Internet DRAFT - draft-ietf-ipsecme-qr-ikev2
draft-ietf-ipsecme-qr-ikev2
Internet Engineering Task Force S. Fluhrer
Internet-Draft P. Kampanakis
Intended status: Standards Track D. McGrew
Expires: July 17, 2020 Cisco Systems
V. Smyslov
ELVIS-PLUS
January 14, 2020
Mixing Preshared Keys in IKEv2 for Post-quantum Security
draft-ietf-ipsecme-qr-ikev2-11
Abstract
The possibility of quantum computers poses a serious challenge to
cryptographic algorithms deployed widely today. IKEv2 is one example
of a cryptosystem that could be broken; someone storing VPN
communications today could decrypt them at a later time when a
quantum computer is available. It is anticipated that IKEv2 will be
extended to support quantum-secure key exchange algorithms; however
that is not likely to happen in the near term. To address this
problem before then, this document describes an extension of IKEv2 to
allow it to be resistant to a quantum computer, by using preshared
keys.
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
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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 July 17, 2020.
Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include 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
1.1. Changes . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Requirements Language . . . . . . . . . . . . . . . . . . 6
2. Assumptions . . . . . . . . . . . . . . . . . . . . . . . . . 6
3. Exchanges . . . . . . . . . . . . . . . . . . . . . . . . . . 6
4. Upgrade procedure . . . . . . . . . . . . . . . . . . . . . . 11
5. PPK . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
5.1. PPK_ID format . . . . . . . . . . . . . . . . . . . . . . 12
5.2. Operational Considerations . . . . . . . . . . . . . . . 13
5.2.1. PPK Distribution . . . . . . . . . . . . . . . . . . 13
5.2.2. Group PPK . . . . . . . . . . . . . . . . . . . . . . 13
5.2.3. PPK-only Authentication . . . . . . . . . . . . . . . 14
6. Security Considerations . . . . . . . . . . . . . . . . . . . 14
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 17
8.1. Normative References . . . . . . . . . . . . . . . . . . 17
8.2. Informational References . . . . . . . . . . . . . . . . 18
Appendix A. Discussion and Rationale . . . . . . . . . . . . . . 19
Appendix B. Acknowledgements . . . . . . . . . . . . . . . . . . 20
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 20
1. Introduction
Recent achievements in developing quantum computers demonstrate that
it is probably feasible to build a cryptographically significant one.
If such a computer is implemented, many of the cryptographic
algorithms and protocols currently in use would be insecure. A
quantum computer would be able to solve DH and ECDH problems in
polynomial time [I-D.hoffman-c2pq], and this would imply that the
security of existing IKEv2 [RFC7296] systems would be compromised.
IKEv1 [RFC2409], when used with strong preshared keys, is not
vulnerable to quantum attacks, because those keys are one of the
inputs to the key derivation function. If the preshared key has
sufficient entropy and the PRF, encryption and authentication
transforms are quantum-secure, then the resulting system is believed
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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
property; assuming that the two end systems share a long secret key,
then the resulting exchange is quantum-secure. By bringing post-
quantum security to IKEv2, this document removes the need to use an
obsolete version of the Internet Key Exchange in order to achieve
that security goal.
The general idea is that we add an additional secret that is shared
between the initiator and the responder; this secret is in addition
to the authentication method that is already provided within IKEv2.
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
secret provides quantum resistance to the IPsec SAs (and any child
IKE SAs). We also stir the secret into the SK_pi, SK_pr values; this
allows both sides to detect a secret mismatch cleanly.
It was considered important to minimize the changes to IKEv2. The
existing mechanisms to do authentication and key exchange remain in
place (that is, we continue to do (EC)DH, and potentially PKI
authentication if configured). This document does not replace the
authentication checks that the protocol does; instead, they are
strengthened by using an additional secret key.
1.1. Changes
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
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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.
