Internet Engineering Task Force (IETF) CJ. Tjhai

Internet Engineering Task Force (IETF) CJ. Tjhai
Request for Comments: 9370 M. Tomlinson
Updates: 7296 Post-Quantum
Category: Standards Track G. Bartlett
ISSN: 2070-1721 Quantum Secret
S. Fluhrer
Cisco Systems
D. Van Geest
ISARA Corporation
O. Garcia-Morchon
Philips
V. Smyslov
ELVIS-PLUS
May 2023

Multiple Key Exchanges in the Internet Key Exchange Protocol Version 2
(IKEv2)

Abstract

This document describes how to extend the Internet Key Exchange
Protocol Version 2 (IKEv2) to allow multiple key exchanges to take
place while computing a shared secret during a Security Association
(SA) setup.

This document utilizes the IKE_INTERMEDIATE exchange, where multiple
key exchanges are performed when an IKE SA is being established. It
also introduces a new IKEv2 exchange, IKE_FOLLOWUP_KE, which is used
for the same purpose when the IKE SA is being rekeyed or is creating
additional Child SAs.

This document updates RFC 7296 by renaming a Transform Type 4 from
"Diffie-Hellman Group (D-H)" to "Key Exchange Method (KE)" and
renaming a field in the Key Exchange Payload from "Diffie-Hellman
Group Num" to "Key Exchange Method". It also renames an IANA
registry for this Transform Type from "Transform Type 4 - Diffie-
Hellman Group Transform IDs" to "Transform Type 4 - Key Exchange
Method Transform IDs". These changes generalize key exchange
algorithms that can be used in IKEv2.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9370.

Copyright Notice

Copyright (c) 2023 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 extracted from this document must
include Revised BSD License text as described in Section 4.e of the
Trust Legal Provisions and are provided without warranty as described
in the Revised BSD License.

Table of Contents

1. Introduction
1.1. Problem Description
1.2. Proposed Extension
1.3. Document Organization
2. Multiple Key Exchanges
2.1. Design Overview
2.2. Protocol Details
2.2.1. IKE_SA_INIT Round: Negotiation
2.2.2. IKE_INTERMEDIATE Round: Additional Key Exchanges
2.2.3. IKE_AUTH Exchange
2.2.4. CREATE_CHILD_SA Exchange
2.2.5. Interaction with IKEv2 Extensions
3. IANA Considerations
4. Security Considerations
5. References
5.1. Normative References
5.2. Informative References
Appendix A. Sample Multiple Key Exchanges
A.1. IKE_INTERMEDIATE Exchanges Carrying Additional Key Exchange
Payloads
A.2. No Additional Key Exchange Used
A.3. Additional Key Exchange in the CREATE_CHILD_SA Exchange
Only
A.4. No Matching Proposal for Additional Key Exchanges
Appendix B. Design Criteria
Appendix C. Alternative Design
Acknowledgements
Authors' Addresses

1. Introduction

1.1. Problem Description

The Internet Key Exchange Protocol version 2 (IKEv2), as specified in
[RFC7296], uses the Diffie-Hellman (DH) or the Elliptic Curve Diffie-
Hellman (ECDH) algorithm, which shall be referred to as "(EC)DH"
collectively, to establish a shared secret between an initiator and a
responder. The security of the (EC)DH algorithms relies on the
difficulty to solve a discrete logarithm problem in multiplicative
(and, respectively, elliptic curve) groups when the order of the
group parameter is large enough. While solving such a problem
remains infeasible with current computing power, it is believed that
general-purpose quantum computers will be able to solve this problem,
implying that the security of IKEv2 is compromised. There are,
however, a number of cryptosystems that are conjectured to be
resistant to quantum-computer attacks. This family of cryptosystems
is known as "post-quantum cryptography" (or "PQC"). It is sometimes
also referred to as "quantum-safe cryptography" (or "QSC") or
"quantum-resistant cryptography" (or "QRC").

It is essential to have the ability to perform one or more post-
quantum key exchanges in conjunction with an (EC)DH key exchange so
that the resulting shared key is resistant to quantum-computer
attacks. Since there is currently no post-quantum key exchange that
is as well-studied as (EC)DH, performing multiple key exchanges with
different post-quantum algorithms along with the well-established
classical key-exchange algorithms addresses this concern, since the
overall security is at least as strong as each individual primitive.

1.2. Proposed Extension

This document describes a method to perform multiple successive key
exchanges in IKEv2. This method allows integration of PQC in IKEv2,
while maintaining backward compatibility, to derive a set of IKE keys
that is resistant to quantum-computer attacks. This extension allows
the negotiation of one or more PQC algorithms to exchange data, in
addition to the existing (EC)DH key exchange data. It is believed
that the feature of using more than one post-quantum algorithm is
important, as many of these algorithms are relatively new, and there
may be a need to hedge the security risk with multiple key exchange
data from several distinct PQC algorithms.

IKE peers perform multiple successive key exchanges to establish an
IKE SA. Each exchange produces some shared secret, and these secrets
are combined in a way such that:

(a) the final shared secret is computed from all of the component
key exchange secrets;

(b) unless both peers support and agree to use the additional key
exchanges introduced in this specification, the final shared
secret equivalent to the shared secret specified in [RFC7296] is
obtained; and

(c) if any part of the component key exchange method is a post-
quantum algorithm, the final shared secret is post-quantum
secure.

Some post-quantum key exchange payloads may have sizes larger than
the standard maximum transmission unit (MTU) size. Therefore, there
could be issues with fragmentation at the IP layer. In order to
allow the use of those larger payload sizes, this mechanism relies on
the IKE_INTERMEDIATE exchange as specified in [RFC9242]. With this
mechanism, the key exchange is initiated using a smaller, possibly
classical primitive, such as (EC)DH. Then, before the IKE_AUTH
exchange, one or more IKE_INTERMEDIATE exchanges are carried out,
each of which contains an additional key exchange. As the
IKE_INTERMEDIATE exchange is encrypted, the IKE fragmentation
protocol [RFC7383] can be used. The IKE SK_* values are updated
after each exchange, as described in Section 2.2.2; thus, the final
IKE SA keys depend on all the key exchanges. Hence, the keys are
secure if any of the key exchanges are secure.

While this extension is primarily aimed at IKE SAs due to the
potential fragmentation issue discussed above, it also applies to
CREATE_CHILD_SA exchanges as illustrated in Section 2.2.4 for
creating/rekeying of Child SAs and rekeying of IKE SAs.

Note that readers should consider the approach defined in this
document as providing a long-term solution in upgrading the IKEv2
protocol to support post-quantum algorithms. A short-term solution
to make IKEv2 key exchange quantum secure is to use post-quantum pre-
shared keys as specified in [RFC8784].

Note also that the proposed approach of performing multiple
successive key exchanges in such a way, when the resulting session
keys depend on all of them, is not limited to only addressing the
threat of quantum computers. It can also be used when all of the
performed key exchanges are classical (EC)DH primitives, where, for
various reasons (e.g., policy requirements), it is essential to
perform multiple key exchanges.

This specification does not attempt to address key exchanges with KE
payloads longer than 64 KB; the current IKE payload format does not
allow such a possibility. At the time of writing, it appears likely
that there are a number of key exchanges available that would not
have such a requirement. [BEYOND-64K] discusses approaches that
could be taken to exchange huge payloads if such a requirement were
needed.

1.3. Document Organization

The remainder of this document is organized as follows. Section 2
describes how multiple key exchanges are performed between two IKE
peers and how keying materials are derived for both SAs and Child
SAs. Section 3 discusses IANA considerations for the namespaces
introduced in this document. Section 4 discusses security
considerations. In the Appendices, some examples of multiple key
exchanges are illustrated in Appendix A. Appendix B summarizes
design criteria and alternative approaches that have been considered.
These approaches are later discarded, as described in Appendix C.

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.

2. Multiple Key Exchanges

2.1. Design Overview

Most post-quantum key agreement algorithms are relatively new. Thus,
they are not fully trusted. There are also many proposed algorithms
that have different trade-offs and that rely on different hard
problems. The concern is that some of these hard problems may turn
out to be easier to solve than anticipated; thus, the key agreement
algorithm may not be as secure as expected. A hybrid solution, when
multiple key exchanges are performed and the calculated shared key
depends on all of them, allows us to deal with this uncertainty by
combining a classical key exchange with a post-quantum one, as well
as leaving open the possibility of combining it with multiple post-
quantum key exchanges.

