draft-ietf-ipsecme-qr-ikev2-11.txt   rfc8784.txt 
Internet Engineering Task Force S. Fluhrer Internet Engineering Task Force (IETF) S. Fluhrer
Internet-Draft P. Kampanakis Request for Comments: 8784 P. Kampanakis
Intended status: Standards Track D. McGrew Category: Standards Track D. McGrew
Expires: July 17, 2020 Cisco Systems ISSN: 2070-1721 Cisco Systems
V. Smyslov V. Smyslov
ELVIS-PLUS ELVIS-PLUS
January 14, 2020 June 2020
Mixing Preshared Keys in IKEv2 for Post-quantum Security Mixing Preshared Keys in the Internet Key Exchange Protocol Version 2
draft-ietf-ipsecme-qr-ikev2-11 (IKEv2) for Post-quantum Security
Abstract Abstract
The possibility of quantum computers poses a serious challenge to The possibility of quantum computers poses a serious challenge to
cryptographic algorithms deployed widely today. IKEv2 is one example cryptographic algorithms deployed widely today. The Internet Key
of a cryptosystem that could be broken; someone storing VPN Exchange Protocol Version 2 (IKEv2) is one example of a cryptosystem
communications today could decrypt them at a later time when a that could be broken; someone storing VPN communications today could
quantum computer is available. It is anticipated that IKEv2 will be decrypt them at a later time when a quantum computer is available.
extended to support quantum-secure key exchange algorithms; however It is anticipated that IKEv2 will be extended to support quantum-
that is not likely to happen in the near term. To address this secure key exchange algorithms; however, that is not likely to happen
problem before then, this document describes an extension of IKEv2 to in the near term. To address this problem before then, this document
allow it to be resistant to a quantum computer, by using preshared describes an extension of IKEv2 to allow it to be resistant to a
keys. quantum computer by using preshared keys.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This is an Internet Standards Track document.
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 This document is a product of the Internet Engineering Task Force
and may be updated, replaced, or obsoleted by other documents at any (IETF). It represents the consensus of the IETF community. It has
time. It is inappropriate to use Internet-Drafts as reference received public review and has been approved for publication by the
material or to cite them other than as "work in progress." Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
This Internet-Draft will expire on July 17, 2020. 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/rfc8784.
Copyright Notice Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction
1.1. Changes . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Requirements Language
1.2. Requirements Language . . . . . . . . . . . . . . . . . . 6 2. Assumptions
2. Assumptions . . . . . . . . . . . . . . . . . . . . . . . . . 6 3. Exchanges
3. Exchanges . . . . . . . . . . . . . . . . . . . . . . . . . . 6 4. Upgrade Procedure
4. Upgrade procedure . . . . . . . . . . . . . . . . . . . . . . 11 5. PPK
5. PPK . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 5.1. PPK_ID Format
5.1. PPK_ID format . . . . . . . . . . . . . . . . . . . . . . 12 5.2. Operational Considerations
5.2. Operational Considerations . . . . . . . . . . . . . . . 13 5.2.1. PPK Distribution
5.2.1. PPK Distribution . . . . . . . . . . . . . . . . . . 13 5.2.2. Group PPK
5.2.2. Group PPK . . . . . . . . . . . . . . . . . . . . . . 13 5.2.3. PPK-Only Authentication
5.2.3. PPK-only Authentication . . . . . . . . . . . . . . . 14 6. Security Considerations
6. Security Considerations . . . . . . . . . . . . . . . . . . . 14 7. IANA Considerations
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16 8. References
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 17 8.1. Normative References
8.1. Normative References . . . . . . . . . . . . . . . . . . 17 8.2. Informative References
8.2. Informational References . . . . . . . . . . . . . . . . 18 Appendix A. Discussion and Rationale
Appendix A. Discussion and Rationale . . . . . . . . . . . . . . 19 Acknowledgements
Appendix B. Acknowledgements . . . . . . . . . . . . . . . . . . 20 Authors' Addresses
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 20
1. Introduction 1. Introduction
Recent achievements in developing quantum computers demonstrate that Recent achievements in developing quantum computers demonstrate that
it is probably feasible to build a cryptographically significant one. it is probably feasible to build one that is cryptographically
If such a computer is implemented, many of the cryptographic significant. If such a computer is implemented, many of the
algorithms and protocols currently in use would be insecure. A cryptographic algorithms and protocols currently in use would be
quantum computer would be able to solve DH and ECDH problems in insecure. A quantum computer would be able to solve Diffie-Hellman
polynomial time [I-D.hoffman-c2pq], and this would imply that the (DH) and Elliptic Curve Diffie-Hellman (ECDH) problems in polynomial
security of existing IKEv2 [RFC7296] systems would be compromised. time [C2PQ], and this would imply that the security of existing IKEv2
IKEv1 [RFC2409], when used with strong preshared keys, is not [RFC7296] systems would be compromised. IKEv1 [RFC2409], when used
vulnerable to quantum attacks, because those keys are one of the with strong preshared keys, is not vulnerable to quantum attacks
inputs to the key derivation function. If the preshared key has because those keys are one of the inputs to the key derivation
sufficient entropy and the PRF, encryption and authentication function. If the preshared key has sufficient entropy and the
transforms are quantum-secure, then the resulting system is believed Pseudorandom Function (PRF), encryption, and authentication
to be quantum-secure, that is, secure against classical attackers of transforms are quantum secure, then the resulting system is believed
today or future attackers with a quantum computer. to be quantum secure -- that is, secure against classical attackers
of today or future attackers with a quantum computer.
This document describes a way to extend IKEv2 to have a similar This document describes a way to extend IKEv2 to have a similar
property; assuming that the two end systems share a long secret key, property; assuming that the two end systems share a long secret key,
then the resulting exchange is quantum-secure. By bringing post- then the resulting exchange is quantum secure. By bringing post-
quantum security to IKEv2, this document removes the need to use an quantum security to IKEv2, this document removes the need to use an
obsolete version of the Internet Key Exchange in order to achieve obsolete version of IKE in order to achieve that security goal.
that security goal.
The general idea is that we add an additional secret that is shared The general idea is that we add an additional secret that is shared
between the initiator and the responder; this secret is in addition between the initiator and the responder; this secret is in addition
to the authentication method that is already provided within IKEv2. to the authentication method that is already provided within IKEv2.
We stir this secret into the SK_d value, which is used to generate We stir this secret into the SK_d value, which is used to generate
the key material (KEYMAT) and the SKEYSEED for the child SAs; this the key material (KEYMAT) for the Child Security Associations (SAs)
secret provides quantum resistance to the IPsec SAs (and any child and the SKEYSEED for the IKE SAs created as a result of the initial
IKE SAs). We also stir the secret into the SK_pi, SK_pr values; this IKE SA rekey. This secret provides quantum resistance to the IPsec
allows both sides to detect a secret mismatch cleanly. SAs and any subsequent IKE SAs. We also stir the secret into the
SK_pi and SK_pr values; this allows both sides to detect a secret
mismatch cleanly.
It was considered important to minimize the changes to IKEv2. The It was considered important to minimize the changes to IKEv2. The
existing mechanisms to do authentication and key exchange remain in existing mechanisms to perform authentication and key exchange remain
place (that is, we continue to do (EC)DH, and potentially PKI in place (that is, we continue to perform (EC)DH and potentially PKI
authentication if configured). This document does not replace the authentication if configured). This document does not replace the
authentication checks that the protocol does; instead, they are authentication checks that the protocol does; instead, they are
strengthened by using an additional secret key. strengthened by using an additional secret key.
1.1. Changes 1.1. Requirements Language
RFC EDITOR PLEASE DELETE THIS SECTION.
