Internet Engineering Task Force S. Fluhrer
Internet-Draft D. McGrew
Intended status: Informational P. Kampanakis
Expires: October 21, 2017 Cisco Systems
April 19, 2017
Postquantum Preshared Keys for IKEv2
draft-fluhrer-qr-ikev2-04
Abstract
The possibility of quantum computers pose a serious challenge to
cryptography algorithms 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
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on October 21, 2017.
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carefully, as they describe your rights and restrictions with respect
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Changes . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Requirements Language . . . . . . . . . . . . . . . . . . 4
2. Assumptions . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Exchanges . . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. PPK ID format . . . . . . . . . . . . . . . . . . . . . . . . 7
5. PPK Distribution . . . . . . . . . . . . . . . . . . . . . . 8
6. Upgrade procedure . . . . . . . . . . . . . . . . . . . . . . 8
7. Security Considerations . . . . . . . . . . . . . . . . . . . 8
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
8.1. Normative References . . . . . . . . . . . . . . . . . . 9
8.2. Informational References . . . . . . . . . . . . . . . . 10
Appendix A. Discussion and Rationale . . . . . . . . . . . . . . 10
Appendix B. Acknowledgement . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
It is an open question whether or not it is feasible to build a
quantum computer (and if so, when might one be implemented), 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
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 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
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to the authentication method that is already provided within IKEv2.
We stir in this secret into the SK_d value, which is used to generate
the key material (KEYMAT) keys and the SKEYSEED for the child SAs;
this secret provides quantum resistance to the IPsec SAs (and any
child IKE SAs). We also stir in 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 a PKI
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-03
- Modified how we stir the PPK into the IKEv2 secret state
- Modified how the use of PPKs is negotiated
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
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- 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].
2. Assumptions
We assume that each IKE peer has a list of Postquantum Preshared Keys
(PPK) along with their identifiers (PPK_id), and any potential IKE
initiator has a selection of which PPK to use with with any specific
responder. In addition, the implementation has a configurable flag
that determines whether this postquantum preshared key is mandatory.
This PPK is independent of the preshared key (if any) that the IKEv2
protocol uses to perform authentication.
3. Exchanges
If the initiator is configured to use a postquantum preshared key
with the responder (whether or not the use of the PPK is optional),
then it will include a notify payload in the initial exchange as
follows:
Initiator Responder
------------------------------------------------------------------
HDR, SAi1, KEi, Ni, N(PPK_SUPPORT) --->
N(PPK_SUPPORT) 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.
If the initiator needs to resend this initial message with a cookie
(because the responder response included a cookie notification), then
the resend would include the PPK_SUPPORT notification if the original
message did.
When the responder receives this initial exchange with the notify,
then it MUST check if has a PPK configured. If it does, it MUST
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reply with the IKE initial exchange including a notification in
response.
Initiator Responder
------------------------------------------------------------------
<--- HDR, SAr1, KEr, Nr, [CERTREQ], N(PPK_SUPPORT)
If the responder does not have a PPK configured, then it continues
with the IKE protocol as normal, not including the notify.
When the initiator receives this reply, it checks whether the
responder included the PPK_SUPPORT notify. If the responder did not,
then the initiator MUST either proceed with the standard IKE
negotiation (without using a PPK), or abort the exchange (for
example, because the initiator has the PPK marked as mandatory). If
the responder did include the PPK_SUPPORT notify, then it selects a
PPK, along with its identifier PPK_id. Then, it computes this
modification of the standard IKE key derivation:
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 IKE key derivation process except that
the three subkeys SK_d, SK_pi, SK_pr are run through the prf again,
this time using the PPK as the key.
The initiator then sends the initial encrypted 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(PPK_IDENITY) is a status notification payload with the type [TBA];
it has a protocol ID of 0, and no SPI and has a notification data
that consists of the identifier PPK_id.
