CoRE Working Group                                            R. Höglund
Internet-Draft                                                 M. Tiloca
Updates: 8613 (if approved)                                      RISE AB
Intended status: Standards Track                            7 March 2022
Expires: 8 September 2022


                     Key Update for OSCORE (KUDOS)
                  draft-ietf-core-oscore-key-update-01

Abstract

   Object Security for Constrained RESTful Environments (OSCORE) uses
   AEAD algorithms to ensure confidentiality and integrity of exchanged
   messages.  Due to known issues allowing forgery attacks against AEAD
   algorithms, limits should be followed on the number of times a
   specific key is used for encryption or decryption.  This document
   defines how two OSCORE peers must follow these limits and what steps
   they must take to preserve the security of their communications.
   Therefore, this document updates RFC8613.  Furthermore, this document
   specifies Key Update for OSCORE (KUDOS), a lightweight procedure that
   two peers can use to update their keying material and establish a new
   OSCORE Security Context.

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
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.







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   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on 8 September 2022.

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   document authors.  All rights reserved.

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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  AEAD Key Usage Limits in OSCORE . . . . . . . . . . . . . . .   4
     2.1.  Problem Overview  . . . . . . . . . . . . . . . . . . . .   4
       2.1.1.  Limits for 'q' and 'v'  . . . . . . . . . . . . . . .   5
     2.2.  Additional Information in the Security Context  . . . . .   6
       2.2.1.  Common Context  . . . . . . . . . . . . . . . . . . .   6
       2.2.2.  Sender Context  . . . . . . . . . . . . . . . . . . .   7
       2.2.3.  Recipient Context . . . . . . . . . . . . . . . . . .   7
     2.3.  OSCORE Messages Processing  . . . . . . . . . . . . . . .   8
       2.3.1.  Protecting a Request or a Response  . . . . . . . . .   8
       2.3.2.  Verifying a Request or a Response . . . . . . . . . .   8
   3.  Current methods for Rekeying OSCORE . . . . . . . . . . . . .   8
   4.  Key Update for OSCORE (KUDOS) . . . . . . . . . . . . . . . .  11
     4.1.  Extensions to the OSCORE Option . . . . . . . . . . . . .  11
     4.2.  Function for Security Context Update  . . . . . . . . . .  12
     4.3.  Establishment of the New OSCORE Security Context  . . . .  14
       4.3.1.  Client-Initiated Key Update . . . . . . . . . . . . .  16
       4.3.2.  Server-Initiated Key Update . . . . . . . . . . . . .  18
     4.4.  Retention Policies  . . . . . . . . . . . . . . . . . . .  22
     4.5.  Discussion  . . . . . . . . . . . . . . . . . . . . . . .  22
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  23
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  23
     6.1.  CoAP Option Numbers Registry  . . . . . . . . . . . . . .  23
     6.2.  OSCORE Flag Bits Registry . . . . . . . . . . . . . . . .  23



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   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  24
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  24
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  25
   Appendix A.  Detailed considerations for AEAD_AES_128_CCM_8 . . .  26
   Appendix B.  Estimation of 'count_q'  . . . . . . . . . . . . . .  27
   Appendix C.  Preserving Observations across Key Updates . . . . .  28
     C.1.  Management of Observations  . . . . . . . . . . . . . . .  29
     C.2.  Signaling to Preserve Observations  . . . . . . . . . . .  30
   Appendix D.  Update of OSCORE Sender/Recipient IDs  . . . . . . .  32
     D.1.  The Recipient-ID Option . . . . . . . . . . . . . . . . .  32
       D.1.1.  Client-Initiated OSCORE IDs Update  . . . . . . . . .  33
       D.1.2.  Server-Initiated OSCORE IDs Update  . . . . . . . . .  35
       D.1.3.  Additional Actions for Stand-Alone Execution  . . . .  38
   Appendix E.  Key Update without Forward Secrecy . . . . . . . . .  38
     E.1.  Handling and use of Keying Material . . . . . . . . . . .  39
       E.1.1.  Actions after Device Reboot . . . . . . . . . . . . .  40
     E.2.  Signaling of FS Mode or Non-FS Mode . . . . . . . . . . .  41
     E.3.  Selection and Negotiation of KUDOS Mode . . . . . . . . .  42
   Appendix F.  Document Updates . . . . . . . . . . . . . . . . . .  45
     F.1.  Version -00 to -01  . . . . . . . . . . . . . . . . . . .  45
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  45
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  45

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.

   In particular, OSCORE uses AEAD algorithms to provide confidentiality
   and integrity of messages exchanged between two peers.  Due to known
   issues allowing forgery attacks against AEAD algorithms, limits
   should be followed on the number of times a specific key is used to
   perform encryption or decryption [I-D.irtf-cfrg-aead-limits].

   Should these limits be exceeded, an adversary may break the security
   properties of the AEAD algorithm, such as message confidentiality and
   integrity, e.g. by performing a message forgery attack.  The original
   OSCORE specification [RFC8613] does not consider such limits.

   This document updates [RFC8613] as follows.








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   *  It defines when a peer must stop using an OSCORE Security Context
      shared with another peer, due to the reached key usage limits.
      When this happens, the two peers have to establish a new Security
      Context with new keying material, in order to continue their
      secure communication with OSCORE.

   *  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].

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 the CoAP [RFC7252] and OSCORE [RFC8613] protocols.

2.  AEAD Key Usage Limits in OSCORE

   The following sections details how key usage limits for AEAD
   algorithms must be considered when using OSCORE.  It covers specific
   limits for common AEAD algorithms used with OSCORE; necessary
   additions to the OSCORE Security Context, updates to the OSCORE
   message processing, and existing methods for rekeying OSCORE.

2.1.  Problem Overview

   The OSCORE security protocol [RFC8613] uses AEAD algorithms to
   provide integrity and confidentiality of messages, as exchanged
   between two peers sharing an OSCORE Security Context.

   When processing messages with OSCORE, each peer should follow
   specific limits as to the number of times it uses a specific key.
   This applies separately to the Sender Key used to encrypt outgoing
   messages, and to the Recipient Key used to decrypt and verify
   incoming protected messages.

   Exceeding these limits may allow an adversary to break the security
   properties of the AEAD algorithm, such as message confidentiality and
   integrity, e.g. by performing a message forgery attack.

   The following refers to the two parameters 'q' and 'v' introduced in
   [I-D.irtf-cfrg-aead-limits], to use when deploying an AEAD algorithm.



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   *  'q': this parameter has as value the number of messages protected
      with a specific key, i.e. the number of times the AEAD algorithm
      has been invoked to encrypt data with that key.

   *  'v': this parameter has as value the number of alleged forgery
      attempts that have been made against a specific key, i.e. the
      amount of failed decryptions that has been done with the AEAD
      algorithm for that key.

   When a peer uses OSCORE:

   *  The key used to protect outgoing messages is its Sender Key, in
      its Sender Context.

   *  The key used to decrypt and verify incoming messages is its
      Recipient Key, in its Recipient Context.

   Both keys are derived as part of the establishment of the OSCORE
   Security Context, as defined in Section 3.2 of [RFC8613].

   As mentioned above, exceeding specific limits for the 'q' or 'v'
   value can weaken the security properties of the AEAD algorithm used,
   thus compromising secure communication requirements.

   Therefore, in order to preserve the security of the used AEAD
   algorithm, OSCORE has to observe limits for the 'q' and 'v' values,
   throughout the lifetime of the used AEAD keys.

2.1.1.  Limits for 'q' and 'v'

   Formulas for calculating the security levels as Integrity Advantage
   (IA) and Confidentiality Advantage (CA) probabilities, are presented
   in [I-D.irtf-cfrg-aead-limits].  These formulas take as input
   specific values for 'q' and 'v' (see section Section 2.1) and for
   'l', i.e., the maximum length of each message (in cipher blocks).

   For the algorithms that can be used as AEAD Algorithm for OSCORE
   shows in Figure 1, the key property to achieve is having IA and CA
   values which are no larger than p = 2^-64, which will ensure a safe
   security level for the AEAD Algorithm.  This can be entailed by using
   the values q = 2^20, v = 2^20, and l = 2^10, that this document
   recommends to use for these algorithms.

   Figure 1 shows the resulting IA and CA probabilities enjoyed by the
   considered algorithms, when taking the value of 'q', 'v' and 'l'
   above as input to the formulas defined in
   [I-D.irtf-cfrg-aead-limits].




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       +------------------------+----------------+----------------+
       | Algorithm name         | IA probability | CA probability |
       |------------------------+----------------+----------------|
       | AEAD_AES_128_CCM       | 2^-64          | 2^-66          |
       | AEAD_AES_128_GCM       | 2^-97          | 2^-89          |
       | AEAD_AES_256_GCM       | 2^-97          | 2^-89          |
       | AEAD_CHACHA20_POLY1305 | 2^-73          | -              |
       +------------------------+----------------+----------------+

     Figure 1: Probabilities for algorithms based on chosen q, v and l
                                  values.

