Extended Key Update for Transport Layer Security (TLS) 1.3
draft-ietf-tls-extended-key-update-09
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft whose latest revision state is "Active".
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|
|---|---|---|---|
| Authors | Hannes Tschofenig , Michael Tüxen , Tirumaleswar Reddy.K , Steffen Fries , Yaroslav Rosomakho | ||
| Last updated | 2026-02-18 (Latest revision 2026-01-13) | ||
| Replaces | draft-tschofenig-tls-extended-key-update | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
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| Additional resources | Mailing list discussion | ||
| Stream | WG state | WG Document | |
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| Send notices to | (None) |
draft-ietf-tls-extended-key-update-09
Transport Layer Security H. Tschofenig
Internet-Draft Siemens
Updates: 9261, 8446 (if approved) M. Tüxen
Intended status: Standards Track Münster Univ. of Applied Sciences
Expires: 23 August 2026 T. Reddy
Nokia
S. Fries
Siemens
Y. Rosomakho
Zscaler
19 February 2026
Extended Key Update for Transport Layer Security (TLS) 1.3
draft-ietf-tls-extended-key-update-09
Abstract
TLS 1.3 ensures forward secrecy by performing an ephemeral Diffie-
Hellman key exchange during the initial handshake, protecting past
communications even if a party's long-term keys (typically a private
key with a corresponding certificate) are later compromised. While
the built-in KeyUpdate mechanism allows application traffic keys to
be refreshed during a session, it does not incorporate fresh entropy
from a new key exchange and therefore does not provide post-
compromise security. This limitation can pose a security risk in
long-lived sessions, such as those found in industrial IoT or
telecommunications environments.
To address this, this specification defines an extended key update
mechanism that performs a fresh Diffie-Hellman exchange within an
active session, thereby ensuring post-compromise security. By
forcing attackers to exfiltrate new key material repeatedly, this
approach mitigates the risks associated with static key compromise.
Regular renewal of session keys helps contain the impact of such
compromises. The extension is applicable to both TLS 1.3 and DTLS
1.3.
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 August 2026.
Copyright Notice
Copyright (c) 2026 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/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
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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. Extended Key Update Messages . . . . . . . . . . . . . . . . 5
5. TLS 1.3 Considerations . . . . . . . . . . . . . . . . . . . 8
5.1. TLS 1.3 Extended Key Update Example . . . . . . . . . . . 10
6. DTLS 1.3 Considerations . . . . . . . . . . . . . . . . . . . 11
6.1. DTLS 1.3 Extended Key Update Example . . . . . . . . . . 14
7. Updating Traffic Keys . . . . . . . . . . . . . . . . . . . . 16
8. Post-Quantum Cryptography Considerations . . . . . . . . . . 18
9. SSLKEYLOGFILE Update . . . . . . . . . . . . . . . . . . . . 19
10. Exporter . . . . . . . . . . . . . . . . . . . . . . . . . . 19
10.1. Post-Compromise Security for the Initial Exporter
Secret . . . . . . . . . . . . . . . . . . . . . . . . . 20
10.2. Exporter Usage After Extended Key Update . . . . . . . . 21
11. Use of Post-Handshake Authentication and Exported
Authenticators with Extended Key Update . . . . . . . . . 22
11.1. Post-Handshake Certificate-Based Client
Authentication . . . . . . . . . . . . . . . . . . . . . 22
11.2. Exported Authenticators . . . . . . . . . . . . . . . . 23
12. Security Considerations . . . . . . . . . . . . . . . . . . . 23
12.1. Scope of Key Compromise . . . . . . . . . . . . . . . . 24
12.2. Post-Compromise Security . . . . . . . . . . . . . . . . 24
12.3. Denial-of-Service (DoS) . . . . . . . . . . . . . . . . 25
12.4. Operational Guidance . . . . . . . . . . . . . . . . . . 25
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13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 25
13.1. TLS Flags . . . . . . . . . . . . . . . . . . . . . . . 25
13.2. TLS HandshakeType . . . . . . . . . . . . . . . . . . . 25
13.3. ExtendedKeyUpdate Message Subtypes Registry . . . . . . 26
13.4. SSLKEYLOGFILE labels . . . . . . . . . . . . . . . . . . 26
14. References . . . . . . . . . . . . . . . . . . . . . . . . . 27
14.1. Normative References . . . . . . . . . . . . . . . . . . 27
14.2. Informative References . . . . . . . . . . . . . . . . . 28
Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . 30
Appendix B. State Machines . . . . . . . . . . . . . . . . . . . 30
B.1. TLS 1.3 State Machines . . . . . . . . . . . . . . . . . 31
B.1.1. Initiator State Machine . . . . . . . . . . . . . . . 31
B.1.2. Responder State Machine . . . . . . . . . . . . . . . 32
B.2. DTLS 1.3 State Machines . . . . . . . . . . . . . . . . . 33
B.2.1. Terms and Abbreviations . . . . . . . . . . . . . . . 34
B.2.2. State Machine (Initiator) . . . . . . . . . . . . . . 34
B.2.3. State Machine (Responder) . . . . . . . . . . . . . . 36
Appendix C. Overview of Security Goals . . . . . . . . . . . . . 38
C.1. Post-Compromise Security (PCS) . . . . . . . . . . . . . 39
C.2. Key Freshness and Cryptographic Independence . . . . . . 39
C.3. Elimination of Standard KeyUpdate . . . . . . . . . . . . 39
C.4. Detecting Divergent Key State . . . . . . . . . . . . . . 39
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 40
1. Introduction
The Transport Layer Security (TLS) 1.3 protocol provides forward
secrecy by using an ephemeral Diffie-Hellman (DHE) key exchange
during the initial handshake. This ensures that encrypted
communication remains confidential even if an attacker later obtains
a party's long-term private key, protecting against passive
adversaries who record encrypted traffic for later decryption.
TLS 1.3 also includes a KeyUpdate mechanism that allows traffic keys
to be refreshed during an established session. However, this
mechanism does not provide post-compromise security, as it applies
only a key derivation function to the previous application traffic
key as input. While this design is generally sufficient for short-
lived connections, it may present security limitations in scenarios
where sessions persist for extended periods, such as in industrial
IoT or telecommunications systems, where continuous availability is
critical and session renegotiation or resumption is impractical.
Earlier versions of TLS supported session renegotiation, which
allowed peers to negotiate fresh keying material, including
performing new Diffie-Hellman exchanges during the session lifetime.
Due to protocol complexity and known vulnerabilities, renegotiation
was first restricted by [TLS-RENEGOTIATION] and ultimately removed in
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TLS 1.3. While the KeyUpdate message was introduced to offer limited
rekeying functionality, it does not fulfill the same cryptographic
role as renegotiation and cannot refresh the TLS main secret and
consequently cannot derive new secrets from fresh DHE input.
Security guidance from national agencies, such as ANSSI (France
[ANSSI]), recommends the periodic renewal of cryptographic keys
during long-lived sessions to limit the impact of key compromise.