Changes in this draft in each version iterations.
draft-ietf-ipsecme-qr-ikev2-11
o Updates the IANA section based on Eric V.'s IESG Review.
o Updates based on IESG Reviews (Alissa, Adam, Barry, Alexey, Mijra,
Roman, Martin.
draft-ietf-ipsecme-qr-ikev2-10
o Addresses issues raised during IETF LC.
draft-ietf-ipsecme-qr-ikev2-09
o Addresses issues raised in AD review.
draft-ietf-ipsecme-qr-ikev2-08
o Editorial changes.
draft-ietf-ipsecme-qr-ikev2-07
o Editorial changes.
draft-ietf-ipsecme-qr-ikev2-06
o Editorial changes.
draft-ietf-ipsecme-qr-ikev2-05
o Addressed comments received during WGLC.
draft-ietf-ipsecme-qr-ikev2-04
o Using Group PPK is clarified based on comment from Quynh Dang.
draft-ietf-ipsecme-qr-ikev2-03
o Editorial changes and minor text nit fixes.
o Integrated Tommy P. text suggestions.
draft-ietf-ipsecme-qr-ikev2-02
o Added note that the PPK is stirred in the initial IKE SA setup
only.
o Added note about the initiator ignoring any content in the
PPK_IDENTITY notification from the responder.
o fixed Tero's suggestions from 2/6/1028
o Added IANA assigned message types where necessary.
o fixed minor text nits
draft-ietf-ipsecme-qr-ikev2-01
o Nits and minor fixes.
o prf is replaced with prf+ for the SK_d and SK_pi/r calculations.
o Clarified using PPK in case of EAP authentication.
o PPK_SUPPORT notification is changed to USE_PPK to better reflect
its purpose.
o Migrated from draft-fluhrer-qr-ikev2-05 to draft-ietf-ipsecme-qr-
ikev2-00 that is a WG item.
draft-fluhrer-qr-ikev2-05
o Nits and editorial fixes.
o Made PPK_ID format and PPK Distributions subsection of the PPK
section. Also added an Operational Considerations section.
o Added comment about Child SA rekey in the Security Considerations
section.
o Added NO_PPK_AUTH to solve the cases where a PPK_ID is not
configured for a responder.
o Various text changes and clarifications.
o Expanded Security Considerations section to describe some security
concerns and how they should be addressed.
draft-fluhrer-qr-ikev2-03
o Modified how we stir the PPK into the IKEv2 secret state.
o Modified how the use of PPKs is negotiated.
draft-fluhrer-qr-ikev2-02
o Simplified the protocol by stirring in the preshared key into the
child SAs; this avoids the problem of having the responder decide
which preshared key to use (as it knows the initiator identity at
that point); it does mean that someone with a quantum computer can
recover the initial IKE negotiation.
o Removed positive endorsements of various algorithms. Retained
warnings about algorithms known to be weak against a quantum
computer.
draft-fluhrer-qr-ikev2-01
o Added explicit guidance as to what IKE and IPsec algorithms are
quantum resistant.
draft-fluhrer-qr-ikev2-00
o We switched from using vendor ID's to transmit the additional data
to notifications.
o We added a mandatory cookie exchange to allow the server to
communicate to the client before the initial exchange.
o We added algorithm agility by having the server tell the client
what algorithm to use in the cookie exchange.
o We have the server specify the PPK Indicator Input, which allows
the server to make a trade-off between the efficiency for the
search of the clients PPK, and the anonymity of the client.
o We now use the negotiated PRF (rather than a fixed HMAC-SHA256) to
transform the nonces during the KDF.
1.2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP "OPTIONAL" in this document are to be interpreted as described in
14 [RFC2119] [RFC8174] when, and only when, they appear in all BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here. capitals, as shown here.
2. Assumptions 2. Assumptions
We assume that each IKE peer has a list of Post-quantum Preshared We assume that each IKE peer has a list of Post-quantum Preshared
Keys (PPK) along with their identifiers (PPK_ID), and any potential Keys (PPKs) along with their identifiers (PPK_ID), and any potential
IKE initiator selects which PPK to use with any specific responder. IKE initiator selects which PPK to use with any specific responder.
In addition, implementations have a configurable flag that determines In addition, implementations have a configurable flag that determines
whether this post-quantum preshared key is mandatory. This PPK is whether this PPK is mandatory. This PPK is independent of the
independent of the preshared key (if any) that the IKEv2 protocol preshared key (if any) that the IKEv2 protocol uses to perform
uses to perform authentication (because the preshared key in IKEv2 is authentication (because the preshared key in IKEv2 is not used for
not used for any key derivation, and thus doesn't protect against any key derivation and thus doesn't protect against quantum
quantum computers). The PPK specific configuration that is assumed computers). The PPK-specific configuration that is assumed to be on
to be on each node consists of the following tuple: each node consists of the following tuple:
Peer, PPK, PPK_ID, mandatory_or_not Peer, PPK, PPK_ID, mandatory_or_not
We assume the reader is familiar with the payload notation defined in
Section 1.2 of [RFC7296].
3. Exchanges 3. Exchanges
If the initiator is configured to use a post-quantum preshared key If the initiator is configured to use a PPK with the responder
with the responder (whether or not the use of the PPK is mandatory), (whether or not the use of the PPK is mandatory), then it MUST
then it MUST include a notification USE_PPK in the IKE_SA_INIT include a notification USE_PPK in the IKE_SA_INIT request message as
request message as follows: follows:
Initiator Responder Initiator Responder
------------------------------------------------------------------ ------------------------------------------------------------------
HDR, SAi1, KEi, Ni, N(USE_PPK) ---> HDR, SAi1, KEi, Ni, N(USE_PPK) --->
N(USE_PPK) is a status notification payload with the type 16435; it N(USE_PPK) is a status notification payload with the type 16435; it
has a protocol ID of 0, no SPI and no notification data associated has a protocol ID of 0, no Security Parameter Index (SPI), and no
with it. notification data associated with it.
If the initiator needs to resend this initial message with a COOKIE If the initiator needs to resend this initial message with a COOKIE
notification, then the resend would include the USE_PPK notification notification, then the resend would include the USE_PPK notification
if the original message did (see Section 2.6 of [RFC7296]). if the original message did (see Section 2.6 of [RFC7296]).
If the responder does not support this specification or does not have If the responder does not support this specification or does not have
any PPK configured, then it ignores the received notification (as any PPK configured, then it ignores the received notification (as
defined in [RFC7296] for unknown status notifications) and continues defined in [RFC7296] for unknown status notifications) and continues
with the IKEv2 protocol as normal. Otherwise the responder replies with the IKEv2 protocol as normal. Otherwise, the responder replies
with the IKE_SA_INIT message including a USE_PPK notification in the with the IKE_SA_INIT message, including a USE_PPK notification in the
response: response:
Initiator Responder Initiator Responder
------------------------------------------------------------------ ------------------------------------------------------------------
<--- HDR, SAr1, KEr, Nr, [CERTREQ,] N(USE_PPK) <--- HDR, SAr1, KEr, Nr, [CERTREQ,] N(USE_PPK)
When the initiator receives this reply, it checks whether the When the initiator receives this reply, it checks whether the
responder included the USE_PPK notification. If the responder did responder included the USE_PPK notification. If the responder did
not and the flag mandatory_or_not indicates that using PPKs is not include the USE_PPK notification and the flag mandatory_or_not
mandatory for communication with this responder, then the initiator indicates that using PPKs is mandatory for communication with this
MUST abort the exchange. This situation may happen in case of responder, then the initiator MUST abort the exchange. This
misconfiguration, when the initiator believes it has a mandatory-to- situation may happen in case of misconfiguration, i.e., when the
use PPK for the responder, while the responder either doesn't support initiator believes it has a mandatory-to-use PPK for the responder
PPKs at all or doesn't have any PPK configured for the initiator. and the responder either doesn't support PPKs at all or doesn't have
See Section 6 for discussion of the possible impacts of this any PPK configured for the initiator. See Section 6 for discussion
situation. of the possible impacts of this situation.
If the responder did not include the USE_PPK notification and using a If the responder did not include the USE_PPK notification and using a
PPK for this particular responder is optional, then the initiator PPK for this particular responder is optional, then the initiator
continues with the IKEv2 protocol as normal, without using PPKs. continues with the IKEv2 protocol as normal, without using PPKs.