In order to be able to use IKE fragmentation [RFC7383] for those key
exchanges that may have long public keys, this specification utilizes
the IKE_INTERMEDIATE exchange defined in [RFC9242]. The initial
IKE_SA_INIT messages do not have any inherent fragmentation support
within IKE. However, IKE_SA_INIT messages can include a relatively
short KE payload. The additional key exchanges are performed using
IKE_INTERMEDIATE messages that follow the IKE_SA_INIT exchange. This
is to allow the standard IKE fragmentation mechanisms (which cannot
be used in IKE_SA_INIT) to be available for the potentially large Key
Exchange payloads with post-quantum algorithm data.

Note that this document assumes that each key exchange method
requires one round trip and consumes exactly one IKE_INTERMEDIATE
exchange. This assumption is valid for all classic key exchange
methods defined so far and for all post-quantum methods currently
known. For hypothetical future key exchange methods that require
multiple round trips to complete, a separate document should define
how such methods are split into several IKE_INTERMEDIATE exchanges.

In order to minimize communication overhead, only the key shares that
are agreed upon are actually exchanged. To negotiate additional key
exchanges, seven new Transform Types are defined. These transforms
and Transform Type 4 share the same Transform IDs.

It is assumed that new Transform Type 4 identifiers will be assigned
later for various post-quantum key exchanges [IKEV2TYPE4ID]. This
specification does not make a distinction between classical (EC)DH
and post-quantum key exchanges, nor between post-quantum algorithms
that are true key exchanges and post-quantum algorithms that act as
key transport mechanisms: all are treated equivalently by the
protocol. This document renames a field in the Key Exchange Payload
from "Diffie-Hellman Group Num" to "Key Exchange Method". This
document also renames Transform Type 4 from "Diffie-Hellman Group
(D-H)" to "Key Exchange Method (KE)". The corresponding renaming to
the IANA registry is described in Section 3.

The fact that newly defined transforms share the same registry for
possible Transform IDs with Transform Type 4 allows additional key
exchanges to be of any type: either post-quantum or classical (EC)DH.
This approach allows any combination of the defined key exchange
methods to take place. This also allows IKE peers to perform a
single post-quantum key exchange in the IKE_SA_INIT without
additional key exchanges, provided that the IP fragmentation is not
an issue and that hybrid key exchange is not needed.

The SA payload in the IKE_SA_INIT message includes one or more newly
defined transforms that represent the extra key exchange policy
required by the initiator. The responder follows the usual IKEv2
negotiation rules: it selects a single transform of each type and
returns all of them in the IKE_SA_INIT response message.

Then, provided that additional key exchanges are negotiated, the
initiator and the responder perform one or more IKE_INTERMEDIATE
exchanges. Following that, the IKE_AUTH exchange authenticates peers
and completes IKE SA establishment.

Initiator Responder
---------------------------------------------------------------------
<-- IKE_SA_INIT (additional key exchanges negotiation) -->

<-- {IKE_INTERMEDIATE (additional key exchange)} -->

...

<-- {IKE_INTERMEDIATE (additional key exchange)} -->

<-- {IKE_AUTH} -->

2.2. Protocol Details

In the simplest case, the initiator starts a single key exchange (and
has no interest in supporting multiple), and it is not concerned with
possible fragmentation of the IKE_SA_INIT messages (because either
the key exchange that it selects is small enough not to fragment or
the initiator is confident that fragmentation will be handled either
by IP fragmentation or by transport via TCP).

In this case, the initiator performs the IKE_SA_INIT for a single key
exchange using a Transform Type 4 (possibly with a post-quantum
algorithm) and including the initiator KE payload. If the responder
accepts the policy, it responds with an IKE_SA_INIT response, and IKE
continues as usual.

If the initiator wants to negotiate multiple key exchanges, then the
initiator uses the protocol behavior listed below.

2.2.1. IKE_SA_INIT Round: Negotiation

Multiple key exchanges are negotiated using the standard IKEv2
mechanism via SA payload. For this purpose, seven new transform
types are defined: Additional Key Exchange 1 (ADDKE1) with IANA-
assigned value 6, Additional Key Exchange 2 (ADDKE2) (7), Additional
Key Exchange 3 (ADDKE3) (8), Additional Key Exchange 4 (ADDKE4) (9),
Additional Key Exchange 5 (ADDKE5) (10), Additional Key Exchange 6
(ADDKE6) (11), and Additional Key Exchange 7 (ADDKE7) (12). They are
collectively called "Additional Key Exchange (ADDKE) Transform Types"
in this document and have slightly different semantics than the
existing IKEv2 Transform Types. They are interpreted as an
indication of additional key exchange methods that peers agree to
perform in a series of IKE_INTERMEDIATE exchanges following the
IKE_SA_INIT exchange. The allowed Transform IDs for these transform
types are the same as the IDs for Transform Type 4, so they all share
a single IANA registry for Transform IDs.

The key exchange method negotiated via Transform Type 4 always takes
place in the IKE_SA_INIT exchange, as defined in [RFC7296].
Additional key exchanges negotiated via newly defined transforms MUST
take place in a series of IKE_INTERMEDIATE exchanges following the
IKE_SA_INIT exchange, performed in an order of the values of their
Transform Types. This is so that the key exchange negotiated using
Additional Key Exchange i always precedes that of Additional Key
Exchange i + 1. Each additional key exchange method MUST be fully
completed before the next one is started.

With these semantics, note that ADDKE Transform Types are not
associated with any particular type of key exchange and do not have
any Transform IDs that are specific per Transform Type IANA registry.
Instead, they all share a single registry for Transform IDs, namely
"Transform Type 4 - Key Exchange Method Transform IDs". All key
exchange algorithms (both classical or post-quantum) should be added
to this registry. This approach gives peers flexibility in defining
the ways they want to combine different key exchange methods.

When forming a proposal, the initiator adds transforms for the
IKE_SA_INIT exchange using Transform Type 4. In most cases, they
will contain classical (EC)DH key exchange methods, but that is not a
requirement. Additional key exchange methods are proposed using
ADDKE Transform Types. All of these transform types are optional;
the initiator is free to select any of them for proposing additional
key exchange methods. Consequently, if none of the ADDKE Transform
Types are included in the proposal, then this proposal indicates the
performing of standard IKEv2, as defined in [RFC7296]. On the other
hand, if the initiator includes any ADDKE Transform Type in the
proposal, the responder MUST select one of the algorithms proposed
using this type. Note that this is not a new requirement; this
behavior is already specified in Section 2.7 of [RFC7296]. A
Transform ID NONE MAY be added to those transform types that contain
key exchange methods which the initiator believes are optional
according to its local policy.

The responder performs the negotiation using the standard IKEv2
procedure described in Section 3.3 of [RFC7296]. However, for the
ADDKE Transform Types, the responder's choice MUST NOT contain
duplicated algorithms (those with an identical Transform ID and
attributes), except for the Transform ID of NONE. An algorithm is
represented as a transform. In some cases, the transform could
include a set of associated attributes that define details of the
algorithm. In this case, two transforms can be the same, but the
attributes must be different. Additionally, the order of the
attributes does not affect the equality of the algorithm, so the
following two transforms define the same algorithm: "ID=alg1,
ATTR1=attr1, ATTR2=attr2" and "ID=alg1, ATTR2=attr2, ATTR1=attr1".
If the responder is unable to select algorithms that are not
duplicated for each proposed key exchange (either because the
proposal contains too few choices or due to the local policy
restrictions on using the proposed algorithms), then the responder
MUST reject the message with an error notification of type
NO_PROPOSAL_CHOSEN. If the responder's message contains one or more
duplicated choices, the initiator should log the error and MUST treat
the exchange as failed. The initiator MUST NOT initiate any
IKE_INTERMEDIATE (or IKE_FOLLOWUP_KE) exchanges so that no new SA is
created. If this happens in the CREATE_CHILD_SA exchange, then the
initiator MAY delete the IKE SA over which the invalid message was
received by sending a Delete payload.

If the responder selects NONE for some ADDKE Transform Types
(provided they are proposed by the initiator), then any corresponding
additional key exchanges MUST NOT take place. Therefore, if the
initiator includes NONE in all of the ADDKE Transform Types and the
responder selects this value for all of them, then no
IKE_INTERMEDIATE exchanges performing additional key exchanges will
take place between the peers. Note that the IKE_INTERMEDIATE
exchanges may still take place for other purposes.

The initiator MAY propose ADDKE Transform Types that are not
consecutive, for example, proposing ADDKE2 and ADDKE5 Transform Types
only. The responder MUST treat all of the omitted ADDKE transforms
as if they were proposed with Transform ID NONE.

Below is an example of the SA payload in the initiator's IKE_SA_INIT
request message. Here, the abbreviation "KE" is used for the Key
Exchange transform, which this document renames from the Diffie-
Hellman Group transform. Additionally, the notations PQ_KEM_1,
PQ_KEM_2, and PQ_KEM_3 are used to represent Transform IDs that have
yet to be defined of some popular post-quantum key exchange methods.

