Extended Key Update for Transport Layer Security (TLS) 1.3
draft-tschofenig-tls-extended-key-update-01
| Document | Type | Active Internet-Draft (individual) | |
|---|---|---|---|
| Authors | Hannes Tschofenig , Michael Tüxen , Tirumaleswar Reddy.K , Steffen Fries | ||
| Last updated | 2024-03-03 | ||
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draft-tschofenig-tls-extended-key-update-01
TLS H. Tschofenig
Internet-Draft Siemens
Intended status: Standards Track M. Tüxen
Expires: 3 September 2024 Münster Univ. of Applied Sciences
T. Reddy
Nokia
S. Fries
Siemens
2 March 2024
Extended Key Update for Transport Layer Security (TLS) 1.3
draft-tschofenig-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
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This Internet-Draft will expire on 3 September 2024.
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
Provisions Relating to IETF Documents (https://trustee.ietf.org/
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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. Negotiating the Extended Key Update . . . . . . . . . . . . . 5
4. Using HPKE . . . . . . . . . . . . . . . . . . . . . . . . . 6
5. Extended Key Update Message . . . . . . . . . . . . . . . . . 8
6. Updating Traffic Secrets . . . . . . . . . . . . . . . . . . 9
7. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
8. DTLS 1.3 Considerations . . . . . . . . . . . . . . . . . . . 12
9. API Considerations . . . . . . . . . . . . . . . . . . . . . 12
9.1. The "Get HPKE Ciphersuite" API . . . . . . . . . . . . . 13
9.2. The "Encapsulate" API . . . . . . . . . . . . . . . . . . 13
9.3. The "Decapsulate" API . . . . . . . . . . . . . . . . . . 13
9.4. The "Update-Prepare" API . . . . . . . . . . . . . . . . 14
9.5. The "Update-Trigger" API . . . . . . . . . . . . . . . . 14
10. Post-Quantum Considerations . . . . . . . . . . . . . . . . . 14
11. Security Considerations . . . . . . . . . . . . . . . . . . . 14
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 16
13.1. Normative References . . . . . . . . . . . . . . . . . . 16
13.2. Informative References . . . . . . . . . . . . . . . . . 16
Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . 17
Appendix B. Design Rational . . . . . . . . . . . . . . . . . . 18
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 19
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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 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
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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
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* 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;
Figure 1: ExtensionType Structure.
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;
Figure 2: HpkeCipherSuites Structure.
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.
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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;
Figure 3: HPKE_Payload Structure.
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.
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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().
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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;
Figure 4: ExtendedKeyUpdate Structure.
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
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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
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* 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.
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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
...
+---------------- 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]
Figure 5: Extended Key Update Message Exchange.
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.
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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.
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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.
9.1. The "Get HPKE Ciphersuite" API
This API allows the application to determine the negotiated HPKE
ciphersuite from the (D)TLS stack. This information is useful for
the application since it needs to exchange or present public keys to
the stack.
It takes a reference to the initial connection as input and returns
the HpkeCipherSuite structure (if the extension was successfully
negotiated) or an empty payload otherwise.
9.2. The "Encapsulate" API
This API allows the application to request the (D)TLS stack to
execute HPKE SealBase operation. It takes the following values as
input:
* a reference to the initial connection
* public key of the recipient
* HPKE ciphersuite
* Random value
It returns the Figure 3 payload.
9.3. The "Decapsulate" API
This API allows the application to request the (D)TLS stack to
execute HPKE OpenBase operation. It takes the following values as
input:
* a reference to the initial connection
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* a reference to the secret key corresponding to the previously
exchanged public key
* the Figure 3 payload
It returns the random value, in case of success.
9.4. The "Update-Prepare" API
This API allows the application to request the (D)TLS stack to
execute HPKE OpenBase operation. It takes the following values as
input:
* a reference to the initial connection
* the random value obtained from the "Decapsulate" API call
It returns the success or failure.
9.5. The "Update-Trigger" API
This API allows the application to request the (D)TLS stack to
initiate an extended key update using the message defined in
Section 5.
It takes an identifier to the public key of the recipient as input
and returns success or failure.
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:
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"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]
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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]
13. References
13.1. 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-09, 7 July 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-tls-
rfc8446bis-09>.
[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>.
[RFC9180] Barnes, R., Bhargavan, K., Lipp, B., and C. Wood, "Hybrid
Public Key Encryption", RFC 9180, DOI 10.17487/RFC9180,
February 2022, <https://www.rfc-editor.org/rfc/rfc9180>.
13.2. 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>.
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Internet-Draft Extended Key Update for TLS March 2024
[I-D.westerbaan-cfrg-hpke-xyber768d00]
Westerbaan, B. and C. A. Wood, "X25519Kyber768Draft00
hybrid post-quantum KEM for HPKE", Work in Progress,
Internet-Draft, draft-westerbaan-cfrg-hpke-xyber768d00-02,
4 May 2023, <https://datatracker.ietf.org/doc/html/draft-
westerbaan-cfrg-hpke-xyber768d00-02>.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008,
<https://www.rfc-editor.org/rfc/rfc5246>.
[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>.
[RFC9261] Sullivan, N., "Exported Authenticators in TLS", RFC 9261,
DOI 10.17487/RFC9261, July 2022,
<https://www.rfc-editor.org/rfc/rfc9261>.
[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, are:
* Marcelo Ricardo Leitner
* Zaheduzzaman Sarker
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* 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
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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
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
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