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draft-ietf-ipsecme-qr-ikev2-00
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
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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",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2. Assumptions
We assume that each IKE peer has a list of Post-quantum Preshared
Keys (PPK) along with their identifiers (PPK_ID), and any potential
IKE initiator selects which PPK to use with any specific responder.
In addition, implementations have a configurable flag that determines
whether this post-quantum preshared key is mandatory. This PPK is
independent of the preshared key (if any) that the IKEv2 protocol
uses to perform authentication (because the preshared key in IKEv2 is
not used for any key derivation, and thus doesn't protect against
quantum computers). The PPK specific configuration that is assumed
to be on each node consists of the following tuple:
Peer, PPK, PPK_ID, mandatory_or_not
3. Exchanges
If the initiator is configured to use a post-quantum preshared key
with the responder (whether or not the use of the PPK is mandatory),
then it MUST include a notification USE_PPK in the IKE_SA_INIT
request message as follows:
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Initiator Responder
------------------------------------------------------------------
HDR, SAi1, KEi, Ni, N(USE_PPK) --->
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
with it.
If the initiator needs to resend this initial message with a COOKIE
notification, then the resend would include the USE_PPK notification
if the original message did (see Section 2.6 of [RFC7296]).
If the responder does not support this specification or does not have
any PPK configured, then it ignores the received notification (as
defined in [RFC7296] for unknown status notifications) and continues
with the IKEv2 protocol as normal. Otherwise the responder replies
with the IKE_SA_INIT message including a USE_PPK notification in the
response:
Initiator Responder
------------------------------------------------------------------
<--- HDR, SAr1, KEr, Nr, [CERTREQ,] N(USE_PPK)
When the initiator receives this reply, it checks whether the
responder included the USE_PPK notification. If the responder did
not and the flag mandatory_or_not indicates that using PPKs is
mandatory for communication with this responder, then the initiator
MUST abort the exchange. This situation may happen in case of
misconfiguration, when the initiator believes it has a mandatory-to-
use PPK for the responder, while the responder either doesn't support
PPKs at all or doesn't have any PPK configured for the initiator.
See Section 6 for discussion of the possible impacts of this
situation.
If the responder did not include the USE_PPK notification and using a
PPK for this particular responder is optional, then the initiator
continues with the IKEv2 protocol as normal, without using PPKs.
If the responder did include the USE_PPK notification, then the
initiator selects a PPK, along with its identifier PPK_ID. Then, it
computes this modification of the standard IKEv2 key derivation from
Section 2.14 of [RFC7296]:
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SKEYSEED = prf(Ni | Nr, g^ir)
{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_pi = prf+ (PPK, SK_pi')
SK_pr = prf+ (PPK, SK_pr')
That is, we use the standard IKEv2 key derivation process except that
the three resulting subkeys SK_d, SK_pi, SK_pr (marked with primes in
the formula above) are then run through the prf+ again, this time
using the PPK as the key. The result is the unprimed versions of
these keys which are then used as inputs to subsequent steps of the
IKEv2 exchange.
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
underlying PRF has output size different from its key size. Note,
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.
For such PRFs only the first iteration of prf+ is needed:
SK_d = prf (PPK, SK_d' | 0x01)
SK_pi = prf (PPK, SK_pi' | 0x01)
SK_pr = prf (PPK, SK_pr' | 0x01)
Note that the PPK is used in SK_d, SK_pi and SK_pr calculation only
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
similar operation.
The initiator then sends the IKE_AUTH request message, including the
PPK_ID value as follows:
Initiator Responder
------------------------------------------------------------------
HDR, SK {IDi, [CERT,] [CERTREQ,]
[IDr,] AUTH, SAi2,
TSi, TSr, N(PPK_IDENTITY, PPK_ID), [N(NO_PPK_AUTH)]} --->
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
identifier PPK_ID.
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
the responder cannot continue with PPK (in particular, it cannot
authenticate the initiator), but the responder could be able to
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continue with normal IKEv2 protocol if the initiator provided its
authentication data computed as in normal IKEv2, without using PPKs.