SA Payload
|
+--- Proposal #1 ( Proto ID = IKE(1), SPI Size = 8,
| 9 transforms, SPI = 0x35a1d6f22564f89d )
|
+-- Transform ENCR ( ID = ENCR_AES_GCM_16 )
| +-- Attribute ( Key Length = 256 )
|
+-- Transform KE ( ID = 4096-bit MODP Group )
|
+-- Transform PRF ( ID = PRF_HMAC_SHA2_256 )
|
+-- Transform ADDKE2 ( ID = PQ_KEM_1 )
|
+-- Transform ADDKE2 ( ID = PQ_KEM_2 )
|
+-- Transform ADDKE3 ( ID = PQ_KEM_1 )
|
+-- Transform ADDKE3 ( ID = PQ_KEM_2 )
|
+-- Transform ADDKE5 ( ID = PQ_KEM_3 )
|
+-- Transform ADDKE5 ( ID = NONE )

In this example, the initiator proposes performing the initial key
exchange using a 4096-bit MODP Group followed by two mandatory
additional key exchanges (i.e., ADDKE2 and ADDKE3 Transform Types)
using PQ_KEM_1 and PQ_KEM_2 methods in any order followed by an
additional key exchange (i.e., ADDKE5 Transform Type) using the
PQ_KEM_3 method that may be omitted.

The responder might return the following SA payload, indicating that
it agrees to perform two additional key exchanges, PQ_KEM_2 followed
by PQ_KEM_1, and that it does not want to additionally perform
PQ_KEM_3.

SA Payload
|
+--- Proposal #1 ( Proto ID = IKE(1), SPI Size = 8,
| 6 transforms, SPI = 0x8df52b331a196e7b )
|
+-- Transform ENCR ( ID = ENCR_AES_GCM_16 )
| +-- Attribute ( Key Length = 256 )
|
+-- Transform KE ( ID = 4096-bit MODP Group )
|
+-- Transform PRF ( ID = PRF_HMAC_SHA2_256 )
|
+-- Transform ADDKE2 ( ID = PQ_KEM_2 )
|
+-- Transform ADDKE3 ( ID = PQ_KEM_1 )
|
+-- Transform ADDKE5 ( ID = NONE )

If the initiator includes any ADDKE Transform Types into the SA
payload in the IKE_SA_INIT exchange request message, then it MUST
also negotiate the use of the IKE_INTERMEDIATE exchange, as described
in [RFC9242] by including an INTERMEDIATE_EXCHANGE_SUPPORTED
notification in the same message. If the responder agrees to use
additional key exchanges while establishing an initial IKE SA, it
MUST also return this notification in the IKE_SA_INIT response
message, confirming that IKE_INTERMEDIATE exchange is supported and
will be used for transferring additional key exchange data. If the
IKE_INTERMEDIATE exchange is not negotiated, then the peers MUST
treat any ADDKE Transform Types in the IKE_SA_INIT exchange messages
as unknown transform types and skip the proposals they appear in. If
no other proposals are present in the SA payload, the peers will
proceed as if no proposal has been chosen (i.e., the responder will
send a NO_PROPOSAL_CHOSEN notification).

Initiator Responder
---------------------------------------------------------------------
HDR, SAi1(.. ADDKE*...), KEi, Ni,
N(INTERMEDIATE_EXCHANGE_SUPPORTED) --->
HDR, SAr1(.. ADDKE*...), KEr, Nr,
[CERTREQ],
<--- N(INTERMEDIATE_EXCHANGE_SUPPORTED)

It is possible for an attacker to manage to send a response to the
initiator's IKE_SA_INIT request before the legitimate responder does.
If the initiator continues to create the IKE SA using this response,
the attempt will fail. Implementers may wish to consider strategies
as described in Section 2.4 of [RFC7296] to handle such an attack.

2.2.2. IKE_INTERMEDIATE Round: Additional Key Exchanges

For each additional key exchange agreed to in the IKE_SA_INIT
exchange, the initiator and the responder perform an IKE_INTERMEDIATE
exchange, as described in [RFC9242].

Initiator Responder
---------------------------------------------------------------------
HDR, SK {KEi(n)} -->
<-- HDR, SK {KEr(n)}

The initiator sends key exchange data in the KEi(n) payload. This
message is protected with the current SK_ei/SK_ai keys. The notation
"KEi(n)" denotes the n-th IKE_INTERMEDIATE KE payload from the
initiator; the integer "n" is sequential starting from 1.

On receiving this, the responder sends back key exchange payload
KEr(n); "KEr(n)" denotes the n-th IKE_INTERMEDIATE KE payload from
the responder. Similar to how the request is protected, this message
is protected with the current SK_er/SK_ar keys.

The former "Diffie-Hellman Group Num" (now called "Key Exchange
Method") field in the KEi(n) and KEr(n) payloads MUST match the n-th
negotiated additional key exchange.

Once this exchange is done, both sides compute an updated keying
material:

SKEYSEED(n) = prf(SK_d(n-1), SK(n) | Ni | Nr)

From this exchange, SK(n) is the resulting shared secret. Ni and Nr
are nonces from the IKE_SA_INIT exchange. SK_d(n-1) is the last
generated SK_d (derived from IKE_SA_INIT for the first use of
IKE_INTERMEDIATE and, otherwise, from the previous IKE_INTERMEDIATE
exchange). The other keying materials, SK_d, SK_ai, SK_ar, SK_ei,
SK_er, SK_pi, and SK_pr, are generated from the SKEYSEED(n) as
follows:

{SK_d(n) | SK_ai(n) | SK_ar(n) | SK_ei(n) | SK_er(n) | SK_pi(n) |
SK_pr(n)} = prf+ (SKEYSEED(n), Ni | Nr | SPIi | SPIr)

Both the initiator and the responder use these updated key values in
the next exchange (IKE_INTERMEDIATE or IKE_AUTH).

2.2.3. IKE_AUTH Exchange

After all IKE_INTERMEDIATE exchanges have completed, the initiator
and the responder perform an IKE_AUTH exchange. This exchange is the
standard IKE exchange, as described in [RFC7296], with the
modification of AUTH payload calculation described in [RFC9242].

2.2.4. CREATE_CHILD_SA Exchange

The CREATE_CHILD_SA exchange is used in IKEv2 for the purposes of
creating additional Child SAs, rekeying these Child SAs, and rekeying
IKE SA itself. When creating or rekeying Child SAs, the peers may
optionally perform a key exchange to add a fresh entropy into the
session keys. In the case of an IKE SA rekey, the key exchange is
mandatory. Peers supporting this specification may want to use
multiple key exchanges in these situations.

Using multiple key exchanges with a CREATE_CHILD_SA exchange is
negotiated in a similar fashion to the initial IKE exchange, see
Section 2.2.1. If the initiator includes any ADDKE Transform Types
in the SA payload (along with Transform Type 4), and if the responder
agrees to perform additional key exchanges, then the additional key
exchanges are performed in a series of new IKE_FOLLOWUP_KE exchanges
that follow the CREATE_CHILD_SA exchange. The IKE_FOLLOWUP_KE
exchange is introduced especially for transferring data of additional
key exchanges following the one performed in the CREATE_CHILD_SA.
Its Exchange Type value is 44.

The key exchange negotiated via Transform Type 4 always takes place
in the CREATE_CHILD_SA exchange, as per the IKEv2 specification
[RFC7296]. Additional key exchanges are performed in an order of the
values of their Transform Types so that the key exchange negotiated
using Additional Key Exchange i always precedes the key exchange
negotiated using Additional Key Exchange i + 1. Each additional key
exchange method MUST be fully completed before the next one is
started. Note that this document assumes that each key exchange
method consumes exactly one IKE_FOLLOWUP_KE exchange. For the
methods that require multiple round trips, a separate document should
define how such methods are split into several IKE_FOLLOWUP_KE
exchanges.

After an IKE SA is created, the window size may be greater than one;
thus, multiple concurrent exchanges may be in progress. Therefore,
it is essential to link the IKE_FOLLOWUP_KE exchanges together with
the corresponding CREATE_CHILD_SA exchange. Once an IKE SA is
created, all IKE exchanges are independent and IKEv2 doesn't have a
built-in mechanism to link an exchange with another one. A new
status type notification called "ADDITIONAL_KEY_EXCHANGE" is
introduced for this purpose. Its Notify Message Type value is 16441,
and the Protocol ID and SPI Size are both set to 0. The data
associated with this notification is a blob meaningful only to the
responder so that the responder can correctly link successive
exchanges. For the initiator, the content of this notification is an
opaque blob.