This approach encourages designs that force an attacker to perform
dynamic key exfiltration, as defined in [CONFIDENTIALITY]. Dynamic
key exfiltration refers to attack scenarios where an adversary must
repeatedly extract fresh keying material to maintain access to
protected data, increasing operational cost and risk for the
attacker. In contrast, static key exfiltration, where the TLS main
secret is extracted once and reused, poses a greater long-term
threat, especially when session keys are not refreshed with fresh key
exchange input rather than key derivation.
This specification defines a TLS extension that introduces an
extended key update mechanism. Unlike the standard key update, this
mechanism allows peers to perform a fresh Diffie-Hellman exchange
within an active session using one of the groups negotiated during
the initial handshake. By periodically rerunning (EC)DHE, this
extension enables the derivation of new traffic keys that are
independent of prior key material. As noted in Appendix F of [TLS],
this approach mitigates the risk of static key exfiltration and
shifts the attacker burden toward dynamic key exfiltration.
The proposed extension is applicable to both TLS 1.3 [TLS] and DTLS
1.3 [DTLS]. For clarity, the term "TLS" is used throughout this
document to refer to both protocols unless otherwise specified.
2. Terminology and Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
To distinguish the key update procedure defined in [TLS] from the key
update procedure specified in this document, we use the terms
"standard key update" and "extended key update", respectively.
In this document, we use the term post-compromise security, as
defined in [CCG16]. We assume that an adversary may obtain access to
the application traffic keys.
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Unless otherwise specified, all references to traffic keys in this
document refer to application traffic keys and because the Extended
Key Update procedure occurs after the handshake phase has completed,
no handshake traffic keys are involved.
In this document, send key refers to the [sender]_write_key, and
receive key refers to the [receiver]_write_key. These keys are
derived from the active client_application_traffic_secret_N and
server_application_traffic_secret_N, as defined in (D)TLS 1.3 [TLS],
and are replaced with new ones after each successful Extended Key
Update.
3. Negotiating the Extended Key Update
Client and servers use the TLS flags extension [TLS-FLAGS] to
indicate support for the functionality defined in this document. We
call this flag "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 post-compromise security will have to initiate a full
handshake.
4. Extended Key Update Messages
If the client and server agree to use the extended key update
mechanism, the standard key update MUST NOT be used. In this case,
the extended key update fully replaces the standard key update
functionality.
Implementations that receive a classic KeyUpdate message after
successfully negotiating the Extended Key Update functionality MUST
terminate the connection with an "unexpected_message" alert.
The extended key update messages are signaled in a new handshake
message named ExtendedKeyUpdate (EKU), with an internal uint8 message
subtype indicating its role. This specification defines three
ExtendedKeyUpdate message subtypes:
* key_update_request (0)
* key_update_response (1)
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* new_key_update (2)
New ExtendedKeyUpdate message subtypes are assigned by IANA as
described in Section 13.3.
A TLS peer which receives a ExtendedKeyUpdate with an unexpected
message subtype MUST abort the connection with an
"unexpected_message" alert.
The extended key update process can be initiated by either peer after
it has sent a Finished message. Implementations that receive an
ExtendedKeyUpdate message prior to the sender having sent Finished
MUST terminate the connection with an "unexpected_message" alert.
The KeyShareEntry carried in a key_update_request and in a
key_update_response MUST use the group that was negotiated by the
client and server during the initial handshake. An implementation
that receives an algorithm other than previously negotiated 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; either the TLS client or the TLS server may act as
initiator.
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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]N <-------> [Application Data]N
...
[EKU(key_update_request)]N -------->
<-------- [EKU(key_update_response)]N
[EKU(new_key_update)]N -------->
...
[Application Data]N+1 <-------> [Application Data]N+1
Legend:
+ Indicates noteworthy extensions sent in the
previously noted message.
- Indicates optional or situation-dependent
messages/extensions that are not always sent.
() Indicates messages protected using keys
derived from a client_early_traffic_secret.
{} Indicates messages protected using keys
derived from a [sender]_handshake_traffic_secret.
[]N Indicates messages protected using keys
derived from [sender]_application_traffic_secret_N.
Figure 1: Extended Key Update Message Exchange in TLS 1.3.
The ExtendedKeyUpdate wire format is:
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enum {
key_update_request(0),
key_update_response(1),
new_key_update(2),
(255)
} ExtendedKeyUpdateType;
struct {
ExtendedKeyUpdateType eku_type;
select (eku_type) {
case key_update_request: {
KeyShareEntry key_share;
}
case key_update_response: {
KeyShareEntry key_share;
}
case new_key_update: {
/* empty */
}
};
} ExtendedKeyUpdate;
Fields:
* eku_type: the subtype of the ExtendedKeyUpdate message.
* key_share: key share information. The contents of this field is
determined by the specified group and its corresponding definition
(see Section 4.2.8 of [TLS]).
5. TLS 1.3 Considerations
The following steps are taken by a TLS 1.3 implementation; the steps
executed with DTLS 1.3 differ slightly.
1. The initiator sends ExtendedKeyUpdate(key_update_request)
carrying a KeyShareEntry. While an extended key update is in
progress, the initiator MUST NOT initiate another key update.
2. Upon receipt, the responder sends its own KeyShareEntry in a
ExtendedKeyUpdate(key_update_response) message. While an
extended key update is in progress, the responder MUST NOT
initiate another key update. The responder MAY defer sending a
response if system load or resource constraints prevent immediate
processing. In such cases, the response MUST be sent once
sufficient resources become available.
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3. After the responder sends ExtendedKeyUpdate(key_update_response)
it MUST update its send keys.
4. Upon receipt of an ExtendedKeyUpdate(key_update_response) the
initiator derives the new secrets from the exchanged key shares.
The initiator then updates its receive keys and sends an empty
ExtendedKeyUpdate(new_key_update) message to complete the
process. The initiator MUST NOT defer derivation of the secrets
and sending the ExtendedKeyUpdate(new_key_update) message as it
would stall the communication.
5. After sending ExtendedKeyUpdate(new_key_update) initiator MUST
update its send keys.
6. After receiving the initiator’s
ExtendedKeyUpdate(new_key_update), the responder MUST update its
receive keys.
Both initiator and responder MUST encrypt their ExtendedKeyUpdate
messages with the old keys. Responder MUST ensure that the
ExtendedKeyUpdate(new_key_update) encrypted with the old key is
received before accepting any messages encrypted with the new key.
If TLS peers independently initiate the extended key update and the
requests cross in flight, the ExtendedKeyUpdate(key_update_request)
with the lower lexicographic order of the key_exchange value in
KeyShareEntry MUST be ignored. This prevents each side from
advancing keys by two generations. If the tie-break comparison
yields equality (an event that should be impossible for genuine
asymmetric key pairs), the endpoint MUST treat this as a protocol
violation, send an "unexpected_message" alert, and close the
connection.
The handshake framing uses a single HandshakeType for this message
(see Figure 2).
