Internet Engineering Task Force S. Fluhrer
Internet-Draft D. McGrew
Intended status: Informational P. Kampanakis
Expires: February 5, 2017 Cisco Systems
August 4, 2016
Postquantum Preshared Keys for IKEv2
draft-fluhrer-qr-ikev2-02
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 . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Exchanges . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Computing SKEYSEED . . . . . . . . . . . . . . . . . . . 6
3.2. Verifying preshared key . . . . . . . . . . . . . . . . . 7
3.3. Child SAs . . . . . . . . . . . . . . . . . . . . . . . . 7
4. Security Considerations . . . . . . . . . . . . . . . . . . . 7
5. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
5.1. Normative References . . . . . . . . . . . . . . . . . . 8
5.2. Informational References . . . . . . . . . . . . . . . . 9
Appendix A. Discussion and Rationale . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
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 IKE keys (along with the
parameters that IKEv2 normally uses); this secret adds quantum
resistance to the exchange.
It was considered important to minimize the changes to IKEv2. The
existing mechanisms to do authentication and key exchange remain in
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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-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].
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), 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.
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In addition, we assume that the initiator knows which PPK to use with
the peer it is initiating to (for instance, if it knows the peer,
then it can determine which PPK will be used).
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 exchange as follows:
Initiator Responder
------------------------------------------------------------------
HDR, SAi1, KEi, Ni, N(PPK_REQUEST) --->
N(PPK_REQUEST) 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 recieves the initial exchange with the notify
payload, then (if it is configured to support PPK), it responds with:
Initiator Responder
------------------------------------------------------------------
<--- HDR, N(COOKIE), N(PPK_ENCODE)
If it is not configured to support PPK, the responder continues with
the standard IKEv2 protocol.
In other words, it asks for the responder to generate and send a
cookie in its responses (as listed in section 2.6 of RFC7296), and in
addition, include a notify that gives details of how the initiator
should indicate what the PPK is. This notification payload has the
type [TBA}; it has a protocol ID of 0, and no SPI; the notification
data is of the format:
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PPK Indicator Algorithm |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PPK Indicator Input (variable) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The PPK Indicator Algorithm is a 4 byte word that states which PPK
indicator to use. That is, it gives the encoding format for the PPK
that should be used is given to the responder. At present, the only
assigned encoding is 0x00000001, which indicates that AES256_SHA256
will be used (as explained below).
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PPK Indicator Input is a data input to the PPK indicator Algorithm;
its length will depend on the PPK indicator; for the indicator
AES256_SHA256, this PPK Indicator Input is 16 bytes.
The contents of this PPK Indicator Input is selected by responder
policy; below we give trade-offs of the various possibilities
When the initiator receives this notification, it responds as
follows:
Initiator Responder
------------------------------------------------------------------
HDR, N(COOKIE), SAi1, KEi, Ni, N(PPK_REQUEST) --->
This is the standard IKEv2 cookie response, with a PPK_REQUEST
notification added
N(PPK_REQUEST) is a status notification payload with the type [TBA];
it has a protocol ID of 0, and no SPI; however this time, the
notification data as as follows:
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PPK Indicator Algorithm |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PPK Indicator Input (variable) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PPK Indicator (variable) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The PPK Indicator Algorithm and PPK Indicator Input are precisely the
same as was given in the PPK_ENCODE format (as is repeated in case
the responder ran this cookie protocol in a stateless manner). The
PPK Indicator is the encoded version of the PPK that the initiator
has. The idea behind this is to allow the responder to select which
PPK it should use when it derives the IKEv2 keys.
For the AES256_SHA256 PPK indicator, the PPK Indicator is 16 bytes.
To compute it, we use HMAC_SHA256(PPK, "A") as the 256 bit AES key to
encrypt the 16 bytes on PPK Indicator Input (in ECB mode), where "A"
is a string consisting of a single 0x41 octet.
When the responder receives this notification payload, it verifies
that the PPK Indicator Algorithm is as it has specified, and it MAY
verify that the PPK Indicator Input is as it has specified. If
everything is on the level, it scans through its list of configured
postquantum preshared keys, and determines which one it is (possibly
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(assuming AES256_SHA256_PPK) by computing AES256(HMAC_SHA256(PPK,
"A"), PPK_Indicator_Input) and comparing that value to the 16 bytes
within the payload. Alternatively, it may have preselected a PPK
Indicator Input, and has precomputed (again assuming
AES256_SHA256_PPK) AES256(HMAC_SHA256(PPK, "A"), PPK_Indicator_Input)
for each PPK it knows about (in which case, this is a simple search).
