Extended Key Update for Transport Layer Security (TLS) 1.3
draft-ietf-tls-extended-key-update-01

Transport Layer Security                                   H. Tschofenig
Internet-Draft                                                   Siemens
Intended status: Standards Track                                M. Tüxen
Expires: 23 April 2025                 Münster Univ. of Applied Sciences
                                                                T. Reddy
                                                                   Nokia
                                                                S. Fries
                                                                 Siemens
                                                            Y. Rosomakho
                                                                 Zscaler
                                                         20 October 2024

       Extended Key Update for Transport Layer Security (TLS) 1.3
                 draft-ietf-tls-extended-key-update-01

Abstract

   The Transport Layer Security (TLS) 1.3 specification offers a
   dedicated message to update cryptographic keys during the lifetime of
   an ongoing session.  The traffic secret and the initialization vector
   are updated directionally but the sender may trigger the recipient,
   via the request_update field, to transmit a key update message in the
   reverse direction.

   In environments where sessions are long-lived, such as industrial IoT
   or telecommunication networks, this key update alone is insufficient
   since forward secrecy is not offered via this mechanism.  Earlier
   versions of TLS allowed the two peers to perform renegotiation, which
   is a handshake that establishes new cryptographic parameters for an
   existing session.  When a security vulnerability with the
   renegotiation mechanism was discovered, RFC 5746 was developed as a
   fix.  Renegotiation has, however, been removed from version 1.3
   leaving a gap in the feature set of TLS.

   This specification defines an extended key update that supports
   forward secrecy.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

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   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 23 April 2025.

Copyright Notice

   Copyright (c) 2024 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   Please review these documents carefully, as they describe your rights
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology and Requirements Language . . . . . . . . . . . .   4
   3.  Key Exfiltration and Forward Secrecy  . . . . . . . . . . . .   4
   4.  Negotiating the Extended Key Update . . . . . . . . . . . . .   4
   5.  Extended Key Update Message . . . . . . . . . . . . . . . . .   5
   6.  Updating Traffic Secrets  . . . . . . . . . . . . . . . . . .   9
   7.  Example . . . . . . . . . . . . . . . . . . . . . . . . . . .  10
   8.  DTLS 1.3 Considerations . . . . . . . . . . . . . . . . . . .  11
   9.  Post-Quantum Cryptogrphy Considerations . . . . . . . . . . .  12
   10. SSLKEYLOGFILE update  . . . . . . . . . . . . . . . . . . . .  12
   11. Exporter  . . . . . . . . . . . . . . . . . . . . . . . . . .  12
   12. Security Considerations . . . . . . . . . . . . . . . . . . .  13
   13. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13
   14. References  . . . . . . . . . . . . . . . . . . . . . . . . .  13
     14.1.  Normative References . . . . . . . . . . . . . . . . . .  14
     14.2.  Informative References . . . . . . . . . . . . . . . . .  14
   Appendix A.  Acknowledgments  . . . . . . . . . . . . . . . . . .  15
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  16

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1.  Introduction

   The features of TLS and DTLS have changed over the years and while
   newer versions optimized the protocol and at the same time enhanced
   features (often with the help of extensions) some functionality was
   removed without replacement.  The ability to update keys and
   initialization vectors has been added in TLS 1.3
   [I-D.ietf-tls-rfc8446bis] using the KeyUpdate message and it intended
   to (partially) replace renegotiation from earlier TLS versions.  The
   renegotiation feature, while complex, offered additional
   functionality that is not supported with TLS 1.3 anymore, including
   the update of master keys with a Diffie-Hellman exchange during the
   lifetime of a session.

   There are use cases of TLS and DTLS where long-lived sessions are
   common.  In those environments, such as industrial IoT and
   telecommunication networks, availability is important and an
   interruption of the communication due to periodic session resumptions
   is not an option.  Re-running a handshake with (EC)DHE and switching
   from the old to the new session may be a solution for some
   applications but introduces complexity, impacts performance and may
   lead to service interruption as well.