   When AEAD_AES_128_CCM_8 is used as AEAD Algorithm for OSCORE, the
   triplet (q, v, l) considered above yields larger values of IA and CA.
   Hence, specifically for AEAD_AES_128_CCM_8, this document recommends
   using the triplet (q, v, l) = (2^20, 2^14, 2^8).  This is appropriate
   since the resulting CA and IA values are not greater than the
   threshold value of 2^-50 defined in [I-D.irtf-cfrg-aead-limits], and
   thus yields an acceptable security level.  Achieving smaller values
   of CA and IA would require to inconveniently reduce 'q', 'v' or 'l',
   with no corresponding increase in terms of security.  This is further
   elaborated in Appendix A.

       +------------------------+----------+----------+-----------+
       | Algorithm name         | l=2^6 in | l=2^8 in | l=2^10 in |
       |                        | bytes    | bytes    | bytes     |
       |------------------------+----------+----------|-----------|
       | AEAD_AES_128_CCM       | 1024     | 4096     | 16384     |
       | AEAD_AES_128_GCM       | 1024     | 4096     | 16384     |
       | AEAD_AES_256_GCM       | 1024     | 4096     | 16384     |
       | AEAD_AES_128_CCM_8     | 1024     | 4096     | 16384     |
       | AEAD_CHACHA20_POLY1305 | 4096     | 16384    | 65536     |
       +------------------------+----------+----------+-----------+

            Figure 2: Maximum length of each message (in bytes)

2.2.  Additional Information in the Security Context

   In addition to what defined in Section 3.1 of [RFC8613], the OSCORE
   Security Context MUST also include the following information.

2.2.1.  Common Context

   The Common Context is extended to include the following parameter.

   *  'exp': with value the expiration time of the OSCORE Security
      Context, as a non-negative integer.  The parameter contains a
      numeric value representing the number of seconds from



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      1970-01-01T00:00:00Z UTC until the specified UTC date/time,
      ignoring leap seconds, analogous to what specified for NumericDate
      in Section 2 of [RFC7519].

      At the time indicated in this field, a peer MUST stop using this
      Security Context to process any incoming or outgoing message, and
      is required to establish a new Security Context to continue
      OSCORE-protected communications with the other peer.

2.2.2.  Sender Context

   The Sender Context is extended to include the following parameters.

   *  'count_q': a non-negative integer counter, keeping track of the
      current 'q' value for the Sender Key. At any time, 'count_q' has
      as value the number of messages that have been encrypted using the
      Sender Key. The value of 'count_q' is set to 0 when establishing
      the Sender Context.

   *  'limit_q': a non-negative integer, which specifies the highest
      value that 'count_q' is allowed to reach, before stopping using
      the Sender Key to process outgoing messages.

      The value of 'limit_q' depends on the AEAD algorithm specified in
      the Common Context, considering the properties of that algorithm.
      The value of 'limit_q' is determined according to Section 2.1.1.

   Note for implementation: it is possible to avoid storing and
   maintaining the counter 'count_q'.  Rather, an estimated value to be
   compared against 'limit_q' can be computed, by leveraging the Sender
   Sequence Number of the peer and (an estimate of) the other peer's.  A
   possible method to achieve this is described in Appendix B.  While
   this relieves peers from storing and maintaining the precise
   'count_q' value, it results in overestimating the number of
   encryptions performed with a Sender Key. This in turn results in
   approaching 'limit_q' sooner and performing a key update procedure
   more frequently.

2.2.3.  Recipient Context

   The Recipient Context is extended to include the following
   parameters.

   *  'count_v': a non-negative integer counter, keeping track of the
      current 'v' value for the Recipient Key. At any time, 'count_v'
      has as value the number of failed decryptions occurred on incoming
      messages using the Recipient Key. The value of 'count_v' is set to
      0 when establishing the Recipient Context.



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   *  'limit_v': a non-negative integer, which specifies the highest
      value that 'count_v' is allowed to reach, before stopping using
      the Recipient Key to process incoming messages.

      The value of 'limit_v' depends on the AEAD algorithm specified in
      the Common Context, considering the properties of that algorithm.
      The value of 'limit_v' is determined according to Section 2.1.1.

2.3.  OSCORE Messages Processing

   In order to keep track of the 'q' and 'v' values and ensure that AEAD
   keys are not used beyond reaching their limits, the processing of
   OSCORE messages is extended as defined in this section.  A limitation
   that is introduced is that, in order to not exceed the selected value
   for 'l', the total size of the COSE plaintext, authentication Tag,
   and possible cipher padding for a message may not exceed the block
   size for the selected algorithm multiplied with 'l'.

   In particular, the processing of OSCORE messages follows the steps
   outlined in Section 8 of [RFC8613], with the additions defined below.

2.3.1.  Protecting a Request or a Response

   Before encrypting the COSE object using the Sender Key, the 'count_q'
   counter MUST be incremented.

   If 'count_q' exceeds the 'limit_q' limit, the message processing MUST
   be aborted.  From then on, the Sender Key MUST NOT be used to encrypt
   further messages.

2.3.2.  Verifying a Request or a Response

   If an incoming message is detected to be a replay (see Section 7.4 of
   [RFC8613]), the 'count_v' counter MUST NOT be incremented.

   If the decryption and verification of the COSE object using the
   Recipient Key fails, the 'count_v' counter MUST be incremented.

   After 'count_v' has exceeded the 'limit_v' limit, incoming messages
   MUST NOT be decrypted and verified using the Recipient Key, and their
   processing MUST be aborted.

3.  Current methods for Rekeying OSCORE

   Before the limit of 'q' or 'v' defined in Section 2.1.1 has 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.



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   In practice, the two peers have to establish new Sender and Recipient
   Keys, as the keys actually used by the AEAD algorithm.  When this
   happens, both peers reset their 'count_q' and 'count_v' values to 0
   (see Section 2.2).

   Other specifications define a number of ways to accomplish this, 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].

   *  The two peers can run the OSCORE profile
      [I-D.ietf-ace-oscore-profile] of the Authentication and
      Authorization for Constrained Environments (ACE) Framework
      [I-D.ietf-ace-oauth-authz].

      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.




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   *  The two peers can run the EDHOC key exchange protocol based on
      Diffie-Hellman and defined in [I-D.ietf-lake-edhoc], 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 especially an OSCORE Master Secret and an OSCORE
      Master Salt, from which an OSCORE Security Context can be
      established.

      This procedure additionally provides forward secrecy.

   *  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.

      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.

   It is RECOMMENDED that the peer initiating the key update procedure
   starts it before reaching the 'q' or 'v' limits.  Otherwise, the AEAD
   keys possibly to be used during the key update procedure itself may
   already be or become invalid before the rekeying is completed, which
   may prevent a successful establishment of the new OSCORE Security
   Context altogether.







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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.

   KUDOS relies on the support function updateCtx() defined in
   Section 4.2 and the message exchange defined in Section 4.3.  The
   following properties are fulfilled.

   *  KUDOS can be initiated by either peer.  In particular, the client
      or the server may start KUDOS by sending the first rekeying
      message.

   *  The new OSCORE Security Context enjoys forward secrecy.

   *  The same ID Context value used in the old OSCORE Security Context
      is preserved in the new Security Context.  Furthermore, the ID
      Context value never changes throughout the KUDOS execution.

   *  KUDOS is robust against a peer rebooting, and it especially avoids
      the reuse of AEAD (nonce, key) pairs.

   *  KUDOS completes in one round trip.  The two peers achieve mutual
      proof-of-possession in the following exchange, which is protected
      with the newly established OSCORE Security Context.

4.1.  Extensions to the OSCORE Option

   In order to support the message exchange for establishing a new
   OSCORE Security Context as defined in Section 4.3, this document
   extends the use of the OSCORE option originally defined in [RFC8613]
   as follows.

   *  This document defines the usage of the seventh least significant
      bit, called "Extension-1 Flag", in the first byte of the OSCORE
      option containing the OSCORE flag bits.  This flag bit is
      specified in Section 6.2.

      When the Extension-1 Flag is set to 1, the second byte of the
      OSCORE option MUST include the set of OSCORE flag bits 8-15.

   *  This document defines the usage of the first least significant bit
      "ID Detail Flag", 'd', in the second byte of the OSCORE option
      containing the OSCORE flag bits.  This flag bit is specified in
      Section 6.2.





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      When it is set to 1, the compressed COSE object contains an 'id
      detail', to be used for the steps defined in Section 4.3.  In
      particular, the 1 byte following 'kid context' (if any) encodes
      the length x of 'id detail', and the following x bytes encode 'id
      detail'.

      Hereafter, this document refers to a message where the 'd' flag is
      set to 0 as "non KUDOS (request/response) message", and to a
      message where the 'd' flag is set to 1 as "KUDOS (request/
      response) message".

   *  The second-to-eighth least significant bits in the second byte of
      the OSCORE option containing the OSCORE flag bits are reserved for
      future use.  These bits SHALL be set to zero when not in use.
      According to this specification, if any of these bits are set to
      1, the message is considered to be malformed and decompression
      fails as specified in item 2 of Section 8.2 of [RFC8613].