The responder MUST include this notification in a CREATE_CHILD_SA or
IKE_FOLLOWUP_KE response message in case the next IKE_FOLLOWUP_KE
exchange is expected, filling it with some data that would allow
linking the current exchange to the next one. The initiator MUST
send back this notification intact in the request message of the next
IKE_FOLLOWUP_KE exchange.

Below is an example of CREATE_CHILD_SA exchange followed by three
additional key exchanges.

Initiator Responder
---------------------------------------------------------------------
HDR(CREATE_CHILD_SA), SK {SA, Ni, KEi} -->
<-- HDR(CREATE_CHILD_SA), SK {SA, Nr, KEr,
N(ADDITIONAL_KEY_EXCHANGE)(link1)}

HDR(IKE_FOLLOWUP_KE), SK {KEi(1),
N(ADDITIONAL_KEY_EXCHANGE)(link1)} -->
<-- HDR(IKE_FOLLOWUP_KE), SK {KEr(1),
N(ADDITIONAL_KEY_EXCHANGE)(link2)}

HDR(IKE_FOLLOWUP_KE), SK {KEi(2),
N(ADDITIONAL_KEY_EXCHANGE)(link2)} -->
<-- HDR(IKE_FOLLOWUP_KE), SK {KEr(2),
N(ADDITIONAL_KEY_EXCHANGE)(link3)}

HDR(IKE_FOLLOWUP_KE), SK {KEi(3),
N(ADDITIONAL_KEY_EXCHANGE)(link3)} -->
<-- HDR(IKE_FOLLOWUP_KE), SK {KEr(3)}

The former "Diffie-Hellman Group Num" (now called "Key Exchange
Method") field in the KEi(n) and KEr(n) payloads MUST match the n-th
negotiated additional key exchange.

Due to some unexpected events (e.g., a reboot), it is possible that
the initiator may lose its state, forget that it is in the process of
performing additional key exchanges, and never start the remaining
IKE_FOLLOWUP_KE exchanges. The responder MUST handle this situation
gracefully and delete the associated state if it does not receive the
next expected IKE_FOLLOWUP_KE request after some reasonable period of
time. Due to various factors such as computational resource and key
exchange algorithm used, note that it is not possible to give
normative guidance on how long this timeout period should be. In
general, 5-20 seconds of waiting time should be appropriate in most
cases.

It may also take too long for the initiator to prepare and to send
the next IKE_FOLLOWUP_KE request, or, due to the network conditions,
the request could be lost and retransmitted. In this case, the
message may reach the responder when it has already deleted the
associated state, following the advice above. If the responder
receives an IKE_FOLLOWUP_KE message for which it does not have a key
exchange state, it MUST send back a new error type notification
called "STATE_NOT_FOUND". This is an error notification that is not
fatal to the IKE SA. Its Notify Message Type value is 47, its
Protocol ID and SPI Size are both set to 0, and the data is empty.
If the initiator receives this notification in response to an
IKE_FOLLOWUP_KE exchange performing an additional key exchange, it
MUST cancel this exchange and MUST treat the whole series of
exchanges started from the CREATE_CHILD_SA exchange as having failed.
In most cases, the receipt of this notification is caused by the
premature deletion of the corresponding state on the responder (the
time period between IKE_FOLLOWUP_KE exchanges appeared to be too long
from the responder's point of view, e.g., due to a temporary network
failure). After receiving this notification, the initiator MAY start
a new CREATE_CHILD_SA exchange, which may eventually be followed by
the IKE_FOLLOWUP_KE exchanges, to retry the failed attempt. If the
initiator continues to receive STATE_NOT_FOUND notifications after
several retries, it MUST treat this situation as a fatal error and
delete the IKE SA by sending a DELETE payload.

It is possible that the peers start rekeying the IKE SA or the Child
SA at the same time, which is called "simultaneous rekeying".
Sections 2.8.1 and 2.8.2 of [RFC7296] describe how IKEv2 handles this
situation. In a nutshell, IKEv2 follows the rule that, in the case
of simultaneous rekeying, if two identical new IKE SAs (or two pairs
of Child SAs) are created, then one of them should be deleted. Which
one to delete is determined by comparing the values of four nonces
that are used in the colliding CREATE_CHILD_SA exchanges. The IKE SA
(or pair of Child SAs) created by the exchange in which the smallest
nonce is used should be deleted by the initiator of this exchange.

With multiple key exchanges, the SAs are not yet created when the
CREATE_CHILD_SA is completed. Instead, they would be created only
after the series of IKE_FOLLOWUP_KE exchanges is finished. For this
reason, if additional key exchanges are negotiated in the
CREATE_CHILD_SA exchange in which the smallest nonce is used, then,
because there is nothing to delete yet, the initiator of this
exchange just stops the rekeying process, and it MUST NOT initiate
the IKE_FOLLOWUP_KE exchange.

In most cases, rekey collisions are resolved in the CREATE_CHILD_SA
exchange. However, a situation may occur when, due to packet loss,
one of the peers receives the CREATE_CHILD_SA message requesting the
rekey of an SA that is already being rekeyed by this peer (i.e., the
CREATE_CHILD_SA exchange initiated by this peer has already been
completed, and the series of IKE_FOLLOWUP_KE exchanges is in
progress). In this case, a TEMPORARY_FAILURE notification MUST be
sent in response to such a request.

If multiple key exchanges are negotiated in the CREATE_CHILD_SA
exchange, then the resulting keys are computed as follows.

In the case of an IKE SA rekey:

SKEYSEED = prf(SK_d, SK(0) | Ni | Nr | SK(1) | ... SK(n))

In the case of a Child SA creation or rekey:

KEYMAT = prf+ (SK_d, SK(0) | Ni | Nr | SK(1) | ... SK(n))

In both cases, SK_d is from the existing IKE SA; SK(0), Ni, and Nr
are the shared key and nonces from the CREATE_CHILD_SA, respectively;
SK(1)...SK(n) are the shared keys from additional key exchanges.

2.2.5. Interaction with IKEv2 Extensions

It is believed that this specification requires no modification to
the IKEv2 extensions defined so far. In particular, the IKE SA
resumption mechanism defined in [RFC5723] can be used to resume IKE
SAs created using this specification.

2.2.5.1. Interaction with Childless IKE SA

It is possible to establish IKE SAs with post-quantum algorithms by
only using IKE_FOLLOWUP_KE exchanges and without the use of
IKE_INTERMEDIATE exchanges. In this case, the IKE SA that is created
from the IKE_SA_INIT exchange, can be immediately rekeyed with
CREATE_CHILD_SA with additional key exchanges, where IKE_FOLLOWUP_KE
messages are used for these additional key exchanges. If the
classical key exchange method is used in the IKE_SA_INIT message, the
very first Child SA created in IKE_AUTH will offer no resistance
against the quantum threats. Consequently, if the peers' local
policy requires all Child SAs to be post-quantum secure, then the
peers can avoid creating the very first Child SA by adopting
[RFC6023]. In this case, the initiator sends two types of proposals
in the IKE_SA_INIT request: one with and another one without ADDKE
Transform Types. The responder chooses the latter proposal type and
includes a CHILDLESS_IKEV2_SUPPORTED notification in the IKE_SA_INIT
response. Assuming that the initiator supports childless IKE SA
extension, both peers perform the modified IKE_AUTH exchange
described in [RFC6023], and no Child SA is created in this exchange.
The peers should then immediately rekey the IKE SA and subsequently
create the Child SAs, all with additional key exchanges using a
CREATE_CHILD_SA exchange.

It is also possible for the initiator to send proposals without any
ADDKE Transform Types in the IKE_SA_INIT message. In this instance,
the responder will have no information about whether or not the
initiator supports the extension in this specification. This may not
be efficient, as the responder will have to wait for the subsequent
CREATE_CHILD_SA request to determine whether or not the initiator's
request is appropriate for its local policy.

The support for childless IKE SA is not negotiated, but it is the
responder that indicates the support for this mode. As such, the
responder cannot enforce that the initiator use this mode.
Therefore, it is entirely possible that the initiator does not
support this extension and sends IKE_AUTH request as per [RFC7296]
instead of [RFC6023]. In this case, the responder may respond with
an error that is not fatal, such as the NO_PROPOSAL_CHOSEN notify
message type.

Note that if the initial IKE SA is used to transfer sensitive
information, then this information will not be protected using the
additional key exchanges, which may use post-quantum algorithms. In
this arrangement, the peers will have to use post-quantum algorithm
in Transform Type 4 in order to mitigate the risk of quantum attack.

