Internet Engineering Task Force S. Fluhrer
Internet-Draft D. McGrew
Intended status: Informational P. Kampanakis
Expires: May 1, 2017 Cisco Systems
October 28, 2016
Postquantum Preshared Keys for IKEv2
draft-fluhrer-qr-ikev2-03
Abstract
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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Changes . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Assumptions . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Exchanges . . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Creating Child SA Keying Material . . . . . . . . . . . . . . 5
5. Security Considerations . . . . . . . . . . . . . . . . . . . 6
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
6.1. Normative References . . . . . . . . . . . . . . . . . . 7
6.2. Informational References . . . . . . . . . . . . . . . . 7
Appendix A. Discussion and Rationale . . . . . . . . . . . . . . 7
Appendix B. Acknowledgement . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
It is an open question whether or not it is feasible to build a
quantum computer, but if it is, 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, and this would imply
that the security of existing IKEv2 systems would be compromised.
IKEv1 when used with preshared keys does not share this
vulnerability, because those keys are one of the inputs to the key
derivation function. If the preshared key have sufficient entropy
and the PRF and encryption and authentication transforms are
postquantum secure, then the resulting system is believed to be
quantum resistant, that is, believed to be invulnerable to an
attacker 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 resistant. By bringing
postquantum security to IKEv2, this note 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 in this secret when generating the key material (KEYMAT) keys
for the child SAs (along with the parameters that IKEv2 normally
uses); this secret provides quantum resistance to the IPsec SAs.
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 a PKI
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authentication if configured). This does not replace the
authentication checks that the protocol does; instead, it is done as
a parallel check.
1.1. Changes
Changes in this draft from the previous versions
draft-02
- 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 negotation.
- Removed positive endorsements of various algorithms. Retained
warnings about algorithms known to be weak against a Quantum Computer
draft-01
- Added explicit guidance as to what IKE and IPsec algorithms are
Quantum Resistant
draft-00
- We switched from using vendor ID's to transmit the additional data
to notifications
- We added a mandatory cookie exchange to allow the server to
communicate to the client before the initial exchange
- We added algorithm agility by having the server tell the client
what algorithm to use in the cookie exchange
- 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.
- 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", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
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2. Assumptions
We assume that each IKE peer (both the initiator and the responder)
has an optional Postquantum Preshared Key (PPK) (potentially on a
per-peer basis, selected by peer identity), and also has a
configurable flag that determines whether this postquantum preshared
key is mandatory. This preshared key is independent of the preshared
key (if any that the IKEv2 protocol uses to perform authentication.
3. Exchanges
If the initiator has a configured postquantum preshared key (whether
or not it is optional), then it will include a notify payload in its
initial encrypted exchange as follows:
Initiator Responder
------------------------------------------------------------------
HDR, SK {IDi, [CERT,] [CERTREQ,]
[IDr,] AUTH, SAi2,
TS, TSr, N(PPK_NOTIFY)} --->
N(PPK_NOTIFY) is a status notification payload with the type [TBA];
it has a protocol ID of 0, and no SPI and no notification data
associated with it.
When the responder receives the initial encrypted exchange, it checks
to see if it received a notify within that exchange, is configured to
support PPK with the initiator's identity, and whether that use is
mandatory. If the notify was received, and the responder does have a
PPK for that identity, then it responds with the standard IKE
response with the PPK_NOTIFY notify message included, namely:
Initiator Responder
------------------------------------------------------------------
<--- HDR, SK {IDr, [CERT,] AUTH,
SAr2, TSi, TSr, N(PPK_NOTIFY)}
If the responder is not configured to support PPK with that identity,
it continues with the standard IKE protocol, not including the
notification.
If the responder is configured to support PPK with that identity, and
it does not receive the notification, then if the PPK usage is
configured as mandatory, it MUST abort the exchange. If the PPK
usage is configured as optional, it continues with the standard IKE
protocol, not including the notification.
This table summarizes the above logic by the responder
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Received Nonce Have PPK PPK Mandatory Action
------------------------------------------------------------------
No No * Standard IKE protocol
No Yes No Standard IKE protocol
No Yes Yes Abort negotiation
Yes No * Standard IKE protocol
Yes Yes * Include PPK_NOTIFY Nonce
When the initiator receives the response, then (if it is configured
to use a PPK with the responder), then it checks for the presense of
the notification. If it receives one, it marks the SA as using the
configured PPK; if it does not receive one, it MUST either abort the
exchange (if the PPK was configured as mandatory), or it MUST
continue without using the PPK (if the PPK was configured as
optional).
The protocol continues as standard until it comes time to compute the
child SA keying material.
4. Creating Child SA Keying Material
When it comes time to generate the keying material for a child SA,
the implementation (both the initiator and the responder) checks to
see if they agreed to use a PPK. If they did, then they look up
(based on the peer's identity) the configured PPK, and then both
sides use one of these alternative formula (based on whether an
optional Diffie-Hellman was included):
Ni' = prf(PPK, Ni)
Nr' = prf(PPK, Nr)
KEYMAT = prf+(SK_d, Ni' | Nr')
or
Ni' = prf(PPK, Ni)
Nr' = prf(PPK, Nr)
KEYMAT = prf+(SK_d, g^ir (new) | Ni' | Nr')
where PPK is the configured postquantum preshared key, Ni, Nr are the
nonces from the IKE_SA_INIT exchange if this require is the first
Child SA created or the fresh Ni and Nr from the CREATE_CHILD_SA
exchange if this is a subsequent creation, and prf is the
pseudorandom function that was negotiated for this SA.