   Figure 3 shows the OSCORE option value including also 'id detail'.

  0 1 2 3 4 5 6 7  8   9   10  11  12  13  14  15 <----- n bytes ----->
 +-+-+-+-+-+-+-+-+---+---+---+---+---+---+---+---+---------------------+
 |0|1|0|h|k|  n  | 0 | 0 | 0 | 0 | 0 | 0 | 0 | d | Partial IV (if any) |
 +-+-+-+-+-+-+-+-+---+---+---+---+---+---+---+---+---------------------+

  <- 1 byte -> <----- s bytes ------> <- 1 byte -> <----- x bytes ---->
 +------------+----------------------+---------------------------------+
 | s (if any) | kid context (if any) | x (if any) | id detail (if any) |
 +------------+----------------------+------------+--------------------+

 +------------------+
 | kid (if any) ... |
 +------------------+

        Figure 3: The OSCORE option value, including 'id detail'

4.2.  Function for Security Context Update

   The updateCtx() function shown in Figure 4 takes as input a nonce N
   as well as an OSCORE Security Context CTX_IN, and returns as output a
   new OSCORE Security Context CTX_OUT.

   As a first step, the updateCtx() function derives the new values of
   the Master Secret and Master Salt for CTX_OUT, according to one of
   the two following methods.  The used method depends on how the two
   peers established their original Security Context, i.e., the Security
   Context that they shared before performing KUDOS with one another for
   the first time.



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   *  If the original Security Context was established by running the
      EDHOC protocol [I-D.ietf-lake-edhoc], the following applies.

      First, the EDHOC key PRK_4x3m shared by the two peers is updated
      using the EDHOC-KeyUpdate() function defined in Section 4.4 of
      [I-D.ietf-lake-edhoc], which takes the nonce N as input.

      After that, the EDHOC-Exporter() function defined in Section 4.3
      of [I-D.ietf-lake-edhoc] is used to derive the new values for the
      Master Secret and Master Salt, consistently with what is defined
      in Appendix A.2 of [I-D.ietf-lake-edhoc].  In particular, the
      context parameter provided as second argument to the EDHOC-
      Exporter() function is the empty CBOR byte string (0x40)
      [RFC8949], which is denoted as h''.

      Note that, compared to the compliance requirements in Section 7 of
      [I-D.ietf-lake-edhoc], a peer MUST support the EDHOC-KeyUpdate()
      function, in case it establishes an original Security Context
      through the EDHOC protocol and intends to perform KUDOS.

   *  If the original Security Context was established through other
      means than the EDHOC protocol, the new Master Secret is derived
      through an HKDF-Expand() step, which takes as input N as well as
      the Master Secret value from the Security Context CTX_IN.
      Instead, the new Master Salt takes N as value.

   In either case, 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.  Finally, the function
   returns the newly derived Security Context CTX_OUT.

   updateCtx(N, CTX_IN) {

     CTX_OUT       // The new Security Context
     MSECRET_NEW   // The new Master Secret
     MSALT_NEW     // The new Master Salt

     if <the original Security Context was established through EDHOC> {

       EDHOC-KeyUpdate(N)
       // This results in updating the key PRK_4x3m of the
       // EDHOC session, i.e., PRK_4x3m = Extract(N, PRK_4x3m)



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       MSECRET_NEW = EDHOC-Exporter("OSCORE_Master_Secret",
                                    h'', key_length)
         = EDHOC-KDF(PRK_4x3m, TH_4,
                     "OSCORE_Master_Secret", h'', key_length)

       MSALT_NEW = EDHOC-Exporter("OSCORE_Master_Salt",
                                  h'', salt_length)
         = EDHOC-KDF(PRK_4x3m, TH_4,
                     "OSCORE_Master_Salt", h'', salt_length)

     }
     else {
       Master Secret Length = < Size of CTX_IN.MasterSecret in bytes >

       MSECRET_NEW = HKDF-Expand-Label(CTX_IN.MasterSecret, Label,
                                       N, Master Secret Length)
                   = HKDF-Expand(CTX_IN.MasterSecret, HkdfLabel,
                                 Master Secret Length)

       MSALT_NEW = N;
     }

     < Derive CTX_OUT using MSECRET_NEW and MSALT_NEW,
       together with other parameters from CTX_IN >

     Return CTX_OUT;

   }

   Where HkdfLabel is defined as

   struct {
       uint16 length = Length;
       opaque label<7..255> = "oscore " + Label;
       opaque context<0..255> = Context;
   } HkdfLabel;

       Figure 4: Function for deriving a new OSCORE Security Context

4.3.  Establishment of the New OSCORE Security Context

   This section defines the actual KUDOS procedure performed by two
   peers to update their OSCORE keying material.  Before starting KUDOS,
   the two peers share the OSCORE Security Context CTX_OLD.  Once
   completed the KUDOS execution, the two peers agree on a newly
   established OSCORE Security Context CTX_NEW.





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   In particular, each peer contributes by generating a fresh value R1
   or R2, and providing it to the other peer.  The byte string
   concatenation of the two values, hereafter denoted as R1 | R2, is
   used as input N by the updateCtx() function, in order to derive the
   new OSCORE Security Context CTX_NEW.  As for any new OSCORE Security
   Context, the Sender Sequence Number and the replay window are re-
   initialized accordingly (see Section 3.2.2 of [RFC8613]).

   Once a peer has successfully derived the new OSCORE Security Context
   CTX_NEW, that peer MUST use CTX_NEW to protect outgoing non KUDOS
   messages.

   Also, that 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 Appendix C.

   Once a peer has successfully decrypted and verified an incoming
   message protected with CTX_NEW, that peer MUST discard the old
   Security Context CTX_OLD.

   KUDOS can be started by the client or the server, as defined in
   Section 4.3.1 and Section 4.3.2, respectively.  The following
   properties hold for both the client- and server-initiated version of
   KUDOS.

   *  The initiator always offers the fresh value R1.

   *  The responder always offers the fresh value R2.

   *  The responder is always the first one deriving the new OSCORE
      Security Context CTX_NEW.

   *  The initiator is always the first one achieving key confirmation,
      hence able to safely discard the old OSCORE Security Context
      CTX_OLD.

   *  Both the initiator and the responder use the same respective
      OSCORE Sender ID and Recipient ID.  Also, they both preserve and
      use the same OSCORE ID Context from CTX_OLD.

   The length of the nonces R1 and R2 is application specific.  The
   application needs to set the length of each nonce such that the
   probability of its value being repeated is negligible.  To this end,
   each nonce is typically at least 8 bytes long.






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   Once a peer acting as initiator (responder) has sent (received) the
   first KUDOS message, that peer MUST NOT send a non KUDOS message to
   the other peer, until having completed the key update process on its
   side.  The initiator completes the key update process when receiving
   the second KUDOS message and successfully verifying it with the new
   OSCORE Security Context CTX_NEW.  The responder completes the key
   update process when sending the second KUDOS message, as protected
   with the new OSCORE Security Context CTX_NEW.

4.3.1.  Client-Initiated Key Update

   Figure 5 shows the KUDOS workflow with the client acting as
   initiator.

                        Client               Server
                     (initiator)          (responder)
                          |                    |
     Generate R1          |                    |
                          |                    |
     CTX_1 =              |                    |
       updateCtx(R1,      |                    |
                 CTX_OLD) |                    |
                          |                    |
                          |     Request #1     |
     Protect with CTX_1   |------------------->|
                          | OSCORE Option:     | CTX_1 =
                          |   ...              |   updateCtx(R1,
                          |   d flag: 1        |             CTX_OLD)
                          |   ...              |
                          |   ID Detail: R1    | Verify with CTX_1
                          |   ...              |
                          |                    | Generate R2
                          |                    |
                          |                    | CTX_NEW =
                          |                    |   updateCtx(R1|R2,
                          |                    |             CTX_OLD)
                          |                    |
                          |     Response #1    |
                          |<-------------------| Protect with CTX_NEW
     CTX_NEW =            | OSCORE Option:     |
       updateCtx(R1|R2,   |   ...              |
                 CTX_OLD) |   d flag: 1        |
                          |   ...              |
     Verify with CTX_NEW  |   ID Detail: R2    |
                          |   ...              |
     Discard CTX_OLD      |                    |
                          |                    |




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     // The actual key update process ends here.
     // The two peers can use the new Security Context CTX_NEW.

                          |                    |
                          |     Request #2     |
     Protect with CTX_NEW |------------------->|
                          |                    | Verify with CTX_NEW
                          |                    |
                          |                    | Discard CTX_OLD
                          |                    |
                          |     Response #2    |
                          |<-------------------| Protect with CTX_NEW
     Verify with CTX_NEW  |                    |
                          |                    |

                 Figure 5: Client-Initiated KUDOS Workflow

   First, the client generates a random value R1, and uses the nonce N =
   R1 together with the old Security Context CTX_OLD, in order to derive
   a temporary Security Context CTX_1.  Then, the client sends an OSCORE
   request to the server, protected with the Security Context CTX_1.  In
   particular, the request has the 'd' flag bit set to 1 and specifies
   R1 as 'id detail' (see Section 4.1).

