOC
The EDHOC protocol defines the transport of additional External
Authorization Data (EAD) within an optional EAD field of the EDHOC
messages (see Section 3.8 of [RFC9528]). An EAD field is composed of
one or multiple EAD items, each of which specifies an identifying
'ead_label' encoded as a CBOR integer, and an optional 'ead_value'
encoded as a CBOR bstr.
This document defines a new EDHOC EAD item KUDOS_EAD and registers
its 'ead_label' in Section 6.3. By including this EAD item in an
outgoing EDHOC message, a sender peer can indicate whether it
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supports KUDOS and in which modes, as well as query the other peer
about its support. Note that peers do not have to use this EDHOC EAD
item to be able to run KUDOS with each other, irrespective of the
modes they support. A KUDOS peer MUST only use the EDHOC EAD item
KUDOS_EAD as non-critical. That is, when included in an EDHOC
message, its 'ead_label' MUST be used with positive sign. The
possible values of the 'ead_value' are as follows:
+======+==========+=============================================+
| Name | Value | Description |
+======+==========+=============================================+
| ASK | h'' | Used only in EDHOC message_1. It asks the |
| | (0x40) | recipient peer to specify in EDHOC |
| | | message_2 whether it supports KUDOS. |
+------+----------+---------------------------------------------+
| NONE | h'00' | Used only in EDHOC message_2 and message_3. |
| | (0x4100) | It specifies that the sender peer does not |
| | | support KUDOS. |
+------+----------+---------------------------------------------+
| FULL | h'01' | Used only in EDHOC message_2 and message_3. |
| | (0x4101) | It specifies that the sender peer supports |
| | | KUDOS in FS mode and no-FS mode. |
+------+----------+---------------------------------------------+
| PART | h'02' | Used only in EDHOC message_2 and message_3. |
| | (0x4102) | It specifies that the sender peer supports |
| | | KUDOS in no-FS mode only. |
+------+----------+---------------------------------------------+
Table 2: Values for the EDHOC EAD item KUDOS_EAD
When the KUDOS_EAD item is included in EDHOC message_1 with
'ead_value' ASK, a recipient peer that supports the KUDOS_EAD item
MUST specify whether it supports KUDOS in EDHOC message_2.
When the KUDOS_EAD item is not included in EDHOC message_1 with
'ead_value' ASK, a recipient peer that supports the KUDOS_EAD item
MAY still specify whether it supports KUDOS in EDHOC message_2.
When the KUDOS_EAD item is included in EDHOC message_2 with
'ead_value' FULL or PART, a recipient peer that supports the
KUDOS_EAD item SHOULD specify whether it supports KUDOS in EDHOC
message_3. An exception applies in case, based on application
policies or other context information, the recipient peer that
receives EDHOC message_2 already knows that the sender peer is
supposed to have such knowledge.
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When the KUDOS_EAD item is included in EDHOC message_2 with
'ead_value' NONE, a recipient peer that supports the KUDOS_EAD item
MUST NOT specify whether it supports KUDOS in EDHOC message_3.
In the following cases, the recipient peer silently ignores the
KUDOS_EAD item specified in the received EDHOC message, and does not
include a KUDOS_EAD item in the next EDHOC message it sends (if any).
* The recipient peer does not support the KUDOS_EAD item.
* The KUDOS_EAD item is included in EDHOC message_1 with 'ead_value'
different than ASK
* The KUDOS_EAD item is included in EDHOC message_2 or message_3
with 'ead_value' ASK.
* The KUDOS_EAD item is included in EDHOC message_4.
That is, by specifying 'ead_value' ASK in EDHOC message_1, a peer A
can indicate to the other peer B that it wishes to know if B supports
KUDOS and in what mode(s). In the following EDHOC message_2, B
indicates whether it supports KUDOS and in what mode(s), by
specifying either NONE, FULL, or PART as 'ead_value'. Specifying the
'ead_value' FULL or PART in EDHOC message_2 also asks A to indicate
whether it supports KUDOS in EDHOC message_3.
To further illustrate the functionality, two examples are presented
below as EDHOC executions where only the new KUDOS_EAD item is shown
when present, and assuming that no other EAD items are used by the
two peers.