3. IANA Considerations

This document adds a new exchange type into the "IKEv2 Exchange
Types" registry:

44 IKE_FOLLOWUP_KE

This document renames Transform Type 4 defined in the "Transform Type
Values" registry from "Diffie-Hellman Group (D-H)" to "Key Exchange
Method (KE)".

This document renames the IKEv2 registry originally titled "Transform
Type 4 - Diffie-Hellman Group Transform IDs" to "Transform Type 4 -
Key Exchange Method Transform IDs".

This document adds the following Transform Types to the "Transform
Type Values" registry:

+======+====================================+===============+
| Type | Description | Used In |
+======+====================================+===============+
| 6 | Additional Key Exchange 1 (ADDKE1) | (optional in |
| | | IKE, AH, ESP) |
+------+------------------------------------+---------------+
| 7 | Additional Key Exchange 2 (ADDKE2) | (optional in |
| | | IKE, AH, ESP) |
+------+------------------------------------+---------------+
| 8 | Additional Key Exchange 3 (ADDKE3) | (optional in |
| | | IKE, AH, ESP) |
+------+------------------------------------+---------------+
| 9 | Additional Key Exchange 4 (ADDKE4) | (optional in |
| | | IKE, AH, ESP) |
+------+------------------------------------+---------------+
| 10 | Additional Key Exchange 5 (ADDKE5) | (optional in |
| | | IKE, AH, ESP) |
+------+------------------------------------+---------------+
| 11 | Additional Key Exchange 6 (ADDKE6) | (optional in |
| | | IKE, AH, ESP) |
+------+------------------------------------+---------------+
| 12 | Additional Key Exchange 7 (ADDKE7) | (optional in |
| | | IKE, AH, ESP) |
+------+------------------------------------+---------------+

Table 1: "Transform Type Values" Registry

This document defines a new Notify Message Type in the "IKEv2 Notify
Message Types - Status Types" registry:

16441 ADDITIONAL_KEY_EXCHANGE

This document also defines a new Notify Message Type in the "IKEv2
Notify Message Types - Error Types" registry:

47 STATE_NOT_FOUND

IANA has added the following instructions for designated experts for
the "Transform Type 4 - Key Exchange Method Transform IDs"
subregistry:

* While adding new Key Exchange (KE) methods, the following
considerations must be applied. A KE method must take exactly one
round-trip (one IKEv2 exchange), and at the end of this exchange,
both peers must be able to derive the shared secret. In addition,
any public value that peers exchanged during a KE method must fit
into a single IKEv2 payload. If these restrictions are not met
for a KE method, then there must be documentation on how this KE
method is used in IKEv2.

IANA has also completed the following changes. It is assumed that
[RFC9370] refers to this specification.

* Added a reference to [RFC9370] in what was the "Transform Type 4 -
Diffie-Hellman Group Transform IDs" registry.

* Replaced the Note on what was the "Transform Type 4 - Diffie-
Hellman Group Transform IDs" registry with the following notes:

This registry was originally named "Transform Type 4 - Diffie-
Hellman Group Transform IDs" and was referenced using that name in
a number of RFCs published prior to [RFC9370], which gave it the
current title.

This registry is used by the "Key Exchange Method (KE)" transform
type and by all "Additional Key Exchange (ADDKE)" transform types.

To find out requirement levels for Key Exchange Methods for IKEv2,
see [RFC8247].

* Appended [RFC9370] to the Reference column of Transform Type 4 in
the "Transform Type Values" registry.

* Added these notes to the "Transform Type Values" registry:

"Key Exchange Method (KE)" transform type was originally named
"Diffie-Hellman Group (D-H)" and was referenced by that name in a
number of RFCs published prior to [RFC9370], which gave it the
current title.

All "Additional Key Exchange (ADDKE)" entries use the same
"Transform Type 4 - Key Exchange Method Transform IDs" registry as
the "Key Exchange Method (KE)" entry.

4. Security Considerations

The extension in this document is intended to mitigate two possible
threats in IKEv2: the compromise of (EC)DH key exchange using Shor's
algorithm while remaining backward compatible and the potential
compromise of existing or future PQC key exchange algorithms. To
address the former threat, this extension allows the establishment of
a shared secret by using multiple key exchanges: typically, one
classical (EC)DH and the other one post-quantum algorithm. In order
to address the latter threat, multiple key exchanges using a post-
quantum algorithm can be performed to form the shared key.

Unlike key exchange methods (Transform Type 4), the Encryption
Algorithm (Transform Type 1), the Pseudorandom Function (Transform
Type 2), and the Integrity Algorithm (Transform Type 3) are not
susceptible to Shor's algorithm. However, they are susceptible to
Grover's attack [GROVER], which allows a quantum computer to perform
a brute force key search, using quadratically fewer steps than the
classical counterpart. Simply increasing the key length can mitigate
this attack. It was previously believed that one needed to double
the key length of these algorithms. However, there are a number of
factors that suggest that it is quite unlikely to achieve the
quadratic speedup using Grover's algorithm. According to NIST
[NISTPQCFAQ], current applications can continue using an AES
algorithm with the minimum key length of 128 bits. Nevertheless, if
the data needs to remain secure for many years to come, one may want
to consider using a longer key size for the algorithms in Transform
Types 1-3.

SKEYSEED is calculated from shared SK(x), using an algorithm defined
in Transform Type 2. While a quantum attacker may learn the value of
SK(x), if this value is obtained by means of a classical key
exchange, other SK(x) values generated by means of a post-quantum
algorithm ensure that the final SKEYSEED is not compromised. This
assumes that the algorithm defined in the Transform Type 2 is quantum
resistant.

The ordering of the additional key exchanges should not matter in
general, as only the final shared secret is of interest.
Nonetheless, because the strength of the running shared secret
increases with every additional key exchange, an implementer may want
to first perform the most secure method (in some metrics) followed by
less secure methods.

The main focus of this document is to prevent a passive attacker from
performing a "harvest-and-decrypt" attack: in other words, attackers
that record messages exchanged today and proceed to decrypt them once
they have access to cryptographically relevant quantum computers.
This attack is prevented due to the hybrid nature of the key
exchange. Other attacks involving an active attacker using a
quantum-computer are not completely solved by this document. This is
for two reasons:

* The first reason is that the authentication step remains
classical. In particular, the authenticity of the SAs established
under IKEv2 is protected by using a pre-shared key or digital
signature algorithms. While the pre-shared key option, provided
the key is long enough, is post-quantum secure, the other
algorithms are not. Moreover, in implementations where
scalability is a requirement, the pre-shared key method may not be
suitable. Post-quantum authenticity may be provided by using a
post-quantum digital signature.

* Secondly, it should be noted that the purpose of post-quantum
algorithms is to provide resistance to attacks mounted in the
future. The current threat is that encrypted sessions are subject
to eavesdropping and are archived with decryption by quantum
computers at some point in the future. Until quantum computers
become available, there is no point in attacking the authenticity
of a connection because there are no possibilities for
exploitation. These only occur at the time of the connection, for
example, by mounting an on-path attack. Consequently, there is
less urgency for post-quantum authenticity compared to post-
quantum confidentiality.

Performing multiple key exchanges while establishing an IKE SA
increases the responder's susceptibility to DoS attacks because of an
increased amount of resources needed before the initiator is
authenticated. This is especially true for post-quantum key exchange
methods, where many of them are more memory and/or CPU intensive than
the classical counterparts.

Responders may consider recommendations from [RFC8019] to deal with
increased DoS-attack susceptibility. It is also possible that the
responder only agrees to create an initial IKE SA without performing
additional key exchanges if the initiator includes such an option in
its proposals. Then, peers immediately rekey the initial IKE SA with
the CREATE_CHILD_SA exchange, and additional key exchanges are
performed via the IKE_FOLLOWUP_KE exchanges. In this case, at the
point when resource-intensive operations are required, the peers have
already authenticated each other. However, in the context of hybrid
post-quantum key exchanges, this scenario would leave the initial IKE
SA (and initial Child SA, if it is created) unprotected against
quantum computers. Nevertheless, the rekeyed IKE SA (and Child SAs
that will be created over it) will have a full protection. This is
similar to the scenario described in [RFC8784]. Depending on the
arrangement and peers' policy, this scenario may or may not be
appropriate. For example, in the G-IKEv2 protocol [G-IKEV2], the
cryptographic materials are sent from the group controller to the
group members when the initial IKE SA is created.

5. References

5.1. Normative References

[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/info/rfc2119>.

[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/info/rfc7296>.

[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/info/rfc8174>.

[RFC9242] Smyslov, V., "Intermediate Exchange in the Internet Key
Exchange Protocol Version 2 (IKEv2)", RFC 9242,
DOI 10.17487/RFC9242, May 2022,
<https://www.rfc-editor.org/info/rfc9242>.