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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: TLS 1.3 Handshake Structure.
5.1. TLS 1.3 Extended Key Update Example
While Figure 1 shows the high-level interaction between a TLS 1.3
client and server, this section shows an example message exchange
with information about the updated keys added.
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
...
[EKU(key_update_request -------->
(with key_share))]
<-------- [EKU(key_update_response
(with key_share))]
# Server derives new secrets
# and updates SEND keys here
# Client derives new secrets
# and updates RECEIVE keys here
[EKU(new_key_update)] -------->
# Client updates SEND keys here
# Server updates RECEIVE keys here
Figure 3: Extended Key Update Example.
6. DTLS 1.3 Considerations
Unlike TLS 1.3, DTLS 1.3 implementations must take into account that
handshake messages are not transmitted over a reliable transport
protocol.
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EKU messages MUST be transmitted reliably, like other DTLS handshake
messages. If necessary, EKU messages MAY be fragmented as described
in Section 5.5 of [DTLS]. Due to the possibility of an
ExtendedKeyUpdate messages being lost and thereby preventing the
sender of that 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 packet loss and/or reordering, DTLS 1.3 peers MAY receive
records from an earlier epoch. If the necessary keys are available,
implementations SHOULD attempt to process such records; however, they
MAY choose to discard them. The exchange has the following steps:
1. The initiator sends an ExtendedKeyUpdate(key_update_request)
message, which contains a key share. While an extended key
update is in progress, the initiator MUST NOT initiate further
key updates. This message is subject to DTLS handshake
retransmission, but delivery is only confirmed when the initiator
either receives the corresponding
ExtendedKeyUpdate(key_update_response) or an ACK.
2. Upon receipt, the responder sends its own KeyShareEntry in a
ExtendedKeyUpdate(key_update_response) message. While an
extended key update is in progress, the responder MUST NOT
initiate further key updates. The responder MAY defer sending a
response if system load or resource constraints prevent immediate
processing. In such cases, the responder MUST acknowledge
receipt of the key_update_request with an ACK and, once
sufficient resources become available, retransmit the
key_update_response until it is acknowledged by the initiator.
3. On receipt of ExtendedKeyUpdate(key_update_response) the
initiator derives a secret key based on the exchanged key shares.
This message also serves as an implicit acknowledgment of the
initiator's ExtendedKeyUpdate(key_update_request), so no separate
ACK is required. The initiator MUST update its receive keys and
epoch value. The initiator MUST NOT defer derivation of the
secrets.
4. The initiator transmits an ExtendedKeyUpdate(new_key_update)
message. This message is subject to DTLS retransmission until
acknowledged.
5. The responder MUST acknowledge the received message by sending an
ACK message.
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6. After the responder receives the initiator's
ExtendedKeyUpdate(new_key_update), the responder MUST update its
send key and epoch value. With the receipt of that message, the
responder MUST also update its receive keys.
7. On receipt of the ACK message, the initiator updates its send key
and epoch value. If this ACK is not received, the initiator re-
transmits its ExtendedKeyUpdate(new_key_update) until ACK is
received. The key update is complete once this ACK is processed
by the initiator.
The handshake framing uses a single HandshakeType for this message
(see Figure 4).
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enum {
client_hello(1),
server_hello(2),
new_session_ticket(4),
end_of_early_data(5),
encrypted_extensions(8),
request_connection_id(9),
new_connection_id(10),
certificate(11),
certificate_request(13),
certificate_verify(15),
finished(20),
key_update(24),
extended_key_update(TBD), /* new */
message_hash(254),
(255)
} HandshakeType;
struct {
HandshakeType msg_type; /* handshake type */
uint24 length; /* bytes in message */
uint16 message_seq; /* DTLS-required field */
uint24 fragment_offset; /* DTLS-required field */
uint24 fragment_length; /* DTLS-required field */
select (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;
case request_connection_id: RequestConnectionId;
case new_connection_id: NewConnectionId;
} body;
} DTLSHandshake;
Figure 4: DTLS 1.3 Handshake Structure.
6.1. DTLS 1.3 Extended Key Update Example
The following example illustrates a successful extended key update,
including how the epochs change during the exchange.
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Client Server
(Initiator) (Responder)
/---------------------------------------\
| Initial Handshake |
\---------------------------------------/
[C: tx=3, rx=3] [S: tx=3, rx=3]
[Application Data] -------->
[C: tx=3, rx=3] [S: tx=3, rx=3]
[C: tx=3, rx=3] [S: tx=3, rx=3]
<-------- [Application Data]
[C: tx=3, rx=3] [S: tx=3, rx=3]
/---------------------------------------\
| Some time later ... |
\---------------------------------------/
[C: tx=3, rx=3] [S: tx=3, rx=3]
[EKU(key_update_request)] -------->
# no epoch change yet
<-------- [EKU(key_update_response)]
# Sent under OLD epoch. Server does NOT bump yet.
# Step 3: initiator bumps RECEIVE epoch on
# receiving key update response-in:
# (rx:=rx+1; tx still old)
[C: tx=3, rx=4] [S: tx=3, rx=3]
[EKU(new_key_update)] -------->
# Sender's NKU is tagged with OLD tx (3).
# Epoch switch point:
# Step 6: responder bumps BOTH tx and rx on NKU-in:
[C: tx=3, rx=4] [S: tx=4, rx=4]
<-------- [ACK] (tag=new)
# Step 7: initiator bumps SEND epoch on ACK-in:
[C: tx=4, rx=4] [S: tx=4, rx=4]
[Application Data] -------->
[C: tx=4, rx=4] [S: tx=4, rx=4]
<-------- [Application Data]
[C: tx=4, rx=4] [S: tx=4, rx=4]
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Figure 5: Example DTLS 1.3 Extended Key Update: Message Exchange.
Figure 6 shows the steps, the message in flight, and the epoch
changes on both sides. The A/B -> X/Y notation indicates the change
of epoch values for tx/rx before and after the message transmission.
+--------------------+----------------+--------------+
| Message | Client tx/rx | Server tx/rx |
+--------------------+----------------+--------------+
| APP ------------> | 3/3 -> 3/3 | 3/3 -> 3/3 |
| <------------ APP | 3/3 -> 3/3 | 3/3 -> 3/3 |
| req -------------> | 3/3 -> 3/3 | 3/3 -> 3/3 |
| <------------ resp | 3/3 -> 3/4 | 3/3 -> 3/3 | <- step 3
| NKU ------------> | 3/4 -> 3/4 | 3/3 -> 4/4 | <- step 6
| <------------- ACK | 3/4 -> 4/4 | 4/4 -> 4/4 | <- step 7
| APP -------------> | 4/4 -> 4/4 | 4/4 -> 4/4 |
| <------------- APP | 4/4 -> 4/4 | 4/4 -> 4/4 |
+--------------------+----------------+--------------+
Figure 6: Example DTLS 1.3 Extended Key Update: Epoch Changes.
