Network Working Group V. Smyslov
Internet-Draft ELVIS-PLUS
Intended status: Standards Track October 17, 2019
Expires: April 19, 2020
An Alternative Approach for Postquantum Preshared Keys in IKEv2
draft-smyslov-ipsecme-ikev2-qr-alt-00
Abstract
An IKEv2 extension defined in [I-D.ietf-ipsecme-qr-ikev2] 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
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This Internet-Draft will expire on April 19, 2020.
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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 . . . . . . . . . . . . . . . . . . . . . 7
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
8.1. Normative References . . . . . . . . . . . . . . . . . . 7
8.2. Informative References . . . . . . . . . . . . . . . . . 7
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction
The Internet Key Exchange Protocol version 2, defined in [RFC7296],
is used in the IPsec architecture to perform authenticated key
exchange. [I-D.ietf-ipsecme-qr-ikev2] 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.yeung-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.yeung-g-ikev2] specifies how to do it), the needed
sequence of actions introduces an additional delay and adds
unnecessary complexity to the protocol.
Since [I-D.ietf-ipsecme-qr-ikev2] 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 Post-Quantum Key Exchanges
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to be used in IKEv2 (which is another long-term approach to protect
against Quantum Computers and is defined in
[I-D.tjhai-ipsecme-hybrid-qske-ikev2]), 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 [I-D.ietf-ipsecme-qr-ikev2], 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 [I-D.ietf-ipsecme-qr-ikev2] 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, she
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 to send PPK identity in the
IKE_INTERMEDIATE exchange or in the IKE_AUTH exchange, as defined in
the [I-D.ietf-ipsecme-qr-ikev2].
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If the initiator decides to use alternative approach, he includes one
or more PPK_IDENTITY notification containing PPK identities, which
the initiator believes can be used for the IKE SA being created, into
the IKE_INTERMEDIATE request. If a series of the IKE_INTERMEDIATE
exchanges take 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 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.
First, if the responder doesn't support the alternative approach, she
will ignore the received PPK_IDENTITY notification(s) and won't
include any 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. Depending on his policy, the initiator
may abort negotiation or may continue with the IKE_AUTH exchange. In
the latter case depending on the policy the initiator may try to
negotiate the use of PPK with conventional approach, as described in
[I-D.ietf-ipsecme-qr-ikev2], or may proceed with the standard
IKE_AUTH exchange, thus giving up using PPK for this IKE SA.
Another situation occurs when the responder supports this extension,
but has no PPK with identity equal to any of the identities provided
by the initiator. Depending on responder's policy the following
scenarios are possible.
If using PPK is mandatory for the responder, then she returns the
AUTHENTICATION_FAILED notification, thus informing the initiator that
the SA cannot be created. In this case the initiator MUST abort the
process of IKE SA establishment.
Initiator Responder
------------------------------------------------------------------
<--- HDR, SK {N(AUTHENTICATION_FAILED)}
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If using PPK is optional for the responder, then she returns the
empty PPK_IDENTITY notification, thus informing the initiator that
the IKE SA can be created only without using PPK.
Initiator Responder
------------------------------------------------------------------
<--- HDR, SK { ... N(PPK_IDENTITY)}
In this case the initiator depending on whether using PPK is
mandatory or not in his own policy may continue establishing IKE SA
without PPK or abort it.
Finally, if the responder supports this extension and is configured
with one of the PPKs which identities were provided by the initiator,
then the responder chooses an appropriate PPK and returns back the
PPK_IDENTITY notification containing its identity.
Initiator Responder
------------------------------------------------------------------
<--- HDR, SK { ... N(PPK_IDENTITY, PPK_ID_i)}
In this case the IKE_AUTH exchange is performed as defined in the
core IKEv2 specification. In particular, neither PPK_IDENTITY nor
NO_PPK_AUTH notifications are included, since it's already known
which PPK to use. However, the keys for the IKE SA are computed
using PPK, as described in Section 4.
Note, that 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.
4. Computing IKE SA Keys
With alternative approach the keys are computed similarly to
[I-D.ietf-ipsecme-qr-ikev2], with the difference, that all SK_* (and
not only SK_d, SK_pi and SK_pr) keys are calculated using PPK:
{SK_d' | SK_ai' | SK_ar' | SK_ei' | SK_er' | SK_pi' | SK_pr' }
= prf+ (SKEYSEED, Ni | Nr | SPIi | SPIr )
SK_d = prf+ (PPK, SK_d')
SK_ai = prf+ (PPK, SK_ai')
SK_ar = prf+ (PPK, SK_ar')
SK_ei = prf+ (PPK, SK_ei')
SK_er = prf+ (PPK, SK_er')
SK_pi = prf+ (PPK, SK_pi')
SK_pr = prf+ (PPK, SK_pr')
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If the last IKE_INTERMEDIATE exchange performs an update of the IKE
SA keys (e.g. as a result of additional key exchange, as described in
[I-D.tjhai-ipsecme-hybrid-qske-ikev2]), then applying PPK MUST be
performed to the result of this update. In other words, it must be
the last action in calculating SK_* keys for the IKE SA being
created.
5. Comparison of the Conventional and the Alternative Approaches
This specification isn't intended to be a replacement for
[I-D.ietf-ipsecme-qr-ikev2]. 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.yeung-g-ikev2].
2. Using 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 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 Postquantum Preshared Keys in the
IKEv2 protocol are discussed in [I-D.ietf-ipsecme-qr-ikev2]. This
specification defines an alternative way of exchanging PPK identity
information.
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7. IANA Considerations
This specification makes no request to IANA.
8. References
8.1. 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>.
[I-D.ietf-ipsecme-qr-ikev2]
Fluhrer, S., McGrew, D., Kampanakis, P., and V. Smyslov,
"Postquantum Preshared Keys for IKEv2", draft-ietf-
ipsecme-qr-ikev2-08 (work in progress), March 2019.
[I-D.ietf-ipsecme-ikev2-intermediate]
Smyslov, V., "Intermediate Exchange in the IKEv2
Protocol", draft-ietf-ipsecme-ikev2-intermediate-02 (work
in progress), July 2019.
8.2. Informative References
[I-D.yeung-g-ikev2]
Weis, B. and V. Smyslov, "Group Key Management using
IKEv2", draft-yeung-g-ikev2-16 (work in progress), July
2019.
[I-D.tjhai-ipsecme-hybrid-qske-ikev2]
Tjhai, C., Tomlinson, M., grbartle@cisco.com, g., Fluhrer,
S., Geest, D., Garcia-Morchon, O., and V. Smyslov,
"Framework to Integrate Post-quantum Key Exchanges into
Internet Key Exchange Protocol Version 2 (IKEv2)", draft-
tjhai-ipsecme-hybrid-qske-ikev2-04 (work in progress),
July 2019.
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Author's Address
Valery Smyslov
ELVIS-PLUS
PO Box 81
Moscow (Zelenograd) 124460
RU
Phone: +7 495 276 0211
Email: svan@elvis.ru
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