EDHOC EDHOC
Initiator Responder
| |
| EAD_1: (TBD_LABEL, ASK) |
+-------------------------------------------------------->|
| message_1 |
| |
| EAD_2: (TBD_LABEL, FULL) |
|<--------------------------------------------------------+
| message_2 |
| |
| EAD_3: (TBD_LABEL, FULL) |
+-------------------------------------------------------->|
| message_3 |
| |
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In the example above, the Initiator asks the EDHOC Responder about
its support for KUDOS ('ead_value' = ASK). In EDHOC message_2, the
Responder indicates that it supports both the FS and no-FS mode of
KUDOS ('ead_value' = FULL). Finally, in EDHOC message_3, the
Initiator indicates that it also supports both the FS and no-FS mode
of KUDOS ('ead_value' = FULL). After the EDHOC execution has
successfully finished, both peers are aware that they both support
KUDOS, in the FS and no-FS modes.
EDHOC EDHOC
Initiator Responder
| |
| EAD_1: (TBD_LABEL, ASK) |
+-------------------------------------------------------->|
| message_1 |
| |
| EAD_2: (TBD_LABEL, NONE) |
|<--------------------------------------------------------+
| message_2 |
| |
+-------------------------------------------------------->|
| message_3 |
| |
In this second example, the Initiator asks the EDHOC Responder about
its support for KUDOS ('ead_value' = ASK). In EDHOC message_2, the
Responder indicates that it does not support KUDOS at all
('ead_value' = NONE). Finally, in EDHOC message_3, the Initiator
does not include the KUDOS_EAD item, since it already knows that
using KUDOS with the other peer will not be possible. After the
EDHOC execution has successfully finished, the Initiator is aware
that the Responder does not support KUDOS, which the two peers are
not going to use with each other.
5. Security Considerations
Depending on the specific key update procedure used to establish a
new OSCORE Security Context, the related security considerations also
apply.
As mentioned in Section 4.3.1, it is RECOMMENDED that the size for
nonces N1 and N2 is 8 bytes. The application needs to set the size
of each nonce such that the probability of its value being repeated
is negligible. Note that the probability of collision of nonce
values is heightened by the birthday paradox. However, considering a
nonce size of 8 bytes there will be a collision on average after
approximately 2^32 instances of Response #1 messages.
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Overall, the size of the nonces N1 and N2 should be set such that the
security level is harmonized with other components of the deployment.
Considering the constraints of embedded implementations, there might
be a need for allowing N1 and N2 values that are smaller in size.
This is acceptable, provided that safety, reliability, and robustness
within the system can still be assured. Although using nonces that
are smaller in size means that there will be a collision on average
after fewer KUDOS messages have been sent, this should not pose
significant problems even for a constrained server operating at a
capacity of one request per second.
The nonces exchanged in the KUDOS messages are sent in the clear, so
using random nonces is preferable for maintaining privacy. If
instead a counter value is used, this can leak some information about
the peers. Specifically, using counters will reveal the frequency of
rekeying procedures performed.
As discussed in [Symmetric-Security], key update methods built on
symmetric key exchange have weaker security properties compared to
methods built on ephemeral Diffie-Hellman key exchange. In fact,
while the two approaches can co-exist, rekeying with symmetric key
exchange is not intended as a substitute for ephemeral Diffie-Hellman
key exchange. Peers should periodically perform a key update based
on ephemeral Diffie-Hellman key exchange (e.g., by running the EDHOC
protocol [RFC9528]). The cadence of such periodic key updates should
be determined based on how much the two peers and their network
environment are constrained, as well as on the maximum amount of time
and of exchanged data that are acceptable between two consecutive key
updates.
6. 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.
6.1. OSCORE Flag Bits Registry
IANA is asked to add the following entries to the "OSCORE Flag Bits"
registry within the "Constrained RESTful Environments (CoRE)
Parameters" registry group.