7. Updating Traffic Keys
When the extended key update message exchange is completed both peers
have successfully updated their application traffic keys. The key
derivation function described in this document is used to perform
this update.
The design of the key derivation function for computing the next
generation of application_traffic_secret is motivated by the desire
to include
* a secret derived from the (EC)DHE exchange (or from the hybrid key
exchange / PQ-KEM exchange),
* a secret that allows the new key exchange to be cryptographically
bound to the previously established secret,
* a transcript hash that is updated after each Extended Key Update
exchange by hashing together the previous transcript hash value
and the current ExtendedKeyUpdate(key_update_request) and
ExtendedKeyUpdate(key_update_response) messages, which contain the
key shares, thereby binding the encapsulated shared secret
ciphertext to the IKM in the case of PQ/T hybrid or PQC key
exchange and cryptographically binding the newly derived secrets
to the prior handshake transcript and all preceding EKU exchanges,
as well as to the current EKU exchange, and
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* new label strings to distinguish it from the key derivation used
in TLS 1.3.
The transcript_hash_N denotes the transcript hash value associated
with generation N. During each Extended Key Update exchange, the
transcript hash value for the next generation is computed as follows:
transcript_hash_N+1 = Transcript-Hash(transcript_hash_N ||
EKU(key_update_request) ||
EKU(key_update_response))
Once transcript_hash_N+1 has been computed, transcript_hash_N can be
deleted. No prior transcript hash values need to be retained for
future EKU exchanges. transcript_hash_0 denotes the transcript hash
of the initial TLS handshake, covering all messages from the
ClientHello up to and including the client Finished message.
The following diagram shows the key derivation hierarchy.
Main Secret N
|
v
Derive-Secret(., "derived", "")
|
v
(EC)DHE -> HKDF-Extract = Main Secret N+1
|
+-----> Derive-Secret(., "c ap traffic",
| transcript_hash_N+1)
| = client_application_traffic_secret_N+1
|
+-----> Derive-Secret(., "s ap traffic",
| transcript_hash_N+1)
| = server_application_traffic_secret_N+1
|
+-----> Derive-Secret(., "exp master",
| transcript_hash_N+1)
| = exporter_secret_N+1
|
+-----> Derive-Secret(., "res master",
| transcript_hash_N+1)
= resumption_main_secret_N+1
During the initial handshake, the Main Secret is generated (see
Section 7.1 of [TLS]). Since the Main Secret is discarded during the
key derivation procedure, a derived value is stored. This stored
value then serves as the input salt to the first key update procedure
that incorporates the ephemeral (EC)DHE- established value as input
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keying material (IKM) to produce main_secret_{N+1}. The derived value
from this new master secret serves as input salt to the subsequent
key update procedure, which also incorporates a fresh ephemeral
(EC)DHE value as IKM. This process is repeated for each additional
key update procedure.
The traffic keys are re-derived from
client_application_traffic_secret_N+1 and
server_application_traffic_secret_N+1, as described in Section 7.3 of
[TLS].
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 as soon as possible. Note: The client_/
server_application_traffic_secret_N and its associated traffic keys
can only be deleted by the responder after receiving the
ExtendedKeyUpdate(new_key_update) message.
Once client_/server_application_traffic_secret_N+1 and the
corresponding traffic keys are in use, all subsequent records,
including alerts and post-handshake messages MUST be protected using
those keys.
When using this extension, it is important to consider its
interaction with ticket-based session resumption. If resumption
occurs without a new (EC)DH exchange that provides forward secrecy,
an attacker could potentially revert the security context to an
earlier state, thereby negating the benefits of the extended key
update. To preserve the security guarantees provided by key updates,
endpoints MUST either invalidate any session tickets issued prior to
the key update or ensure that resumption always involves a fresh
(EC)DH exchange.
If session tickets cannot be stored securely, developers SHOULD
consider disabling ticket-based resumption in their deployments.
While this approach may impact performance, it provides improved
security properties.
8. Post-Quantum Cryptography Considerations
Hybrid key exchange refers to the simultaneous use of multiple key
exchange algorithms, with the resulting shared secret derived by
combining the outputs of each. The goal of this approach is to
maintain security even if all but one of the component algorithms are
later found to be vulnerable.
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The transition to post-quantum cryptography has motivated the
adoption of hybrid key exchanges in TLS, as described in
[TLS-HYBRID]. Specific hybrid groups have been registered in
[TLS-ECDHE-MLKEM]. When hybrid key exchange is used, the
key_exchange field of each KeyShareEntry in the initial handshake is
formed by concatenating the key_exchange fields of the constituent
algorithms. This same approach is reused during the Extended Key
Update, when new key shares are exchanged.
The specification in [TLS-MLKEM] registers the lattice-based ML-KEM
algorithm and its variants, such as ML-KEM-512, ML-KEM-768 and ML-
KEM-1024. The KEM encapsulation key or KEM ciphertext is represented
as a 'KeyShareEntry' field. This same approach is reused during the
Extended Key Update, when new key shares are exchanged.
9. SSLKEYLOGFILE Update
As a successful extended key update exchange invalidates previous
secrets, SSLKEYLOGFILE [TLS-KEYLOGFILE] needs to be populated with
new entries. As a result, two additional secret labels are utilized
in the SSLKEYLOGFILE:
1. CLIENT_TRAFFIC_SECRET_N+1: identifies the
client_application_traffic_secret_N+1 in the key schedule
2. SERVER_TRAFFIC_SECRET_N+1: identifies the
server_application_traffic_secret_N+1 in the key schedule
3. EXPORTER_SECRET_N+1: identifies the exporter_secret_N+1 in the
key schedule
Similar to other entries in the SSLKEYLOGFILE, the label is followed
by the 32-byte value of the Random field from the ClientHello message
that established the TLS connection, and the corresponding secret
encoded in hexadecimal.
SSLKEYLOGFILE entries for the 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_, SERVER_TRAFFIC_SECRET_ and EXPORTER_SECRET_.
10. Exporter
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10.1. Post-Compromise Security for the Initial Exporter Secret
The TLS 1.3 Key Schedule, see Figure 5 of [TLS], derives the
exporter_secret from the main secret. This exporter_secret is static
for the lifetime of the connection and is not updated by a standard
key update.
A core design goal of this specification is not met if the
exporter_secret does not change. Therefore, this document defines an
exporter interface that derives a fresh exporter secret whenever new
application traffic keys are updated through the EKU.
If the initial exporter secret for this new interface were identical
to exporter_secret, then compromising exporter_secret at any point
during the lifetime of the connection would enable an attacker to
recompute all exporter outputs derived from it. This would break
post-compromise security for exported keying material.
Therefore, the initial exporter secret used by the exporter interface
defined in this document, i.e., the exporter output available prior
to the first Extended Key Update, MUST be distinct from the
exporter_secret. This separation ensures that compromise of the TLS
exporter interface does not compromise outputs derived from the
exporter interface defined in this document.