If the responder finds a value that matches the payload for a
particular PPK, that indicates that the intiator and responder share
a PPK and can make use of this extension. Upon finding such a
preshared key, the responder includes a notification payload with the
response:
Initiator Responder
------------------------------------------------------------------
<--- HDR, SAr1, Ker, Nr, [CERTREQ], N(PPK_ACK)
N(PPK_ACK) 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. This notification serves as a postquantum
preshared key confirmation.
If the responder does not find such a PPK, then it MAY continue with
the protocol without including a notification ID (if it is configured
to not have mandatory preshared keys), or it MAY abort the exchange
(if it configured to make preshared keys mandatory).
When the initiator receives the response, it MUST check for the
presence of the notification. If it receives one, it marks the SA as
using the configured preshared key; if it does not receive one, it
MAY either abort the exchange (if the preshared key was configured as
mandatory), or it MAY continue without using the preshared key (if
the preshared key was configured as optional).
3.1. Computing SKEYSEED
When it comes time to generate the keying material during the initial
Exchange, the implementation (both the initiator and the responder)
checks to see if there was an agreed-upon preshared key. If there
was, then both sides use this alternative formula:
SKEYSEED = prf(prf(PPK, Ni) | prf(PPK, Nr), g^ir)
(SK_d | SK_ai | SK_ar | SK_ei | SK_er | SK_pi | SK_pr) =
prf+(SKEYSEED, prf(PPK, Ni) | prf(PPK, Nr) |
SPIi | SPIr)
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where PPK is the postquantum preshared key, Ni, Nr are the nonces
exchanged in the IKEv2 exchange, and prf is the pseudorandom function
that was negotiated for this SA.
We reuse the negotiated PRF to transform the received nonces. We use
this 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.
3.2. Verifying preshared key
Once both the initiator and the responder have exchanged identities,
they both double-check with their policy database to verify that they
were configured to use those preshared keys when negotiating with the
peer. If they are not, they MUST abort the exchange.
3.3. Child SAs
When you create a child SA, 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 one of the
alternative derivations (depending on whether an optional Diffie-
Hellman was included):
KEYMAT = prf+(SK_d, prf(PPK, Ni) | prf(PPK, Nr))
or
KEYMAT = prf+(SK_d, g^ir (new) |
prf(PPK, Ni) | prf(PPK, Nr))
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:
SKEYSEED = prf( SK_d (old), g^ir (new) |
prf(PPK, Ni) | prf(PPK, Nr))
(SK_d | SK_ai | SK_ar | SK_ei | SK_er | SK_pi | SK_pr) =
prf+(SKEYSEED, prf(PPK, Ni) | prf(PPK, Nr) |
SPIi | SPIr)
4. Security Considerations
The PPK Indicator Input within the PPK_ENCODE notification are there
to prevent anyone from deducing whether two different exchanges use
the same PPK values. To prevent such a leakage, servers are
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encouraged to vary them as much as possible (however, they may want
to repeat values to speed up the search for the PPK). Repeating
these values places the anonymity at risk; however it has no other
security implication.
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.
In addition, the policy SHOULD be set to negotiate only quantum-
resistant symmetric algorithms; here is a list of defined IKEv2 (and
IPsec) algorithms which are believed to be Quantum Resistant
IKE Encryption algorithm: assuming that the negotiated keysize is >=
256, then all of: ENCR_AES_CBC, ENCR_AES_CTR, ENCR_AES_CCM_*,
ENCR_AES-GCM, ENCR_CHACHA20_POLY1305, ENCR_CAMELLIA, ENCR_RC5,
ENCR_BLOWFISH
IKE PRF: PRF_HMAC_SHA2_256, PRF_HMAC_SHA2_384, PRF_SHA2_512. Note
that PRF_AES128_XCBC and PRF_AES128_CBC are not on this list, even
though they can use larger keys, because they use a 128 bit key
internally
IKE Integrity algorithm: AUTH_HMAC_SHA2_256, AUTH_HMAC_SHA2_384,
AUTH_HMAC_SHA2_512, AUTH_AES_256_GMAC
AH Transforms: AH-SHA2-256, AH-SHA2-384, AH-SHA2-512, AH-AES-256-GMAC
ESP Transforms: assuming that the negotiated keysize is >= 256, then
all of: ESP_AES-CBC, ESP_AES-CR, ESP_AES-CCM, ESP_AES-GCM,
ESP_CAMELLIA, ESP_RC5, ESP_BLOWFISH, ESP_NULL_AUTH_AES-GMAC
ESP Authentication algorithms: HMAC-SHA2-256, HMAC-SHA2-384, HMAC-
SHA2-512, AES-256-GMAC
5. References
5.1. Normative References
[AES] National Institute of Technology, "Specification for the
Advanced Encryption Standard (AES)", 2001, <FIPS 197>.