5.2. Informative References

[BEYOND-64K]
Tjhai, CJ., Heider, T., and V. Smyslov, "Beyond 64KB Limit
of IKEv2 Payloads", Work in Progress, Internet-Draft,
draft-tjhai-ikev2-beyond-64k-limit-03, 28 July 2022,
<https://datatracker.ietf.org/doc/html/draft-tjhai-ikev2-
beyond-64k-limit-03>.

[G-IKEV2] Smyslov, V. and B. Weis, "Group Key Management using
IKEv2", Work in Progress, Internet-Draft, draft-ietf-
ipsecme-g-ikev2-09, 19 April 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-ipsecme-
g-ikev2-09>.

[GROVER] Grover, L., "A fast quantum mechanical algorithm for
database search", Proc. of the Twenty-Eighth Annual ACM
Symposium on the Theory of Computing (STOC), pp. 212-219,
DOI 10.48550/arXiv.quant-ph/9605043, May 1996,
<https://doi.org/10.48550/arXiv.quant-ph/9605043>.

[IKEV2TYPE4ID]
IANA, "Internet Key Exchange Version 2 (IKEv2) Parameters:
Transform Type 4 - Diffie-Hellman Group Transform IDs",
<https://www.iana.org/assignments/ikev2-parameters/>.

[NISTPQCFAQ]
NIST, "Post-Quantum Cryptography Standard", January 2023,
<https://csrc.nist.gov/Projects/post-quantum-cryptography/
faqs>.

[RFC5723] Sheffer, Y. and H. Tschofenig, "Internet Key Exchange
Protocol Version 2 (IKEv2) Session Resumption", RFC 5723,
DOI 10.17487/RFC5723, January 2010,
<https://www.rfc-editor.org/info/rfc5723>.

[RFC6023] Nir, Y., Tschofenig, H., Deng, H., and R. Singh, "A
Childless Initiation of the Internet Key Exchange Version
2 (IKEv2) Security Association (SA)", RFC 6023,
DOI 10.17487/RFC6023, October 2010,
<https://www.rfc-editor.org/info/rfc6023>.

[RFC7383] Smyslov, V., "Internet Key Exchange Protocol Version 2
(IKEv2) Message Fragmentation", RFC 7383,
DOI 10.17487/RFC7383, November 2014,
<https://www.rfc-editor.org/info/rfc7383>.

[RFC8019] Nir, Y. and V. Smyslov, "Protecting Internet Key Exchange
Protocol Version 2 (IKEv2) Implementations from
Distributed Denial-of-Service Attacks", RFC 8019,
DOI 10.17487/RFC8019, November 2016,
<https://www.rfc-editor.org/info/rfc8019>.

[RFC8247] Nir, Y., Kivinen, T., Wouters, P., and D. Migault,
"Algorithm Implementation Requirements and Usage Guidance
for the Internet Key Exchange Protocol Version 2 (IKEv2)",
RFC 8247, DOI 10.17487/RFC8247, September 2017,
<https://www.rfc-editor.org/info/rfc8247>.

[RFC8784] Fluhrer, S., Kampanakis, P., McGrew, D., and V. Smyslov,
"Mixing Preshared Keys in the Internet Key Exchange
Protocol Version 2 (IKEv2) for Post-quantum Security",
RFC 8784, DOI 10.17487/RFC8784, June 2020,
<https://www.rfc-editor.org/info/rfc8784>.

Appendix A. Sample Multiple Key Exchanges

This appendix shows some examples of multiple key exchanges. These
examples are not normative, and they describe some message flow
scenarios that may occur in establishing an IKE or Child SA. Note
that some payloads that are not relevant to multiple key exchanges
may be omitted for brevity.

A.1. IKE_INTERMEDIATE Exchanges Carrying Additional Key Exchange
Payloads

The exchanges below show that the initiator proposes the use of
additional key exchanges to establish an IKE SA. The initiator
proposes three sets of additional key exchanges, all of which are
optional. Therefore, the responder can choose NONE for some or all
of the additional exchanges if the proposed key exchange methods are
not supported or for whatever reasons the responder decides not to
perform the additional key exchange.

Initiator Responder
---------------------------------------------------------------------
HDR(IKE_SA_INIT), SAi1(.. ADDKE*...), --->
KEi(Curve25519), Ni, N(IKEV2_FRAG_SUPPORTED),
N(INTERMEDIATE_EXCHANGE_SUPPORTED)
Proposal #1
Transform ECR (ID = ENCR_AES_GCM_16,
256-bit key)
Transform PRF (ID = PRF_HMAC_SHA2_512)
Transform KE (ID = Curve25519)
Transform ADDKE1 (ID = PQ_KEM_1)
Transform ADDKE1 (ID = PQ_KEM_2)
Transform ADDKE1 (ID = NONE)
Transform ADDKE2 (ID = PQ_KEM_3)
Transform ADDKE2 (ID = PQ_KEM_4)
Transform ADDKE2 (ID = NONE)
Transform ADDKE3 (ID = PQ_KEM_5)
Transform ADDKE3 (ID = PQ_KEM_6)
Transform ADDKE3 (ID = NONE)
<--- HDR(IKE_SA_INIT), SAr1(.. ADDKE*...),
KEr(Curve25519), Nr, N(IKEV2_FRAG_SUPPORTED),
N(INTERMEDIATE_EXCHANGE_SUPPORTED)
Proposal #1
Transform ECR (ID = ENCR_AES_GCM_16,
256-bit key)
Transform PRF (ID = PRF_HMAC_SHA2_512)
Transform KE (ID = Curve25519)
Transform ADDKE1 (ID = PQ_KEM_2)
Transform ADDKE2 (ID = NONE)
Transform ADDKE3 (ID = PQ_KEM_5)

HDR(IKE_INTERMEDIATE), SK {KEi(1)(PQ_KEM_2)} -->
<--- HDR(IKE_INTERMEDIATE), SK {KEr(1)(PQ_KEM_2)}
HDR(IKE_INTERMEDIATE), SK {KEi(2)(PQ_KEM_5)} -->
<--- HDR(IKE_INTERMEDIATE), SK {KEr(2)(PQ_KEM_5)}

HDR(IKE_AUTH), SK{ IDi, AUTH, SAi2, TSi, TSr } --->
<--- HDR(IKE_AUTH), SK{ IDr, AUTH, SAr2,
TSi, TSr }

In this particular example, the responder chooses to perform two
additional key exchanges. It selects PQ_KEM_2, NONE, and PQ_KEM_5
for the first, second, and third additional key exchanges,
respectively. As per [RFC7296], a set of keying materials is
derived, in particular SK_d, SK_a[i/r], and SK_e[i/r]. Both peers
then perform an IKE_INTERMEDIATE exchange, carrying PQ_KEM_2 payload,
which is protected with SK_e[i/r] and SK_a[i/r] keys. After the
completion of this IKE_INTERMEDIATE exchange, the SKEYSEED is updated
using SK(1), which is the PQ_KEM_2 shared secret, as follows.

SKEYSEED(1) = prf(SK_d, SK(1) | Ni | Nr)

The updated SKEYSEED value is then used to derive the following
keying materials.

{SK_d(1) | SK_ai(1) | SK_ar(1) | SK_ei(1) | SK_er(1) | SK_pi(1) |
SK_pr(1)} = prf+ (SKEYSEED(1), Ni | Nr | SPIi | SPIr)

As per [RFC9242], both peers compute IntAuth_i1 and IntAuth_r1 using
the SK_pi(1) and SK_pr(1) keys, respectively. These values are
required in the IKE_AUTH phase of the exchange.

In the next IKE_INTERMEDIATE exchange, the peers use SK_e[i/r](1) and
SK_a[i/r](1) keys to protect the PQ_KEM_5 payload. After completing
this exchange, keying materials are updated as follows:

SKEYSEED(2) = prf(SK_d(1), SK(2) | Ni | Nr)
{SK_d(2) | SK_ai(2) | SK_ar(2) | SK_ei(2) | SK_er(2) | SK_pi(2) |
SK_pr(2)} = prf+ (SKEYSEED(2), Ni | Nr | SPIi | SPIr)

In this update, SK(2) is the shared secret from the third additional
key exchange, i.e., PQ_KEM_5. Then, both peers compute the values of
IntAuth_[i/r]2 using the SK_p[i/r](2) keys.

After the completion of the second IKE_INTERMEDIATE exchange, both
peers continue to the IKE_AUTH exchange phase. As defined in
[RFC9242], the values IntAuth_[i/r]2 are used to compute IntAuth,
which, in turn, is used to calculate InitiatorSignedOctets and
ResponderSignedOctets blobs (see Section 3.3.2 of [RFC9242]).