This is the standard IKE KEYMAT generation, except that the nonces
are transformed (via the negotiated PRF function) using the preshared
PPK value
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We use this negotiated PRF, rather than negotiating a separate one,
because this PRF is agreed by both sides to have sufficient security
properties (otherwise, they would have negotiated something else),
and so that we don't need to specify a separate negotiation
procedure.
When you rekey an IKE SA (generating a fresh SKEYSEED), the initiator
and the responder will transform the nonces using the same PPK as
they used during the original IKE SA negotiation. That is, they will
use the alternate derivation:
Ni' = prf(PPK, Ni)
Nr' = prf(PPK, Nr)
SKEYSEED = prf( SK_d (old), g^ir (new) | Ni' | Nr' )
(SK_d | SK_ai | SK_ar | SK_ei | SK_er | SK_pi | SK_pr) =
prf+(SKEYSEED, Ni' | Nr' | SPIi | SPIr)
An implementation MAY rekey the initial IKE SA immediately after
negotiating it; this would reduce the amount of data available to an
attacker with a Quantum Computer
5. Security Considerations
Quantum computers are able to perform Grover's algorithm; that
effectively halves the size of a symmetric key. Because of this, the
user SHOULD ensure that the postquantum preshared key used has at
least 256 bits of entropy, in order to provide a 128 bit security
level.
Although this protocol preserves all the security properties of IKE
against adversaries with conventional computers, this protocol allows
an adversary with a Quantum Computer to decrypt all traffic encrypted
with the initial IKE SA. In particular, it allows the adversary to
recover the identities of both sides. If there is IKE traffic other
than the identities that need to be protected against such an
adversary, one suggestion would be to form an initial IKE SA (which
is used to exchange identities), perhaps by using the protocol
documented in RFC6023. Then, you would immediately create a child
IKE SA (which is used to exchange everything else). Because the
child IKE SA keys are a function of the PPK (among other things),
traffic protected by that SA is secure against Quantum capable
adversaries.
In addition, the policy SHOULD be set to negotiate only quantum-
resistant symmetric algorithms; while this RFC doesn't claim to give
advise as to what algorithms are secure (as that may change based on
future cryptographical results), here is a list of defined IKEv2 and
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IPsec algorithms that should NOT be used, as they are known not to be
Quantum Resistant
Any IKE Encryption algorithm, PRF or Integrity algorithm with key
size <256 bits
Any ESP Transform with key size <256 bits
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
6. References
6.1. Normative References
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104,
DOI 10.17487/RFC2104, February 1997,
<http://www.rfc-editor.org/info/rfc2104>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://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, <http://www.rfc-editor.org/info/rfc7296>.
6.2. Informational References
[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,
<http://www.rfc-editor.org/info/rfc6023>.
[SPDP] McGrew, D., "A Secure Peer Discovery Protocol (SPDP)",
2001, <http://www.mindspring.com/~dmcgrew/spdp.txt>.
Appendix A. Discussion and Rationale
The idea behind this 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 makes the
IPsec KEYMAT and any child SA's SKEYSEED depend on both the symmetric
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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 PPK's, 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 values for the
initial IKE exchange) unless they can find the PPK, and that's too
difficult if the PPK has enough entropy (say, 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 translating
the nonces, it is hoped that this would be implementable, even on
systems that perform much of the IKEv2 processing is 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, and also if we're rolling this 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 goals of
IKEv2.
The third and fourth goals are in partial conflict. In order to
achieve postquantum security, we need to stir in the PPK when the
keys are computed, however the keys are computed before we know who
we're talking to (and so which PPK we should use). And, we can't
just tell the other side which PPK to use, as we might use different
PPK's for different peers, and so that would violate the anonymity
goal. If we just (for example) included a hash of the PPK, someone
listening in could easily tell when we're using the same PPK for
different exchanges, and thus deduce that the systems are related.
The compromise we selected was to stir in the PPK in all the derived
keys except the initial IKE SA keys, While this allows an attacker
with a Quantum Computer to recover the identities, a poll on the
IPsecME mailing list indicated that the majority of the people on the
list did not think anonymity was an important property within IKE.
We stir in the shared secret within the Child SA keying material;
this allows an implementation that wants to protect the other IKE-
based traffic to create an initial IKE SA to exchange identities, and
then immediately create a Child SA, and use that Child SA to exchange
the rest of the negotiation.
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In addition, when we stir in the PPK, we always use it to modify a
nonce (using the negotiated PRF). We modify the nonce (rather than,
say, including the PPK in with the prf or prf+ computation directly)
so that this would be easier to implement on an hardware-based IKE
implementation; the prf computations might be built-in, but the
nonces would be external inputs, and so modifying those would
minimize the changes.
Appendix B. Acknowledgement
The idea of stirring in the PPK into the IPsec key generation process
was originally suggested on the list by Tero Kivinen.
Authors' Addresses
Scott Fluhrer
Cisco Systems
Email: sfluhrer@cisco.com
David McGrew
Cisco Systems
Email: mcgrew@cisco.com
Panos Kampanakis
Cisco Systems
Email: pkampana@cisco.com
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