   Some deployments have used IPsec in the past to secure their
   communication protocol and have now decided to switch to TLS or DTLS
   instead.  The requirement for updates of cryptographic keys for an
   existing session has become a requirement.  For IPsec, US NIST,
   German BSI, and French ANSSI recommend to re-run Diffie-Hellman
   exchanges frequently to provide forward secrecy and force attackers
   to perform a dynamic key extraction [RFC7624].  ANSSI writes "It is
   recommended to force the periodic renewal of the keys, e.g., every
   hour and every 100 GB of data, in order to limit the impact of a key
   compromise."  [ANSSI-DAT-NT-003].  While IPsec/IKEv2 [RFC7296] offers
   the desired functionality, developers often decide to use TLS/DTLS to
   simplify integration with cloud-based environments.

   This specification defines a new, extended key update message
   supporting perfect forward secrecy.  It does so by utilizing a
   Diffie-Hellman exchange using one of the groups negotiated during the
   initial exchange.  The support for this extension is signaled using
   the TLS flags extension mechanism.  The frequent re-running of
   extended key update forces an attacker to do dynamic key
   exfiltration.

   This specification is applicable to both TLS 1.3
   [I-D.ietf-tls-rfc8446bis] and DTLS 1.3 [RFC9147].  Throughout the
   specification we do not distinguish between these two protocols
   unless necessary for better understanding.

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2.  Terminology and Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

   To distinguish the key update procedure defined in
   [I-D.ietf-tls-rfc8446bis] from the key update procedure specified in
   this document, we use the terms "key update" and "extended key
   update", respectively.

3.  Key Exfiltration and Forward Secrecy

   [RFC9325] provides a good summary of what (perfect) forward secrecy
   is and how it relates to the TLS protocol.  In summary, it says:

   "Forward secrecy (also called "perfect forward secrecy" or "PFS") is
   a defense against an attacker who records encrypted conversations
   where the session keys are only encrypted with the communicating
   parties' long-term keys.  Should the attacker be able to obtain these
   long-term keys at some point later in time, the session keys and thus
   the entire conversation could be decrypted."

   Appendix F of [I-D.ietf-tls-rfc8446bis] goes into details of
   explaining the security properties of the TLS 1.3 protocol and notes
   "... forward secrecy without rerunning (EC)DHE does not stop an
   attacker from doing static key exfiltration".  It concludes with a
   recommendation by saying: "Frequently rerunning (EC)DHE forces an
   attacker to do dynamic key exfiltration (or content exfiltration)."
   The terms static and dynamic key exfiltration are defined in
   [RFC7624].  Dynamic key exfiltration, refers to attacks in which the
   collaborator delivers keying material to the attacker frequently,
   e.g., on a per-session basis.  Static key exfiltration means that the
   transfer of keys happens once or rarely and that the transferred key
   is typically long-lived.

4.  Negotiating the Extended Key Update

   Client and servers use the TLS flags extension
   [I-D.ietf-tls-tlsflags] to indicate support for the functionality
   defined in this document.  We call this the "extended_key_update"
   extension and the corresponding flag is called "Extended_Key_Update"
   flag.

   The "Extended_Key_Update" flag proposed by the client in the
   ClientHello (CH) MUST be acknowledged in the EncryptedExtensions
   (EE), if the server also supports the functionality defined in this
   document and is configured to use it.

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   If the "Extended_Key_Update" flag is not set, servers ignore any of
   the functionality specified in this document and applications that
   require perfect forward security will have to initiate a full
   handshake.

5.  Extended Key Update Message

   The ExtendedKeyUpdate handshake message is used to indicate an update
   of cryptographic keys.  This key update process can be sent by either
   peer after it has sent a Finished message.  Implementations that
   receive a ExtendedKeyUpdate message prior to receiving a Finished
   message MUST terminate the connection with an "unexpected_message"
   alert.

   The KeyShare entry in the ExtendedKeyUpdate message MUST be the same
   group mutually supported by the client and server during the initial
   handshake.  The peers MUST NOT send a KeyShare Entry in the
   ExtendedKeyUpdate message that is not mutually supported by the
   client and server during the initial handshake.  An implementation
   that receives any other value MUST terminate the connection with an
   "illegal_parameter" alert.

   Figure 1 shows the interaction graphically.  First, support for the
   functionality in this specification is negotiated in the ClientHello
   and the EncryptedExtensions messages.  Then, the ExtendedKeyUpdate
   exchange is sent to update the application traffic secrets.

   The extended key update exchange is performed between the initiator
   and the responder whereby the initiator may be the TLS client or the
   TLS server.

