Key Usage Limits for OSCORE
draft-ietf-core-oscore-key-limits-02

CoRE Working Group                                            R. Höglund
Internet-Draft                                                 M. Tiloca
Intended status: Informational                                   RISE AB
Expires: 13 July 2024                                    10 January 2024

                      Key Usage Limits for OSCORE
                  draft-ietf-core-oscore-key-limits-02

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.  Among other
   reasons, approaching key usage limits requires updating the OSCORE
   keying material before communications can securely continue.  This
   document defines how two OSCORE peers can follow these key usage
   limits and what steps they should take to preserve the security of
   their communications.

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-limits.

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
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   This Internet-Draft will expire on 13 July 2024.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  AEAD Key Usage Limits in OSCORE . . . . . . . . . . . . . . .   3
     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  . . . . . . . . . . . . . . . . . . .   7
       2.2.2.  Sender Context  . . . . . . . . . . . . . . . . . . .   7
       2.2.3.  Recipient Context . . . . . . . . . . . . . . . . . .   8
     2.3.  OSCORE Message Processing . . . . . . . . . . . . . . . .   8
       2.3.1.  Protecting a Request or a Response  . . . . . . . . .   9
       2.3.2.  Verifying a Request or a Response . . . . . . . . . .   9
   3.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   5.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     5.1.  Normative References  . . . . . . . . . . . . . . . . . .  10
     5.2.  Informative References  . . . . . . . . . . . . . . . . .  10
   Appendix A.  Detailed considerations for AEAD_AES_128_CCM_8 . . .  11
   Appendix B.  Estimation of 'count_q'  . . . . . . . . . . . . . .  12
   Appendix C.  Document Updates . . . . . . . . . . . . . . . . . .  13
     C.1.  Version -01 to -02  . . . . . . . . . . . . . . . . . . .  13
     C.2.  Version -00 to -01  . . . . . . . . . . . . . . . . . . .  13
     C.3.  Version -00 . . . . . . . . . . . . . . . . . . . . . . .  13
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  14
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  14

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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].

   The original OSCORE specification [RFC8613] does not consider such
   key usage limits.  However, should they 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.  Among other reasons, approaching the key usage limits
   requires updating the OSCORE keying material before communications
   can securely continue.  This document defines what steps an OSCORE
   peer should take to preserve the security of its communications, by
   stopping to use the OSCORE Security Context shared with another peer
   when approaching the key usage limits.

1.1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   Readers are expected to be familiar with the terms and concepts
   related to CoAP [RFC7252] and OSCORE [RFC8613].

2.  AEAD Key Usage Limits in OSCORE

   This section details how key usage limits for AEAD algorithms can be
   considered when using OSCORE.  In particular, it discusses specific
   limits for common AEAD algorithms used with OSCORE; parameters to
   track associated to an OSCORE Security Context; and additions to the
   OSCORE message processing.

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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.

   *  '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 have occurred with the AEAD
      algorithm for that key.

   When a peer uses OSCORE:

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

   *  The key used to decrypt and verify incoming messages is its
      Recipient Key from 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.

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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 shown in Figure 1 that can be used as AEAD
   Algorithm for OSCORE, 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 also 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].

       +------------------------+----------------+----------------+
       | 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, as further elaborated in Appendix A.

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       +------------------------+----------+----------+-----------+
       | 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)

   With regards to the limit for 'l', the recommended 'l' value for the
   algorithms shown in Figure 1, and for AEAD_AES_128_CCM_8, is 2^10
   (16384 bytes) and 2^8 (4096 bytes) respectively.  Considering a
   typical MTU size of 1500 bytes, and the fact that the maximum block
   size when using block-wise transfers with CoAP is 1024 bytes (see
   Section 2 of [RFC7959]), it is unlikely that a larger size of 'l'
   than what is recommended makes sense to use in typical network
   setups.

   However, although under typical circumstances an 'l' limit of 2^8
   (4096 bytes) is acceptable, exceptional cases can warrant a higher
   value of 'l'.  For instance, Block-wise Extension for Reliable
   Transport (BERT) extends the CoAP Block-Wise tranfer functionality,
   enabling use of larger messages over reliable transports such as TCP
   or WebSockets (see [RFC8323]).  In case the OSCORE peers wish to take
   full advantage of BERT functionality and the large message sizes it
   allows for, the OSCORE peers must use higher values of 'l'.

   An alternative means of allowing for larger values of 'l', while
   still maintaining the security properties of the used AEAD algorithm,
   is to adjust the 'q' and 'v' values to compensate.  In practice, this
   means reducing the value of 'q' and 'v' considering the new value of
   'l', to ensure an acceptably low value of the IA and CA
   probabilities.  A reasonable target for the IA and CA probability
   values is the threshold value of 2^-50 defined in
   [I-D.irtf-cfrg-aead-limits].

2.2.  Additional Information in the Security Context

   In addition to what is defined in Section 3.1 of [RFC8613], the
   following parameters associated with a OSCORE Security Context can be
   used for keeping track of the expiration of that OSCORE Security
   Context and maintaining key usage below safe limits.