When the responder receives this encrypted exchange, it first
computes the values:
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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 )
It then uses the SK_ei value to decrypt the message; and then finds
the PPK_id value attached to the notify. It then scans through the
payload for the PPK_id attached to the N(PPK_IDENTITY); if it has no
such PPK, it fails the negotiation. If it does have a PPK with that
identity, it further computes:
SK_d = prf(PPK, SK_d')
SK_pi = prf(PPK, SK_pi')
SK_pr = prf(PPK, SK_pr')
And computes the enchange (validating the AUTH payload that the
initiator included) as standard.
This table summarizes the above logic by the responder
Received PPK_SUPPORT 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_SUPPORT
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 to generate SK_d, SK_pi, SK_pr (as shown above); 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).
If the initial exchange had PPK_SUPPORT sent by both the initiator
and the responder, and the initiator does not include a PPK_NOTIFY
notification, then the responder SHOULD fail the exchange.
With this protocol, the computed SK_d is a function of the PPK, and
assuming that the PPK has sufficient entropy (for example, at least
2**256 possible values), then even if an attacker were 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, every child SA key is a function
of SK_d, hence all the keys for all the child SAs are also quantum
resistant (assuming that the PPK was high entropy and secret, and
that all the subkeys are sufficiently long). However, this quantum
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resistance does not extend to the initial SK_ei, SK_er keys; 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.
4. 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 initial standard, both the
initator and the responder are configured with fixed PPK and PPK_ID
values, and do the look up based on that. It is anticipated that
later standards will extend this technique to allow dynamically
changing PPK values. To facilitate such an extension, we specify
that the PPK_ID that the initiator sends will have its first octet be
the PPK ID Type value, which is encoded as follows:
PPK ID Type Value
PPK_ID_OPAQUE 0
PPK_ID_FIXED 1
RESERVED TO IANA 2-127
Reserved for private use 128-255
For PPK_ID_OPAQUE, 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 responder. This PPK ID
type is intended for those implementations that choose not to
disclose the type of PPK to active attackers.
For PPK_ID_FIXED, 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 to do a look up the passed PPK_id
value to determine 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, namely the 64 characters 0-9, A-Z, a-z,
+ and /.
The PPK ID type values 2-127 are reserved for IANA; values 128-255
are for private use among mutually consenting parties.
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5. PPK Distribution
PPK_id's 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, one
suggestion would be to reuse the format within [RFC6030], with the
Key Id field being the PPK_ID (without the 0x01 prefix for a
PPK_ID_FIXED), and with the PPK being the secret, and the algorithm
as PIN ("Algorithm=urn:ietf:params:xml:ns:keyprov:pskc:pin").
6. 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:
- The set of PPKs (and corresponding PPK_id's) that this node would
need to know
- For each peer that this node would initiate to, which PPK that we
would use
- That the use of PPK is currently optional
With this configuration, the node will continue to operate with nodes
that have not yet been upgraded. This is due to the PPK_SUPPORT
notify; if the initiator has not been upgraded, it will not send the
PPK_SUPPORT notify (and so the responder will know that we will not
use a PPK); if the responder has not been upgraded, it will not send
the PPK_SUPPORT notify (and so the initiator will know not to use a
PPK). And, if both peers have been upgraded, they will both realize
it, and in that case, the link will be quantum secure
As an optional second step, after all nodes have been upgraded, then
the administrator may then go back through the nodes, and mark the
use of PPK as mandatory. This will not affect the strength against a
passive attacker; it would mean that an 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).
7. 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
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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 SK_d, which is 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
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
8. References
8.1. 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>.
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[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>.
8.2. 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>.
[RFC6030] Hoyer, P., Pei, M., and S. Machani, "Portable Symmetric
Key Container (PSKC)", RFC 6030, DOI 10.17487/RFC6030,
October 2010, <http://www.rfc-editor.org/info/rfc6030>.
[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
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 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_ei,
SK_er, SK_ai, SK_ar values for the initial IKE exchange) unless they
can find the PPK, and that's 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 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
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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.
Appendix B. Acknowledgement
We would like to thank Tero Kivine, Valery Smyslov, Paul Wouters 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
David McGrew
Cisco Systems
Email: mcgrew@cisco.com
Panos Kampanakis
Cisco Systems
Email: pkampana@cisco.com
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