For this purpose, if using PPKs for communication with this responder
is optional for the initiator (based on the mandatory_or_not flag),
then the initiator MUST include a NO_PPK_AUTH notification in the
above message. This notification informs the responder that PPK is
optional and allows for authenticating the initiator without using
PPK.
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
the initiator's authentication data computed using SK_pi', which has
been computed without using PPKs. This is the same data that would
normally be placed in the Authentication Data field of an AUTH
payload. Since the Auth Method field is not present in the
notification, the authentication method used for computing the
authentication data MUST be the same as method indicated in the AUTH
payload. Note that if the initiator decides to include the
NO_PPK_AUTH notification, the initiator needs to perform
authentication data computation twice, which may consume computation
power (e.g., if digital signatures are involved).
When the responder receives this encrypted exchange, it first
computes the values:
SKEYSEED = prf(Ni | Nr, g^ir)
{SK_d' | SK_ai | SK_ar | SK_ei | SK_er | SK_pi' | SK_pr' }
= prf+ (SKEYSEED, Ni | Nr | SPIi | SPIr )
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
to the PPK_IDENTITY notification. If no PPK_IDENTITY notification is
found and the peers successfully exchanged USE_PPK notifications in
the IKE_SA_INIT exchange, then the responder MUST send back
AUTHENTICATION_FAILED notification and then fail the negotiation.
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
IDi payload, then the responder checks whether using PPKs for this
initiator is mandatory and whether the initiator included NO_PPK_AUTH
notification in the message. If using PPKs is mandatory or no
NO_PPK_AUTH notification is found, then then the responder MUST send
back AUTHENTICATION_FAILED notification and then fail the
negotiation. Otherwise (when PPK is optional and the initiator
included NO_PPK_AUTH notification) the responder MAY continue regular
IKEv2 protocol, except that it uses the data from the NO_PPK_AUTH
notification as the authentication data (which usually resides in the
AUTH payload), for the purpose of the initiator authentication.
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Note, that Authentication Method is still indicated in the AUTH
payload.
This table summarizes the above logic for the responder:
Received Received Configured PPK is
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
and computes the following values:
SK_d = prf+ (PPK, SK_d')
SK_pi = prf+ (PPK, SK_pi')
SK_pr = prf+ (PPK, SK_pr')
The responder then continues with the IKE_AUTH exchange (validating
the AUTH payload that the initiator included) as usual and sends back
a response, which includes the PPK_IDENTITY notification with no data
to indicate that the PPK is used in the exchange:
Initiator Responder
------------------------------------------------------------------
<-- HDR, SK {IDr, [CERT,]
AUTH, SAr2,
TSi, TSr, N(PPK_IDENTITY)}
When the initiator receives the response, then it checks for the
presence of the PPK_IDENTITY notification. If it receives one, it
marks the SA as using the configured PPK to generate SK_d, SK_pi,
SK_pr (as shown above); the content of the received PPK_IDENTITY (if
any) MUST be ignored. If the initiator does not receive the
PPK_IDENTITY, it MUST either fail the IKE SA negotiation sending the
AUTHENTICATION_FAILED notification in the Informational exchange (if
the PPK was configured as mandatory), or continue without using the
PPK (if the PPK was not configured as mandatory and the initiator
included the NO_PPK_AUTH notification in the request).
If EAP is used in the IKE_AUTH exchange, then the initiator doesn't
include AUTH payload in the first request message, however the
responder sends back AUTH payload in the first reply. The peers then
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exchange AUTH payloads after EAP is successfully completed. As a
result, the responder sends AUTH payload twice - in the first
IKE_AUTH reply message and in the last one, while the initiator sends
AUTH payload 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
EAP authentication case too, is that the initiator includes
PPK_IDENTITY (and optionally NO_PPK_AUTH) notification in the request
message containing AUTH payload. Therefore, in case of EAP the
responder always computes the AUTH payload in the first IKE_AUTH
reply message without using PPK (by means of SK_pr'), since PPK_ID is
not yet known to the responder. Once the IKE_AUTH request message
containing the PPK_IDENTITY notification is received, the responder
follows the rules described above for the non-EAP authentication
case.