Prior to the first Extended Key Update, the exporter interface
provides an initial exporter secret, denoted exporter_secret_0. This
secret is derived from the TLS main secret and the handshake
transcript, but is cryptographically independent of the TLS
exporter_secret. It is computed as follows:
exporter_secret_0 =
Derive-Secret(Main Secret,
"exporter eku",
Transcript-Hash(ClientHello..server Finished))
Applications that require post-compromise security MUST use the
exporter interface defined in this document. This exporter interface
is independent of the TLS exporter defined in Section 7.5 of [TLS],
which continues to use a static exporter_secret for the lifetime of
the connection for compatiblity with "legacy" applications.
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10.2. Exporter Usage After Extended Key Update
Protocols such as DTLS-SRTP and DTLS-over-SCTP rely on TLS or DTLS
for key establishment, but reuse portions of the derived keying
material for their own specific purposes. These protocols use the
TLS exporter defined in Section 7.5 of [TLS]. Exporters are also
used for deriving authentication related values such as nonces, as
described in [RFC9729].
Once the Extended Key Update mechanism is complete, such protocols
would need to use the newly derived exporter secret to generate
Exported Keying Material (EKM) to protect packets. The "sk" derived
in the Section 7 will be used as the "Secret" in the exporter
function, defined in Section 7.5 of [TLS], to generate EKM, ensuring
that the exported keying material is aligned with the updated
security context. The newly derived exporter secret is
cryptographically independent of previous exporter secrets.
When a new exporter secret becomes active following a successful
Extended Key Update, the TLS or DTLS implementation would have to
provide an asynchronous notification to the application indicating
that:
* A new epoch has become active, and the (D)TLS implementation can
include the corresponding epoch identifier. Applications
receiving an epoch identifier can use it to request keying
material for that specific epoch through an epoch-aware exporter
interface. In TLS, this identifier represents a local logical
counter that may differ between peers.
Applications are notified that a new epoch is active only after both
peers have completed the Extended Key Update exchange and switched to
the new traffic keys.
* In TLS, the initiator triggers notification after Step 5 in
Section 5, and the responder triggers notification after Step 4 in
Section 5.
* In DTLS, the initiator triggers notification to the application
after Step 7 in Section 6, and the responder triggers notification
after Step 9 in Section 6.
The corresponding EKM is obtained by the application through the TLS/
DTLS exporter interface using its chosen label and context values as
defined in Section 4 of [RFC5705].
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To prevent desynchronization, the application will have to retain
both the previous and the newly derived exporter secrets for a short
period. For TLS, the previous exporter secret would be discarded
once data derived from the new exporter has been successfully
processed, and no records protected with the old exporter secret are
expected to arrive. For DTLS, the previous exporter secret needs to
be retained until the retention timer expires for the prior epoch, to
allow for processing of packets that may arrive out of order. The
retention policy for exporter secrets is application-specific. For
example, in DTLS-SRTP, the application might retain the previous
exporter secret until its replay window no longer accepts packets
protected with keys derived from that secret, as described in
Section 3.3.2 of [RFC3711].
11. Use of Post-Handshake Authentication and Exported Authenticators
with Extended Key Update
EKU provides fresh traffic secrets, but EKU alone does not
authenticate that both endpoints derived the same updated keys. An
attacker that temporarily compromises an endpoint may later act as an
active MitM capable of interfering with the EKU exchange. Such an
attacker can cause the peers to transition to divergent traffic
secrets without detection, but cannot compromise the endpoint to
derive secrets after the new epoch is established. To confirm that
both peers transitioned to the same new key state, TLS 1.3 provides
two mechanisms: Post-Handshake Certificate-Based Client
Authentication and Exported Authenticators [RFC9261].
11.1. Post-Handshake Certificate-Based Client Authentication
When Post-Handshake Certificate-Based Client Authentication
(Section 4.6.2 of [TLS]) is performed after an Extended Key Update
(EKU) is complete, the Handshake Context used for the transcript hash
is updated. It consists of transcript_hash_N+1 concatenated with the
CertificateRequest message. The Finished message is computed using a
MAC key derived from the Base Key of the new epoch
(client_application_traffic_secret_N+1). This confirms that both
peers are operating with the same updated traffic keys and completes
an authenticated transition after the EKU.
To prevent cryptographic state ambiguity, the following constraints
apply:
* A TLS server MUST NOT initiate a post-handshake CertificateRequest
while an EKU exchange is in progress.
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* If a CertificateRequest has been sent but the corresponding
Finished message has not yet been received, the peers MUST NOT
initiate an EKU exchange.
11.2. Exported Authenticators
This document updates Section 5.1 of [RFC9261] to specify that, after
an Extended Key Update has completed, the Handshake Context and
Finished MAC Key used for Exported Authenticators MUST be derived
from the exporter secret associated with the current epoch.
Implementations that support the epoch-aware Exported Authenticators
interface MUST provide a means for applications to request the
generation or validation of Exported Authenticators using the
exporter secret for a specific epoch.
The Handshake Context and Finished MAC Key used in both the
CertificateVerify message (Section 5.2.2 of [RFC9261]) and the
Finished message (Section 5.2.3 of [RFC9261]) are derived from the
exporter secret associated with the current epoch. If a MitM
interferes with the EKU exchange and causes the peers to derive
different traffic and exporter secrets, their Handshake Contexts and
Finished MAC Keys will differ. As a result, validation procedures
specified in Section 5.2.4 of [RFC9261] will fail, thereby detecting
the divergence of key state between peers.
A new optional API SHOULD be defined to permit applications to
request or verify Exported Authenticators for a specific exporter
epoch. As discussed in Section 7 of [RFC9261], this can, as an
exception, be implemented at the application layer when the epoch-
aware TLS exporter is available. The APIs defined in [RFC9261]
remain unchanged, so existing applications continue to operate
without modification. The epoch-aware API accepts an epoch
identifier; when present, the (D)TLS implementation MUST derive the
Handshake Context and Finished MAC Key from the exporter secret
associated with that epoch. When Exported Authenticators are
generated using the epoch-aware Exported Authenticators interface,
the epoch identifier used for their derivation can be conveyed in the
certificate_request_context field, allowing the peer, particularly in
DTLS where records may be reordered, to determine the correct
exporter secret for validation.
12. Security Considerations
This section discusses additional security and operational aspects
introduced by the Extended Key Update mechanism. All security
considerations of TLS 1.3 [TLS] and DTLS.13 [DTLS] continue to apply.
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12.1. Scope of Key Compromise
Extended Key Update assumes a transient compromise of the current
application traffic keys, not a persistent attacker with ongoing
access to key material. The procedure itself does not rely on long-
term private keys; those are assumed to remain secure, as they are
typically stored in a secure element, such as a Trusted Execution
Environment (TEE), Hardware Security Module (HSM), or Trusted
Platform Module (TPM). In contrast, application traffic keys are
stored within the rich operating system, where short-term exposure
due to memory disclosure or transient compromise may occur. Post-
compromise security can be re-established, provided the compromise is
no longer active when an Extended Key Update is performed.
