Alternative Approach for Mixing Preshared Keys in IKEv2 for Post-quantum Security
draft-smyslov-ipsecme-ikev2-qr-alt-03
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Valery Smyslov
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2021-02-03
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Network Working Group V. Smyslov
Internet-Draft ELVIS-PLUS
Intended status: Standards Track February 3, 2021
Expires: August 7, 2021
Alternative Approach for Mixing Preshared Keys in IKEv2 for Post-quantum
Security
draft-smyslov-ipsecme-ikev2-qr-alt-03
Abstract
An IKEv2 extension defined in [RFC8784] allows IPsec traffic to be
protected against someone storing VPN communications today and
decrypting it later, when (and if) quantum computers are available.
However, this protection doesn't cover an initial IKEv2 SA, which
might be unacceptable in some scenarios. This specification defines
an alternative way get the same protection against quantum computers,
which allows to extend it on the initial IKEv2 SA.
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/.
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 August 7, 2021.
Copyright Notice
Copyright (c) 2021 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
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include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology and Notation . . . . . . . . . . . . . . . . . . 3
3. Alternative Approach Description . . . . . . . . . . . . . . 3
4. Computing IKE SA Keys . . . . . . . . . . . . . . . . . . . . 5
5. Comparison of the Conventional and the Alternative Approaches 6
6. Security Considerations . . . . . . . . . . . . . . . . . . . 6
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 6
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
9.1. Normative References . . . . . . . . . . . . . . . . . . 7
9.2. Informative References . . . . . . . . . . . . . . . . . 7
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 7
1. Introduction
The Internet Key Exchange Protocol version 2, defined in [RFC7296],
is used in the IPsec architecture to perform authenticated key
exchange. [RFC8784] defines an extension of IKEv2 for protecting
today's VPN traffic against future quantum computers. At the time
this extension was being developed, it was a consensus in the IPSECME
WG that only IPsec traffic needs to have such a protection. It was
believed that no sensitive information is transferred over IKE SA and
extending the protection to also cover IKE SA traffic would require
serious modifications to core IKEv2 protocol, that contradicted to
one of the goals to minimize such changes. For the cases when this
protection is needed it was suggested to immediately rekey IKE SA
once it is created.
In some situations it is desirable to have this protection for IKE SA
from the very beginning, when an initial IKE SA is created. An
example of such situation is Group Key Management protocol using
IKEv2, defined in [I-D.ietf-ipsecme-g-ikev2]. In this protocol
session keys are transferred from Group Controller/Key Server (GCKS)
to Group Members (GM) immediately once an initial IKE SA is created.
While it is possible to postpone transfer of the keys until the IKE
SA is rekeyed (and [I-D.ietf-ipsecme-g-ikev2] specifies how to do
it), the needed sequence of actions introduces an additional delay
and adds unnecessary complexity to the protocol.
Since [RFC8784] was written, a new IKE_INTERMEDIATE exchange for
IKEv2 was defined in [I-D.ietf-ipsecme-ikev2-intermediate]. While
the primary motivation for developing this exchange was to allow
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multiple key exchanges to be used in IKEv2 (which is defined in
[I-D.ietf-ipsecme-ikev2-multiple-ke]), the IKE_INTERMEDIATE exchange
itself can be used for other purposes too.
This specification makes use of the IKE_INTERMEDIATE exchange to
define an alternative approach to [RFC8784], which allows getting
protection against quantum computers for initial IKE SA.
2. Terminology and Notation
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.
We will use a term Conventional Approach in the content of using PPK
to refer to the [RFC8784] and a term Alternative Approach to refer to
this specification.
3. Alternative Approach Description
IKE initiator who supports the IKE_INTERMEDIATE exchange and wants to
use PPK includes both the INTERMEDIATE_EXCHANGE_SUPPORTED and the
USE_PPK notifications in the IKE_SA_INIT request. If responder
supports the IKE_INTERMEDIATE exchange and is willing to use PPK, it
includes both these notifications in the response.