Initiator Responder
----------------------------------------------------------------
HDR, SK {IDi, [CERTREQ,]
[IDr,] SAi2,
TSi, TSr} -->
<-- HDR, SK {IDr, [CERT,] AUTH,
EAP}
HDR, SK {EAP} -->
<-- HDR, SK {EAP (success)}
HDR, SK {AUTH,
N(PPK_IDENTITY, PPK_ID)
[, N(NO_PPK_AUTH)]} -->
<-- HDR, SK {AUTH, SAr2, TSi, TSr
[, N(PPK_IDENTITY)]}
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
has included NO_PPK_AUTH notification), and thus the responder's
PPK_IDENTITY notification is marked as optional. Also, note that the
IKE_SA_INIT exchange in case of PPK is as described above (including
exchange of the USE_PPK notifications), regardless whether EAP is
employed in the IKE_AUTH or not.
4. Upgrade procedure
This algorithm was designed so that someone can introduce PPKs into
an existing IKE network without causing network disruption.
In the initial phase of the network upgrade, the network
administrator would visit each IKE node, and configure:
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o The set of PPKs (and corresponding PPK_IDs) that this node would
need to know.
o For each peer that this node would initiate to, which PPK will be
used.
o That the use of PPK is currently not mandatory.
With this configuration, the node will continue to operate with nodes
that have not yet been upgraded. This is due to the USE_PPK
notification and the NO_PPK_AUTH notification; if the initiator has
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
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
both peers have been upgraded, but the responder isn't yet configured
with the PPK for the initiator, then the responder could do standard
IKEv2 protocol if the initiator sent NO_PPK_AUTH notification. If
both the responder and initiator have been upgraded and properly
configured, they will both realize it, and the Child SAs will be
quantum-secure.
As an optional second step, after all nodes have been upgraded, then
the administrator should then go back through the nodes, and mark the
use of PPK as mandatory. This will not affect the strength against 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
(EC)DH in real-time) would not be able to perform a downgrade attack.
5. PPK
5.1. PPK_ID format
This standard requires that both the initiator and the responder have
a secret PPK value, with the responder selecting the PPK based on the
PPK_ID that the initiator sends. In this standard, both the
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
that later specifications will extend this technique to allow
dynamically changing PPK values. To facilitate such an extension, we
specify that the PPK_ID the initiator sends will have its first octet
be the PPK_ID Type value. This document defines two values for
PPK_ID Type:
o 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
to be mutually intelligible by both by initiator and the
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responder. This PPK_ID type is intended for those implementations
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 are fixed octet strings; the remaining bytes of the PPK_ID are
a configured value. We assume that there is a fixed mapping
between PPK_ID and PPK, which is configured locally to both the
initiator and the responder. The responder can use the PPK_ID to
look up the corresponding PPK value. Not all implementations are
able to configure arbitrary octet strings; to improve the
potential interoperability, it is recommended that, in the
PPK_ID_FIXED case, both the PPK and the PPK_ID strings be limited
to the Base64 character set [RFC4648].
5.2. Operational Considerations
The need to maintain several independent sets of security credentials
can significantly complicate a security administrator's job, and can
potentially slow down widespread adoption of this specification. It
is anticipated, that administrators will try to simplify their job by
decreasing the number of credentials they need to maintain. This
section describes some of the considerations for PPK management.
5.2.1. PPK Distribution
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
fashion. While the method of distribution is a local matter and out
of scope of this document or IKEv2, [RFC6030] describes a format for
for the transport and provisioning of symmetric keys. That format
could be reused using the PIN profile (defined in Section 10.2 of
[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
<Secret> element containing the PPK.
