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 keys with a Diffie-Hellman exchange during the lifetime of a session. If a traffic secret (referred as application_traffic_secret_N) has been compromised, an attacker can passively eavesdrop on all future data sent on the connection, including data encrypted with application_traffic_secret_N+1, application_traffic_secret_N+2, etc.¶

While such a feature is less relevant in environments with shorter-lived sessions, such as transactions on the web, there are uses 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, NIST, BSI, and 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 key update mechanism supporting forward secrecy. It does so by re-using the design approach introduced by the "Exported Authenticators" specification [RFC9261], which uses the application layer protocol to exchange post-handshake messages. This approach minimizes the impact on the TLS state machine but places more burden on application layer protocol designer. To achieve interoperability the payloads exchanged via the application layer are specified in this document and we make use of Hybrid Public Key Encryption (HPKE) [RFC9180], which offers an easy migration path for the integration of post quantum cryptography with its key encapsulation construction (KEM). Since HPKE requires the sender to possess the recipient's public key, those public keys need to be exchanged upfront. This specification is silent about when and how often these public keys are exchanged by the application layer protocol. Note: To accomplish forward secrecy the public key of the recipient can be only used once.¶

To leave the exchange of the public keys up to the application is an intentional design decision to offer flexibility for developers and there is experience with such an approach already from secure end-to-end messaging protocols. To synchronize the switch to the new traffic secret, the key updates are directional and accomplished with a new key update message. The trigger to switch to the new traffic secrets is necessary since the TLS record layer conveys no key identifier like an epoch or a Connection Identifier (CID).¶

The support for the functionality described in this specification is signaled using the TLS extension mechanism. Using the extended key update message frequently forces an attacker to perform 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.¶

This document re-uses the Key Encapsulation Mechanism (KEM) terminology from RFC 9180 [RFC9180].¶

The following abbreviations are used in this document:¶

• KDF: Key Derivation Function¶

• AEAD: Authenticated Encryption with Associated Data¶

• HPKE: Hybrid Public Key Encryption¶

3. Negotiating the Extended Key Update

The "extended_key_update" extension is used by the client and the server to negotiate an HPKE ciphersuite to use, which refers to the combination of a KEM, KDF, AEAD combination. These HPKE ciphersuites are communicated in the ClientHello and EncryptedExtensions messages. The values for the KEM, the KDF, and the AEAD algorithms are taken from the IANA registry created by [RFC9180].¶

This extension is only supported with TLS 1.3 [I-D.ietf-tls-rfc8446bis] and newer; if TLS 1.2 [RFC5246] or earlier is negotiated, the peers MUST ignore this extension.¶

This document defines a new extension type, the extended_key_update(TBD1), as shown in Figure 1, which can be used to signal the supported HPKE ciphersuites for the extended key update message to the peer.¶

   enum {
       extended_key_update(TBD1), (65535)
   } ExtensionType;

This new extension is populated with the structure shown in Figure 2.¶

struct {
    uint16 kdf_id;
    uint16 aead_id;
    uint16 kem_id;
} HpkeCipherSuite;

struct {
    HpkeCipherSuite cipher_suites<4..2^16-4>;
} HpkeCipherSuites;

Whenever it is sent by the client as a ClientHello message extension ([I-D.ietf-tls-rfc8446bis], Section 4.1.2), it indicates what HPKE ciphersuites it supports.¶

A server that supports and wants to use the extended key update feature MUST send the "extended_key_update" extension in the EncryptedExtensions message indicating what HPKE ciphersuites it prefers to use. The extension, shown in Figure 2, contains a list of supported ciphersuites in preference order, with the most preferred version first.¶

The server MUST select one of the ciphersuites from the list offered by the client. If no suitable ciphersuite is found, the server MUST NOT return an "extended_key_update" extension to the client.¶

If this extension is not present, as with any TLS extensions, servers ignore any the functionality specified in this document and applications have to rely on the features offered by the TLS 1.3-specified KeyUpdate instead.¶