   Upon receiving the OSCORE request, the server retrieves the value R1
   from the 'id detail' of the request, and uses the nonce N = R1
   together with the old Security Context CTX_OLD, in order to derive
   the temporary Security Context CTX_1.  Then, the server verifies the
   request by using the Security Context CTX_1.

   After that, the server generates a random value R2, and uses the
   nonce N = R1 | R2 together with the old Security Context CTX_OLD, in
   order to derive the new Security Context CTX_NEW.  Then, the server
   sends an OSCORE response to the client, protected with the new
   Security Context CTX_NEW.  In particular, the response has the 'd'
   flag bit set to 1 and specifies R2 as 'id detail'.

   Upon receiving the OSCORE response, the client retrieves the value R2
   from the 'id detail' of the response.  Since the client has received
   a response to an OSCORE request it made with the 'd' flag bit set to
   1, the client uses the nonce N = R1 | R2 together with the old
   Security Context CTX_OLD, in order to derive the new Security Context
   CTX_NEW.  Finally, the client verifies the response by using the
   Security Context CTX_NEW and deletes the old Security Context
   CTX_OLD.






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   After that, the client can send a new OSCORE request protected with
   the new Security Context CTX_NEW.  When successfully verifying the
   request using the Security Context CTX_NEW, the server deletes the
   old Security Context CTX_OLD and can reply with an OSCORE response
   protected with the new Security Context CTX_NEW.

   From then on, the two peers can protect their message exchanges by
   using the new Security Context CTX_NEW.

   Note that the server achieves key confirmation only when receiving a
   message from the client as protected with the new Security Context
   CTX_NEW.  If the server sends a non KUDOS request to the client
   protected with CTX_NEW before then, and the server receives a 4.01
   (Unauthorized) error response as reply, the server SHOULD delete the
   new Security Context CTX_NEW and start a new client-initiated key
   update process, by taking the role of initiator as per Figure 5.

   Also note that, if both peers reboot simultaneously, they will run
   the client-initiated version of KUDOS defined in this section.  That
   is, one of the two peers implementing a CoAP client will send KUDOS
   Request #1 in Figure 5.

4.3.1.1.  Avoiding In-Transit Requests During a Key Update

   Before sending the KUDOS message Request #1 in Figure 5, the client
   MUST ensure that it has no ouststanding interactions with the server
   (see Section 4.7 of [RFC7252]), with the exception of ongoing
   observations [RFC7641] with that server.

   If there are any, the client MUST NOT initiate 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
   server.

   Later on, this prevents a non KUDOS response protected with the new
   Security Context CTX_NEW to cryptographically match 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.

4.3.2.  Server-Initiated Key Update

   Figure 6 shows the KUDOS workflow with the server acting as
   initiator.






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                        Client               Server
                     (responder)          (initiator)
                          |                    |
                          |     Request #1     |
     Protect with CTX_OLD |------------------->|
                          |                    | Verify with CTX_OLD
                          |                    |
                          |                    | Generate R1
                          |                    |
                          |                    | CTX_1 =
                          |                    |   updateCtx(R1,
                          |                    |             CTX_OLD)
                          |                    |
                          |     Response #1    |
                          |<-------------------| Protect with CTX_1
     CTX_1 =              | OSCORE Option:     |
       updateCtx(R1,      |   ...              |
                 CTX_OLD) |   d flag: 1        |
                          |   ...              |
     Verify with CTX_1    |   ID Detail: R1    |
                          |   ...              |
     Generate R2          |                    |
                          |                    |
     CTX_NEW =            |                    |
       updateCtx(R1|R2,   |                    |
                 CTX_OLD) |                    |
                          |                    |
                          |     Request #2     |
     Protect with CTX_NEW |------------------->|
                          | OSCORE Option:     | CTX_NEW =
                          |   ...              |   updateCtx(R1|R2,
                          |   d flag: 1        |             CTX_OLD)
                          |   ...              |
                          |   ID Detail: R1|R2 | Verify with CTX_NEW
                          |   ...              |
                          |                    | Discard CTX_OLD
                          |                    |

     // The actual key update process ends here.
     // The two peers can use the new Security Context CTX_NEW.

                          |     Response #2    |
                          |<-------------------| Protect with CTX_NEW
     Verify with CTX_NEW  |                    |
                          |                    |
     Discard CTX_OLD      |                    |
                          |                    |




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                 Figure 6: Server-Initiated KUDOS Workflow

   First, the client sends a normal OSCORE request to the server,
   protected with the old Security Context CTX_OLD and with the 'd' flag
   bit set to 0.

   Upon receiving the OSCORE request and after having verified it with
   the old Security Context CTX_OLD as usual, the server generates a
   random value R1 and uses the nonce N = R1 together with the old
   Security Context CTX_OLD, in order to derive a temporary Security
   Context CTX_1.  Then, the server sends an OSCORE response to the
   client, protected with the Security Context CTX_1.  In particular,
   the response has the 'd' flag bit set to 1 and specifies R1 as 'id
   detail' (see Section 4.1).

   Upon receiving the OSCORE response, the client retrieves the value R1
   from the 'id detail' of the response, and uses the nonce N = R1
   together with the old Security Context CTX_OLD, in order to derive
   the temporary Security Context CTX_1.  Then, the client verifies the
   response by using the Security Context CTX_1.

   After that, the client generates a random value R2, and uses the
   nonce N = R1 | R2 together with the old Security Context CTX_OLD, in
   order to derive the new Security Context CTX_NEW.  Then, the client
   sends an OSCORE request to the server, protected with the new
   Security Context CTX_NEW.  In particular, the request has the 'd'
   flag bit set to 1 and specifies R1 | R2 as 'id detail'.

   Upon receiving the OSCORE request, the server retrieves the value
   R1 | R2 from the request.  Then, the server verifies that: i) the
   value R1 is identical to the value R1 specified in a previous OSCORE
   response with the 'd' flag bit set to 1; and ii) the value R1 | R2
   has not been received before in an OSCORE request with the 'd' flag
   bit set to 1.  If the verification succeeds, the server uses the
   nonce N = R1 | R2 together with the old Security Context CTX_OLD, in
   order to derive the new Security Context CTX_NEW.  Finally, the
   server verifies the request by using the Security Context CTX_NEW and
   deletes the old Security Context CTX_OLD.

   After that, the server can send an OSCORE response protected with the
   new Security Context CTX_NEW.  When successfully verifying the
   response using the Security Context CTX_NEW, the client deletes the
   old Security Context CTX_OLD.

   From then on, the two peers can protect their message exchanges by
   using the new Security Context CTX_NEW.





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   Note that the client achieves key confirmation only when receiving a
   message from the server as protected with the new Security Context
   CTX_NEW.  If the client sends a non KUDOS request to the server
   protected with CTX_NEW before then, and the client receives a 4.01
   (Unauthorized) error response as reply, the client SHOULD delete the
   new Security Context CTX_NEW and start a new client-initiated key
   update process, by taking the role of initiator as per Figure 5 in
   Section 4.3.1.

4.3.2.1.  Avoiding In-Transit Requests During a Key Update

   Before sending the KUDOS message Request #2 in Figure 6, the client
   MUST ensure that it has no ouststanding interactions with the server
   (see Section 4.7 of [RFC7252]), with the exception of ongoing
   observations [RFC7641] with that server.

   If there are any, the client MUST NOT initiate 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
   server.

   Later on, this prevents a non KUDOS response protected with the new
   Security Context CTX_NEW to cryptographically match 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.

4.3.2.2.  Preventing Deadlock Situations

   When the server-initiated version of KUDOS is used, the two peers
   risk to run into a deadlock, if all the following conditions hold.

   *  The client is a client-only device, i.e., it is not capable to act
      as CoAP server and thus does not listen for incoming requests.

   *  The server needs to execute KUDOS, which, due to the previous
      point, can only be performed in its server-initiated version as
      per Figure 6.  That is, the server has to wait for an incoming non
      KUDOS request, in order to initiate KUDOS by replying with the
      first KUDOS message as a response.

   *  The client sends only Non-confirmable CoAP requests to the server
      and does not expect responses sent back as reply, hence freeing up
      a request's Token value once the request is sent.






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   In such a case, in order to avoid experiencing a deadlock situation
   where the server needs to execute KUDOS but cannot practically
   initiate it, a client-only device that supports KUDOS SHOULD
   intersperse Non-confirmable requests it sends to that server with
   confirmable requests.

4.4.  Retention Policies

   Applications MAY define policies that allows a peer to also
   temporarily keep the old Security Context CTX_OLD, rather than simply
   overwriting it to become CTX_NEW.  This allows the peer to decrypt
   late, still on-the-fly incoming 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.

   *  When an amount of time defined by the policy has elapsed since the
      establishment of CTX_NEW, the peer deletes CTX_OLD.

4.5.  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 3).  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.

   Since the JRC uses ID Context values as identifiers of network nodes,
   namely "pledge identifiers", the above implies that the JRC does not
   have anymore to perform 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.