A.2. No Additional Key Exchange Used

The initiator proposes two sets of optional additional key exchanges,
but the responder does not support any of them. The responder
chooses NONE for each set. Consequently, the IKE_INTERMEDIATE
exchange does not take place, and the exchange proceeds to the
IKE_AUTH phase. The resulting keying materials are the same as those
derived with [RFC7296].

Initiator Responder
---------------------------------------------------------------------
HDR(IKE_SA_INIT), SAi1(.. ADDKE*...), --->
KEi(Curve25519), Ni, N(IKEV2_FRAG_SUPPORTED),
N(INTERMEDIATE_EXCHANGE_SUPPORTED)
Proposal #1
Transform ECR (ID = ENCR_AES_GCM_16,
256-bit key)
Transform PRF (ID = PRF_HMAC_SHA2_512)
Transform KE (ID = Curve25519)
Transform ADDKE1 (ID = PQ_KEM_1)
Transform ADDKE1 (ID = PQ_KEM_2)
Transform ADDKE1 (ID = NONE)
Transform ADDKE2 (ID = PQ_KEM_3)
Transform ADDKE2 (ID = PQ_KEM_4)
Transform ADDKE2 (ID = NONE)
<--- HDR(IKE_SA_INIT), SAr1(.. ADDKE*...),
KEr(Curve25519), Nr, N(IKEV2_FRAG_SUPPORTED),
N(INTERMEDIATE_EXCHANGE_SUPPORTED)
Proposal #1
Transform ECR (ID = ENCR_AES_GCM_16,
256-bit key)
Transform PRF (ID = PRF_HMAC_SHA2_512)
Transform KE (ID = Curve25519)
Transform ADDKE1 (ID = NONE)
Transform ADDKE2 (ID = NONE)

HDR(IKE_AUTH), SK{ IDi, AUTH, SAi2, TSi, TSr } --->
<--- HDR(IKE_AUTH), SK{ IDr, AUTH, SAr2,
TSi, TSr }

A.3. Additional Key Exchange in the CREATE_CHILD_SA Exchange Only

The exchanges below show that the initiator does not propose the use
of additional key exchanges to establish an IKE SA, but they are
required in order to establish a Child SA. In order to establish a
fully quantum-resistant IPsec SA, the responder includes a
CHILDLESS_IKEV2_SUPPORTED notification in their IKE_SA_INIT response
message. The initiator understands and supports this notification,
exchanges a modified IKE_AUTH message with the responder, and rekeys
the IKE SA immediately with additional key exchanges. Any Child SA
will have to be created via a subsequent CREATED_CHILD_SA exchange.

Initiator Responder
---------------------------------------------------------------------
HDR(IKE_SA_INIT), SAi1, --->
KEi(Curve25519), Ni, N(IKEV2_FRAG_SUPPORTED)
<--- HDR(IKE_SA_INIT), SAr1,
KEr(Curve25519), Nr, N(IKEV2_FRAG_SUPPORTED),
N(CHILDLESS_IKEV2_SUPPORTED)

HDR(IKE_AUTH), SK{ IDi, AUTH } --->
<--- HDR(IKE_AUTH), SK{ IDr, AUTH }

HDR(CREATE_CHILD_SA),
SK{ SAi(.. ADDKE*...), Ni, KEi(Curve25519) } --->
Proposal #1
Transform ECR (ID = ENCR_AES_GCM_16,
256-bit key)
Transform PRF (ID = PRF_HMAC_SHA2_512)
Transform KE (ID = Curve25519)
Transform ADDKE1 (ID = PQ_KEM_1)
Transform ADDKE1 (ID = PQ_KEM_2)
Transform ADDKE2 (ID = PQ_KEM_5)
Transform ADDKE2 (ID = PQ_KEM_6)
Transform ADDKE2 (ID = NONE)
<--- HDR(CREATE_CHILD_SA), SK{ SAr(.. ADDKE*...),
Nr, KEr(Curve25519),
N(ADDITIONAL_KEY_EXCHANGE)(link1) }
Proposal #1
Transform ECR (ID = ENCR_AES_GCM_16,
256-bit key)
Transform PRF (ID = PRF_HMAC_SHA2_512)
Transform KE (ID = Curve25519)
Transform ADDKE1 (ID = PQ_KEM_2)
Transform ADDKE2 (ID = PQ_KEM_5)

HDR(IKE_FOLLOWUP_KE), SK{ KEi(1)(PQ_KEM_2), --->
N(ADDITIONAL_KEY_EXCHANGE)(link1) }
<--- HDR(IKE_FOLLOWUP_KE), SK{ KEr(1)(PQ_KEM_2),
N(ADDITIONAL_KEY_EXCHANGE)(link2) }

HDR(IKE_FOLLOWUP_KE), SK{ KEi(2)(PQ_KEM_5), --->
N(ADDITIONAL_KEY_EXCHANGE)(link2) }
<--- HDR(IKE_FOLLOWUP_KE), SK{ KEr(2)(PQ_KEM_5) }

A.4. No Matching Proposal for Additional Key Exchanges

The initiator proposes the combination of PQ_KEM_1, PQ_KEM_2,
PQ_KEM_3, and PQ_KEM_4 as the additional key exchanges. The
initiator indicates that either PQ_KEM_1 or PQ_KEM_2 must be used to
establish an IKE SA, but ADDKE2 Transform Type is optional.
Therefore, the responder can either select PQ_KEM_3 or PQ_KEM_4 or
omit this key exchange by selecting NONE. Although the responder
supports the optional PQ_KEM_3 and PQ_KEM_4 methods, it does not
support either the PQ_KEM_1 or the PQ_KEM_2 mandatory method;
therefore, it responds with a NO_PROPOSAL_CHOSEN notification.

Initiator Responder
---------------------------------------------------------------------
HDR(IKE_SA_INIT), SAi1(.. ADDKE*...), --->
KEi(Curve25519), Ni, N(IKEV2_FRAG_SUPPORTED),
N(INTERMEDIATE_EXCHANGE_SUPPORTED)
Proposal #1
Transform ECR (ID = ENCR_AES_GCM_16,
256-bit key)
Transform PRF (ID = PRF_HMAC_SHA2_512)
Transform KE (ID = Curve25519)
Transform ADDKE1 (ID = PQ_KEM_1)
Transform ADDKE1 (ID = PQ_KEM_2)
Transform ADDKE2 (ID = PQ_KEM_3)
Transform ADDKE2 (ID = PQ_KEM_4)
Transform ADDKE2 (ID = NONE)
<--- HDR(IKE_SA_INIT), N(NO_PROPOSAL_CHOSEN)

Appendix B. Design Criteria

The design of the extension is driven by the following criteria:

1) Need for PQC in IPsec

Quantum computers, which might become feasible in the near
future, pose a threat to our classical public key cryptography.
PQC, a family of public key cryptography that is believed to be
resistant to these computers, needs to be integrated into the
IPsec protocol suite to restore confidentiality and
authenticity.

2) Hybrid

There is currently no post-quantum key exchange that is trusted
at the level that (EC)DH is trusted for defending against
conventional (non-quantum) adversaries. A hybrid post-quantum
algorithm to be introduced, along with the well-established
primitives, addresses this concern, since the overall security
is at least as strong as each individual primitive.

3) Focus on post-quantum confidentiality

A passive attacker can store all monitored encrypted IPsec
communication today and decrypt it once a quantum computer is
available in the future. This attack can have serious
consequences that will not be visible for years to come. On the
other hand, an attacker can only perform active attacks, such as
impersonation of the communicating peers, once a quantum
computer is available sometime in the future. Thus, this
specification focuses on confidentiality due to the urgency of
this problem and presents a defense against the serious attack
described above, but it does not address authentication because
it is less urgent at this stage.

4) Limit the amount of exchanged data

The protocol design should be such that the amount of exchanged
data, such as public keys, is kept as small as possible, even if
the initiator and the responder need to agree on a hybrid group
or if multiple public keys need to be exchanged.

5) Not post-quantum specific

Any cryptographic algorithm could be potentially broken in the
future by currently unknown or impractical attacks. Quantum
computers are merely the most concrete example of this. The
design does not categorize algorithms as "post-quantum" or "non-
post-quantum", nor does it create assumptions about the
properties of the algorithms; meaning that if algorithms with
different properties become necessary in the future, this
extension can be used unchanged to facilitate migration to those
algorithms.

6) Limited amount of changes

A key goal is to limit the number of changes required when
enabling a post-quantum handshake. This ensures easier and
quicker adoption in existing implementations.

7) Localized changes

Another key requirement is that changes to the protocol are
limited in scope, in particular, limiting changes in the
exchanged messages and in the state machine, so that they can be
easily implemented.