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        Client                                           Server

 Key  ^ ClientHello
 Exch | + key_share
      | + signature_algorithms
      v + Extended_Key_Update       -------->
                                                   ServerHello  ^ Key
                                                   + key_share  | Exch
                                                                v
                                         {EncryptedExtensions   ^ Server
                                        + Extended_Key_Update}  | Params
                                          {CertificateRequest}  v
                                                 {Certificate}  ^
                                           {CertificateVerify}  | Auth
                                                    {Finished}  v
                                <--------
      ^ {Certificate}
 Auth | {CertificateVerify}
      v {Finished}              -------->
        [Application Data]      <------->  [Application Data]
                                   ...
 [ExtendedKeyUpdateRequest]     -------->
                                <--------  [ExtendedKeyUpdateResponse]
                                   ...
             [NewKeyUpdate]     <-------
                                -------->  [NewKeyUpdate]
                                   ...
        [Application Data]      <------->  [Application Data]

            Figure 1: Extended Key Update Message Exchange.

   The structure of the ExtendedKeyUpdate message is shown below.

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   struct {
     KeyShareEntry key_share;
   } ExtendedKeyUpdateRequest;

   enum {
     accepted(0),
     retry(1),
     rejected(2),
     clashed(3),
     (255)
   } ExtendedKeyUpdateResponseStatus;

   struct {
     ExtendedKeyUpdateResponseStatus status;
     select (ExtendedKeyUpdateResponse.status) {
        case accepted: KeyShareEntry key_share;
        case retry: uint8 delay;
     }
   } ExtendedKeyUpdateResponse;

   struct {
   } NewKeyUpdate;

   key_share: Key share information.  The contents of this field are
   determined by the specified group and its corresponding definition.
   The structures are defined in [I-D.ietf-tls-rfc8446bis].

   status: Response to ExtendedKeyUpdateRequest.  This status field
   indicates whether responder accepted or declined Extended Key Update
   Request.

   delay: Delay in seconds for the initiator to retry the request.

   There are three rejection reasons defined:

   1.  retry: request was declined temporarily as responder is too busy.
       In this case ExtendedKeyUpdateResponse contains delay in seconds
       for initiator to retry.  Initiator MUST NOT retry within this
       interval and SHOULD retry after it lapsed.  Note that responder
       MAY apply an overall rate limit to extended key update that would
       not be specific to given TLS session.  If initiator cannot
       proceed without immediate Extended Key Update it MUST terminate
       the connection with TLS alert "extended_key_update_required"
       (alert number TBD).

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   2.  rejected: request was declined permanently.  Initiator MUST NOT
       retry and if it cannot proceed without Extended Key Update it
       MUST terminate the connection with alert
       "extended_key_update_required" (alert number TBD).

   3.  clashed: request was declined because responder already initiated
       its own extended key update.

   The exchange has the following steps:

   1.  Initiator sends a ExtendedKeyUpdateRequest message, which
       contains a key share.  While an extended key update is in
       progress, the initiator MUST NOT initiate further key updates.

   2.  On receipt of the ExtendedKeyUpdateRequest message, the responder
       sends the ExtendedKeyUpdateResponse message.  If the responder
       accepts the request, it sets the status to accepted and includes
       its own key share.  If the responder declines the request, it
       sets the status accordingly and does not include the key share.
       While an extended key update is in progress, the responder MUST
       NOT initiate further key updates.

   3.  On receipt of the ExtendedKeyUpdateResponse message with accepted
       status, the initiator is able to derive a secret key based on the
       exchanged key shares.  After sending a NewKeyUpdate message, the
       initiator MUST update its traffic keys and MUST send all its
       traffic using the next generation of keys.

   4.  On receipt of the NewKeyUpdate message by the responder, it MUST
       update its receive keys.  In response, the responder transmits a
       NewKeyUpdate message and MUST update its sending keys.

   Both sender and receiver MUST encrypt their NewKeyUpdate messages
   with the old keys.  Additionally, both sides MUST enforce that a
   NewKeyUpdate with the old key is received before accepting any
   messages encrypted with the new key.

   If TLS peers independently initiate the extended key update procedure
   and the requests cross in flight, the ExtendedKeyUpdateRequest
   message with the lower lexicographic order for the key_exchange value
   in the KeyShareEntry will be rejected by the responder using clashed
   status in ExtendedKeyUpdateResponse message.  This approach prevents
   each side incrementing keys by two generations.