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5.  Security Considerations

   This document mainly covers security considerations about using AEAD
   keys in OSCORE and their usage limits, in addition to the security
   considerations of [RFC8613].

   Depending on the specific key update procedure used to establish a
   new OSCORE Security Context, the related security considerations also
   apply.

   [TODO: Add more considerations.]

6.  IANA Considerations

   RFC Editor: Please replace "[this document]" with the RFC number of
   this document and delete this paragraph.

   This document has the following actions for IANA.

6.1.  CoAP Option Numbers Registry

   IANA is asked to enter the following option number to the "CoAP
   Option Numbers" registry within the "CoRE Parameters" registry group.

                +--------+--------------+-----------------+
                | Number |     Name     |    Reference    |
                +--------+--------------+-----------------+
                |  TBD   | Recipient-ID | [this document] |
                +--------+--------------+-----------------+

   The number suggested to IANA for the Recipient-ID option is 24.

6.2.  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 |                  |                        |           |
   +----------+------------------+------------------------+-----------+
   |    1     | Extension-1 Flag | Set to 1 if the OSCORE | [this     |
   |          |                  | Option specifies a     | document] |
   |          |                  | second byte of OSCORE  |           |
   |          |                  | flag bits              |           |
   +----------+------------------+------------------------+-----------+
   |    15    |  ID Detail Flag  | Set to 1 if the        | [this     |
   |          |                  | compressed COSE object | document] |
   |          |                  | contains 'id detail'   |           |
   +----------+------------------+------------------------+-----------+

7.  References

7.1.  Normative References

   [I-D.ietf-lake-edhoc]
              Selander, G., Mattsson, J. P., and F. Palombini,
              "Ephemeral Diffie-Hellman Over COSE (EDHOC)", Work in
              Progress, Internet-Draft, draft-ietf-lake-edhoc-12, 20
              October 2021, <https://www.ietf.org/archive/id/draft-ietf-
              lake-edhoc-12.txt>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC7252]  Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
              Application Protocol (CoAP)", RFC 7252,
              DOI 10.17487/RFC7252, June 2014,
              <https://www.rfc-editor.org/info/rfc7252>.

   [RFC7641]  Hartke, K., "Observing Resources in the Constrained
              Application Protocol (CoAP)", RFC 7641,
              DOI 10.17487/RFC7641, September 2015,
              <https://www.rfc-editor.org/info/rfc7641>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8613]  Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
              "Object Security for Constrained RESTful Environments
              (OSCORE)", RFC 8613, DOI 10.17487/RFC8613, July 2019,
              <https://www.rfc-editor.org/info/rfc8613>.



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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/info/rfc8949>.

7.2.  Informative References

   [I-D.ietf-ace-oauth-authz]
              Seitz, L., Selander, G., Wahlstroem, E., Erdtman, S., and
              H. Tschofenig, "Authentication and Authorization for
              Constrained Environments (ACE) using the OAuth 2.0
              Framework (ACE-OAuth)", Work in Progress, Internet-Draft,
              draft-ietf-ace-oauth-authz-46, 8 November 2021,
              <https://www.ietf.org/archive/id/draft-ietf-ace-oauth-
              authz-46.txt>.

   [I-D.ietf-ace-oscore-profile]
              Palombini, F., Seitz, L., Selander, G., and M. Gunnarsson,
              "OSCORE Profile of the Authentication and Authorization
              for Constrained Environments Framework", Work in Progress,
              Internet-Draft, draft-ietf-ace-oscore-profile-19, 6 May
              2021, <https://www.ietf.org/archive/id/draft-ietf-ace-
              oscore-profile-19.txt>.

   [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-03, 12 July 2021,
              <https://www.ietf.org/archive/id/draft-irtf-cfrg-aead-
              limits-03.txt>.

   [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,
              <http://www.openmobilealliance.org/release/LightweightM2M/
              V1_2-20201110-A/OMA-TS-LightweightM2M_Transport-
              V1_2-20201110-A.pdf>.





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   [RFC7519]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
              (JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,
              <https://www.rfc-editor.org/info/rfc7519>.

   [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/info/rfc7554>.

   [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/info/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/info/rfc8446>.

   [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/info/rfc9031>.

Appendix A.  Detailed considerations for AEAD_AES_128_CCM_8

   For the AEAD_AES_128_CCM_8 algorithm when used as AEAD Algorithm for
   OSCORE, larger IA and CA values are achieved, depending on the value
   of 'q', 'v' and 'l'.  Figure 7 shows the resulting IA and CA
   probabilities enjoyed by AEAD_AES_128_CCM_8, when taking different
   values of 'q', 'v' and 'l' as input to the formulas defined in
   [I-D.irtf-cfrg-aead-limits].

   As shown in Figure 7, it is especially possible to achieve the lowest
   IA = 2^-50 and a good CA = 2^-70 by considering the largest possible
   value of the (q, v, l) triplet equal to (2^20, 2^10, 2^8), while
   still keeping a good security level.  Note that the value of 'l' does
   not impact on IA, while CA displays good values for every considered
   value of 'l'.












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        +-----------------------+----------------+----------------+
        | 'q', 'v' and 'l'      | IA probability | CA probability |
        |-----------------------+----------------+----------------|
        | q=2^20, v=2^20, l=2^8 | 2^-44          | 2^-70          |
        | q=2^15, v=2^20, l=2^8 | 2^-44          | 2^-80          |
        | q=2^10, v=2^20, l=2^8 | 2^-44          | 2^-90          |
        | q=2^20, v=2^15, l=2^8 | 2^-49          | 2^-70          |
        | q=2^15, v=2^15, l=2^8 | 2^-49          | 2^-80          |
        | q=2^10, v=2^15, l=2^8 | 2^-49          | 2^-90          |
        | q=2^20, v=2^14, l=2^8 | 2^-50          | 2^-70          |
        | q=2^15, v=2^14, l=2^8 | 2^-50          | 2^-80          |
        | q=2^10, v=2^14, l=2^8 | 2^-50          | 2^-90          |
        | q=2^20, v=2^10, l=2^8 | 2^-54          | 2^-70          |
        | q=2^15, v=2^10, l=2^8 | 2^-54          | 2^-80          |
        | q=2^10, v=2^10, l=2^8 | 2^-54          | 2^-90          |
        |-----------------------+----------------+----------------|
        | q=2^20, v=2^20, l=2^6 | 2^-44          | 2^-74          |
        | q=2^15, v=2^20, l=2^6 | 2^-44          | 2^-84          |
        | q=2^10, v=2^20, l=2^6 | 2^-44          | 2^-94          |
        | q=2^20, v=2^15, l=2^6 | 2^-49          | 2^-74          |
        | q=2^15, v=2^15, l=2^6 | 2^-49          | 2^-84          |
        | q=2^10, v=2^15, l=2^6 | 2^-49          | 2^-94          |
        | q=2^20, v=2^14, l=2^6 | 2^-50          | 2^-74          |
        | q=2^15, v=2^14, l=2^6 | 2^-50          | 2^-84          |
        | q=2^10, v=2^14, l=2^6 | 2^-50          | 2^-94          |
        | q=2^20, v=2^10, l=2^6 | 2^-54          | 2^-74          |
        | q=2^15, v=2^10, l=2^6 | 2^-54          | 2^-84          |
        | q=2^10, v=2^10, l=2^6 | 2^-54          | 2^-94          |
        +-----------------------+----------------+----------------+

     Figure 7: Probabilities for AEAD_AES_128_CCM_8 based on chosen q,
                              v and l values.

Appendix B.  Estimation of 'count_q'

   This section defines a method to compute an estimate of the counter
   'count_q' (see Section 2.2.2), hence not requiring a peer to store it
   in its own Sender Context.

   This method relies on the fact that, at any point in time, a peer has
   performed _at most_ ENC = (SSN + SSN*) encryptions using its own
   Sender Key, where:

   *  SSN is the current value of this peer's Sender Sequence Number.

   *  SSN* is the current value of other peer's Sender Sequence Number.
      That is, SSN* is an overestimation of the responses without
      Partial IV that this peer has sent.



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   Thus, when protecting an outgoing message (see Section 2.3.1), the
   peer aborts the message processing if the estimated est_q > limit_q,
   where est_q = (SSN + X) and X is determined as follows.

   *  If the outgoing message is a response, X is the Partial IV
      specified in the corresponding request that this peer is
      responding to.  Note that X < SSN* always holds.

   *  If the outgoing message is a request, X is the highest Partial IV
      value marked as received in this peer's Replay Window plus 1, or 0
      if it has not accepted any protected message from the other peer
      yet.  That is, X is the highest Partial IV specified in message
      received from the other peer, i.e., the highest seen Sender
      Sequence Number of the other peer.  Note that, also in this case,
      X < SSN* always holds.

Appendix C.  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
   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,
   after having completed 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, i.e., an Observe request protected with CTX_OLD and an
   Observe request protected with CTX_NEW.

   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 Appendix C.2.  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, as defined
   in Section 4.3.