If the responder did include the USE_PPK notification, then the If the responder did include the USE_PPK notification, then the
initiator selects a PPK, along with its identifier PPK_ID. Then, it initiator selects a PPK, along with its identifier PPK_ID. Then, it
computes this modification of the standard IKEv2 key derivation from computes this modification of the standard IKEv2 key derivation from
Section 2.14 of [RFC7296]: Section 2.14 of [RFC7296]:
SKEYSEED = prf(Ni | Nr, g^ir) SKEYSEED = prf(Ni | Nr, g^ir)
{SK_d' | SK_ai | SK_ar | SK_ei | SK_er | SK_pi' | SK_pr' ) {SK_d' | SK_ai | SK_ar | SK_ei | SK_er | SK_pi' | SK_pr'}
= prf+ (SKEYSEED, Ni | Nr | SPIi | SPIr } = prf+ (SKEYSEED, Ni | Nr | SPIi | SPIr)
SK_d = prf+ (PPK, SK_d') SK_d = prf+ (PPK, SK_d')
SK_pi = prf+ (PPK, SK_pi') SK_pi = prf+ (PPK, SK_pi')
SK_pr = prf+ (PPK, SK_pr') SK_pr = prf+ (PPK, SK_pr')
That is, we use the standard IKEv2 key derivation process except that That is, we use the standard IKEv2 key derivation process, except
the three resulting subkeys SK_d, SK_pi, SK_pr (marked with primes in that the three resulting subkeys SK_d, SK_pi, and SK_pr (marked with
the formula above) are then run through the prf+ again, this time primes in the formula above) are then run through the prf+ again,
using the PPK as the key. The result is the unprimed versions of this time using the PPK as the key. The result is the unprimed
these keys which are then used as inputs to subsequent steps of the versions of these keys, which are then used as inputs to subsequent
IKEv2 exchange. steps of the IKEv2 exchange.
Using a prf+ construction ensures that it is always possible to get Using a prf+ construction ensures that it is always possible to get
the resulting keys of the same size as the initial ones, even if the the resulting keys of the same size as the initial ones, even if the
underlying PRF has output size different from its key size. Note, underlying PRF has an output size different from its key size. Note
that at the time of this writing, all PRFs defined for use in IKEv2 that at the time of this writing, all PRFs defined for use in IKEv2
[IKEV2-IANA-PRFS] had output size equal to the (preferred) key size. (see the "Transform Type 2 - Pseudorandom Function Transform IDs"
For such PRFs only the first iteration of prf+ is needed: subregistry [IANA-IKEV2]) have an output size equal to the
(preferred) key size. For such PRFs, only the first iteration of
prf+ is needed:
SK_d = prf (PPK, SK_d' | 0x01) SK_d = prf (PPK, SK_d' | 0x01)
SK_pi = prf (PPK, SK_pi' | 0x01) SK_pi = prf (PPK, SK_pi' | 0x01)
SK_pr = prf (PPK, SK_pr' | 0x01) SK_pr = prf (PPK, SK_pr' | 0x01)
Note that the PPK is used in SK_d, SK_pi and SK_pr calculation only Note that the PPK is used in SK_d, SK_pi, and SK_pr calculations only
during the initial IKE SA setup. It MUST NOT be used when these during the initial IKE SA setup. It MUST NOT be used when these
subkeys are calculated as result of IKE SA rekey, resumption or other subkeys are calculated as result of IKE SA rekey, resumption, or
similar operation. other similar operations.
The initiator then sends the IKE_AUTH request message, including the The initiator then sends the IKE_AUTH request message, including the
PPK_ID value as follows: PPK_ID value as follows:
Initiator Responder Initiator Responder
------------------------------------------------------------------ ------------------------------------------------------------------
HDR, SK {IDi, [CERT,] [CERTREQ,] HDR, SK {IDi, [CERT,] [CERTREQ,]
[IDr,] AUTH, SAi2, [IDr,] AUTH, SAi2,
TSi, TSr, N(PPK_IDENTITY, PPK_ID), [N(NO_PPK_AUTH)]} ---> TSi, TSr, N(PPK_IDENTITY, PPK_ID), [N(NO_PPK_AUTH)]} --->
PPK_IDENTITY is a status notification with the type 16436; it has a PPK_IDENTITY is a status notification with the type 16436; it has a
protocol ID of 0, no SPI and a notification data that consists of the protocol ID of 0, no SPI, and notification data that consists of the
identifier PPK_ID. identifier PPK_ID.
A situation may happen when the responder has some PPKs, but doesn't A situation may happen when the responder has some PPKs but doesn't
have a PPK with the PPK_ID received from the initiator. In this case have a PPK with the PPK_ID received from the initiator. In this
the responder cannot continue with PPK (in particular, it cannot case, the responder cannot continue with the PPK (in particular, it
authenticate the initiator), but the responder could be able to cannot authenticate the initiator), but the responder could be able
continue with normal IKEv2 protocol if the initiator provided its to continue with the normal IKEv2 protocol if the initiator provided
authentication data computed as in normal IKEv2, without using PPKs. its authentication data computed as in the normal IKEv2 without using
For this purpose, if using PPKs for communication with this responder PPKs. For this purpose, if using PPKs for communication with this
is optional for the initiator (based on the mandatory_or_not flag), responder is optional for the initiator (based on the
then the initiator MUST include a NO_PPK_AUTH notification in the mandatory_or_not flag), then the initiator MUST include a NO_PPK_AUTH
above message. This notification informs the responder that PPK is notification in the above message. This notification informs the
optional and allows for authenticating the initiator without using responder that PPKs are optional and allows for authenticating the
PPK. initiator without using PPKs.
NO_PPK_AUTH is a status notification with the type 16437; it has a NO_PPK_AUTH is a status notification with the type 16437; it has a
protocol ID of 0 and no SPI. The Notification Data field contains protocol ID of 0 and no SPI. The Notification Data field contains
the initiator's authentication data computed using SK_pi', which has the initiator's authentication data computed using SK_pi', which has
been computed without using PPKs. This is the same data that would been computed without using PPKs. This is the same data that would
normally be placed in the Authentication Data field of an AUTH normally be placed in the Authentication Data field of an AUTH
payload. Since the Auth Method field is not present in the payload. Since the Auth Method field is not present in the
notification, the authentication method used for computing the notification, the authentication method used for computing the
authentication data MUST be the same as method indicated in the AUTH authentication data MUST be the same as the method indicated in the
payload. Note that if the initiator decides to include the AUTH payload. Note that if the initiator decides to include the
NO_PPK_AUTH notification, the initiator needs to perform NO_PPK_AUTH notification, the initiator needs to perform
authentication data computation twice, which may consume computation authentication data computation twice, which may consume computation
power (e.g., if digital signatures are involved). power (e.g., if digital signatures are involved).
When the responder receives this encrypted exchange, it first When the responder receives this encrypted exchange, it first
computes the values: computes the values:
SKEYSEED = prf(Ni | Nr, g^ir) SKEYSEED = prf(Ni | Nr, g^ir)
{SK_d' | SK_ai | SK_ar | SK_ei | SK_er | SK_pi' | SK_pr' } {SK_d' | SK_ai | SK_ar | SK_ei | SK_er | SK_pi' | SK_pr'}
= prf+ (SKEYSEED, Ni | Nr | SPIi | SPIr ) = prf+ (SKEYSEED, Ni | Nr | SPIi | SPIr)
The responder then uses the SK_ei/SK_ai values to decrypt/check the The responder then uses the SK_ei/SK_ai values to decrypt/check the
message and then scans through the payloads for the PPK_ID attached message and then scans through the payloads for the PPK_ID attached
to the PPK_IDENTITY notification. If no PPK_IDENTITY notification is to the PPK_IDENTITY notification. If no PPK_IDENTITY notification is
found and the peers successfully exchanged USE_PPK notifications in found and the peers successfully exchanged USE_PPK notifications in
the IKE_SA_INIT exchange, then the responder MUST send back the IKE_SA_INIT exchange, then the responder MUST send back an
AUTHENTICATION_FAILED notification and then fail the negotiation. AUTHENTICATION_FAILED notification and then fail the negotiation.