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2.2.1.  Common Context

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

      At the time indicated by this parameter, 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.  That is, the
      expiration of an OSCORE Security Context means that the current
      Sender Key must no longer be used for protecting outgoing
      messages, and the Recipient Key must no longer be used for
      unprotecting incoming messages.

      The value of 'exp' must be set upon installing the OSCORE Security
      Context, namely at time t_1, considering a lifetime value t_l.  In
      particular, t_l can be a default value (potentially differing
      between the two peers sharing the OSCORE Security Context), or can
      alternatively be agreed by the two peers during the establishment
      of the OSCORE Security Context.  For instance, this value may be
      stored and/or transported in an OSCORE LwM2M object, or specified
      as part of an EDHOC Application Profile [I-D.ietf-lake-edhoc] used
      when running EDHOC for establishing the OSCORE Security Context.
      Regardless of how the lifetime value is determined, the 'exp'
      parameters is set to indicate the point in time corresponding to
      t_1 offset by t_l.

2.2.2.  Sender Context

   The Sender Context has the following associated 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.

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      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 thus in performing a key update
   procedure more frequently.

2.2.3.  Recipient Context

   The Recipient Context has the following associated 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.

   *  '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 Message Processing

   In order to keep track of the 'q' and 'v' values and ensure that AEAD
   keys are not used beyond reaching their limits, OSCORE peers protect
   messages with OSCORE 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 must
   not exceed the block size for the selected algorithm multiplied with
   'l‘. The size of the COSE plaintext is calculated as described in
   Section 5.3 of [RFC8613].

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   If OSCORE peers need to transmit messages exceeding the maximum
   recommended size caclulated from 'l', CoAP Block-Wise transfers
   [RFC7959] may be used as a means to split the whole, large content
   into smaller segments.  The following steps can be adopted by a
   client or server to determine whether the usage of block-wise
   transfer is necessary for the transmission of a specific OSCORE
   protected message.

   1.  The CoAP message to transmit is first produced.

   2.  The sum of the total size of the COSE plaintext, the length of
       the authentication tag, and the length of any potential
       ciphertext padding should be computed to produce a value T.  It
       should be noted that the size of the padding and the length of
       the authentication tag depend on the used AEAD algorithm.

   3.  If the value of T exceeds the 'l' value for the used AEAD
       algorithm, block-wise transfer is to be used with the CoAP
       message before protecting it with OSCORE.

   The processing of CoAP messages with OSCORE 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 is incremented.

   If 'count_q' exceeds the 'limit_q' limit, the message processing is
   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 is not incremented.

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

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3.  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].

   [TODO: Add more considerations.]

4.  IANA Considerations

   This document has no IANA actions.

5.  References

5.1.  Normative References

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

   [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>.

5.2.  Informative References

   [I-D.ietf-lake-edhoc]
              Selander, G., Mattsson, J. P., and F. Palombini,
              "Ephemeral Diffie-Hellman Over COSE (EDHOC)", Work in
              Progress, Internet-Draft, draft-ietf-lake-edhoc-22, 25
              August 2023, <https://datatracker.ietf.org/doc/html/draft-
              ietf-lake-edhoc-22>.

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   [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-07, 31 May 2023,
              <https://datatracker.ietf.org/doc/html/draft-irtf-cfrg-
              aead-limits-07>.

   [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>.

   [RFC7959]  Bormann, C. and Z. Shelby, Ed., "Block-Wise Transfers in
              the Constrained Application Protocol (CoAP)", RFC 7959,
              DOI 10.17487/RFC7959, August 2016,
              <https://www.rfc-editor.org/info/rfc7959>.

   [RFC8323]  Bormann, C., Lemay, S., Tschofenig, H., Hartke, K.,
              Silverajan, B., and B. Raymor, Ed., "CoAP (Constrained
              Application Protocol) over TCP, TLS, and WebSockets",
              RFC 8323, DOI 10.17487/RFC8323, February 2018,
              <https://www.rfc-editor.org/info/rfc8323>.

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 3 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 3, 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 3: 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.  Document Updates

   RFC EDITOR: PLEASE REMOVE THIS SECTION.

C.1.  Version -01 to -02

   *  Updated references

C.2.  Version -00 to -01

   *  Extended discussion on setting the lifetime of OSCORE Security
      Contexts.

   *  Mention adjusting the 'q' and 'v' values to compensate for a
      larger 'l' value.

   *  Specify how to perform pre-calculation of message size to
      determine need for block-wise.

   *  Cover exceptional cases where the 'l' value needs to be larger
      than 2^8.

   *  Note on relevance of 'l' limit considering maximum block size and
      typical MTU.

C.3.  Version -00

   *  Editorial improvements.

   *  Extended terminology.

   *  Recommendation on limits for CCM_8.  Details in Appendix.

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   *  Example of method to estimate and not store 'count_q'.

   *  Split out material from Key Update for OSCORE draft into this new
      document.

Acknowledgments

   The authors sincerely thank Christian Amsüss, Carsten Bormann, John
   Preuß Mattsson, Göran Selander and Rafa Marin-Lopez 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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