Extended Key Update can restore confidentiality only if the attacker
no longer has access to either peer. If an adversary retains access
to current application traffic keys and can act as a man-in-the-
middle during the Extended Key Update, then the update cannot restore
security unless Section 11 is used.
If one of the mechanisms defined in Section 11 is not used, the
attacker can impersonate each endpoint, substitute EKU messages, and
maintain control of the communication. When Post-handshake
Certificate-Based Client Authentication or the modified Exported
Authenticator mechanism is used, the authentication messages are
bound to the keys established after the EKU. Any modification or
substitution of EKU messages therefore becomes detectable, preventing
this attack.
If a compromise occurs before the handshake completes, the ephemeral
key exchange, client_handshake_traffic_secret,
server_handshake_traffic_secret, and the initial client_/
server_application_traffic_secret could be exposed. In that case,
only the initial handshake messages and the application data
encrypted under the initial client_/server_application_traffic_secret
can be decrypted until the Extended Key Update procedure completes.
The Extended Key Update procedure derives fresh
application_traffic_secrets from a new ephemeral key exchange,
ensuring that all subsequent application data remains confidential.
12.2. Post-Compromise Security
Extended Key Update provides post-compromise security for long-lived
TLS sessions. To ensure post-compromise security guarantees:
* Each update MUST use freshly generated ephemeral key-exchange
material. Implementations MUST NOT reuse ephemeral key-exchange
material across updates or across TLS sessions.
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12.3. Denial-of-Service (DoS)
The Extended Key Update mechanism increases computational and state-
management overhead. A malicious peer could attempt to exhaust CPU
or memory resources by initiating excessive update requests.
Implementations SHOULD apply the following mitigations:
* Limit the frequency of accepted Extended Key Update requests per
session.
* A peer that has sent an Extended Key Update MUST NOT initiate
another until the previous update completes. If a peer violates
this rule, the receiving peer MUST treat it as a protocol
violation, send an "unexpected_message" alert, and terminate the
connection.
12.4. Operational Guidance
Deployments SHOULD evaluate Extended Key Update performance under
load and fault conditions, such as high-frequency or concurrent
updates. TLS policies SHOULD define explicit rate limits that
balance post-compromise security benefits against potential DoS
exposure.
13. IANA Considerations
13.1. TLS Flags
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]
13.2. TLS HandshakeType
IANA is requested to add the following entry to the "TLS
HandshakeType" registry [TLS-Ext-Registry]:
* Value: TBD2
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* Description: extended_key_update
* DTLS-OK: Y
* Reference: [This document]
13.3. ExtendedKeyUpdate Message Subtypes Registry
IANA is requested to create a new registry "TLS ExtendedKeyUpdate
Message Subtypes", within the existing "Transport Layer Security
(TLS) Parameters" registry [TLS-Ext-Registry]. This new registry
reserves types used for Extended Key Update entries. The initial
contents of this registry are as follows.
+=======+=====================+=========+===============+
| Value | Description | DTLS-OK | Reference |
+=======+=====================+=========+===============+
| 0 | key_update_request | Y | This document |
+-------+---------------------+---------+---------------+
| 1 | key_update_response | Y | This document |
+-------+---------------------+---------+---------------+
| 2 | new_key_update | Y | This document |
+-------+---------------------+---------+---------------+
| 3-255 | Unassigned | | |
+-------+---------------------+---------+---------------+
Table 1
New assignments in the "TLS ExtendedKeyUpdate Types" registry will be
administered by IANA through Specification Required procedure
[RFC8126]. The role of the designated expert is described in
Section 17 of [RFC8447]. The designated expert [RFC8126] ensures
that the specification is publicly available. It is sufficient to
have an Internet-Draft (that is posted and never published as an RFC)
or to cite a document from another standards body, industry
consortium, or any other location. An expert may provide more in-
depth reviews, but their approval should not be taken as an
endorsement of the ExtendedKeyUpdate Message Subtype.
13.4. SSLKEYLOGFILE labels
IANA is requested to add the following entries to the "TLS
SSLKEYLOGFILE Labels" extension registry [TLS-Ext-Registry]:
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+===========================+=============+===========+============+
| Value | Description | Reference | Comment |
+===========================+=============+===========+============+
| CLIENT_TRAFFIC_SECRET_N+1 | Secret | This | N |
| | protecting | document | represents |
| | client | | iteration |
| | records | | of |
| | after | | Extended |
| | Extended | | Key Update |
| | Key Update | | |
+---------------------------+-------------+-----------+------------+
| SERVER_TRAFFIC_SECRET_N+1 | Secret | This | N |
| | protecting | document | represents |
| | server | | iteration |
| | records | | of |
| | after | | Extended |
| | Extended | | Key Update |
| | Key Update | | |
+---------------------------+-------------+-----------+------------+
| EXPORTER_SECRET_N+1 | Exporter | This | N |
| | secret | document | represents |
| | after | | iteration |
| | Extended | | of |
| | Key Update | | Extended |
| | | | Key Update |
+---------------------------+-------------+-----------+------------+
Table 2
14. References
14.1. Normative References
[DTLS] 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>.
[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>.
[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, DOI 10.17487/RFC3711, March 2004,
<https://www.rfc-editor.org/rfc/rfc3711>.
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[RFC5705] Rescorla, E., "Keying Material Exporters for Transport
Layer Security (TLS)", RFC 5705, DOI 10.17487/RFC5705,
March 2010, <https://www.rfc-editor.org/rfc/rfc5705>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.
[RFC9261] Sullivan, N., "Exported Authenticators in TLS", RFC 9261,
DOI 10.17487/RFC9261, July 2022,
<https://www.rfc-editor.org/rfc/rfc9261>.
[RFC9729] Schinazi, D., Oliver, D., and J. Hoyland, "The Concealed
HTTP Authentication Scheme", RFC 9729,
DOI 10.17487/RFC9729, February 2025,
<https://www.rfc-editor.org/rfc/rfc9729>.
[TLS] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", Work in Progress, Internet-Draft, draft-
ietf-tls-rfc8446bis-14, 13 September 2025,
<https://datatracker.ietf.org/doc/html/draft-ietf-tls-
rfc8446bis-14>.
[TLS-FLAGS]
Nir, Y., "A Flags Extension for TLS 1.3", Work in
Progress, Internet-Draft, draft-ietf-tls-tlsflags-16, 14
September 2025, <https://datatracker.ietf.org/doc/html/
draft-ietf-tls-tlsflags-16>.
14.2. Informative References
[ANSSI] ANSSI, "Recommendations for securing networks with IPsec,
Technical Report", August 2015,
<https://cyber.gouv.fr/sites/default/files/2012/09/
NT_IPsec_EN.pdf>.
[CCG16] IEEE, "On Post-compromise Security", August 2016,
<https://doi.org/10.1109/csf.2016.19>.
[CONFIDENTIALITY]
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>.