4. Using HPKE

To support interoperability between the two endpoints, the following payload structure is defined.¶

struct {
    opaque kid<0..2^16-1>;
    opaque enc<0..2^16-1>;
    opaque ct<32..2^8-1>;
} HPKE_Payload;

The fields have the following meaning:¶

• kid: The identifier of the recipient public key used for the HPKE computation. This allows the sender to indicate what public key it used in case it has multiple public keys for a given recipient.¶

• enc: The HPKE encapsulated key, used by the peers to decrypt the corresponding payload field.¶

• ct: The ciphertext, which is the result of encrypting a random value, RAND, with HPKE, as described in [RFC9180] using the HPKE SealBase() operation. RAND MUST be at least 32 bytes long but the maximum length MUST NOT exceed 255 bytes. This RAND value is input to the application_traffic_secret generation, as described in Section 6.¶

This specification MUST use the HPKE Base mode; authenticated HPKE modes are not supported.¶

The SealBase() operation requires four inputs, namely¶

• the public key of the recipient,¶

• context information (info),¶

• associated data (aad), and¶

• plaintext.¶

SealBase() will return two outputs, "enc" and "ct", which will be stored in the HPKE_Payload structure.¶

Two input values for the SealBase() operation require further explanation:¶

• The info value MUST be set to the empty string.¶

• The aad value MUST be populated with the TLS exporter secret. The exporter interface is described in Section 7.5 of [I-D.ietf-tls-rfc8446bis]. For (D)TLS 1.3, the exporter_master_secret MUST be used, not the early_exporter_master_secret.¶

The exporter value is computed as:¶

   TLS-Exporter(label, context_value, key_length) =
       HKDF-Expand-Label(Derive-Secret(Secret, label, ""),
                         "exporter", Hash(context_value), key_length)

The following values are used for the TLS-Exporter function:¶

• the label is set to "extended key update client" and "extended key update server" for extended key updates sent by the client or server, respectively¶

• the context_value is set to a zero length value.¶

• the length of the exported value is equal to the length of the output of the hash function associated with the selected ciphersuite.¶

The recipient will use the OpenBase() operation with the "enc" and the "ct" parameters received from the sender. The "aad" and the "info" parameters are constructed as previously described for SealBase().¶

The OpenBase function will, if successful, decrypt "ct". When decrypted, the result will either return the random value or an error.¶

5. Extended Key Update Message

The ExtendedKeyUpdate handshake message is used to indicate that the sender is updating its sending cryptographic keys. This message can be sent by either peer after it has sent a Finished message and exchanged the necessary public key(s) and HPKE payload(s) by the application layer protocol. Implementations that receive a ExtendedKeyUpdate message prior to receiving a Finished message or prior to the exchange of the needed application layer payloads (public key and HPKE) MUST terminate the connection with an "unexpected_message" alert.¶

After sending the ExtendedKeyUpdate message, the sender MUST send all its traffic using the next generation of keys, computed as described in Section 6. Upon receiving an ExtendedKeyUpdate message, the receiver MUST update its receiving traffic keys.¶

enum {
    update_not_requested(0), update_requested(1), (255)
} KeyUpdateRequest;

struct {
    opaque kid<0..2^16-1>;
    KeyUpdateRequest request_update;
} ExtendedKeyUpdate;

The kid field indicates the public key of the recipient that was used by HPKE to encrypt the random value.¶

The request_update field indicates whether the recipient of the ExtendedKeyUpdate should respond with its own ExtendedKeyUpdate. If an implementation receives any other value, it MUST terminate the connection with an "illegal_parameter" alert.¶

If the request_update field is set to "update_requested", the receiver MUST send an ExtendedKeyUpdate of its own with request_update set to "update_not_requested" prior to sending its next Application Data record. This mechanism allows either side to force an update to the entire connection, but causes an implementation which receives multiple ExtendedKeyUpdates while it is silent to respond with a single update. Note that implementations may receive an arbitrary number of messages between sending a ExtendedKeyUpdate with request_update set to "update_requested" and receiving the peer's ExtendedKeyUpdate, because those messages may already be in flight.¶