If the PPK_IDENTITY notification contains a PPK_ID that is not known If the PPK_IDENTITY notification contains a PPK_ID that is not known
to the responder or is not configured for use for the identity from to the responder or is not configured for use for the identity from
IDi payload, then the responder checks whether using PPKs for this the IDi payload, then the responder checks whether using PPKs for
initiator is mandatory and whether the initiator included NO_PPK_AUTH this initiator is mandatory and whether the initiator included a
notification in the message. If using PPKs is mandatory or no NO_PPK_AUTH notification in the message. If using PPKs is mandatory
NO_PPK_AUTH notification is found, then then the responder MUST send or no NO_PPK_AUTH notification is found, then the responder MUST send
back AUTHENTICATION_FAILED notification and then fail the back an AUTHENTICATION_FAILED notification and then fail the
negotiation. Otherwise (when PPK is optional and the initiator negotiation. Otherwise (when a PPK is optional and the initiator
included NO_PPK_AUTH notification) the responder MAY continue regular included a NO_PPK_AUTH notification), the responder MAY continue the
IKEv2 protocol, except that it uses the data from the NO_PPK_AUTH regular IKEv2 protocol, except that it uses the data from the
notification as the authentication data (which usually resides in the NO_PPK_AUTH notification as the authentication data (which usually
AUTH payload), for the purpose of the initiator authentication. resides in the AUTH payload) for the purpose of the initiator
authentication. Note that the authentication method is still
indicated in the AUTH payload.
Note, that Authentication Method is still indicated in the AUTH Table 1 summarizes the above logic for the responder:
payload.
This table summarizes the above logic for the responder: +==========+=============+============+===========+================+
| Received | Received | Configured | PPK is | Action |
| USE_PPK | NO_PPK_AUTH | with PPK | Mandatory | |
+==========+=============+============+===========+================+
| No | * | No | * | Standard IKEv2 |
| | | | | protocol |
+----------+-------------+------------+-----------+----------------+
| No | * | Yes | No | Standard IKEv2 |
| | | | | protocol |
+----------+-------------+------------+-----------+----------------+
| No | * | Yes | Yes | Abort |
| | | | | negotiation |
+----------+-------------+------------+-----------+----------------+
| Yes | No | No | * | Abort |
| | | | | negotiation |
+----------+-------------+------------+-----------+----------------+
| Yes | Yes | No | Yes | Abort |
| | | | | negotiation |
+----------+-------------+------------+-----------+----------------+
| Yes | Yes | No | No | Standard IKEv2 |
| | | | | protocol |
+----------+-------------+------------+-----------+----------------+
| Yes | * | Yes | * | Use PPK |
+----------+-------------+------------+-----------+----------------+
Received Received Configured PPK is Table 1
USE_PPK NO_PPK_AUTH with PPK Mandatory Action
---------------------------------------------------------------------
No * No * Standard IKEv2 protocol
No * Yes No Standard IKEv2 protocol
No * Yes Yes Abort negotiation
Yes No No * Abort negotiation
Yes Yes No Yes Abort negotiation
Yes Yes No No Standard IKEv2 protocol
Yes * Yes * Use PPK
If PPK is in use, then the responder extracts the corresponding PPK If a PPK is in use, then the responder extracts the corresponding PPK
and computes the following values: and computes the following values:
SK_d = prf+ (PPK, SK_d') SK_d = prf+ (PPK, SK_d')
SK_pi = prf+ (PPK, SK_pi') SK_pi = prf+ (PPK, SK_pi')
SK_pr = prf+ (PPK, SK_pr') SK_pr = prf+ (PPK, SK_pr')
The responder then continues with the IKE_AUTH exchange (validating The responder then continues with the IKE_AUTH exchange (validating
the AUTH payload that the initiator included) as usual and sends back the AUTH payload that the initiator included) as usual and sends back
a response, which includes the PPK_IDENTITY notification with no data a response, which includes the PPK_IDENTITY notification with no data
to indicate that the PPK is used in the exchange: to indicate that the PPK is used in the exchange:
Initiator Responder Initiator Responder
------------------------------------------------------------------ ------------------------------------------------------------------
<-- HDR, SK {IDr, [CERT,] <-- HDR, SK {IDr, [CERT,]
AUTH, SAr2, AUTH, SAr2,
TSi, TSr, N(PPK_IDENTITY)} TSi, TSr, N(PPK_IDENTITY)}
When the initiator receives the response, then it checks for the When the initiator receives the response, it checks for the presence
presence of the PPK_IDENTITY notification. If it receives one, it of the PPK_IDENTITY notification. If it receives one, it marks the
marks the SA as using the configured PPK to generate SK_d, SK_pi, SA as using the configured PPK to generate SK_d, SK_pi, and SK_pr (as
SK_pr (as shown above); the content of the received PPK_IDENTITY (if shown above); the content of the received PPK_IDENTITY (if any) MUST
any) MUST be ignored. If the initiator does not receive the be ignored. If the initiator does not receive the PPK_IDENTITY, it
PPK_IDENTITY, it MUST either fail the IKE SA negotiation sending the MUST either fail the IKE SA negotiation sending the
AUTHENTICATION_FAILED notification in the Informational exchange (if AUTHENTICATION_FAILED notification in the INFORMATIONAL exchange (if
the PPK was configured as mandatory), or continue without using the the PPK was configured as mandatory) or continue without using the
PPK (if the PPK was not configured as mandatory and the initiator PPK (if the PPK was not configured as mandatory and the initiator
included the NO_PPK_AUTH notification in the request). included the NO_PPK_AUTH notification in the request).
If EAP is used in the IKE_AUTH exchange, then the initiator doesn't If the Extensible Authentication Protocol (EAP) is used in the
include AUTH payload in the first request message, however the IKE_AUTH exchange, then the initiator doesn't include the AUTH
responder sends back AUTH payload in the first reply. The peers then payload in the first request message; however, the responder sends
exchange AUTH payloads after EAP is successfully completed. As a back the AUTH payload in the first reply. The peers then exchange
result, the responder sends AUTH payload twice - in the first AUTH payloads after EAP is successfully completed. As a result, the
IKE_AUTH reply message and in the last one, while the initiator sends responder sends the AUTH payload twice -- in the first and last
AUTH payload only in the last IKE_AUTH request. See more details IKE_AUTH reply message -- while the initiator sends the AUTH payload
about EAP authentication in IKEv2 in Section 2.16 of [RFC7296]. only in the last IKE_AUTH request. See more details about EAP
authentication in IKEv2 in Section 2.16 of [RFC7296].
The general rule for using PPK in the IKE_AUTH exchange, which covers The general rule for using a PPK in the IKE_AUTH exchange, which
EAP authentication case too, is that the initiator includes covers the EAP authentication case too, is that the initiator
PPK_IDENTITY (and optionally NO_PPK_AUTH) notification in the request includes a PPK_IDENTITY (and optionally a NO_PPK_AUTH) notification
message containing AUTH payload. Therefore, in case of EAP the in the request message containing the AUTH payload. Therefore, in
responder always computes the AUTH payload in the first IKE_AUTH case of EAP, the responder always computes the AUTH payload in the
reply message without using PPK (by means of SK_pr'), since PPK_ID is first IKE_AUTH reply message without using a PPK (by means of
not yet known to the responder. Once the IKE_AUTH request message SK_pr'), since PPK_ID is not yet known to the responder. Once the
containing the PPK_IDENTITY notification is received, the responder IKE_AUTH request message containing the PPK_IDENTITY notification is
follows the rules described above for the non-EAP authentication received, the responder follows the rules described above for the
case. non-EAP authentication case.
Initiator Responder Initiator Responder
---------------------------------------------------------------- ----------------------------------------------------------------
HDR, SK {IDi, [CERTREQ,] HDR, SK {IDi, [CERTREQ,]
[IDr,] SAi2, [IDr,] SAi2,
TSi, TSr} --> TSi, TSr} -->
<-- HDR, SK {IDr, [CERT,] AUTH, <-- HDR, SK {IDr, [CERT,] AUTH,
EAP} EAP}
HDR, SK {EAP} --> HDR, SK {EAP} -->
<-- HDR, SK {EAP (success)} <-- HDR, SK {EAP (success)}
HDR, SK {AUTH, HDR, SK {AUTH,
N(PPK_IDENTITY, PPK_ID) N(PPK_IDENTITY, PPK_ID)
[, N(NO_PPK_AUTH)]} --> [, N(NO_PPK_AUTH)]} -->
<-- HDR, SK {AUTH, SAr2, TSi, TSr <-- HDR, SK {AUTH, SAr2, TSi, TSr
[, N(PPK_IDENTITY)]} [, N(PPK_IDENTITY)]}
Note that the diagram above shows both the cases when the responder Note that the diagram above shows both the cases when the responder
uses PPK and when it chooses not to use it (provided the initiator uses a PPK and when it chooses not to use it (provided the initiator
has included NO_PPK_AUTH notification), and thus the responder's has included the NO_PPK_AUTH notification); thus, the responder's
PPK_IDENTITY notification is marked as optional. Also, note that the PPK_IDENTITY notification is marked as optional. Also, note that the
IKE_SA_INIT exchange in case of PPK is as described above (including IKE_SA_INIT exchange using a PPK is as described above (including
exchange of the USE_PPK notifications), regardless whether EAP is exchange of the USE_PPK notifications), regardless of whether or not
employed in the IKE_AUTH or not. EAP is employed in the IKE_AUTH.