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         struct {
             HandshakeType msg_type;    /* handshake type */
             uint24 length;             /* bytes in message */
             select (Handshake.msg_type) {
                 case client_hello:          ClientHello;
                 case server_hello:          ServerHello;
                 case end_of_early_data:     EndOfEarlyData;
                 case encrypted_extensions:  EncryptedExtensions;
                 case certificate_request:   CertificateRequest;
                 case certificate:           Certificate;
                 case certificate_verify:    CertificateVerify;
                 case finished:              Finished;
                 case new_session_ticket:    NewSessionTicket;
                 case key_update:            KeyUpdate;
                 case extended_key_update:   ExtendedKeyUpdate;
             };
         } Handshake;

                       Figure 2: Handshake Structure.

   The ExtendedKeyUpdate and the KeyUpdates MAY be used in combination
   over the lifetime of a TLS communication session, depending on the
   desired security properties.

6.  Updating Traffic Secrets

   The ExtendedKeyUpdate handshake message is used to indicate that the
   sender is updating its sending cryptographic keys.  This message can
   be sent by either endpoint after the Finished messages have been
   exchanged.

   The design of the key derivation function for computing the next
   generation of application_traffic_secret is motivated by the desire
   to include

   *  the old traffic secret as well as a secret derived from the DH
      exchange or from the hybrid key exchange,

   *  the concatenation of the ExtendedKeyUpdateRequest and the
      ExtendedKeyUpdateResponse messages, which contain the key shares,
      binding the encapsulated shared secret ciphertext to IKM in case
      of hybrid key exchange, providing MAL-BIND-K-CT security (see
      [CDM23]), and

   *  a new label string to distinguish it from the application traffic
      secret computation defined in [I-D.ietf-tls-rfc8446bis] for use
      with the regular KeyUpdate.

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   sk = HKDF-Extract(Transcript-Hash(KeyUpdateMessages), secret)

   application_traffic_secret_N+1 =
       HKDF-Expand-Label(sk,
                         "traffic up2", application_traffic_secret_N,
                         Hash.length)

   The traffic keys are re-derived from the client_/
   server_application_traffic_secret_N+1 as described in Section 7.3 of
   [I-D.ietf-tls-rfc8446bis].

   Once client_/server_application_traffic_secret_N+1 and its associated
   traffic keys have been computed, implementations SHOULD delete
   client_/server_application_traffic_secret_N and its associated
   traffic keys.  Note: The client_/server_application_traffic_secret_N
   and its associated traffic keys can only be deleted after receiving
   the NewKeyUpdate message.

7.  Example

   Figure 1 shows the interaction between a TLS 1.3 client and server
   graphically.  This section shows an example message exchange where a
   client updates its sending keys.

   There are two phases:

   1.  The support for the functionality in this specification is
       negotiated in the ClientHello and the EncryptedExtensions
       messages.

   2.  Once the initial handshake is completed, a key update can be
       triggered.

   Figure 3 provides an overview of the exchange starting with the
   initial negotiation followed by the key update.

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        Client                                           Server

 Key  ^ ClientHello
 Exch | + key_share
      | + signature_algorithms
      v + extended_key_update
                              -------->
                                                   ServerHello  ^ Key
                                                   + key_share  | Exch
                                                                v
                                         {EncryptedExtensions   ^ Server
                                        + extended_key_update}  | Params
                                          {CertificateRequest}  v
                                                 {Certificate}  ^
                                           {CertificateVerify}  | Auth
                                                    {Finished}  v
                                <--------
      ^ {Certificate}
 Auth | {CertificateVerify}
      v {Finished}              -------->
                                   ...
                               some time later
                                   ...
  [ExtendedKeyUpdateRequest]    -------->
   (with KeyShare)
                                <-------- [ExtendedKeyUpdateResponse]
                                            (with KeyShare)
  [NewKeyUpdate]                -------->
                                <-------- [NewKeyUpdate]

            Figure 3: Extended Key Update Message Exchange.

8.  DTLS 1.3 Considerations

   Due to the possibility of a NewKeyUpdate message being lost and
   thereby preventing the sender of the NewKeyUpdate message from
   updating its keying material, receivers MUST retain the pre-update
   keying material until receipt and successful decryption of a message
   using the new keys.