If implementations independently send their own ExtendedKeyUpdate with request_update set to "update_requested", and they cross in flight, then each side will also send a response, with the result that each side increments by two generations.¶

The sender MUST encrypt ExtendedKeyUpdate messages with the old keys and the receiver MUST decrypt ExtendedKeyUpdate messages with the old keys. Senders MUST enforce that ExtendedKeyUpdate encrypted with the old key is received before accepting any messages encrypted with the new key.¶

If a sending implementation receives a ExtendedKeyUpdate with request_update set to "update_requested", it MUST NOT send its own ExtendedKeyUpdate if that would cause it to exceed these limits and SHOULD instead ignore the "update_requested" flag.¶

The ExtendedKeyUpdate and the KeyUpdates MAY be used in combination.¶

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 three conditions are met:¶

• The endpoint has sent a Finished message.¶

• The endpoint is configured with a public key of the recipient. The process for exchanging and updating these public keys is application-specific.¶

• The endpoint has conveyed the HPKE payload at the application layer to the peer. HPKE is used to securely exchange a random number using a KEM.¶

The next generation of traffic keys is computed as described in this section. The traffic keys are derived, as described in Section 7.3 of [I-D.ietf-tls-rfc8446bis].¶

There are two changes to the application_traffic_secret computation described in [I-D.ietf-tls-rfc8446bis], namely¶

• the label is adjusted to distinguish it from the regular KeyUpdate message, and¶

• the random value, which was securely exchanged between the two endpoints, is included in the generation of the application traffic secret.¶

The next generation application_traffic_secret is computed as:¶

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

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.¶

7. Example

Figure 5 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 three phases worthwhile to highlight:¶

1. First, the support for the functionality in this specification is negotiated in the ClientHello and the EncryptedExtensions messages. As a result, the two peers have a shared understanding of the negotiated HPKE ciphersuite, which includes a KEM, a KDF, and an AEAD.¶

2. Once the initial handshake is completed, application layer payloads can be exchanged. The two peers exchange public keys suitable for use with the HPKE KEM and subsequently an HPKE- encrypted random value.¶

3. When a key update needs to be triggered by the application, it instructs the (D)TLS stack to transmit an ExtendedKeyUpdate message.¶

Figure 5 provides an overview of the exchange starting with the initial negotiation followed by the key update, which involves the application layer interaction.¶

       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
                                  ...
  +---------------- Application Layer Exchange --------------+
  |                                                          |
  |     (a)  Sender sends public key to the client           |
  |                                                          |
  |     (b)  Client uses HPKE to generate enc, and ct        |
  |                                                          |
  |     (c)  Client sents enc, and ct to the server          |
  |                                                          |
  |     (d)  Client triggers the extended key update         |
  |          at the TLS layer                                |
  |                                                          |
  +---------------- Application Layer Exchange --------------+

       [ExtendedKeyUpdate]     -------->
                               <--------  [ExtendedKeyUpdate]

For the server to generate and transmit a public key it is necessary to determine whether the extended key update extension has been negotiated success and what HPKE ciphersuite was selected. This information can be obtained by the application by using the "Get HPKE Ciphersuite" API.¶

Once the public key has been sent to the client, it can use the "Encapsulate" API with SealBase(pk, info, aad, rand) to produce enc, and ct. A random value has to be passed into the API call.¶

The client transmit the enc, and ct values to the server, which performs the reverse operation using the "Decapsulate" API with OpenBase(enc, skR, info, aad, ct) returning the random value.¶

The server uses the "Update-Prepare" API to get the (D)TLS stack ready for a key update.¶

When the client wants to switch to the new sending key it uses the "Update-Trigger" API to inform the (D)TLS library to trigger the transmission of the ExtendedKeyUpdate message.¶

8. DTLS 1.3 Considerations

As with other handshake messages with no built-in response, the ExtendedKeyUpdate MUST be acknowledged. In order to facilitate epoch reconstruction implementations MUST NOT send records with the new keys or send a new ExtendedKeyUpdate until the previous ExtendedKeyUpdate has been acknowledged (this avoids having too many epochs in active use).¶