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+==========+=============+============================+============+
| Bit | Name | Description | Reference |
| Position | | | |
+==========+=============+============================+============+
| 0 | Extension-1 | Set to 1 if the OSCORE | [RFC-XXXX] |
| | Flag | Option specifies a second | |
| | | byte, which includes the | |
| | | OSCORE flag bits 8-15 | |
+----------+-------------+----------------------------+------------+
| 8 | Extension-2 | Set to 1 if the OSCORE | [RFC-XXXX] |
| | Flag | Option specifies a third | |
| | | byte, which includes the | |
| | | OSCORE flag bits 16-23 | |
+----------+-------------+----------------------------+------------+
| 15 | Nonce Flag | Set to 1 if nonce is | [RFC-XXXX] |
| | | present in the compressed | |
| | | COSE object | |
+----------+-------------+----------------------------+------------+
| 16 | Extension-3 | Set to 1 if the OSCORE | [RFC-XXXX] |
| | Flag | Option specifies a fourth | |
| | | byte, which includes the | |
| | | OSCORE flag bits 24-31 | |
+----------+-------------+----------------------------+------------+
| 24 | Extension-4 | Set to 1 if the OSCORE | [RFC-XXXX] |
| | Flag | Option specifies a fifth | |
| | | byte, which includes the | |
| | | OSCORE flag bits 32-39 | |
+----------+-------------+----------------------------+------------+
| 32 | Extension-5 | Set to 1 if the OSCORE | [RFC-XXXX] |
| | Flag | Option specifies a sixth | |
| | | byte, which includes the | |
| | | OSCORE flag bits 40-47 | |
+----------+-------------+----------------------------+------------+
| 40 | Extension-6 | Set to 1 if the OSCORE | [RFC-XXXX] |
| | Flag | Option specifies a seventh | |
| | | byte, which includes the | |
| | | OSCORE flag bits 48-55 | |
+----------+-------------+----------------------------+------------+
| 48 | Extension-7 | Set to 1 if the OSCORE | [RFC-XXXX] |
| | Flag | Option specifies an eighth | |
| | | byte, which includes the | |
| | | OSCORE flag bits 56-63 | |
+----------+-------------+----------------------------+------------+
Table 3: Registrations in the OSCORE Flag Bits Registry
In the same registry, IANA is asked to mark as 'Unassigned' the entry
with Bit Position of 1, i.e., to update the entry as follows.
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+==============+============+=============+===========+
| Bit Position | Name | Description | Reference |
+==============+============+=============+===========+
| 1 | Unassigned | | |
+--------------+------------+-------------+-----------+
Table 4: Update in the OSCORE Flag Bits Registry
6.2. CoAP Option Numbers Registry
IANA is asked to add this document as a reference for the OSCORE
Option in the "CoAP Option Numbers" registry within the "Constrained
RESTful Environments (CoRE) Parameters" registry group.
6.3. EDHOC External Authorization Data Registry
IANA is asked to add the following entries to the "EDHOC External
Authorization Data" registry defined in Section 10.5 of [RFC9528]
within the "Ephemeral Diffie-Hellman Over COSE (EDHOC)" registry
group.
+=======+=====================================+============+
| Label | Description | Reference |
+=======+=====================================+============+
| TBD1 | Indicates whether this peer | [RFC-XXXX] |
| | supports KUDOS and in which mode(s) | |
+-------+-------------------------------------+------------+
Table 5: Registrations in the EDHOC External
Authorization Data Registry
6.4. The Well-Known URI Registry
IANA is asked to add the 'kudos' well-known URI to the Well-Known
URIs registry as defined by [RFC8615].
* URI suffix: kudos
* Change controller: IETF
* Specification document(s): [RFC-XXXX]
* Related information: None
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6.5. Resource Type (rt=) Link Target Attribute Values Registry
IANA is requested to add the resource type "core.kudos" to the
"Resource Type (rt=) Link Target Attribute Values" registry under the
registry group "Constrained RESTful Environments (CoRE) Parameters".
* Value: "core.kudos"
* Description: KUDOS resource.
* Reference: [RFC-XXXX]
7. References
7.1. Normative References
[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/rfc/rfc2119>.
[RFC5869] Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
Key Derivation Function (HKDF)", RFC 5869,
DOI 10.17487/RFC5869, May 2010,
<https://www.rfc-editor.org/rfc/rfc5869>.
[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/rfc/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/rfc/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/rfc/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/rfc/rfc8613>.
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[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/rfc/rfc8949>.