   Due to loss and/or reordering, DTLS 1.3 implementations may receive a
   record with an older epoch than the current one.  They SHOULD attempt
   to process those records with that epoch but MAY opt to discard such
   out-of-epoch records.

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9.  Post-Quantum Cryptogrphy Considerations

   Hybrid key exchange refers to using multiple key exchange algorithms
   simultaneously and combining the result with the goal of providing
   security even if all but one of the component algorithms is broken.
   The transition to post-quantum cryptography motivates the
   introduction of hybrid key exchanges to TLS, as described in
   [I-D.ietf-tls-hybrid-design].  When the hybrid key exchange is used,
   then the key_exchange field of a KeyShareEntry in the initial
   exchange is the concatenation of the key_exchange field for each of
   the algorithms.  The same approach is then re-used in the extended
   key update when key shares are exchanged.

10.  SSLKEYLOGFILE update

   As Extended Key Update invalidates previous secrets, SSLKEYLOGFILE
   [I-D.ietf-tls-keylogfile] needs to be populated with new entries.
   Each completed Extended Key Update results in two additional secret
   labels in SSLKEYLOGFILE:

   1.  CLIENT_TRAFFIC_SECRET_N+1: identified as
       client_application_traffic_secret_N+1 in the key schedule

   2.  SERVER_TRAFFIC_SECRET_N+1: identified as
       server_application_traffic_secret_N+1 in the key schedule

   Similarly to other records in SSLKEYLOGFILE label is followed by
   32-byte value of the Random field from the ClientHello message that
   established the TLS connection and corresponding secret encoded in
   hexadecimal.

   SSLKEYLOGFILE entries for Extended Key Update MUST NOT be produced if
   SSLKEYLOGFILE was not used for other secrets in the handshake.

   Note that each successful Extended Key Update invalidates all
   previous SSLKEYLOGFILE secrets including past iterations of
   CLIENT_TRAFFIC_SECRET_ and SERVER_TRAFFIC_SECRET_.

11.  Exporter

   Protocols like DTLS-SRTP and DTLS-over-SCTP utilize TLS or DTLS for
   key establishment but repurpose some of the keying material for their
   own purpose.  These protocols use the TLS exporter defined in
   Section 7.5 of [I-D.ietf-tls-rfc8446bis].

   Once the Extended Key Update mechanism is complete, such protocols
   would need to use the newly derived key to generate Exported Keying
   Material (EKM) to protect packets.  The "sk" derived in the Section 6

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   will be used as the "Secret" in the exporter function, defined in
   Section 7.5 of [I-D.ietf-tls-rfc8446bis], to generate EKM, ensuring
   that the exported keying material is aligned with the updated
   security context.

12.  Security Considerations

   This entire document is about security.

   When utilizing this extension it is important to understand the
   interaction with ticket-based resumption since resumption without the
   execution of a Diffie-Hellman exchange offering forward secrecy will
   potentially undo updates to the application traffic secret
   derivation, depending on when tickets have been exchanged.

13.  IANA Considerations

   IANA is requested to allocate value TBD for the
   "too_many_extendedkeyupdate_requested" alert in the "TLS Alerts"
   registry.  The value for the "DTLS-OK" column is "Y".

   IANA is requested to add the following entry to the "TLS Flags"
   extension registry [TLS-Ext-Registry]:

   *  Value: TBD1

   *  Flag Name: extended_key_update

   *  Messages: CH, EE

   *  Recommended: Y

   *  Reference: [This document]

   IANA is requested to add the following entry to the "TLS
   HandshakeType" registry [TLS-Ext-Registry]:

   *  Value: TBD2

   *  Description: extended_key_update

   *  DTLS-OK: Y

   *  Reference: [This document]

   *  Comment:

14.  References

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14.1.  Normative References

   [I-D.ietf-tls-rfc8446bis]
              Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", Work in Progress, Internet-Draft, draft-
              ietf-tls-rfc8446bis-11, 14 September 2024,
              <https://datatracker.ietf.org/doc/html/draft-ietf-tls-
              rfc8446bis-11>.

   [I-D.ietf-tls-tlsflags]
              Nir, Y., "A Flags Extension for TLS 1.3", Work in
              Progress, Internet-Draft, draft-ietf-tls-tlsflags-14, 13
              September 2024, <https://datatracker.ietf.org/doc/html/
              draft-ietf-tls-tlsflags-14>.