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[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/rfc/rfc8126>.
[RFC8447] Salowey, J. and S. Turner, "IANA Registry Updates for TLS
and DTLS", RFC 8447, DOI 10.17487/RFC8447, August 2018,
<https://www.rfc-editor.org/rfc/rfc8447>.
[TLS-ECDHE-MLKEM]
Kwiatkowski, K., Kampanakis, P., Westerbaan, B., and D.
Stebila, "Post-quantum hybrid ECDHE-MLKEM Key Agreement
for TLSv1.3", Work in Progress, Internet-Draft, draft-
ietf-tls-ecdhe-mlkem-04, 8 February 2026,
<https://datatracker.ietf.org/doc/html/draft-ietf-tls-
ecdhe-mlkem-04>.
[TLS-Ext-Registry]
IANA, "Transport Layer Security (TLS) Extensions",
November 2023, <https://www.iana.org/assignments/tls-
extensiontype-values>.
[TLS-HYBRID]
Stebila, D., Fluhrer, S., and S. Gueron, "Hybrid key
exchange in TLS 1.3", Work in Progress, Internet-Draft,
draft-ietf-tls-hybrid-design-16, 7 September 2025,
<https://datatracker.ietf.org/doc/html/draft-ietf-tls-
hybrid-design-16>.
[TLS-KEYLOGFILE]
Thomson, M., Rosomakho, Y., and H. Tschofenig, "The
SSLKEYLOGFILE Format for TLS", Work in Progress, Internet-
Draft, draft-ietf-tls-keylogfile-05, 9 June 2025,
<https://datatracker.ietf.org/doc/html/draft-ietf-tls-
keylogfile-05>.
[TLS-MLKEM]
Connolly, D., "ML-KEM Post-Quantum Key Agreement for TLS
1.3", Work in Progress, Internet-Draft, draft-ietf-tls-
mlkem-07, 12 February 2026,
<https://datatracker.ietf.org/doc/html/draft-ietf-tls-
mlkem-07>.
[TLS-RENEGOTIATION]
Rescorla, E., Ray, M., Dispensa, S., and N. Oskov,
"Transport Layer Security (TLS) Renegotiation Indication
Extension", RFC 5746, DOI 10.17487/RFC5746, February 2010,
<https://www.rfc-editor.org/rfc/rfc5746>.
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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,
Matthijs van Duin, Rifaat Shekh-Yusef, Joe Birr-Pixton, Eric
Rescorla, and Thom Wiggers for their review comments.
Appendix B. State Machines
The sections below describe the state machines for the extended key
update operation for TLS 1.3 and DTLS 1.3.
The state machine diagrams in the Appendix are provided for
illustrative purposes only to aid understanding of the protocol flow.
They are not normative. In case of any discrepancy between the
Appendix diagrams and the protocol behavior specified in the main
body of this document, the text in the draft takes precedence.
For editorial reasons we abbreviate the protocol message types:
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* Req - ExtendedKeyUpdate(request)
* Resp - ExtendedKeyUpdate(response)
* NKU - ExtendedKeyUpdate(new_key_update)
* ACK - Acknowledgement message from Section 7 of [DTLS]
* APP - application data payloads
In the (D)TLS 1.3 state machines discussed below, the terms SEND and
RECEIVE keys refer to the send and receive key variables defined in
Section 2.
The TLS state machine diagrams use the following notation:
* Numbered labels (1), (2), … denote normal protocol transitions in
the order they occur.
* Lettered labels (A), (B), … denote exceptional or conditional
transitions.
* Self-loops indicate that the state does not change as a result of
the event.
B.1. TLS 1.3 State Machines
This section describes the initiator and responder state machines.
B.1.1. Initiator State Machine
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+----------------------+
| START |
| send_key=current, |
| receive_key=current |
| updating=0 |
+----------------------+
|
(1) send Req with local KeyShareEntry
set updating=1
v
+----------------------+
| WAIT_RESP |
| updating=1 |
+----------------------+
| \
| \ (A) recv peer Req (crossed updates):
| \ compare peer.key_exchange to local.key_exchange
| \ - if peer < local: IGNORE peer Req (send nothing)
| \ - if peer == local: ABORT "unexpected_message"
| \ - if peer > local: ABANDON local update:
| * set updating=0
| * act as RESPONDER: send Resp with KeyShareEntry,
| derive new secrets, then proceed as in responder flow
|
| (2) recv Resp with key_share:
| derive new secrets
| send NKU (encrypted under old keys)
| update SEND key (send_key := new)
| update RECEIVE key (receive_key := new)
| set updating=0
v
+----------------------+
| FINISHED |
+----------------------+
Notes:
* NKU message is sent under old keys.
* If a classic KeyUpdate arrives (EKU negotiated), ABORT
"unexpected_message".
* Crossed-requests: ignore the request with LOWER lexicographic
key_exchange; if equal, abort.
B.1.2. Responder State Machine
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+----------------------+
| START |
| send_key=current, |
| receive_key=current |
| updating=0 |
+----------------------+
|
(1) recv Req
set updating=1
v
+----------------------+
| RESPOND |
+----------------------+
| send Resp with KeyShareEntry
| (may defer sending if under load;
| must send once resources free)
| derive new secrets
| update SEND keys (send_key := new)
| --> WAIT_I_NKU
v
+----------------------+
| WAIT_I_NKU |
| updating=1 |
+----------------------+
|
(2) recv NKU (encrypted under old keys)
update RECEIVE keys (receive_key := new)
set updating=0
v
+----------------------+
| FINISHED |
+----------------------+
Notes:
* Responder may defer Resp under load (no status reply), then must
send it once resources are free.
* If a classic KeyUpdate arrives (EKU negotiated), ABORT
"unexpected_message".
B.2. DTLS 1.3 State Machines
This section describes the initiator and responder state machines.
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B.2.1. Terms and Abbreviations
The following variables and abbreviations are used in the state
machine diagrams.
* rx - current, accepted receive epoch.
* tx - current transmit epoch used for tagging outgoing messages.
* E - initial epoch value.
* updating - true while a key-update handshake is in progress.
* old_rx - the previous receive epoch remembered during retention.
* retain_old - when true, receiver accepts tags old_rx and rx.
* tag=... - the TX-epoch value written on an outgoing message.
* e==... - the epoch tag carried on an incoming message (what the
peer sent).
* FINISHED / START / WAIT_RESP / WAIT_I_NKU / WAIT_R_NKU / ACTIVATE
RETENTION / RESPOND / WAIT_ACK - diagram states; FINISHED denotes
the steady state after success.
Crossed requests. If both peers independently initiate the extended
key update and the key_update_request messages cross in flight,
compare the KeyShareEntry.key_exchange values. The request with the
lower lexicographic value must be ignored. If the values are equal,
the endpoint must abort with an "unexpected_message" alert. If the
peer's value is higher than the local one, the endpoint abandons its
in-flight update and processes the peer's request as responder.