[RFC9528] Selander, G., Preuß Mattsson, J., and F. Palombini,
"Ephemeral Diffie-Hellman Over COSE (EDHOC)", RFC 9528,
DOI 10.17487/RFC9528, March 2024,
<https://www.rfc-editor.org/rfc/rfc9528>.
7.2. Informative References
[I-D.ietf-ace-edhoc-oscore-profile]
Selander, G., Mattsson, J. P., Tiloca, M., and R. Höglund,
"Ephemeral Diffie-Hellman Over COSE (EDHOC) and Object
Security for Constrained Environments (OSCORE) Profile for
Authentication and Authorization for Constrained
Environments (ACE)", Work in Progress, Internet-Draft,
draft-ietf-ace-edhoc-oscore-profile-06, 21 October 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-ace-
edhoc-oscore-profile-06>.
[I-D.ietf-core-oscore-key-limits]
Höglund, R. and M. Tiloca, "Key Usage Limits for OSCORE",
Work in Progress, Internet-Draft, draft-ietf-core-oscore-
key-limits-04, 8 January 2025,
<https://datatracker.ietf.org/doc/html/draft-ietf-core-
oscore-key-limits-04>.
[I-D.irtf-cfrg-aead-limits]
Günther, F., Thomson, M., and C. A. Wood, "Usage Limits on
AEAD Algorithms", Work in Progress, Internet-Draft, draft-
irtf-cfrg-aead-limits-09, 9 October 2024,
<https://datatracker.ietf.org/doc/html/draft-irtf-cfrg-
aead-limits-09>.
[LwM2M] Open Mobile Alliance, "Lightweight Machine to Machine
Technical Specification - Core, Approved Version 1.2, OMA-
TS-LightweightM2M_Core-V1_2-20201110-A", November 2020,
<http://www.openmobilealliance.org/release/LightweightM2M/
V1_2-20201110-A/OMA-TS-LightweightM2M_Core-
V1_2-20201110-A.pdf>.
[LwM2M-Transport]
Open Mobile Alliance, "Lightweight Machine to Machine
Technical Specification - Transport Bindings, Approved
Version 1.2, OMA-TS-LightweightM2M_Transport-
V1_2-20201110-A", November 2020,
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<http://www.openmobilealliance.org/release/LightweightM2M/
V1_2-20201110-A/OMA-TS-LightweightM2M_Transport-
V1_2-20201110-A.pdf>.
[RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106, RFC 4086,
DOI 10.17487/RFC4086, June 2005,
<https://www.rfc-editor.org/rfc/rfc4086>.
[RFC7554] Watteyne, T., Ed., Palattella, M., and L. Grieco, "Using
IEEE 802.15.4e Time-Slotted Channel Hopping (TSCH) in the
Internet of Things (IoT): Problem Statement", RFC 7554,
DOI 10.17487/RFC7554, May 2015,
<https://www.rfc-editor.org/rfc/rfc7554>.
[RFC7967] Bhattacharyya, A., Bandyopadhyay, S., Pal, A., and T.
Bose, "Constrained Application Protocol (CoAP) Option for
No Server Response", RFC 7967, DOI 10.17487/RFC7967,
August 2016, <https://www.rfc-editor.org/rfc/rfc7967>.
[RFC8180] Vilajosana, X., Ed., Pister, K., and T. Watteyne, "Minimal
IPv6 over the TSCH Mode of IEEE 802.15.4e (6TiSCH)
Configuration", BCP 210, RFC 8180, DOI 10.17487/RFC8180,
May 2017, <https://www.rfc-editor.org/rfc/rfc8180>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/rfc/rfc8446>.
[RFC8615] Nottingham, M., "Well-Known Uniform Resource Identifiers
(URIs)", RFC 8615, DOI 10.17487/RFC8615, May 2019,
<https://www.rfc-editor.org/rfc/rfc8615>.
[RFC8724] Minaburo, A., Toutain, L., Gomez, C., Barthel, D., and JC.
Zuniga, "SCHC: Generic Framework for Static Context Header
Compression and Fragmentation", RFC 8724,
DOI 10.17487/RFC8724, April 2020,
<https://www.rfc-editor.org/rfc/rfc8724>.