5.2.2. Group PPK
This document doesn't explicitly require that PPK is unique for each
pair of peers. If it is the case, then this solution provides full
peer authentication, but it also means that each host must have as
many independent PPKs as the peers it is going to communicate with.
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
peer uses classical public key cryptography in addition to PPK for
key exchange and authentication, members of the group can neither
impersonate each other nor read other's traffic, unless they use
quantum computers to break public key operations. However group
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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
quantum computer.
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
equipped with a quantum computer gets access to a group PPK, all
communications inside the group are revealed.
For these reasons using group PPK is NOT RECOMMENDED.
5.2.3. PPK-only Authentication
If quantum computers become a reality, classical public key
cryptography will provide little security, so administrators may find
it attractive not to use it at all for authentication. This will
reduce the number of credentials they need to maintain to PPKs only.
Combining group PPK and PPK-only authentication is NOT RECOMMENDED,
since in this case any member of the group can impersonate any other
member even without help of quantum computers.
PPK-only authentication can be achieved in IKEv2 if the NULL
Authentication method [RFC7619] is employed. Without PPK the NULL
Authentication method provides no authentication of the peers,
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
mandatory for the peers if they advertise support for PPK in
IKE_SA_INIT and use NULL Authentication. Additionally, since the
peers are authenticated via PPK, the ID Type in the IDi/IDr payloads
SHOULD NOT be ID_NULL, despite using the NULL Authentication method.
6. Security Considerations
Quantum computers are able to perform Grover's algorithm [GROVER];
that effectively halves the size of a symmetric key. Because of
this, the user SHOULD ensure that the post-quantum preshared key used
has at least 256 bits of entropy, in order to provide 128 bits of
post-quantum security. That provides security equivalent to Level 5
as defined in the NIST PQ Project Call For Proposals [NISTPQCFP].
With this protocol, the computed SK_d is a function of the PPK.
Assuming that the PPK has sufficient entropy (for example, at least
2^256 possible values), then even if an attacker was able to recover
the rest of the inputs to the PRF function, it would be infeasible to
use Grover's algorithm with a quantum computer to recover the SK_d
value. Similarly, all keys that are a function of SK_d, which
include all Child SAs keys and all keys for subsequent IKE SAs
(created when the initial IKE SA is rekeyed), are also quantum-secure
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(assuming that the PPK was of high enough entropy, and that all the
subkeys are sufficiently long).
An attacker with a quantum computer that can decrypt the initial IKE
SA has access to all the information exchanged over it, such as
identities of the peers, configuration parameters and all negotiated
IPsec SAs information (including traffic selectors), with the
exception of the cryptographic keys used by the IPsec SAs which are
protected by the PPK.
Deployments that treat this information as sensitive or that send
other sensitive data (like cryptographic keys) over IKE SA MUST rekey
the IKE SA before the sensitive information is sent to ensure this
information is protected by the PPK. It is possible to create a
childless IKE SA as specified in [RFC6023]. This prevents Child SA
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 sent in the IKE_AUTH exchange, such as peer identities,
feature notifications, Vendor ID's etc. cannot be hidden from the
attack described above, even if the additional IKE SA rekey is
performed.
In addition, the policy SHOULD be set to negotiate only quantum-
secure symmetric algorithms; while this RFC doesn't claim to give
advice as to what algorithms are secure (as that may change based on
future cryptographical results), below is a list of defined IKEv2 and
IPsec algorithms that should not be used, as they are known to
provide less than 128 bits of post-quantum security
o Any IKEv2 Encryption algorithm, PRF or Integrity algorithm with
key size less than 256 bits.
o Any ESP Transform with key size less than 256 bits.
o PRF_AES128_XCBC and PRF_AES128_CBC; even though they are defined
to be able to use an arbitrary key size, they convert it into a
128-bit key internally.