8) Deterministic operation

This requirement means that the hybrid post-quantum exchange
and, thus, the computed keys will be based on algorithms that
both client and server wish to support.

9) Fragmentation support

Some PQC algorithms could be relatively bulky and might require
fragmentation. Thus, a design goal is the adaptation and
adoption of an existing fragmentation method or the design of a
new method that allows for the fragmentation of the key shares.

10) Backward compatibility and interoperability

This is a fundamental requirement to ensure that hybrid post-
quantum IKEv2 and standard IKEv2 implementations as per
[RFC7296] are interoperable.

11) Compliance with USA Federal Information Processing Standards
(FIPS)

IPsec is widely used in Federal Information Systems, and FIPS
certification is an important requirement. However, at the time
of writing, none of the algorithms that is believed to be post-
quantum is yet FIPS compliant. Nonetheless, it is possible to
combine this post-quantum algorithm with a FIPS-compliant key
establishment method so that the overall design remains FIPS
compliant [NISTPQCFAQ].

12) Ability to use this method with multiple classical (EC)DH key
exchanges

In some situations, peers have no single, mutually trusted, key
exchange algorithm (e.g., due to local policy restrictions).
The ability to combine two (or more) key exchange methods in
such a way that the resulting shared key depends on all of them
allows peers to communicate in this situation.

Appendix C. Alternative Design

This section gives an overview on a number of alternative approaches
that have been considered but later discarded. These approaches are
as follows.

* Sending the classical and post-quantum key exchanges as a single
transform

A method to combine the various key exchanges into a single large
KE payload was considered. This effort is documented in a
previous version of this document (draft-tjhai-ipsecme-hybrid-
qske-ikev2-01). This method allows us to cleanly apply hybrid key
exchanges during the Child SA. However, it does add considerable
complexity and requires an independent fragmentation solution.

* Sending post-quantum proposals and policies in the KE payload only

With the objective of not introducing unnecessary notify payloads,
a method to communicate the hybrid post-quantum proposal in the KE
payload during the first pass of the protocol exchange was
considered. Unfortunately, this design is susceptible to the
following downgrade attack. Consider the scenario where there is
an on-path attacker sitting between an initiator and a responder.
Through the SAi payload, the initiator proposes using a hybrid
post-quantum group and, as a fallback, a Diffie-Hellman group; and
through the KEi payload, the initiator proposes a list of hybrid
post-quantum proposals and policies. The on-path attacker
intercepts this traffic and replies with N(INVALID_KE_PAYLOAD),
suggesting a downgrade to the fallback Diffie-Hellman group
instead. The initiator then resends the same SAi payload and the
KEi payload containing the public value of the fallback Diffie-
Hellman group. Note that the attacker may forward the second
IKE_SA_INIT message only to the responder. Therefore, at this
point in time, the responder will not have the information that
the initiator prefers the hybrid group. Of course, it is possible
for the responder to have a policy to reject an IKE_SA_INIT
message that (a) offers a hybrid group but does not offer the
corresponding public value in the KEi payload and (b) the
responder has not specifically acknowledged that it does not
support the requested hybrid group. However, the checking of this
policy introduces unnecessary protocol complexity. Therefore, in
order to fully prevent any downgrade attacks, using a KE payload
alone is not sufficient, and the initiator MUST always indicate
its preferred post-quantum proposals and policies in a notify
payload in the subsequent IKE_SA_INIT messages following an
N(INVALID_KE_PAYLOAD) response.

* New payload types to negotiate hybrid proposals and to carry post-
quantum public values

Semantically, it makes sense to use a new payload type, which
mimics the SA payload, to carry a hybrid proposal. Likewise,
another new payload type that mimics the KE payload could be used
to transport hybrid public value. Although, in theory, a new
payload type could be made backward compatible by not setting its
critical flag as per Section 2.5 of [RFC7296], it is believed that
it may not be that simple in practice. Since the original release
of IKEv2 in RFC 4306, no new payload type has ever been proposed;
therefore, this creates a potential risk of having a backward-
compatibility issue from nonconformant IKEv2 implementations.
Since there appears to be no other compelling advantages apart
from a semantic one, the existing Transform Type and notify
payloads are used instead.

* Hybrid public value payload

One way to transport the negotiated hybrid public payload, which
contains one classical Diffie-Hellman public value and one or more
post-quantum public values, is to bundle these into a single KE
payload. Alternatively, these could also be transported in a
single new hybrid public value payload. However, following the
same reasoning as above may not be a good idea from a backward-
compatibility perspective. Using a single KE payload would
require encoding or formatting to be defined so that both peers
are able to compose and extract the individual public values.
However, it is believed that it is cleaner to send the hybrid
public values in multiple KE payloads: one for each group or
algorithm. Furthermore, at this point in the protocol exchange,
both peers should have indicated support for handling multiple KE
payloads.

* Fragmentation

The handling of large IKE_SA_INIT messages has been one of the
most challenging tasks. A number of approaches have been
considered, and the two prominent ones that have been discarded
are outlined as follows.

The first approach is to treat the entire IKE_SA_INIT message as a
stream of bytes, which is then split into a number of fragments,
each of which is wrapped onto a payload that will fit into the
size of the network MTU. The payload that wraps each fragment has
a new payload type, and it is envisaged that this new payload type
will not cause a backward-compatibility issue because, at this
stage of the protocol, both peers should have indicated support of
fragmentation in the first pass of the IKE_SA_INIT exchange. The
negotiation of fragmentation is performed using a notify payload,
which also defines supporting parameters, such as the size of
fragment in octets and the fragment identifier. The new payload
that wraps each fragment of the messages in this exchange is
assigned the same fragment identifier. Furthermore, it also has
other parameters, such as a fragment index and total number of
fragments. This approach has been discarded due to its blanket
approach to fragmentation. In cases where only a few payloads
need to be fragmented, this approach appears to be overly
complicated.

Another idea that has been discarded is fragmenting an individual
payload without introducing a new payload type. The idea is to
use the 9-th bit (the bit after the critical flag in the RESERVED
field) in the generic payload header as a flag to mark that this
payload is fragmented. As an example, if a KE payload is to be
fragmented, it may look as follows.

1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Payload |C|F| RESERVED | Payload Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Diffie-Hellman Group Number | Fragment Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Fragment Index | Total Fragments |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Total KE Payload Data Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Fragmented KE Payload ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 1: Example of How to Fragment a KE Payload

When the flag F is set, the current KE payload is a fragment of a
larger KE payload. The Payload Length field denotes the size of
this payload fragment in octets: including the size of the generic
payload header. The 2-octet RESERVED field following Diffie-
Hellman Group Number was to be used as a fragment identifier to
help the assembly and disassembly of fragments. The Fragment
Index and Total Fragments fields are self-explanatory. The Total
KE Payload Data Length indicates the size of the assembled KE
payload data in octets. Finally, the actual fragment is carried
in Fragment KE Payload field.

This approach has been discarded because it is believed that the
working group may not want to use the RESERVED field to change the
format of a packet, and that implementers may not like the added
complexity from checking the fragmentation flag in each received
payload. More importantly, fragmenting the messages in this way
may leave the system to be more prone to denial-of-service (DoS)
attacks. This issue can be solved using IKE_INTERMEDIATE
[RFC9242] to transport the large post-quantum key exchange
payloads and using the generic IKEv2 fragmentation protocol
[RFC7383].

* Group sub-identifier

As discussed before, each group identifier is used to distinguish
a post-quantum algorithm. Further classification could be made on
a particular post-quantum algorithm by assigning an additional
value alongside the group identifier. This sub-identifier value
may be used to assign different security-parameter sets to a given
post-quantum algorithm. However, this level of detail does not
fit the principles of the document where it should deal with
generic hybrid key exchange protocol and not a specific
ciphersuite. Furthermore, there are enough Diffie-Hellman group
identifiers should this be required in the future.

Acknowledgements

The authors would like to thank Frederic Detienne and Olivier Pelerin
for their comments and suggestions, including the idea to negotiate
the post-quantum algorithms using the existing KE payload. The
authors are also grateful to Tobias Heider and Tobias Guggemos for
valuable comments. Thanks to Paul Wouters for reviewing the
document.

Authors' Addresses

Cen Jung Tjhai
Post-Quantum
Email: [email protected]

Martin Tomlinson
Post-Quantum
Email: [email protected]

Graham Bartlett
Quantum Secret
Email: [email protected]

Scott Fluhrer
Cisco Systems
Email: [email protected]

Daniel Van Geest
ISARA Corporation
Email: [email protected]

Oscar Garcia-Morchon
Philips
Email: [email protected]

Valery Smyslov
ELVIS-PLUS
Email: [email protected]