[RFC8824] Minaburo, A., Toutain, L., and R. Andreasen, "Static
Context Header Compression (SCHC) for the Constrained
Application Protocol (CoAP)", RFC 8824,
DOI 10.17487/RFC8824, June 2021,
<https://www.rfc-editor.org/rfc/rfc8824>.
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[RFC9031] Vučinić, M., Ed., Simon, J., Pister, K., and M.
Richardson, "Constrained Join Protocol (CoJP) for 6TiSCH",
RFC 9031, DOI 10.17487/RFC9031, May 2021,
<https://www.rfc-editor.org/rfc/rfc9031>.
[RFC9176] Amsüss, C., Ed., Shelby, Z., Koster, M., Bormann, C., and
P. van der Stok, "Constrained RESTful Environments (CoRE)
Resource Directory", RFC 9176, DOI 10.17487/RFC9176, April
2022, <https://www.rfc-editor.org/rfc/rfc9176>.
[RFC9200] Seitz, L., Selander, G., Wahlstroem, E., Erdtman, S., and
H. Tschofenig, "Authentication and Authorization for
Constrained Environments Using the OAuth 2.0 Framework
(ACE-OAuth)", RFC 9200, DOI 10.17487/RFC9200, August 2022,
<https://www.rfc-editor.org/rfc/rfc9200>.
[RFC9203] Palombini, F., Seitz, L., Selander, G., and M. Gunnarsson,
"The Object Security for Constrained RESTful Environments
(OSCORE) Profile of the Authentication and Authorization
for Constrained Environments (ACE) Framework", RFC 9203,
DOI 10.17487/RFC9203, August 2022,
<https://www.rfc-editor.org/rfc/rfc9203>.
[Symmetric-Security]
Mattsson, J. P., "Security of Symmetric Ratchets and Key
Chains - Implications for Protocols like TLS 1.3, Signal,
and PQ3", February 2024,
<https://eprint.iacr.org/2024/220>.
Appendix A. Examples
The following sections shows two examples of KUDOS being executed and
successfully completing.
A.1. Successful KUDOS Initiated with a Request
The following shows a succesful execution of KUDOS where KUDOS is
started by the client sending a divergent KUDOS message.
KUDOS status: KUDOS status:
- CTX_OLD: -,- - CTX_OLD: -,-
- State: IDLE (0,0) - State: IDLE (0,0)
Client Server
| |
Generate N1, X1 | |
| |
CTX_TEMP = updateCtx( | |
X1 | N1, | |
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0x, | |
CTX_OLD ) | |
| |
| Request #1 |
Protect with CTX_TEMP +--------------------->| /.well-known/kudos
| OSCORE { |
KUDOS status: | ... | CTX_TEMP = updateCtx(
CTX_OLD: X1, N1 | Partial IV: 0 | 0x,
State: BUSY (1,0) | ... | X1 | N1,
| d flag: 1 | CTX_OLD )
| x: X1 = b'00000111' |
| nonce: N1 | Verify with CTX_TEMP
| ... |
| } |
| Encrypted Payload { |
| ... |
| } |
| |
| | KUDOS status:
| | CTX_OLD: -, -
| | State: BUSY (0,1)
| |
| |
| | Generate N2, X2
| |
| | CTX_NEW = updateCtx(
| | X2 | N2),
| | X1 | N1),
| | CTX_OLD )
| |
| Response #1 |
|<---------------------+ Protect with CTX_NEW
| OSCORE { |
| ... | KUDOS status:
CTX_NEW = updateCtx( | Partial IV: 0 | CTX_OLD: X2, N2
X1 | N1, | ... | State: PENDING (1,1)
X2 | N2 , | d flag: 1 |
CTX_OLD ) | x: X2 = b'01000111' |
| nonce: N2 |
Verify with CTX_NEW | ... |
| } |
/ key confirmation / | Encrypted Payload { |
| ... |
Pre-IDLE steps: | } |
Delete CTX_TEMP | |
Delete CTX_OLD, X1, N1 | |
| |
KUDOS status: | |
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CTX_NEW: -, - | |
State: IDLE (0,0) | |
| |
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 { |
| ... |
| } | Verify with CTX_NEW
| Encrypted Payload { |
| Application Payload | / key confirmation /
| ... |
| } | Pre-IDLE steps:
| | Delete CTX_TEMP
| | Delete CTX_OLD, X2, N2
| |
| | KUDOS status:
| | CTX_NEW: -, -
| | State: IDLE (0,0)
| Response #2 |
|<---------------------+ Protect with CTX_NEW
Verify with CTX_NEW | OSCORE { |
| ... |
| } |
| Encrypted Payload { |
| ... |
| Application Payload |
| } |
| |
A.2. Successful KUDOS Initiated with a Response
The following shows a succesful execution of KUDOS where KUDOS is
started by the Server sending a divergent KUDOS message.