   If a peer supporting KUDOS is generally interested in preserving
   ongoing observations across a key update, the peer maintains a
   counter EPOCH for each ongoing observation it participates in.  At
   any point in time, (EPOCH + 1) is the number of KUDOS executions
   performed by the peer since the sucessful registration of the
   associated observation.  That is, EPOCH indicates the lifetime of an
   observation measured in keying material epochs, and is bounded by the
   configuration parameter MAX_EPOCH.




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   [ NOTE:

   MAX_EPOCH really has to be the same for any two peers.  As a start,
   it can be assumed that a TBD default value applies, unless a
   different one is provided.  It is possible to enable an actual
   negotiation between two peers running KUDOS, see Appendix C.2.

   ]

   The following sections specify the different actions taken by the
   peer depending on whether it acts as client or server in an ongoing
   observation, as well as the signaling method used in KUDOS to agree
   on preserving the ongoing observations beyond the current KUDOS
   execution.

   [ NOTE:

   This method may be of more general applicability, i.e, also in case
   an update of the OSCORE keying material is performed through a
   different means than KUDOS.

   ]

C.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 sends back a successful response also including the
   Observe option, hence confirming that the observation has been
   registered, then the server initializes to 0 the counter EPOCH
   associated with the just confirmed observation.

   If the client receives back the successful response from the server,
   then the client initializes to 0 the counter EPOCH associated with
   the just confirmed observation.

   If, later on, the client is not interested in the observation
   anymore, it MUST NOT simply forget about it.  Rather, 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.




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   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
   across the key update, then A explicitly indicates it by using the
   signaling approach embedded in KUDOS and defined in Appendix C.2.

   After having successfully completed the KUDOS execution (i.e., after
   having successfully derived the new OSCORE Security Context CTX_NEW),
   if the other peer B has confirmed its interest in preserving those
   ongoing observations also by using the signaling approach defined in
   Appendix C.2, then the peer A performs the following actions.

   1.  For each ongoing observation X that A has with B and for which
       following notifications are going to be protected with CTX_NEW:

       a.  The peer A increments the counter EPOCH associated with X.

       b.  If the updated value of EPOCH associated with X has reached
       MAX_EPOCH, then the peer A MUST terminate the observation.

   2.  For each still ongoing observation X that A has with B after the
       previous step, such that A acts as client in X and for which
       following notifications are going to be protected with CTX_NEW:

       a.  The peer A MUST attempt again to cancel X, if A previously
       tried to do that but had not received a response from the other
       peer B as confirmation.  As specified above, such an observation
       cancellation MUST be performed by sending a cancellation request.

       b.  The peer A considers all the OSCORE Partial IV values used in
       the Observe registration request associated with any of the still
       ongoing observations with the other peer B.  Then, the peer A
       determines the value PIV* as the highest OSCORE Partial IV among
       those considered at the previous step.

       c.  In the Sender Context within CTX_NEW, the peer A sets its own
       Sender Sequence Number to (PIV* + 1), rather than to 0.

C.2.  Signaling to Preserve Observations

   When performing KUDOS, a peer can indicate to the other peer its
   interest in preserving the ongoing observations that they have with
   one another and are bound to the OSCORE Security Context to renew.
   To this end, the extended OSCORE option shown in Figure 3 and
   included in a KUDOS message is further extended as follows.

   [ NOTE:

   This is an early proposal with many details to be refined.



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   ]

   An additional bit "Preserve Observations", 'b', is set to 1 by the
   sender peer to indicate that it wishes to preserve ongoing
   observations with the other peer.

   While 'b' can be a bit in the second byte of the OSCORE option
   containing the OSCORE flag bits, 'b' can rather be one bit in the 1
   byte 'x' following 'kid context' (if any) and originally encoding the
   size of 'id detail'.  Since, the recommended size of 'id detail' is 8
   bytes, the number of bits left available in the 'x' byte is amply
   sufficient to still indicate the size of 'id detail'.

   It is fundamental to integrity-protect the value of the bit 'b' set
   in the two KUDOS messages.  This can be achieved by taking also the
   whole byte 'x' including the bit 'b' as input in the derivation of
   the new OSCORE Security Context CTX_NEW.

   That is, the updateCtx() function defined in Figure 4 would be
   invoked as follows:

   *  CTX_1 = updateCtx(X1|R1, CTX_OLD), when deriving CTX_1 for
      processing the first KUDOS message in the KUDOS execution.

   *  CTX_NEW = updateCtx(X1|X2|R1|R2, CTX_OLD), when deriving CTX_NEW
      for processing the second KUDOS message in the KUDOS execution.

   where X1 and X2 are the values of the 'x' byte specified in the
   OSCORE option of the first and second KUDOS message in the KUDOS
   execution, respectively.

   [ NOTE:

   The single bit 'b' can actually be replaced by three bits 'b1', 'b2'
   and 'b3' still within the byte 'x'.  These can be used by the two
   peers performing KUDOS to negotiate the value of MAX_EPOCH (see
   Appendix C.  Then, the two peers agree to use as MAX_EPOCH the
   smallest of the two values exchanged during the execution of KUDOS.

   The final encoding of the 'x' byte specified in Section 4.1 will be
   affected.  In particular, a smarter encoding would be convenient for
   the bits left to use to indicate the size in bytes of 'id detail'.

   ]







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Appendix D.  Update of OSCORE Sender/Recipient IDs

   This section defines an optional procedure that two peers can execute
   to update the OSCORE Sender/Recipient IDs that they use in their
   shared OSCORE Security Context.

   This procedure can be initiated by either peer.  In particular, the
   client or the server may start it by sending the first OSCORE ID
   update message.  When sending an OSCORE ID update message, a peer
   provides its new intended OSCORE Recipient ID to the other peer.

   Furthermore, this procedure can be executed stand-alone, or rather
   seamlessly integrated in an execution of KUDOS (see Section 4).

   *  In the former stand-alone case, updating the OSCORE Sender/
      Recipient IDs effectively results in updating part of the current
      OSCORE Security Context.

      That is, a new Sender Key, Recipient Key and Common IV are derived
      as defined in Section 3.2 of [RFC8613].  Also, the Sender Sequence
      Number and the replay window are re-initialized accordingly, as
      defined in Section 3.2.2 of [RFC8613].  Since the same Master
      Secret is preserved, forward secrecy is not achieved.

      Finally, as defined in Appendix D.1.3, the two peers must take
      additional actions to ensure a safe execution of the OSCORE IDs
      update procedure.

   *  In the latter integrated case, the KUDOS initiator (responder)
      also acts as initiator (responder) for the ID update procedure.

   [TODO: think about the possibility of safely preserving ongoing
   observations following an update of OSCORE IDs alone.]

D.1.  The Recipient-ID Option

   The Recipient ID Option defined in this section has the properties
   summarized in Figure 8, which extends Table 4 of [RFC7252].  That is,
   the option is elective, safe to forward, part of the cache key and
   non repeatable.











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    +------+---+---+---+---+--------------+--------+--------+---------+
    | No.  | C | U | N | R | Name         | Format | Length | Default |
    +------+---+---+---+---+--------------+--------+--------+---------+
    |      |   |   |   |   |              |        |        |         |
    | TBD1 |   |   |   |   | Recipient-ID | opaque |  0-7   | (none)  |
    |      |   |   |   |   |              |        |        |         |
    +------+---+---+---+---+--------------+--------+--------+---------+
             C=Critical, U=Unsafe, N=NoCacheKey, R=Repeatable

                     Figure 8: The Recipient-ID Option.

   This document particularly defines how this option is used in
   messages protected with OSCORE.  That is, when the option is included
   in an outgoing message, the option value specifies the new OSCORE
   Recipient ID that the sender endpoint intends to use with the other
   endpoint sharing the OSCORE Security Context.

   The Recipient-ID Option is of class E in terms of OSCORE processing
   (see Section 4.1 of [RFC8613]).

D.1.1.  Client-Initiated OSCORE IDs Update

   Figure 9 shows the stand-alone OSCORE IDs update workflow, with the
   client acting as initiator.

   On each peer, SID and RID denote the OSCORE Sender ID and Recipient
   ID of that peer, respectively.