Section 3 requires the initiator to abort the initial exchange if
using PPKs is mandatory for it, but the responder does not include
the USE_PPK notification in the response. In this situation, when
the initiator aborts negotiation it leaves a half-open IKE SA on the
responder (because IKE_SA_INIT completes successfully from the
responder's point of view). This half-open SA will eventually expire
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
a Denial-of-Service (DoS) attack and take protection measures (see
[RFC8019] for more detail). In this situation, it is RECOMMENDED
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that the initiator caches the negative result of the negotiation and
doesn't make attempts to create it again for some time. This period
of time may vary, but it is believed that waiting for at least few
minutes will not cause the responder to treat it as DoS attack.
Note, that this situation would most likely be a result of
misconfiguration and some re-configuration of the peers would
probably be needed.
If using PPKs is optional for both peers and they authenticate
themselves using digital signatures, then an attacker in between,
equipped with a quantum computer capable of breaking public key
operations in real time, is able to mount downgrade attack by
removing USE_PPK notification from the IKE_SA_INIT and forging
digital signatures in the subsequent exchange. If using PPKs is
mandatory for at least one of the peers or PSK is used for
authentication, then the attack will be detected and the SA won't be
created.
If using PPKs is mandatory for the initiator, then an attacker able
to eavesdrop and to inject packets into the network can prevent
creating an IKE SA by mounting the following attack. The attacker
intercepts the initial request containing the USE_PPK notification
and injects a forged response containing no USE_PPK. If the attacker
manages to inject this packet before the responder sends a genuine
response, then the initiator would abort the exchange. To thwart
this kind of attack it is RECOMMENDED, that if using PPKs is
mandatory for the initiator and the received response doesn't contain
the USE_PPK notification, then the initiator doesn't abort the
exchange immediately. Instead it waits for more response messages
retransmitting the request as if no responses were received at all,
until either the received message contains the USE_PPK or the
exchange times out (see section 2.4 of [RFC7296] for more details
about retransmission timers in IKEv2). If neither of the received
responses contains USE_PPK, then the exchange is aborted.
If using PPK is optional for both peers, then in case of
misconfiguration (e.g., mismatched PPK_ID) the IKE SA will be created
without protection against quantum computers. It is advised that if
PPK was configured, but was not used for a particular IKE SA, then
implementations SHOULD audit this event.
7. IANA Considerations
This document defines three new Notify Message Types in the "Notify
Message Types - Status Types" registry
(https://www.iana.org/assignments/ikev2-parameters/
ikev2-parameters.xhtml#ikev2-parameters-16):
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16435 USE_PPK [THIS RFC]
16436 PPK_IDENTITY [THIS RFC]
16437 NO_PPK_AUTH [THIS RFC]
This document also creates a new IANA registry "IKEv2 Post-quantum
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
----------- ----- ---------
Reserved 0 [THIS RFC]
PPK_ID_OPAQUE 1 [THIS RFC]
PPK_ID_FIXED 2 [THIS RFC]
Unassigned 3-127 [THIS RFC]
Private Use 128-255 [THIS RFC]
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
parties. To register new PPK_IDs in the unassigned range, a Type
name, a Value between 3 and 127 and a Reference specification need to
be defined. Changes and additions to the unassigned range of this
registry are by the Expert Review Policy [RFC8126]. Changes and
additions to the private use range of this registry are by the
Private Use Policy [RFC8126].
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>.
[RFC7296] Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T.
Kivinen, "Internet Key Exchange Protocol Version 2
(IKEv2)", STD 79, RFC 7296, DOI 10.17487/RFC7296, October
2014, <https://www.rfc-editor.org/info/rfc7296>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
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8.2. Informational References
[GROVER] Grover, L., "A Fast Quantum Mechanical Algorithm for
Database Search", Proc. of the Twenty-Eighth Annual ACM
Symposium on the Theory of Computing (STOC 1996), 1996.
[I-D.hoffman-c2pq]
Hoffman, P., "The Transition from Classical to Post-
Quantum Cryptography", draft-hoffman-c2pq-06 (work in
progress), November 2019.