Initiator Responder
------------------------------------------------------------------
HDR, SAi1, KEi, Ni,
N(INTERMEDIATE_EXCHANGE_SUPPORTED),
N(USE_PPK) --->
<--- HDR, SAr1, KEr, Nr, [CERTREQ,]
N(INTERMEDIATE_EXCHANGE_SUPPORTED),
N(USE_PPK)
If the responder returned both these notifications, then the
initiator MAY choose to use the IKE_INTERMEDIATE exchange to
negotiate PPK identity with the responder. Note, that it is up to
the initiator whether to use the alternative or conventional
approaches, i.e. whether to send PPK identity in the
IKE_INTERMEDIATE exchange or in the IKE_AUTH exchange, as defined in
the [RFC8784].
If the initiator decides to use alternative approach, it includes one
or more PPK_IDENTITY notification containing PPK identities the
initiator believes are appropriate for the IKE SA being created, into
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the IKE_INTERMEDIATE request. If a series of the IKE_INTERMEDIATE
exchanges takes place, the PPK_IDENTITY notification(s) MUST be sent
in the last one, i.e. in the IKE_INTERMEDIATE exchange immediately
preceding the IKE_AUTH exchange. If the last IKE_INTERMEDIATE
exchange contains other payloads aimed for some other purpose, then
the notification(s) MAY be piggybacked with these payloads.
Initiator Responder
------------------------------------------------------------------
HDR, SK { ... N(PPK_IDENTITY, PPK_ID_1)
[, N(PPK_IDENTITY, PPK_ID_2)] ...
[, N(PPK_IDENTITY, PPK_ID_n)]} --->
Depending on the responder's capabilities and policy the following
situations are possible.
If the responder doesn't support the alternative approach, it will
ignore the received PPK_IDENTITY notification(s) and won't include
any additional notifications in the response. If the responder
doesn't have any of the PPKs which IDs were sent by the initiator,
then it MUST behave as if it doesn't support the alternative
approach, i.e. include no additional notifications in the response.
Initiator Responder
------------------------------------------------------------------
<--- HDR, SK { ... }
In this case the initiator cannot make an initial IKE SA to be a
quantum computer resistant, so if this is a requirement for the
initiator, then it MUST abort creating IKE SA. Otherwise, the
initiator continues with the IKE_AUTH exchange and tries to use PPK
as described in [RFC8784].
If the responder supports this extension and is configured with one
of the PPKs which IDs were sent by the initiator, then the responder
chooses one of these PPKs and returns back its identity in the
PPK_IDENTITY notification.
Initiator Responder
------------------------------------------------------------------
<--- HDR, SK { ... N(PPK_IDENTITY, PPK_ID_i)}
In this case the IKE_AUTH exchange is performed as defined in
[RFC7296], so that neither PPK_IDENTITY nor NO_PPK_AUTH notifications
are sent, since it's already known which PPK to use. The keys for
the IKE SA are computed using PPK, as described in Section 4.
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If the responder returns PPK identity that was not suggested by the
initiator, then the initiator must treat this as a fatal error and
MUST abort the IKE SA establishment.
Since the responder selects PPK before it knows identity of the
initiator, a situation may occur, when the responder agrees to use
some PPK in the IKE_INTERMEDIATE exchange, but later discovers during
the IKE_AUTH exchange that this particular PPK is not associated with
the initiator's identity in its local policy. Note, that the
responder does have this PPK, but it is just not listed among the
PPKs for using with this initiator. In this case the responder
SHOULD abort negotiation and return back the AUTHENTICATION_FAILED
notification to be consistent with its policy. However, if using PPK
with this initiator is marked optional in the local policy, then the
responder MAY continue creating IKE SA using the negotiated "wrong"
PPK.