4. Upgrade procedure 4. Upgrade Procedure
This algorithm was designed so that someone can introduce PPKs into This algorithm was designed so that someone can introduce PPKs into
an existing IKE network without causing network disruption. an existing IKE network without causing network disruption.
In the initial phase of the network upgrade, the network In the initial phase of the network upgrade, the network
administrator would visit each IKE node, and configure: administrator would visit each IKE node and configure:
o The set of PPKs (and corresponding PPK_IDs) that this node would * The set of PPKs (and corresponding PPK_IDs) that this node would
need to know. need to know.
o For each peer that this node would initiate to, which PPK will be * The PPK that will be used for each peer that this node would
used. initiate to.
o That the use of PPK is currently not mandatory. * The value "false" for the mandatory_or_not flag for each peer that
this node would initiate to (thus indicating that the use of PPKs
is not mandatory).
With this configuration, the node will continue to operate with nodes With this configuration, the node will continue to operate with nodes
that have not yet been upgraded. This is due to the USE_PPK that have not yet been upgraded. This is due to the USE_PPK
notification and the NO_PPK_AUTH notification; if the initiator has notification and the NO_PPK_AUTH notification; if the initiator has
not been upgraded, it will not send the USE_PPK notification (and so not been upgraded, it will not send the USE_PPK notification (and so
the responder will know that the peers will not use a PPK). If the the responder will know that the peers will not use a PPK). If the
responder has not been upgraded, it will not send the USE_PPK responder has not been upgraded, it will not send the USE_PPK
notification (and so the initiator will know to not use a PPK). If notification (and so the initiator will know to not use a PPK). If
both peers have been upgraded, but the responder isn't yet configured both peers have been upgraded but the responder isn't yet configured
with the PPK for the initiator, then the responder could do standard with the PPK for the initiator, then the responder could continue
IKEv2 protocol if the initiator sent NO_PPK_AUTH notification. If with the standard IKEv2 protocol if the initiator sent a NO_PPK_AUTH
both the responder and initiator have been upgraded and properly notification. If both the responder and initiator have been upgraded
configured, they will both realize it, and the Child SAs will be and properly configured, they will both realize it, and the Child SAs
quantum-secure. will be quantum secure.
As an optional second step, after all nodes have been upgraded, then As an optional second step, after all nodes have been upgraded, the
the administrator should then go back through the nodes, and mark the administrator should then go back through the nodes and mark the use
use of PPK as mandatory. This will not affect the strength against a of a PPK as mandatory. This will not affect the strength against a
passive attacker, but it would mean that an active attacker with a passive attacker, but it would mean that an active attacker with a
quantum computer (which is sufficiently fast to be able to break the quantum computer (which is sufficiently fast to be able to break the
(EC)DH in real-time) would not be able to perform a downgrade attack. (EC)DH in real time) would not be able to perform a downgrade attack.
5. PPK 5. PPK
5.1. PPK_ID format 5.1. PPK_ID Format
This standard requires that both the initiator and the responder have This standard requires that both the initiator and the responder have
a secret PPK value, with the responder selecting the PPK based on the a secret PPK value, with the responder selecting the PPK based on the
PPK_ID that the initiator sends. In this standard, both the PPK_ID that the initiator sends. In this standard, both the
initiator and the responder are configured with fixed PPK and PPK_ID initiator and the responder are configured with fixed PPK and PPK_ID
values, and do the look up based on PPK_ID value. It is anticipated values and perform the lookup based on the PPK_ID value. It is
that later specifications will extend this technique to allow anticipated that later specifications will extend this technique to
dynamically changing PPK values. To facilitate such an extension, we allow dynamically changing PPK values. To facilitate such an
specify that the PPK_ID the initiator sends will have its first octet extension, we specify that the PPK_ID the initiator sends will have
be the PPK_ID Type value. This document defines two values for its first octet be the PPK_ID type value. This document defines two
PPK_ID Type: values for the PPK_ID type:
o PPK_ID_OPAQUE (1) - for this type the format of the PPK_ID (and * PPK_ID_OPAQUE (1) - For this type, the format of the PPK_ID (and
the PPK itself) is not specified by this document; it is assumed the PPK itself) is not specified by this document; it is assumed
to be mutually intelligible by both by initiator and the to be mutually intelligible by both the initiator and the
responder. This PPK_ID type is intended for those implementations responder. This PPK_ID type is intended for those implementations
that choose not to disclose the type of PPK to active attackers. that choose not to disclose the type of PPK to active attackers.
o PPK_ID_FIXED (2) - in this case the format of the PPK_ID and the * PPK_ID_FIXED (2) - In this case, the format of the PPK_ID and the
PPK are fixed octet strings; the remaining bytes of the PPK_ID are PPK are fixed octet strings; the remaining bytes of the PPK_ID are
a configured value. We assume that there is a fixed mapping a configured value. We assume that there is a fixed mapping
between PPK_ID and PPK, which is configured locally to both the between PPK_ID and PPK, which is configured locally to both the
initiator and the responder. The responder can use the PPK_ID to initiator and the responder. The responder can use the PPK_ID to
look up the corresponding PPK value. Not all implementations are look up the corresponding PPK value. Not all implementations are
able to configure arbitrary octet strings; to improve the able to configure arbitrary octet strings; to improve the
potential interoperability, it is recommended that, in the potential interoperability, it is recommended that, in the
PPK_ID_FIXED case, both the PPK and the PPK_ID strings be limited PPK_ID_FIXED case, both the PPK and the PPK_ID strings be limited
to the Base64 character set [RFC4648]. to the base64 character set [RFC4648].
5.2. Operational Considerations 5.2. Operational Considerations
The need to maintain several independent sets of security credentials The need to maintain several independent sets of security credentials
can significantly complicate a security administrator's job, and can can significantly complicate a security administrator's job and can
potentially slow down widespread adoption of this specification. It potentially slow down widespread adoption of this specification. It
is anticipated, that administrators will try to simplify their job by is anticipated that administrators will try to simplify their job by
decreasing the number of credentials they need to maintain. This decreasing the number of credentials they need to maintain. This
section describes some of the considerations for PPK management. section describes some of the considerations for PPK management.
5.2.1. PPK Distribution 5.2.1. PPK Distribution
PPK_IDs of the type PPK_ID_FIXED (and the corresponding PPKs) are PPK_IDs of the type PPK_ID_FIXED (and the corresponding PPKs) are
assumed to be configured within the IKE device in an out-of-band assumed to be configured within the IKE device in an out-of-band
fashion. While the method of distribution is a local matter and out fashion. While the method of distribution is a local matter and is
of scope of this document or IKEv2, [RFC6030] describes a format for out of scope of this document or IKEv2, [RFC6030] describes a format
for the transport and provisioning of symmetric keys. That format for the transport and provisioning of symmetric keys. That format
could be reused using the PIN profile (defined in Section 10.2 of could be reused using the PIN profile (defined in Section 10.2 of
[RFC6030]) with the "Id" attribute of the <Key> element being the [RFC6030]) with the "Id" attribute of the <Key> element being the
PPK_ID (without the PPK_ID Type octet for a PPK_ID_FIXED) and the PPK_ID (without the PPK_ID type octet for a PPK_ID_FIXED) and the
<Secret> element containing the PPK. <Secret> element containing the PPK.
5.2.2. Group PPK 5.2.2. Group PPK
This document doesn't explicitly require that PPK is unique for each This document doesn't explicitly require that the PPK be unique for
pair of peers. If it is the case, then this solution provides full each pair of peers. If this is the case, then this solution provides
peer authentication, but it also means that each host must have as full peer authentication, but it also means that each host must have
many independent PPKs as the peers it is going to communicate with. as many independent PPKs as peers it is going to communicate with.