[IKEV2-IANA-PRFS]
"Internet Key Exchange Version 2 (IKEv2) Parameters,
Transform Type 2 - Pseudorandom Function Transform IDs",
<https://www.iana.org/assignments/ikev2-parameters/
ikev2-parameters.xhtml#ikev2-parameters-6>.
[NISTPQCFP]
NIST, "NIST Post-Quantum Cryptography Call for Proposals",
2016, <https://csrc.nist.gov/CSRC/media/Projects/Post-
Quantum-Cryptography/documents/call-for-proposals-final-
dec-2016.pdf>.
[RFC2409] Harkins, D. and D. Carrel, "The Internet Key Exchange
(IKE)", RFC 2409, DOI 10.17487/RFC2409, November 1998,
<https://www.rfc-editor.org/info/rfc2409>.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
<https://www.rfc-editor.org/info/rfc4648>.
[RFC6023] Nir, Y., Tschofenig, H., Deng, H., and R. Singh, "A
Childless Initiation of the Internet Key Exchange Version
2 (IKEv2) Security Association (SA)", RFC 6023,
DOI 10.17487/RFC6023, October 2010,
<https://www.rfc-editor.org/info/rfc6023>.
[RFC6030] Hoyer, P., Pei, M., and S. Machani, "Portable Symmetric
Key Container (PSKC)", RFC 6030, DOI 10.17487/RFC6030,
October 2010, <https://www.rfc-editor.org/info/rfc6030>.
[RFC7619] Smyslov, V. and P. Wouters, "The NULL Authentication
Method in the Internet Key Exchange Protocol Version 2
(IKEv2)", RFC 7619, DOI 10.17487/RFC7619, August 2015,
<https://www.rfc-editor.org/info/rfc7619>.
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[RFC8019] Nir, Y. and V. Smyslov, "Protecting Internet Key Exchange
Protocol Version 2 (IKEv2) Implementations from
Distributed Denial-of-Service Attacks", RFC 8019,
DOI 10.17487/RFC8019, November 2016,
<https://www.rfc-editor.org/info/rfc8019>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.
Appendix A. Discussion and Rationale
The idea behind this document is that while a quantum computer can
easily reconstruct the shared secret of an (EC)DH exchange, they
cannot as easily recover a secret from a symmetric exchange. This
document makes the SK_d, and hence the IPsec KEYMAT and any child
SA's SKEYSEED, depend on both the symmetric PPK, and also the Diffie-
Hellman exchange. If we assume that the attacker knows everything
except the PPK during the key exchange, and there are 2^n plausible
PPKs, then a quantum computer (using Grover's algorithm) would take
O(2^(n/2)) time to recover the PPK. So, even if the (EC)DH can be
trivially solved, the attacker still can't recover any key material
(except for the SK_ei, SK_er, SK_ai and SK_ar values for the initial
IKE exchange) unless they can find the PPK, which is too difficult if
the PPK has enough entropy (for 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
within the IKEv2 protocol, and in particular, within the cryptography
of IKEv2. By limiting our changes to notifications, and only
adjusting the SK_d, SK_pi, SK_pr, it is hoped that this would be
implementable, even on systems that perform most of the IKEv2
processing in hardware.
A third goal was to be friendly to incremental deployment in
operational networks, for which we might not want to have a global
shared key, or quantum-secure IKEv2 is rolled out incrementally.
This is why we specifically try to allow the PPK to 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
provided by IKEv2.
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Appendix B. Acknowledgements
We would like to thank Tero Kivinen, Paul Wouters, Graham Bartlett,
Tommy Pauly, Quynh Dang and the rest of the IPSecME Working Group for
their feedback and suggestions for the scheme.
Authors' Addresses
Scott Fluhrer
Cisco Systems
Email: sfluhrer@cisco.com
Panos Kampanakis
Cisco Systems
Email: pkampana@cisco.com
David McGrew
Cisco Systems
Email: mcgrew@cisco.com
Valery Smyslov
ELVIS-PLUS
Phone: +7 495 276 0211
Email: svan@elvis.ru
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