[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>.
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[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>.
5.2. Informational References
[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
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
unless they can find the PPK, and that's too difficult if the PPK has
enough entropy (say, 256 bits).
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. One such goal is anonymity; that someone listening into the
exchanges cannot easily determine who is negotiating with whom.
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
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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 allow the responder to make the
trade-off between anonymity and efficiency (by including the PPK
Indicator Input, which varies how the PPK is encoded, and allowing
the responder to specify it).
A responder who values anonymitity may select a random PPK Indicator
Input each time; in this case, the responder needs to do a linear
scan over all PPK's it has been configured with
A responder who can't afford a linear scan could precompute a small
(possibly rolling) set of the PPK Indicator Inputs; in this case, it
would precompute how each PPK would be indicated. If it reissues the
same PPK Indicator Input to two different exchanges, someone would be
able to verify whether the same PPK was used; this is some loss of
anonymity; but is considerably more efficient.
An alternative approach to solve this problem would be to do a normal
(non-QR) IKEv2 exchange, and when the two sides obtain identities,
see if they need to be QR, and if so, create an immediate IKEv2 child
SA (using the PPK). One issue with this is that someone with a
quantum computer could deduce the identities used; another issue is
the added complexity required by the IKE state machines.
A slightly different approach to try to make this even more friendly
to IKEv2-based cryptographic hardware might be to use invertible
cryptography when we present the nonces to the kdf. The idea here is
in case we have IKEv2 hardware that insists on selecting its own
nonces (and so we won't be able to give a difference nonce to the
KDF); instead, we encrypt the nonce that we send (and decrypt the
nonce that we get). Of course, this means that the responder will
need to figure out which PPK we're using up front (based on the
notifications); we're not sure if this idea would be a net
improvement (especially since the transform we're proposing now is
cryptographically secure and simple).
The reasoning behind the cryptography used: the values we use in the
AES256_SHA256 PPK Indicator Algorithm are cryptographically
independent of the values used during the SKEYSEED generation
(because, even if we use HMAC_256 as our PRF, HMAC_SHA256(PPK, A) is
independent of HMAC_SHA256(PPK, B) if A and B are different strings
(and as any real nonce must be longer than a single byte, there is
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never a collision between that and "A". This independent stems from
the assumption that HMAC_SHA256 is a secure MAC.
The method of encoding the PPK within the notification (using AES-
256) was chosen as it met two goals:
o Anonymity; given A, AES256_K1(A), B, AES256_K2(B), it's fairly
obvious that gives someone (even if they have a quantum computer)
no clue about whether K1==K2 (unless either A==B or AES256_K1(A)==
AES256_K2(B); both highly unlikely events if A and B are chosen
randomly).
o Performance during the linear search; a responder could preexpand
the AES keys, and so comparing a potential PPK against a
notification from the initiator would amount to performing a
single AES block encryption and then doing a 16 byte comparison.
The first goal is considered important; one of the goals of IKEv2 is
to provide anonymity. The second is considered important because the
linear scan directly affects scalability. While this draft allows
the server to gain performance at the cost of anonymity, it was
considered useful if we make the fully-anonymous method as attractive
as possible. This use of AES makes this linear scan as cheap as
possible (while preserving security).
We allow the responder to specify the PPK Indicator Algorithm; this
was in response to requests for algorithm agility. At present, it
appears unlikely that there would be a need for an additional
encoding (as the current one is extremely conservative
cryptographically); however the option is there.
The current draft forces a cookie exchange, and hence adds a round
trip over the normal IKEv2 operation. This was done to allow the
server to specify the PPK Indicator algorithm. While as additional
round trip may seem costly, it does not invalidate this proposal, The
reason for this proposal is to give an alternative to IKEv1 with
preshared keys. While this additional round trip may seem costly, it
is important to note that, even with the additional round trip, this
proposal is still cheaper than IKEv1. Thus the mechanisms specified
in this note meet the goal of providing a better alternative than
relying on an obsolete version of the protocol for post quantum
security.
One issue that is currently open: what should happen if the initiator
guesses at the PPK Indicator Algorithm, selects a random PPK
Indicator Input, and includes that in the initial message? After
all, if the server follows the recommendation that the cookie
exchange is stateless, and if the server chooses the PPK Indicator
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Input In randomly, it has no way to know that the client isn't
running this protocol as specified. If the responder supports that
PPK Indicator Algorithm, it could very well respond without forcing a
cookie exchange (which would eliminate a message exchange round).
It's not clear is whether we should endorse this mode of operation,
and explicitly state that if the server recieves such an initial
request, and it doesn't recognize the PPK Indicator Input, it should
act like it recieved an iniital PPK_REQUEST.
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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