As the number of peers grows the PPKs will not scale. As the number of peers grows, the PPKs will not scale.
It is possible to use a single PPK for a group of users. Since each It is possible to use a single PPK for a group of users. Since each
peer uses classical public key cryptography in addition to PPK for peer uses classical public key cryptography in addition to a PPK for
key exchange and authentication, members of the group can neither key exchange and authentication, members of the group can neither
impersonate each other nor read other's traffic, unless they use impersonate each other nor read each other's traffic unless they use
quantum computers to break public key operations. However group quantum computers to break public key operations. However, group
members can record any traffic they have access to that comes from members can record any traffic they have access to that comes from
other group members and decrypt it later, when they get access to a other group members and decrypt it later, when they get access to a
quantum computer. quantum computer.
In addition, the fact that the PPK is known to a (potentially large) In addition, the fact that the PPK is known to a (potentially large)
group of users makes it more susceptible to theft. When an attacker group of users makes it more susceptible to theft. When an attacker
equipped with a quantum computer gets access to a group PPK, all equipped with a quantum computer gets access to a group PPK, all
communications inside the group are revealed. communications inside the group are revealed.
For these reasons using group PPK is NOT RECOMMENDED. For these reasons, using a group PPK is NOT RECOMMENDED.
5.2.3. PPK-only Authentication 5.2.3. PPK-Only Authentication
If quantum computers become a reality, classical public key If quantum computers become a reality, classical public key
cryptography will provide little security, so administrators may find cryptography will provide little security, so administrators may find
it attractive not to use it at all for authentication. This will it attractive not to use it at all for authentication. This will
reduce the number of credentials they need to maintain to PPKs only. reduce the number of credentials they need to maintain because they
Combining group PPK and PPK-only authentication is NOT RECOMMENDED, only need to maintain PPK credentials. Combining group PPK and PPK-
since in this case any member of the group can impersonate any other only authentication is NOT RECOMMENDED since, in this case, any
member even without help of quantum computers. member of the group can impersonate any other member, even without
the help of quantum computers.
PPK-only authentication can be achieved in IKEv2 if the NULL PPK-only authentication can be achieved in IKEv2 if the NULL
Authentication method [RFC7619] is employed. Without PPK the NULL Authentication method [RFC7619] is employed. Without PPK, the NULL
Authentication method provides no authentication of the peers, Authentication method provides no authentication of the peers;
however since a PPK is stirred into the SK_pi and the SK_pr, the however, since a PPK is stirred into the SK_pi and the SK_pr, the
peers become authenticated if a PPK is in use. Using PPKs MUST be peers become authenticated if a PPK is in use. Using PPKs MUST be
mandatory for the peers if they advertise support for PPK in mandatory for the peers if they advertise support for PPKs in
IKE_SA_INIT and use NULL Authentication. Additionally, since the IKE_SA_INIT and use NULL Authentication. Additionally, since the
peers are authenticated via PPK, the ID Type in the IDi/IDr payloads peers are authenticated via PPKs, the ID Type in the IDi/IDr payloads
SHOULD NOT be ID_NULL, despite using the NULL Authentication method. SHOULD NOT be ID_NULL, despite using the NULL Authentication method.
6. Security Considerations 6. Security Considerations
Quantum computers are able to perform Grover's algorithm [GROVER]; A critical consideration is how to ensure the randomness of this
that effectively halves the size of a symmetric key. Because of post-quantum preshared key. Quantum computers are able to perform
this, the user SHOULD ensure that the post-quantum preshared key used Grover's algorithm [GROVER]; that effectively halves the size of a
has at least 256 bits of entropy, in order to provide 128 bits of symmetric key. In addition, an adversary impersonating the server,
post-quantum security. That provides security equivalent to Level 5 even with a conventional computer, can perform a dictionary search
as defined in the NIST PQ Project Call For Proposals [NISTPQCFP]. over plausible post-quantum preshared key values. The strongest
practice is to ensure that any post-quantum preshared key contains at
least 256 bits of entropy; this will provide 128 bits of post-quantum
security, while providing security against conventional dictionary
attacks. That provides the security equivalent to Category 5 as
defined in the NIST Post-Quantum Cryptography Call for Proposals
[NISTPQCFP]. Deriving a post-quantum preshared key from a password,
name, or other low-entropy source is not secure because of these
known attacks.
With this protocol, the computed SK_d is a function of the PPK. With this protocol, the computed SK_d is a function of the PPK.
Assuming that the PPK has sufficient entropy (for example, at least Assuming that the PPK has sufficient entropy (for example, at least
2^256 possible values), then even if an attacker was able to recover 2^(256) possible values), even if an attacker was able to recover the
the rest of the inputs to the PRF function, it would be infeasible to rest of the inputs to the PRF function, it would be infeasible to use
use Grover's algorithm with a quantum computer to recover the SK_d Grover's algorithm with a quantum computer to recover the SK_d value.
value. Similarly, all keys that are a function of SK_d, which Similarly, all keys that are a function of SK_d, which include all
include all Child SAs keys and all keys for subsequent IKE SAs Child SA keys and all keys for subsequent IKE SAs (created when the
(created when the initial IKE SA is rekeyed), are also quantum-secure initial IKE SA is rekeyed), are also quantum secure (assuming that
(assuming that the PPK was of high enough entropy, and that all the the PPK was of high enough entropy and that all the subkeys are
subkeys are sufficiently long). sufficiently long).
An attacker with a quantum computer that can decrypt the initial IKE An attacker with a quantum computer that can decrypt the initial IKE
SA has access to all the information exchanged over it, such as SA has access to all the information exchanged over it, such as
identities of the peers, configuration parameters and all negotiated identities of the peers, configuration parameters, and all negotiated
IPsec SAs information (including traffic selectors), with the IPsec SA information (including traffic selectors), with the
exception of the cryptographic keys used by the IPsec SAs which are exception of the cryptographic keys used by the IPsec SAs, which are
protected by the PPK. protected by the PPK.
Deployments that treat this information as sensitive or that send Deployments that treat this information as sensitive or that send
other sensitive data (like cryptographic keys) over IKE SA MUST rekey other sensitive data (like cryptographic keys) over IKE SAs MUST
the IKE SA before the sensitive information is sent to ensure this rekey the IKE SA before the sensitive information is sent to ensure
information is protected by the PPK. It is possible to create a this information is protected by the PPK. It is possible to create a
childless IKE SA as specified in [RFC6023]. This prevents Child SA childless IKE SA as specified in [RFC6023]. This prevents Child SA
configuration information from being transmitted in the original IKE configuration information from being transmitted in the original IKE
SA that is not protected by a PPK. Some information related to IKE SA that is not protected by a PPK. Some information related to IKE
SA, that is sent in the IKE_AUTH exchange, such as peer identities, SA that is sent in the IKE_AUTH exchange, such as peer identities,
feature notifications, Vendor ID's etc. cannot be hidden from the feature notifications, vendor IDs, etc., cannot be hidden from the
attack described above, even if the additional IKE SA rekey is attack described above, even if the additional IKE SA rekey is
performed. performed.
In addition, the policy SHOULD be set to negotiate only quantum- In addition, the policy SHOULD be set to negotiate only quantum-
secure symmetric algorithms; while this RFC doesn't claim to give secure symmetric algorithms; while this RFC doesn't claim to give
advice as to what algorithms are secure (as that may change based on advice as to what algorithms are secure (as that may change based on
future cryptographical results), below is a list of defined IKEv2 and future cryptographical results), below is a list of defined IKEv2 and
IPsec algorithms that should not be used, as they are known to IPsec algorithms that should not be used, as they are known to
provide less than 128 bits of post-quantum security provide less than 128 bits of post-quantum security:
o Any IKEv2 Encryption algorithm, PRF or Integrity algorithm with * Any IKEv2 encryption algorithm, PRF, or integrity algorithm with a
key size less than 256 bits. key size less than 256 bits.
o Any ESP Transform with key size less than 256 bits. * Any ESP transform with a key size less than 256 bits.
o PRF_AES128_XCBC and PRF_AES128_CBC; even though they are defined * PRF_AES128_XCBC and PRF_AES128_CBC: even though they can use as
to be able to use an arbitrary key size, they convert it into a input a key of arbitrary size, such input keys are converted into
128-bit key internally. a 128-bit key for internal use.