KUDOS status: KUDOS status:
- CTX_OLD: -,- - CTX_OLD: -,-
- State: IDLE (0,0) - State: IDLE (0,0)
Client Server
| |
| Request #1 |
Protect with CTX_OLD +--------------------->| /temp
| OSCORE { |
| ... |
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| } | Verify with CTX_OLD
| Encrypted Payload { |
| ... | Generate N1, X1
| Application Payload |
| } | CTX_TEMP = updateCtx(
| | X1 | N1,
| | 0x,
| | CTX_OLD )
| |
| Response #1 |
|<---------------------+ Protect with CTX_TEMP
| OSCORE { |
| ... | KUDOS status:
CTX_TEMP = updateCtx( | Partial IV: 0 | CTX_OLD: X1, N1
0x, | ... | State: BUSY (1,0)
X1 | N1, | d flag: 1 |
CTX_OLD ) | x: X1 = b'00000111' |
| nonce: N1 |
Verify with CTX_TEMP | ... |
| } |
| Encrypted Payload { |
| ... |
| } |
| |
KUDOS status: | |
- CTX_OLD: -,- | |
- State: BUSY (0,1) | |
| |
| |
Generate N2, X2 | |
| |
CTX_NEW = updateCtx( | |
X2 | N2, | |
X1 | N1, | |
CTX_OLD ) | |
| |
| Request #2 |
Protect with CTX_NEW +--------------------->| /.well-known/kudos
| OSCORE { |
KUDOS status: | ... |
- CTX_OLD: X2, N2 | d flag: 1 | CTX_NEW = updateCtx(
- State: PENDING (1,1) | x: X2 = b'01000111' | X1 | N1,
| nonce: N2 | X2 | N2,
| ... | CTX_OLD )
| } |
| Encrypted Payload { | Verify with CTX_NEW
| ... |
| Application Payload | / key confirmation /
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| } |
| | Pre-IDLE steps:
| | Delete CTX_TEMP
| | Delete CTX_OLD, X1, N1
| |
| | KUDOS status:
| | - CTX_NEW: -,-
| | - State: IDLE (0,0)
| |
| |
| |
| |
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 { |
/ key confirmation / | ... |
| Application Payload |
Pre-IDLE steps: | } |
Delete CTX_TEMP | |
Delete CTX_OLD, X1, N1 | |
| |
KUDOS status: | |
CTX_NEW: -, - | |
State: IDLE (0,0) | |
| |
Appendix B. Document Updates
This section is to be removed before publishing as an RFC.
B.1. Version -09 to -10
* Major re-design building on a state machine driving the KUDOS
execution.
B.2. Version -08 to -09
* Merge text about avoiding in-transit requests during a key update
into a single subsection.
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* Improved error handling.
* Editorial improvements and clarifications.
* State that the EDHOC EAD item must be used as non-critical.
* Extended description and updates values for KUDOS communication
overhead.
* Introduce special case when non-CAPABLE devices may operate in FS
Mode.
* Add parameter for signaling KUDOS support when using the ACE
OSCORE profile.
* Enable using the reverse message flow for peers that are only CoAP
servers.
* Further clarifications about achieving key confirmation and
deletion of old contexts.
* Restructure distribution of content about FS and no-FS mode.
* Warn of consequences of running KUDOS with insufficient margin.
* Stressed usefulness of core.kudos for safe KUDOS requests without
side effects.
B.3. Version -07 to -08
* Add note about usage of the CoAP No-Response Option.
* Avoid problems for two simultaneously started key updates.
* Set Notification Number to be uninitialized for new OSCORE
Security Contexts.
* Handle corner case for responder that reached its key usage
limits.
* Re-organizing main section about Forward Secrecy mode into
subsections.