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

   [RFC9147]  Rescorla, E., Tschofenig, H., and N. Modadugu, "The
              Datagram Transport Layer Security (DTLS) Protocol Version
              1.3", RFC 9147, DOI 10.17487/RFC9147, April 2022,
              <https://www.rfc-editor.org/rfc/rfc9147>.

14.2.  Informative References

   [ANSSI-DAT-NT-003]
              ANSSI, "Recommendations for securing networks with IPsec,
              Technical Report", August 2015,
              <https://www.ssi.gouv.fr/uploads/2015/09/NT_IPsec_EN.pdf>.

   [CDM23]    ACM, "Keeping Up with the KEMs: Stronger Security Notions
              for KEMs and automated analysis of KEM-based protocols",
              November 2023, <https://eprint.iacr.org/2023/1933.pdf>.

   [I-D.ietf-tls-hybrid-design]
              Stebila, D., Fluhrer, S., and S. Gueron, "Hybrid key
              exchange in TLS 1.3", Work in Progress, Internet-Draft,
              draft-ietf-tls-hybrid-design-11, 7 October 2024,
              <https://datatracker.ietf.org/doc/html/draft-ietf-tls-
              hybrid-design-11>.

   [I-D.ietf-tls-keylogfile]
              Thomson, M., "The SSLKEYLOGFILE Format for TLS", Work in
              Progress, Internet-Draft, draft-ietf-tls-keylogfile-02, 29
              April 2024, <https://datatracker.ietf.org/doc/html/draft-
              ietf-tls-keylogfile-02>.

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   [RFC7296]  Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T.
              Kivinen, "Internet Key Exchange Protocol Version 2
              (IKEv2)", STD 79, RFC 7296, DOI 10.17487/RFC7296, October
              2014, <https://www.rfc-editor.org/rfc/rfc7296>.

   [RFC7624]  Barnes, R., Schneier, B., Jennings, C., Hardie, T.,
              Trammell, B., Huitema, C., and D. Borkmann,
              "Confidentiality in the Face of Pervasive Surveillance: A
              Threat Model and Problem Statement", RFC 7624,
              DOI 10.17487/RFC7624, August 2015,
              <https://www.rfc-editor.org/rfc/rfc7624>.

   [RFC9325]  Sheffer, Y., Saint-Andre, P., and T. Fossati,
              "Recommendations for Secure Use of Transport Layer
              Security (TLS) and Datagram Transport Layer Security
              (DTLS)", BCP 195, RFC 9325, DOI 10.17487/RFC9325, November
              2022, <https://www.rfc-editor.org/rfc/rfc9325>.

   [TLS-Ext-Registry]
              IANA, "Transport Layer Security (TLS) Extensions",
              November 2023, <https://www.iana.org/assignments/tls-
              extensiontype-values>.

Appendix A.  Acknowledgments

   We would like to thank the members of the "TSVWG DTLS for SCTP
   Requirements Design Team" for their discussion.  The members, in no
   particular order, were:

   *  Marcelo Ricardo Leitner

   *  Zaheduzzaman Sarker

   *  Magnus Westerlund

   *  John Mattsson

   *  Claudio Porfiri

   *  Xin Long

   *  Michael Tüxen

   *  Hannes Tschofenig

   *  K Tirumaleswar Reddy

   *  Bertrand Rault

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   Additionally, we would like to thank the chairs of the Transport and
   Services Working Group (tsvwg) Gorry Fairhurst and Marten Seemann as
   well as the responsible area director Martin Duke.

   Finally, we would like to thank Martin Thomson, Ilari Liusvaara,
   Benjamin Kaduk, Scott Fluhrer, Dennis Jackson, David Benjamin, and
   Thom Wiggers for their review comments.

Authors' Addresses

   Hannes Tschofenig
   Siemens
   Email: hannes.tschofenig@gmx.net

   Michael Tüxen
   Münster Univ. of Applied Sciences
   Email: tuexen@fh-muenster.de

   Tirumaleswar Reddy
   Nokia
   Email: kondtir@gmail.com

   Steffen Fries
   Siemens
   Email: steffen.fries@siemens.com

   Yaroslav Rosomakho
   Zscaler
   Email: yrosomakho@zscaler.com

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