Section 3 requires the initiator to abort the initial exchange if Section 3 requires the initiator to abort the initial exchange if
using PPKs is mandatory for it, but the responder does not include using PPKs is mandatory for it but the responder does not include the
the USE_PPK notification in the response. In this situation, when USE_PPK notification in the response. In this situation, when the
the initiator aborts negotiation it leaves a half-open IKE SA on the initiator aborts the negotiation, it leaves a half-open IKE SA on the
responder (because IKE_SA_INIT completes successfully from the responder (because IKE_SA_INIT completes successfully from the
responder's point of view). This half-open SA will eventually expire responder's point of view). This half-open SA will eventually expire
and be deleted, but if the initiator continues its attempts to create and be deleted, but if the initiator continues its attempts to create
IKE SA with a high enough rate, then the responder may consider it as IKE SA with a high enough rate, then the responder may consider it a
a Denial-of-Service (DoS) attack and take protection measures (see denial-of-service (DoS) attack and take protective measures (see
[RFC8019] for more detail). In this situation, it is RECOMMENDED [RFC8019] for more details). In this situation, it is RECOMMENDED
that the initiator caches the negative result of the negotiation and that the initiator cache the negative result of the negotiation and
doesn't make attempts to create it again for some time. This period not attempt to create it again for some time. This period of time
of time may vary, but it is believed that waiting for at least few may vary, but it is believed that waiting for at least few minutes
minutes will not cause the responder to treat it as DoS attack. will not cause the responder to treat it as a DoS attack. Note that
Note, that this situation would most likely be a result of this situation would most likely be a result of misconfiguration, and
misconfiguration and some re-configuration of the peers would some reconfiguration of the peers would probably be needed.
probably be needed.
If using PPKs is optional for both peers and they authenticate If using PPKs is optional for both peers and they authenticate
themselves using digital signatures, then an attacker in between, themselves using digital signatures, then an attacker in between,
equipped with a quantum computer capable of breaking public key equipped with a quantum computer capable of breaking public key
operations in real time, is able to mount downgrade attack by operations in real time, is able to mount a downgrade attack by
removing USE_PPK notification from the IKE_SA_INIT and forging removing the USE_PPK notification from the IKE_SA_INIT and forging
digital signatures in the subsequent exchange. If using PPKs is digital signatures in the subsequent exchange. If using PPKs is
mandatory for at least one of the peers or PSK is used for mandatory for at least one of the peers or if a preshared key mode is
authentication, then the attack will be detected and the SA won't be used for authentication, then the attack will be detected and the SA
created. won't be created.
If using PPKs is mandatory for the initiator, then an attacker able If using PPKs is mandatory for the initiator, then an attacker able
to eavesdrop and to inject packets into the network can prevent to eavesdrop and inject packets into the network can prevent creation
creating an IKE SA by mounting the following attack. The attacker of an IKE SA by mounting the following attack. The attacker
intercepts the initial request containing the USE_PPK notification intercepts the initial request containing the USE_PPK notification
and injects a forged response containing no USE_PPK. If the attacker and injects a forged response containing no USE_PPK. If the attacker
manages to inject this packet before the responder sends a genuine manages to inject this packet before the responder sends a genuine
response, then the initiator would abort the exchange. To thwart response, then the initiator would abort the exchange. To thwart
this kind of attack it is RECOMMENDED, that if using PPKs is this kind of attack, it is RECOMMENDED that, if using PPKs is
mandatory for the initiator and the received response doesn't contain mandatory for the initiator and the received response doesn't contain
the USE_PPK notification, then the initiator doesn't abort the the USE_PPK notification, the initiator not abort the exchange
exchange immediately. Instead it waits for more response messages immediately. Instead, it waits for more response messages,
retransmitting the request as if no responses were received at all, retransmitting the request as if no responses were received at all,
until either the received message contains the USE_PPK or the until either the received message contains the USE_PPK notification
exchange times out (see section 2.4 of [RFC7296] for more details or the exchange times out (see Section 2.4 of [RFC7296] for more
about retransmission timers in IKEv2). If neither of the received details about retransmission timers in IKEv2). If none of the
responses contains USE_PPK, then the exchange is aborted. received responses contains USE_PPK, then the exchange is aborted.
If using PPK is optional for both peers, then in case of If using a PPK is optional for both peers, then in case of
misconfiguration (e.g., mismatched PPK_ID) the IKE SA will be created misconfiguration (e.g., mismatched PPK_ID), the IKE SA will be
without protection against quantum computers. It is advised that if created without protection against quantum computers. It is advised
PPK was configured, but was not used for a particular IKE SA, then that if a PPK was configured but was not used for a particular IKE
implementations SHOULD audit this event. SA, then implementations SHOULD audit this event.
7. IANA Considerations 7. IANA Considerations
This document defines three new Notify Message Types in the "Notify This document defines three new Notify Message Types in the "IKEv2
Message Types - Status Types" registry Notify Message Types - Status Types" subregistry under the "Internet
(https://www.iana.org/assignments/ikev2-parameters/ Key Exchange Version 2 (IKEv2) Parameters" registry [IANA-IKEV2]:
ikev2-parameters.xhtml#ikev2-parameters-16):
16435 USE_PPK [THIS RFC] +=======+================================+===========+
16436 PPK_IDENTITY [THIS RFC] | Value | NOTIFY MESSAGES - STATUS TYPES | Reference |
16437 NO_PPK_AUTH [THIS RFC] +=======+================================+===========+
| 16435 | USE_PPK | RFC 8784 |
+-------+--------------------------------+-----------+
| 16436 | PPK_IDENTITY | RFC 8784 |
+-------+--------------------------------+-----------+
| 16437 | NO_PPK_AUTH | RFC 8784 |
+-------+--------------------------------+-----------+
This document also creates a new IANA registry "IKEv2 Post-quantum Table 2
Preshared Key ID Types" in IKEv2 IANA registry
(https://www.iana.org/assignments/ikev2-parameters/) for the PPK_ID
types used in the PPK_IDENTITY notification defined in this
specification. The initial values of the new registry are:
PPK_ID Type Value Reference Per this document, IANA has created a new subregistry titled "IKEv2
----------- ----- --------- Post-quantum Preshared Key ID Types" under the "Internet Key Exchange
Reserved 0 [THIS RFC] Version 2 (IKEv2) Parameters" registry [IANA-IKEV2]. This new
PPK_ID_OPAQUE 1 [THIS RFC] subregistry is for the PPK_ID types used in the PPK_IDENTITY
PPK_ID_FIXED 2 [THIS RFC] notification defined in this specification. The initial contents of
Unassigned 3-127 [THIS RFC] the new subregistry are as follows:
Private Use 128-255 [THIS RFC]
+=========+==========================+===========+
| Value | PPK_ID Type | Reference |
+=========+==========================+===========+
| 0 | Reserved | RFC 8784 |
+---------+--------------------------+-----------+
| 1 | PPK_ID_OPAQUE | RFC 8784 |
+---------+--------------------------+-----------+
| 2 | PPK_ID_FIXED | RFC 8784 |
+---------+--------------------------+-----------+
| 3-127 | Unassigned | RFC 8784 |
+---------+--------------------------+-----------+
| 128-255 | Reserved for Private Use | RFC 8784 |
+---------+--------------------------+-----------+
Table 3
The PPK_ID type value 0 is reserved; values 3-127 are to be assigned The PPK_ID type value 0 is reserved; values 3-127 are to be assigned
by IANA; values 128-255 are for private use among mutually consenting by IANA; and values 128-255 are for private use among mutually
parties. To register new PPK_IDs in the unassigned range, a Type consenting parties. To register new PPK_IDs in the Unassigned range,
name, a Value between 3 and 127 and a Reference specification need to a type name, a value between 3 and 127, and a reference specification
be defined. Changes and additions to the unassigned range of this need to be defined. Changes and additions to the Unassigned range of
registry are by the Expert Review Policy [RFC8126]. Changes and this registry are made using the Expert Review policy [RFC8126].
additions to the private use range of this registry are by the Changes and additions to the Reserved for Private Use range of this
Private Use Policy [RFC8126]. registry are made using the Private Use policy [RFC8126].