B.2.2. State Machine (Initiator)
The initiator starts in the START state with matching epochs (rx :=
E; tx := E). It sends a Req and enters WAIT_RESP (updating := 1).
While waiting, APP data may be sent at any time (tagged with the
current tx) and received according to the APP acceptance rule below.
Once the responder returns Resp with a tag matching the current rx,
the initiator derives new key material. It then sends NKU still
tagged with the old tx. The initiator activates retention mode: the
old epoch is remembered, the receive epoch is incremented, and
application data is accepted under both epochs for a transition
period. Initiator moves to WAIT_ACK.
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If a peer key_update_request arrives while in WAIT_RESP (crossed
updates), apply the crossed-request rule above. If the peer's
key_exchange is higher, abandon the local update (updating := 0) and
continue as responder: send key_update_response, derive new secrets,
then proceed with the responder flow. If lower, ignore the peer's
request; if equal, abort with "unexpected_message".
Upon receiving the responder's ACK matching the updated epoch, the
initiator completes the transition by synchronizing transmit and
receive epochs (tx := rx), disabling retention, and clearing the
update flag. The state machine returns to FINISHED, ready for
subsequent updates.
Throughout the process:
* Duplicate messages are tolerated (for retransmission handling).
* Temporary epoch mismatches are permitted while an update is in
progress.
* Application data flows continuously, subject to epoch acceptance
rules.
+---------------------------------+
| START |
| rx := E; tx := E, updating := 0 |
+---------------------------------+
|
(1) send Req [tag=tx]
set updating := 1
v
+------------------------------+
| WAIT_RESP |
+------------------------------+
^ ^ ^ ^
| | | |
| | | |
| | | +-- (A) recv peer Req (crossed):
| | | if peer < local -> IGNORE (Self-Loop)
| | | if peer == local -> ABORT (Error)
| | | if peer > local -> ABANDON update;
| | | updating := 0;
| | | act as RESPONDER
| | |
| | +-- APP send (anytime) (Self-Loop)
| |
| +-- APP recv: retain_old && e == old_rx (Self-Loop)
|
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+-- APP recv: e == rx (Self-Loop)
| recv Resp [e == rx]
| derive secrets
v
+---------------------+
| ACTIVATE RETENTION |
| old_rx=rx; |
| retain_old=1; rx++ |
+---------------------+
|
(2) send NKU [tag=old tx]
v
+------------------------------+
| WAIT_ACK (updating = 1) |
+------------------------------+
|
| APP send/recv allowed
|
(3) recv ACK [e==rx]
tx=rx; retain_old=0; updating := 0
v
+------------------------------------------+
| FINISHED |
+------------------------------------------+
B.2.3. State Machine (Responder)
The responder starts in the START state with synchronized transmit
and receive epochs (rx := E; tx := E) and no update in progress.
Application data can be transmitted at any time using the sender's
current transmit epoch. A receiver must accept application data if
the epoch tag on the DTLS record equals the receiver's current
receive epoch. If the receiver has retention active (retain_old ==
true), the receiver must also accept DTLS records whose epoch tag
equals the remembered previous epoch.
Upon receiving an ExtendedKeyUpdate(key_update_request) (Req), the
responder transitions to RESPOND. The responder may defer sending
ExtendedKeyUpdate(key_update_response) under load; in that case it
must acknowledge the request with an ACK and retransmit the response
until it is acknowledged by the initiator, as specified in DTLS
considerations. When sent, ExtendedKeyUpdate(key_update_response) is
tagged with the current tx.
After an ExtendedKeyUpdate(new_key_update) (NKU) is received with the
correct epoch, the responder:
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1. activates retention (old_rx := rx; retain_old := 1),
2. increments both epochs (rx++, tx++),
3. sends ACK tagged with the new tx (which now equals the new rx),
4. clears updating and enters FINISHED.
Retention at the responder ends automatically on the first APP
received under the new rx (then retain_old := 0). APP traffic is
otherwise permitted at any time; reordering is tolerated by the
acceptance rule.
APP acceptance rule (receiver): accept if e == rx or (retain_old && e
== old_rx). If retain_old is set and an APP with the new rx arrives,
clear retain_old.
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+---------------------------------+
| START |
| rx := E; tx := E, updating := 0 |
+---------------------------------+
|
(1) recv Req [e==rx]
set updating := 1
v
+----------------------+
| RESPOND |
+----------------------+
|
| send Resp [tag=tx]
| (may defer; if deferred, ACK Req and later retransmit Resp
| until acknowledged by the initiator)
v
+---------------+
| WAIT_I_NKU |
| (updating=1) |
+-------+-------+
|
(2) recv NKU [e==rx] (assert accepted)
|
+---------------------+
| ACTIVATE RETENTION |
| old_rx=rx; |
| retain_old=1; |
| rx=rx+1; tx=tx+1 |
+----------+----------+
|
(3) send ACK [tag=tx]
set updating := 0; assert tx==rx
v
+------------------------------------------+
| FINISHED |
| (retain_old=0 after first APP at new rx) |
+------------------------------------------+
Appendix C. Overview of Security Goals
A complete security analysis of the EKU is outside the scope of this
document. This appendix provides an informal description of the
primary security goals that EKU is designed to achieve.
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C.1. Post-Compromise Security (PCS)
Extended Key Update supports post-compromise security under the
assumptions described in Section 12.1. If an attacker temporarily
compromises an endpoint and obtains the traffic keys in use before an
Extended Key Update takes place, but the compromise does not persist
after the EKU completes, the attacker cannot derive the new keying
material established by EKU. This property follows from the use of
fresh ephemeral key exchange material during each Extended Key
Update, which produces new traffic keys that are independent of the
previous ones. This property provides only best-effort post-
compromise security, as it assumes the attacker is not acting as a
MiTM during the Extended Key Update.
As a result, confidentiality of application data encrypted after the
Extended Key Update is preserved even if the earlier traffic keys
were exposed.
C.2. Key Freshness and Cryptographic Independence
Each Extended Key Update derives new traffic keys from ephemeral key
exchange material. This ensures strong separation between successive
traffic keys:
* The new traffic keys established by an Extended Key Update are
independent of the previous traffic keys.
* Compromise of one of traffic keys does not allow recovery of any
earlier or later traffic keys.
* Application data protected under one of the traffic keys cannot be
decrypted using keys from another.
C.3. Elimination of Standard KeyUpdate
Once Extended Key Update has been negotiated for a session, peers
rely exclusively on EKU rather than the standard TLS 1.3 KeyUpdate
mechanism. Relying solely on Extended Key Update helps maintain PCS
properties throughout the lifetime of the TLS session.
C.4. Detecting Divergent Key State
As described in Section 11, both Post-handshake Certificate-Based
Client Authentication and Exported Authenticators can be used after
an Extended Key Update to confirm that both endpoints derived the
same traffic keys. Because the authentication messages produced by
these mechanisms depend on values derived from the updated traffic
keys, any divergence in those traffic keys causes validation to fail,
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revealing interference by an active attacker.
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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