Extended Key Update for Transport Layer Security (TLS) 1.3

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Table of Contents

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.¶

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.¶

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.¶

       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]

The structure of the ExtendedKeyUpdate message is shown below.¶

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).¶

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.¶

      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;

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.¶

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.¶

       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]

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.¶

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 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 "extended_key_update_required" 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

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¶

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