8. References 8. References
8.1. Normative References 8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>. <https://www.rfc-editor.org/info/rfc2119>.
[RFC7296] Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T. [RFC7296] Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T.
Kivinen, "Internet Key Exchange Protocol Version 2 Kivinen, "Internet Key Exchange Protocol Version 2
(IKEv2)", STD 79, RFC 7296, DOI 10.17487/RFC7296, October (IKEv2)", STD 79, RFC 7296, DOI 10.17487/RFC7296, October
2014, <https://www.rfc-editor.org/info/rfc7296>. 2014, <https://www.rfc-editor.org/info/rfc7296>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>. May 2017, <https://www.rfc-editor.org/info/rfc8174>.
8.2. Informational References 8.2. Informative References
[GROVER] Grover, L., "A Fast Quantum Mechanical Algorithm for [C2PQ] Hoffman, P., "The Transition from Classical to Post-
Database Search", Proc. of the Twenty-Eighth Annual ACM Quantum Cryptography", Work in Progress, Internet-Draft,
Symposium on the Theory of Computing (STOC 1996), 1996. draft-hoffman-c2pq-07, 26 May 2020,
<https://tools.ietf.org/html/draft-hoffman-c2pq-07>.
[I-D.hoffman-c2pq] [GROVER] Grover, L., "A Fast Quantum Mechanical Algorithm for
Hoffman, P., "The Transition from Classical to Post- Database Search", STOC '96: Proceedings of the Twenty-
Quantum Cryptography", draft-hoffman-c2pq-06 (work in Eighth Annual ACM Symposium on the Theory of Computing,
progress), November 2019. pp. 212-219", DOI 10.1145/237814.237866, July 1996,
<https://doi.org/10.1145/237814.237866>.
[IKEV2-IANA-PRFS] [IANA-IKEV2]
"Internet Key Exchange Version 2 (IKEv2) Parameters, IANA, "Internet Key Exchange Version 2 (IKEv2)
Transform Type 2 - Pseudorandom Function Transform IDs", Parameters",
<https://www.iana.org/assignments/ikev2-parameters/ <https://www.iana.org/assignments/ikev2-parameters/>.
ikev2-parameters.xhtml#ikev2-parameters-6>.
[NISTPQCFP] [NISTPQCFP]
NIST, "NIST Post-Quantum Cryptography Call for Proposals", NIST, "Submission Requirements and Evaluation Criteria for
2016, <https://csrc.nist.gov/CSRC/media/Projects/Post- the Post-Quantum Cryptography Standardization Process",
Quantum-Cryptography/documents/call-for-proposals-final- December 2016, <https://csrc.nist.gov/CSRC/media/Projects/
dec-2016.pdf>. Post-Quantum-Cryptography/documents/call-for-proposals-
final-dec-2016.pdf>.
[RFC2409] Harkins, D. and D. Carrel, "The Internet Key Exchange [RFC2409] Harkins, D. and D. Carrel, "The Internet Key Exchange
(IKE)", RFC 2409, DOI 10.17487/RFC2409, November 1998, (IKE)", RFC 2409, DOI 10.17487/RFC2409, November 1998,
<https://www.rfc-editor.org/info/rfc2409>. <https://www.rfc-editor.org/info/rfc2409>.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data [RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006, Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
<https://www.rfc-editor.org/info/rfc4648>. <https://www.rfc-editor.org/info/rfc4648>.
[RFC6023] Nir, Y., Tschofenig, H., Deng, H., and R. Singh, "A [RFC6023] Nir, Y., Tschofenig, H., Deng, H., and R. Singh, "A
skipping to change at page 19, line 18 skipping to change at line 777
DOI 10.17487/RFC8019, November 2016, DOI 10.17487/RFC8019, November 2016,
<https://www.rfc-editor.org/info/rfc8019>. <https://www.rfc-editor.org/info/rfc8019>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for [RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26, Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017, RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>. <https://www.rfc-editor.org/info/rfc8126>.
Appendix A. Discussion and Rationale Appendix A. Discussion and Rationale
The idea behind this document is that while a quantum computer can The primary goal of this document is to augment the IKEv2 protocol to
easily reconstruct the shared secret of an (EC)DH exchange, they provide protection against quantum computers without requiring novel
cannot as easily recover a secret from a symmetric exchange. This cryptographic algorithms. The idea behind this document is that
document makes the SK_d, and hence the IPsec KEYMAT and any child while a quantum computer can easily reconstruct the shared secret of
SA's SKEYSEED, depend on both the symmetric PPK, and also the Diffie- an (EC)DH exchange, it cannot as easily recover a secret from a
Hellman exchange. If we assume that the attacker knows everything symmetric exchange. This document makes the SK_d (and thus also the
except the PPK during the key exchange, and there are 2^n plausible IPsec KEYMAT and any subsequent IKE SA's SKEYSEED) depend on both the
PPKs, then a quantum computer (using Grover's algorithm) would take symmetric PPK and the Diffie-Hellman exchange. If we assume that the
O(2^(n/2)) time to recover the PPK. So, even if the (EC)DH can be attacker knows everything except the PPK during the key exchange and
trivially solved, the attacker still can't recover any key material there are 2^(n) plausible PPKs, then a quantum computer (using
(except for the SK_ei, SK_er, SK_ai and SK_ar values for the initial Grover's algorithm) would take O(2^(n/2)) time to recover the PPK.
IKE exchange) unless they can find the PPK, which is too difficult if So, even if the (EC)DH can be trivially solved, the attacker still
the PPK has enough entropy (for example, 256 bits). Note that we do can't recover any key material (except for the SK_ei, SK_er, SK_ai,
allow an attacker with a quantum computer to rederive the keying and SK_ar values for the initial IKE exchange) unless they can find
material for the initial IKE SA; this was a compromise to allow the the PPK, which is too difficult if the PPK has enough entropy (for
responder to select the correct PPK quickly. example, 256 bits). Note that we do allow an attacker with a quantum
computer to rederive the keying material for the initial IKE SA; this
was a compromise to allow the responder to select the correct PPK
quickly.
Another goal of this protocol is to minimize the number of changes Another goal of this protocol is to minimize the number of changes
within the IKEv2 protocol, and in particular, within the cryptography within the IKEv2 protocol, in particular, within the cryptography of
of IKEv2. By limiting our changes to notifications, and only IKEv2. By limiting our changes to notifications and only adjusting
adjusting the SK_d, SK_pi, SK_pr, it is hoped that this would be the SK_d, SK_pi, and SK_pr, it is hoped that this would be
implementable, even on systems that perform most of the IKEv2 implementable, even on systems that perform most of the IKEv2
processing in hardware. processing in hardware.
A third goal was to be friendly to incremental deployment in A third goal is to be friendly to incremental deployment in
operational networks, for which we might not want to have a global operational networks for which we might not want to have a global
shared key, or quantum-secure IKEv2 is rolled out incrementally. shared key or for which quantum-secure IKEv2 is rolled out
This is why we specifically try to allow the PPK to be dependent on incrementally. This is why we specifically try to allow the PPK to
the peer, and why we allow the PPK to be configured as optional. be dependent on the peer and why we allow the PPK to be configured as
optional.
A fourth goal was to avoid violating any of the security properties A fourth goal is to avoid violating any of the security properties
provided by IKEv2. provided by IKEv2.
Appendix B. Acknowledgements Acknowledgements
We would like to thank Tero Kivinen, Paul Wouters, Graham Bartlett, We would like to thank Tero Kivinen, Paul Wouters, Graham Bartlett,
Tommy Pauly, Quynh Dang and the rest of the IPSecME Working Group for Tommy Pauly, Quynh Dang, and the rest of the IPSECME Working Group
their feedback and suggestions for the scheme. for their feedback and suggestions for the scheme.
Authors' Addresses Authors' Addresses
Scott Fluhrer Scott Fluhrer
Cisco Systems Cisco Systems
Email: sfluhrer@cisco.com Email: sfluhrer@cisco.com
Panos Kampanakis Panos Kampanakis
Cisco Systems Cisco Systems
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