                 Client                             Server
              (initiator)                         (responder)
                   |                                   |
       CTX_A {     |                                   | CTX_A {
        SID = 1    |                                   |  SID = 0
        RID = 0    |                                   |  RID = 1
       }           |                                   | }
                   |                                   |
                   |            Request #1             |
       Protect     |---------------------------------->|
       with CTX_A  | OSCORE Option: ..., kid:1         | Verify
                   | Encrypted_Payload {               | with CTX_A
                   |    ...                            |
                   |    RecipientID: 42                |
                   |    ...                            |
                   |    Application Payload            |
                   | }                                 |
                   |                                   |

                 // When embedded in KUDOS, CTX_1 is CTX_A,



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                 // and there cannot be application payload.

                   |                                   |
                   |            Response #1            |
                   |<----------------------------------| Protect
       Verify      | OSCORE Option: ...                | with CTX_A
       with CTX_A  | Encrypted_Payload {               |
                   |    ...                            |
                   |    Recipient-ID: 78               |
                   |    ...                            |
                   |    Application Payload            |
                   | }                                 |
                   |                                   |

                  // When embedded in KUDOS, this message
                  // is protected using CTX_NEW, and there
                  // there cannot be application payload.
                  //
                  // Then, CTX_B builds on CTX_NEW by updating
                  // the new Sender/Recipient IDs

                   |                                   |
       CTX_B {     |                                   | CTX_B {
        SID = 78   |                                   |  SID = 42
        RID = 42   |                                   |  RID = 78
       }           |                                   | }
                   |                                   |
                   |            Request #2             |
       Protect     |---------------------------------->|
       with CTX_B  | OSCORE Option: ..., kid:78        | Verify
                   | Encrypted_Payload {               | with  CTX_B
                   |    ...                            |
                   |    Application Payload            |
                   | }                                 |
                   |                                   |
                   |            Response #2            |
                   |<----------------------------------| Protect
       Verify      | OSCORE Option: ...                | with CTX_B
       with CTX_B  | Encrypted_Payload {               |
                   |    ...                            |
                   |    Application Payload            |
                   | }                                 |
                   |                                   |
       Discard     |                                   |
       CTX_A       |                                   |
                   |                                   |
                   |            Request #3             |
       Protect     |---------------------------------->|



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       with CTX_B  | OSCORE Option: ..., kid:78        | Verify
                   | Encrypted_Payload {               | with CTX_B
                   |    ...                            |
                   |    Application Payload            |
                   | }                                 |
                   |                                   | Discard
                   |                                   | CTX_A
                   |                                   |

           Figure 9: Client-Initiated OSCORE IDs Update Workflow

   [TODO: discuss the example]

D.1.2.  Server-Initiated OSCORE IDs Update

   Figure 10 shows the stand-alone OSCORE IDs update workflow, with the
   server acting as initiator.

   On each peer, SID and RID denote the OSCORE Sender ID and Recipient
   ID of that peer, respectively.

                 Client                             Server
              (responder)                         (initiator)
                   |                                   |
       CTX_A {     |                                   | CTX_A {
        SID = 1    |                                   |  SID = 0
        RID = 0    |                                   |  RID = 1
       }           |                                   | }
                   |                                   |
                   |            Request #1             |
       Protect     |---------------------------------->|
       with CTX_A  | OSCORE Option: ..., kid:1         | Verify
                   | Encrypted_Payload {               | with CTX_A
                   |    ...                            |
                   |    Application Payload            |
                   | }                                 |
                   |                                   |

                 // When (to be) embedded in KUDOS,
                 // CTX_OLD is CTX_A

                   |                                   |
                   |            Response #1            |
                   |<----------------------------------| Protect
       Verify      | OSCORE Option: ...                | with CTX_A
       with CTX_A  | Encrypted_Payload {               |
                   |    ...                            |
                   |    Recipient-ID: 78               |



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                   |    Application Payload            |
                   | }                                 |

                 // When embedded in KUDOS, this message is
                 // protected with CTX_1 instead, and
                 // there cannot be application payload.

                   |                                   |
       CTX_A {     |                                   | CTX_A {
        SID = 1    |                                   |  SID = 0
        RID = 0    |                                   |  RID = 1
       }           |                                   | }
                   |                                   |
                   |            Request #2             |
       Protect     |---------------------------------->|
       with CTX_A  | OSCORE Option: ..., kid:1         | Verify
                   | Encrypted_Payload {               | with CTX_A
                   |    ...                            |
                   |    Recipient-ID: 42               |
                   |    Application Payload            |
                   | }                                 |
                   |                                   |

                 // When embedded in KUDOS, this message is
                 // protected with CTX_NEW instead, and
                 // there cannot be application payload.

                   |                                   |
                   |            Response #2            |
                   |<----------------------------------| Protect
       Verify      | OSCORE Option: ...                | with CTX_A
       with CTX_A  | Encrypted_Payload {               |
                   |    ...                            |
                   |    Application Payload            |
                   | }                                 |
                   |                                   |

                 // When embedded in KUDOS, this message is
                 // protected with CTX_NEW instead, and
                 // there cannot be application payload.

                   |                                   |
       CTX_B {     |                                   | CTX_B {
        SID = 78   |                                   |  SID = 42
        RID = 42   |                                   |  RID = 78
       }           |                                   | }
                   |                                   |
                   |            Request #3             |



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       Protect     |---------------------------------->|
       with CTX_B  | OSCORE Option: ..., kid:78        | Verify
                   | Encrypted_Payload {               | with CTX_B
                   |    ...                            |
                   |    Application Payload            |
                   | }                                 |
                   |                                   |
                   |            Response #3            |
                   |<----------------------------------| Protect
       Verify      | OSCORE Option: ...                | with CTX_B
       with CTX_B  | Encrypted_Payload {               |
                   |    ...                            |
                   |    Application Payload            |
                   | }                                 |
                   |                                   |
       Discard     |                                   |
       CTX_A       |                                   |
                   |                                   |
                   |            Request #4             |
       Protect     |---------------------------------->|
       with CTX_B  | OSCORE Option: ..., kid:78        | Verify
                   | Encrypted_Payload {               | with CTX_B
                   |    ...                            |
                   |    Application Payload            |
                   | }                                 |
                   |                                   |
                   |                                   | Discard
                   |                                   | CTX_A
                   |                                   |
                   |            Response #4            |
                   |<----------------------------------| Protect
       Verify      | OSCORE Option: ...                | with CTX_B
       with CTX_B  | Encrypted_Payload {               |
                   |    ...                            |
                   |    Application Payload            |
                   | }                                 |
                   |                                   |

           Figure 10: Server-Initiated OSCORE IDs Update Workflow

   [TODO: discuss the example]










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D.1.3.  Additional Actions for Stand-Alone Execution

   After having experienced a loss of state, a peer MUST NOT participate
   in a stand-alone OSCORE IDs update procedure with another peer, until
   having performed a full-fledged establishment/renewal of an OSCORE
   Security Context with the other peer (e.g., through KUDOS or EDHOC
   [I-D.ietf-lake-edhoc]).

   More precisely, a peer has experienced a loss of state if it cannot
   access the latest snapshot of the latest OSCORE Security Context
   CTX_OLD or the whole set of OSCORE Sender/Recipient IDs that have
   been used with the triplet (Master Secret, Master Salt ID Context) of
   CTX_OLD.  This can happen, for instance, following a device reboot.

   Furthermore, when participating in a stand-alone OSCORE IDs update
   procedure, a peer perform the following additional steps.

   *  When sending an OSCORE ID update message, the peer MUST specify
      its new intended OSCORE Recipient ID as value of the Recipient-ID
      option only if such a Recipient ID is not only available (see
      Section 3.3 of [RFC8613], but it has also never been used as
      Recipient ID with the current triplet (Master Secret, Master Salt
      ID Context).

   *  When receiving an OSCORE ID update message, the peer MUST abort
      the procedure if it has already used the identifier specified in
      the Recipient-ID Option as its own Sender ID with current triplet
      (Master Secret, Master Salt ID Context).

   In order to fulfill the conditions above, a peer has to keep track of
   the OSCORE Sender/Recipient IDs that it has used with the current
   triplet (Master Secret, Master Salt ID Context), since the latest
   update of OSCORE Master Secret (e.g, performed through KUDOS).

Appendix E.  Key Update without Forward Secrecy

   The main version of the KUDOS procedure defined in Section 4 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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   However, 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 not more after that.  Therefore, these devices cannot
   perform a stateful key update procedure, which prevents running the
   main version of KUDOS and ensuring forward secrecy.

   In order to address these limitations, this section defines an
   alternative, stateless version of the KUDOS procedure.  This allows
   two peers to achieve the same results as when running the main
   version of KUDOS defined in Section 4, with the difference that no
   forward secrecy is achieved and no state information is required to
   be dynamically written in non-volatile memory.

   Hereafter, "FS mode" and "non-FS mode" refer to the main version of
   KUDOS defined in Section 4 and the alternative version of KUDOS
   defined in this section, respectively.  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_OLD
   provided as input to the updateCtx() function (see Section 4).

   In order to run KUDOS in FS mode, both peers have to be able to write
   in non-volatile memory the OSCORE Master Secret and OSCORE Master
   Salt from the newly derived Security Context CTX_NEW.  If this is not
   the case, the two peers have to run KUDOS in non-FS mode.

E.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-volatible memory (e.g., on disk), beyond a
   one-time-only writing occurring at manufacturing or
   (re-)commissioning time.

   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:




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   *  A peer running KUDOS can have none of the pairs above associated
      with another peer, only one or both.

   *  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.

   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 EDHOC
   [I-D.ietf-lake-edhoc]), a peer considers the Master Secret and Master
   Salt of CTX as Latest Master Secret and Latest Master Salt.  After
   that:

   *  If the peer is a CAPABLE device, it SHOULD store Latest Master
      Secret and Latest Master Salt on disk.