* IANA considerations for CoAP Option Numbers Registry to refer to
this draft for the OSCORE option.
* Use AASVG in diagrams.
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* Use actual tables instead of figures.
* Clarifications and editorial improvements.
* Extended security considerations with reference to relevant paper.
B.4. Version -06 to -07
* Removed material about the ID update procedure, which has been
split out into a separate draft.
* Allow non-random nonces for CAPABLE devices.
* Editorial improvements.
* Permit flexible message flow with KUDOS messages as any request/
response.
* Enable sending KUDOS messages as regular application messages.
B.5. Version -05 to -06
* Mandate support for both the forward and reverse message flow.
* Mention the EDHOC and OSCORE profile of ACE as method for
rekeying.
* Clarify definition of KUDOS (request/response) message.
* Further extend the OSCORE option to transport N1 in the second
KUDOS message as a request.
* Mandate support for the no-FS mode on CAPABLE devices.
* Explain when KUDOS fails during selection of mode.
* Explicitly forbid using old keying material after reboot.
* Editorial improvements.
B.6. Version -04 to -05
* Note on client retransmissions if KUDOS execution fails in reverse
message flow.
* Specify what information needs to be written to non-volatile
memory to handle reboots.
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* Extended recommendations and considerations on minimum size of
nonces N1 & N2.
* Arbitrary maximum size of the Recipient-ID Option.
* Detailed lifecycle of the OSCORE IDs update procedure.
* Described examples of OSCORE IDs update procedure.
* Examples of OSCORE IDs update procedure integrated in KUDOS.
* Considerations about using SCHC for CoAP with OSCORE.
* Clarifications and editorial improvements.
B.7. Version -03 to -04
* Removed content about key usage limits.
* Use of "forward message flow" and "reverse message flow".
* Update to RFC 8613 extended to include protection of responses.
* Include EDHOC_KeyUpdate() in the methods for rekeying.
* Describe reasons for using the OSCORE ID update procedure.
* Clarifications on deletion of CTX_OLD and CTX_NEW.
* Added new section on preventing deadlocks.
* Clarified that peers can decide to run KUDOS at any point.
* Defined preservation of observations beyond OSCORE ID updates.
* Revised discussion section, including also communication overhead.
* Defined a well-known KUDOS resource and a KUDOS resource type.
* Editorial improvements.
B.8. Version -02 to -03
* Use of the OSCORE flag bit 0 to signal more flag bits.
* In UpdateCtx(), open for future key derivation different than
HKDF.
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* Simplified updateCtx() to use only Expand(); used to be METHOD 2.
* Included the Partial IV if the second KUDOS message is a response.
* Added signaling of support for KUDOS in EDHOC.
* Clarifications on terminology and reasons for rekeying.
* Updated IANA considerations.
* Editorial improvements.
B.9. Version -01 to -02
* Extended terminology.
* Moved procedure for preserving observations across key updates to
main body.
* Moved procedure to update OSCORE Sender/Recipient IDs to main
body.
* Moved key update without forward secrecy section to main body.
* Define signaling bits present in the 'x' byte.
* Modifications and alignment of updateCtx() with EDHOC.
* Rules for deletion of old EDHOC keys PRK_out and PRK_exporter.
* Describe CBOR wrapping of involved nonces with examples.
* Renamed 'id detail' to 'nonce'.
* Editorial improvements.
B.10. Version -00 to -01
* Recommendation on limits for CCM_8. Details in Appendix.
* Improved message processing, also covering corner cases.
* Example of method to estimate and not store 'count_q'.
* Added procedure to update OSCORE Sender/Recipient IDs.
* Added method for preserving observations across key updates.
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* Added key update without forward secrecy.
Acknowledgments
The authors sincerely thank Christian Amsüss, Carsten Bormann, Simon
Bouget, Rafa Marin-Lopez, John Preuß Mattsson, and Göran Selander for
their feedback and comments.
The work on this document has been partly supported by the Sweden's
Innovation Agency VINNOVA and the Celtic-Next projects CRITISEC and
CYPRESS; and by the H2020 projects SIFIS-Home (Grant agreement
952652) and ARCADIAN-IoT (Grant agreement 101020259).
Authors' Addresses
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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