4. Computing IKE SA Keys
Once the PPK is negotiated in the last IKE_INTERMEDIATE exchange, the
IKE SA keys are recalculated. Note that if the IKE SA keys are also
recalculated as the result of the other actions performed in the
IKE_INTERMEDIATE exchange (for example, as defined in
[I-D.ietf-ipsecme-ikev2-multiple-ke], then applying PPK MUST be done
after all of them, so that recalculating IKE SA keys with PPK is the
last action before they are used in the IKE_AUTH exchange.
The IKE SA keys are computed as follows. A new SKEYSEED' value is
computed using the negotiated PPK and the most recently computed SK_d
key. Note, that PPK is applied to SK_d exactly how specified in
[RFC8784], and the result is used as SKEYSEED'.
SKEYSEED' = prf+ (PPK, SK_d)
Then the SKEYSEED' is used to recalculate all SK_* keys as defined in
Section 2.14 of [RFC7296].
{SK_d | SK_ai | SK_ar | SK_ei | SK_er | SK_pi | SK_pr}
= prf+ (SKEYSEED', Ni | Nr | SPIi | SPIr )
In the formula above Ni and Nr are nonces from the IKE_SA_INIT
exchange and SPIi, SPIr - SPIs of the IKE SA being created.
The resulting keys are then used in the IKE_AUTH exchange and in the
created IKE SA.
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5. Comparison of the Conventional and the Alternative Approaches
This specification isn't intended to be a replacement for [RFC8784].
Instead, it is supposed to be used in situations where the
conventional approach has a significant shortcomings. However, if
the partners support both approaches, then the alternative approach
MAY also be used in situations where convenient approach suffices.
The alternative approach has the following advantages:
1. The main advantage of the alternative approach is that it allows
an initial IKE SA to be protected against quantum computers.
This is important for those IKE extensions which transfer
sensitive information, e.g. cryptographic keys, over initial IKE
SA. The prominent example of such extensions is
[I-D.ietf-ipsecme-g-ikev2].
2. Using the alternative approach allows the initiator to specify
several appropriate PPKs and the responder to choose one of them.
This feature could simplify PPK rollover.
3. With the alternative approach there is no need for the initiator
to calculate the content of the AUTH payload twice (with and
without PPK) to support a situation when using PPK is optional
for both sides.
The main disadvantage of the alternative approach is that it requires
an additional round trip (the IKE_INTERMEDIATE exchange) to set up
IKE SA. However, if the IKE_INTERMEDIATE exchange has to be used for
some other purposes in any case, then PPK stuff can be piggybacked
with other payloads, thus eliminating this penalty.
6. Security Considerations
Security considerations of using Post-quantum Preshared Keys in the
IKEv2 protocol are discussed in [RFC8784]. This specification
defines an alternative way of exchanging PPK identity information.
7. IANA Considerations
This specification makes no request to IANA.
8. Acknowledgements
The author would like to thank Paul Wouters for valuable comments.
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9. References
9.1. 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>.
[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>.
[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>.
[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>.
[I-D.ietf-ipsecme-ikev2-intermediate]
Smyslov, V., "Intermediate Exchange in the IKEv2
Protocol", draft-ietf-ipsecme-ikev2-intermediate-05 (work
in progress), September 2020.
9.2. Informative References
[I-D.ietf-ipsecme-g-ikev2]
Smyslov, V. and B. Weis, "Group Key Management using
IKEv2", draft-ietf-ipsecme-g-ikev2-02 (work in progress),
January 2021.
[I-D.ietf-ipsecme-ikev2-multiple-ke]
Tjhai, C., Tomlinson, M., Bartlett, G., Fluhrer, S.,
Geest, D., Garcia-Morchon, O., and V. Smyslov, "Multiple
Key Exchanges in IKEv2", draft-ietf-ipsecme-ikev2-
multiple-ke-02 (work in progress), January 2021.
Author's Address
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Valery Smyslov
ELVIS-PLUS
PO Box 81
Moscow (Zelenograd) 124460
RU
Phone: +7 495 276 0211
Email: svan@elvis.ru
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