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

Due to the possibility of an ACK message for an ExtendedKeyUpdate being lost and thereby preventing the sender of the ExtendedKeyUpdate 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.¶

9. API Considerations

The creation and processing of the extended key update messages SHOULD be implemented inside the (D)TLS library even if it is possible to implement it at the application layer. (D)TLS implementations supporting the use of the extended key update SHOULD provide application programming interfaces by which clients and server may request and process the extended key update messages.¶

It is also possible to implement this API outside of the (D)TLS library. This may be preferable in cases where the application does not have access to a TLS library with these APIs or when TLS is handled independently of the application-layer protocol.¶

All APIs MUST fail if the connection uses a (D)TLS version of 1.2 or earlier.¶

The following sub-sections describe APIs that are considered necessary to implement the extended key update functionality but the description is informative only.¶

10. Post-Quantum 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. It is motivated by transition to post-quantum cryptography. HPKE can be extended to support hybrid post-quantum Key Encapsulation Mechanisms (KEMs), as defined in [I-D.westerbaan-cfrg-hpke-xyber768d00]¶

11. Security Considerations

[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 term key exfiltration is defined in [RFC7624].)¶

This specification re-uses public key encryption to update application traffic secrets in one direction. Hence, updates of these application traffic secrets in both directions requires two ExtendedKeyUpdate messages.¶

To perform public key encryption the sender needs to have access to the public key of the recipient. This document makes the assumption that the public key in the exchanged end-entity certificate can be used with the HPKE KEM. The use of HPKE, and the recipients long-term public key, in the ephemeral-static Diffie-Hellman exchange provides perfect forward secrecy of the ongoing connection and demonstrates possession of the long-term secret key.¶

12. IANA Considerations

IANA is also requested to allocate a new value in the "TLS ExtensionType Values" subregistry of the "Transport Layer Security (TLS) Extensions" registry [TLS-Ext-Registry], as follows:¶

• Value: TBD1¶

• Extension Name: extended_key_update¶

• TLS 1.3: CH, EE¶

• DTLS-Only: N¶

• Recommended: Y¶

• Reference: [This document]¶

IANA is also requested to allocate a new value in the "TLS HandshakeType" subregistry of the "Transport Layer Security (TLS) Extensions" registry [TLS-Ext-Registry], as follows:¶

• Value: TBD2¶

• Description: ExtendedKeyUpdate¶

• DTLS-OK: Y¶

• Reference: [This document]¶

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, are:¶

• Marcelo Ricardo Leitner¶

• Zaheduzzaman Sarker¶

• Magnus Westerlund¶

• John Mattsson¶

• Claudio Porfiri¶

• Xin Long¶

• Michael Tuexen¶

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 a review of an initial version of this specification.¶

Appendix B. Design Rational

The design in this document is motivated by long-lived TLS connections, which can be observed in, at least, two use cases: industrial IoT environments and telecommunication operator networks. In the discussions the desire to develop a design that is also compatible with the ongoing work on PQC algorithm and the use of KEMs in particular.¶

HPKE was selected as a building block due to its popularity in IETF protocols and the availability of implementations. The core building blocks of HPKE (a KEM and a key derivation function) could, howerver, be used directly as well.¶

The design presented in this document utilizes HPKE with the Seal/Open API calls instead of utilizing Encap/Decap API calls directly. Available HPKE libraries expose the former API calls and this simplifies the implementation of the solution described in this document. As a side-effect, context information can also be passed into these API calls.¶

The downside of using the currently documented approach is the need to additionally encrypt plaintext, which in our case is a random value. It may also introduce complexity with the integration of hybrid approach.¶

The use of application layer protocol messages to exchange TLS handshake messages is motiviated by the desire to reduce the impact on the TLS state machine but also by the prior work on post-handshake authentication using "Exported Authenticators". A design that exchanges messages at the TLS layer is possible but raises the question about whether post-handshake authentication messages should also be exchanged at the TLS layer to accomplish some level of uniformity. Even the re- introduction of session renegotation, a feature removed with TLS 1.3, may seem worthwhile to consider.¶

Authors' Addresses