      As an exception, this does not apply to possible temporary OSCORE
      Security Contexts used during a key update procedure, such as
      CTX_1 used during the KUDOS execution.  That is, the OSCORE Master
      Secret and Master Salt from such temporary Security Contexts MUST
      NOT be 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.

E.1.1.  Actions after Device Reboot

   Building on the above, after having experienced a reboot, a peer A
   checks whether it has a pair P1 = (Latest Master Secret, Latest
   Master Salt) associated with any another peer B stored on disk.

   *  If a pair P1 is found, the peer A performs the following actions.




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      -  The peer A loads the Latest Master Secret and Latest Master
         Salt to volatile memory, and uses them to derive an OSCORE
         Security Context CTX_OLD.

      -  The peer A runs KUDOS with the other peer B, acting as
         initiator.  If the peer A is a CAPABLE device, it stores on
         disk the Master Secret and Master Salt from the newly
         established OSCORE Security Context CTX_NEW, as Latest Master
         Secret and Latest Master Salt, respectively.

   *  If a pair P1 is not found, the peer A checks whether it has a pair
      P2 = (Bootstrap Master Secret, Bootstrap Master Salt) associated
      with the other peer B stored on disk.

      -  If a pair P2 is found, the peer A performs the following
         actions.

         o  The peer A loads the Bootstrap Master Secret and Bootstrap
            Master Salt to volatile memory, and uses them to derive an
            OSCORE Security Context CTX_OLD.

         o  If the peer A is a CAPABLE device, it stores on disk
            Bootstrap Master Secret and Bootstrap Master Salt as Latest
            Master Secret and Latest Master Salt, respectively.  This
            supports the situation where A is a CAPABLE device and has
            never run KUDOS with the other peer B before.

         o  The peer A runs KUDOS with the other peer B, acting as
            initiator.  If the peer A is a CAPABLE device, it stores on
            disk the Master Secret and Master Salt from the newly
            established OSCORE Security Context CTX_NEW, as Latest
            Master Secret and Latest Master Salt, respectively.

      -  If a pair P2 is not found, the peer A has to use alternative
         ways to establish a first OSCORE Security Context CTX_NEW with
         the other peer B, e.g., by running EDHOC.  After that, if A is
         a CAPABLE device, it stores on disk the Master Secret and
         Master Salt from the newly established OSCORE Security Context
         CTX_NEW, as Latest Master Secret and Latest Master Salt,
         respectively.

E.2.  Signaling of FS Mode or Non-FS Mode

   This section defines the signaling method that two peers use to agree
   on whether to run KUDOS in FS or non-FS mode.  To this end, the
   extended OSCORE option shown in Figure 3 and included in a KUDOS
   message is further extended as follows.




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   An additional bit "No Forward Secrecy", 'p', is set to 1 by the
   sender peer to indicate that it wishes to run KUDOS in no-FS mode, or
   to 0 if it wishes to run KUDOS in FS mode.

   While 'p' can be a bit in the second byte of the OSCORE option
   containing the OSCORE flag bits, 'p' can rather be one bit in the 1
   byte 'x' following 'kid context' (if any) and originally encoding the
   size of 'id detail'.  Since, the recommended size of 'id detail' is 8
   bytes, the number of bits left available in the 'x' byte is amply
   sufficient to still indicate the size of 'id detail'.

   [ NOTE:

   The design currently in Appendix C.2 considers also the 'x' byte as
   input to the updateCtx() function.  Preserving this approach would
   integrity-protect the bit 'p' as well, in addition to the protection
   it already enjoys as a by-product from key confirmation.

   ]

   If the second byte of the OSCORE option containing flag bits is
   present and the 'd' flag is set to 0 (i.e., the message is not a
   KUDOS message), the bit 'p' MUST be set to 0.

   In a KUDOS message (i.e., the 'd' bit is set to 1), the 'p' bit
   practically determines what OSCORE Security Context to use as CTX_OLD
   during the KUDOS execution, consistently with the indicated mode.

   *  If the 'p' bit is set to 0, the updateCtx() function used to
      derive CTX_1 or CTX_NEW considers as input CTX_OLD the current
      OSCORE Security Context shared with the other peer as is.  In
      particular, CTX_OLD includes Latest Master Secret as Master Secret
      and Latest Master Salt as Master Salt.

   *  If the 'p' bit is set to 1, the updateCtx() function used to
      derive CTX_1 or CTX_NEW considers as input CTX_OLD the current
      OSCORE Security Context shared with the other peer, with the
      following difference: Bootstrap Master Secret is used as Master
      Secret and Boostrap Master Salt is used as Master Salt.  That is,
      every execution of KUDOS in no-FS mode between these two peers
      considers the same pair (Master Secret, Master Salt) in the OSCORE
      Security Context CTX_OLD provided as input to the updateCtx()
      function, hence the impossibility to achieve forward secrecy.

E.3.  Selection and Negotiation of KUDOS Mode

   This section defines how a peer determines to run KUDOS either in FS
   or no-FS mode with another peer.



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   *  If a peer A is not a CAPABLE device, it MUST run KUDOS only in no-
      FS mode.  That is, when sending a KUDOS message, it MUST set the
      'p' bit to 1 in the OSCORE option (see Appendix E.2).

   *  If a peer A is a CAPABLE device, it SHOULD run KUDOS only in FS
      mode and SHOULD NOT run KUDOS as initiator in no-FS mode.  That
      is, when sending a KUDOS message, it SHOULD set the 'p' bit to 0
      in the OSCORE option (see Appendix E.2).  An exception applies in
      the following cases.

      -  The peer A is running KUDOS with another peer B, which A has
         learned to not be a 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 acting as responder and running KUDOS with
         another peer B without knowing its capabilities, and A receives
         a KUDOS message with the 'p' bit set to 1.

   *  If the 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 non-
      FS mode.  This also means that, if the peer A acts as responder
      when running KUDOS with the peer B, the peer A MUST terminate the
      KUDOS execution if it receives a KUDOS message from the peer B
      with the 'p' bit set to 1.

      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.
      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 not a
      CAPABLE device, and thus to run KUDOS in no-FS mode although the
      FS mode is actually supported by both peers.

   Within the limitations above, two peers running KUDOS generate the
   new OSCORE Security Context CTX_NEW according to the mode indicated
   per the bit 'p' set by the responder in the second KUDOS message.

   If, after having received the first KUDOS message, the responder can
   continue performing KUDOS, the bit 'p' in the reply message has the
   same value as in the bit 'p' set by the initiator, unless the value
   is 0 and the responder is not a CAPABLE device.  More specifically:



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   *  If both peers are CAPABLE devices, they will run KUDOS in FS mode.
      That is, both initiator and responder sets the 'p' bit to 0 in the
      respective sent KUDOS message.

   *  If both peers are not CAPABLE devices or only the peer acting as
      initiator is not a CAPABLE device, they will run KUDOS in no-FS
      mode.  That is, both initiator and responder sets the 'p' bit to 1
      in the respective sent KUDOS message.

   *  If only the peer acting as initiator is a CAPABLE device and it
      has knowledge of the other peer being a not CAPABLE device, they
      will run KUDOS in no-FS mode.  That is, both initiator and
      responder sets the 'p' bit to 1 in the respective sent KUDOS
      message.

   *  If only the peer acting as initiator is a CAPABLE device and it
      has no knowledge of the other peer being a not CAPABLE device,
      they will not run KUDOS in FS mode and will rather set to ground
      for possibly retrying in no-FS mode.  In particular, the initiator
      sets the 'p' bit of its sent KUDOS message to 0.  Then:

      -  If the responder is a server, it MUST reply with a 5.03
         (Service Unavailable) error response.  The response is
         protected with the newly derived OSCORE Security Context
         CTX_NEW.  The diagnostic payload MAY provide additional
         information.  The 'p' bit in the error response MUST be set to
         1.

         When receiving the error response, the initiator learns that
         the responder is not a CAPABLE device (and hence not able to
         run KUDOS in FS mode).  The initiator MAY try running KUDOS
         again, by setting the 'p' bit to 1 when sending a new request
         as first KUDOS message.

      -  If the responder is a client, it sends to the initiator the
         second KUDOS message protected with the newly derived OSCORE
         Security Context CTX_NEW.  The 'p' bit in the request MUST be
         set to 1.

         When receiving the request above (i.e., with the 'p' bit set to
         1 as a follow-up to the previous KUDOS response having the 'p'
         bit set to 0), the initiator learns that the responder is not a
         CAPABLE device (and hence not able to run KUDOS in FS mode).

   In either case, both KUDOS peers delete the OSCORE Security Contexts
   CTX_1 and CTX_NEW.





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Appendix F.  Document Updates

   RFC EDITOR: PLEASE REMOVE THIS SECTION.

F.1.  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.

   *  Added key update without forward secrecy.

Acknowledgments

   The authors sincerely thank Christian Amsuess, John Mattsson and
   Goeran Selander for their feedback and comments.

   The work on this document has been partly supported by VINNOVA and
   the Celtic-Next project CRITISEC; and by the H2020 project SIFIS-Home
   (Grant agreement 952652).

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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