draft-fluhrer-qr-ikev2-02.txt   draft-fluhrer-qr-ikev2-03.txt 
Internet Engineering Task Force S. Fluhrer Internet Engineering Task Force S. Fluhrer
Internet-Draft D. McGrew Internet-Draft D. McGrew
Intended status: Informational P. Kampanakis Intended status: Informational P. Kampanakis
Expires: February 5, 2017 Cisco Systems Expires: May 1, 2017 Cisco Systems
August 4, 2016 October 28, 2016
Postquantum Preshared Keys for IKEv2 Postquantum Preshared Keys for IKEv2
draft-fluhrer-qr-ikev2-02 draft-fluhrer-qr-ikev2-03
Abstract Abstract
This document describes an extension of IKEv2 to allow it to be This document describes an extension of IKEv2 to allow it to be
resistant to a Quantum Computer, by using preshared keys resistant to a Quantum Computer, by using preshared keys
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
skipping to change at page 1, line 32 skipping to change at page 1, line 32
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
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Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on February 5, 2017. This Internet-Draft will expire on May 1, 2017.
Copyright Notice Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of (http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
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to this document. Code Components extracted from this document must to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Changes . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Changes . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Requirements Language . . . . . . . . . . . . . . . . . . 3 1.2. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Assumptions . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Assumptions . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Exchanges . . . . . . . . . . . . . . . . . . . . . . . . . . 4 3. Exchanges . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Computing SKEYSEED . . . . . . . . . . . . . . . . . . . 6 4. Creating Child SA Keying Material . . . . . . . . . . . . . . 5
3.2. Verifying preshared key . . . . . . . . . . . . . . . . . 7 5. Security Considerations . . . . . . . . . . . . . . . . . . . 6
3.3. Child SAs . . . . . . . . . . . . . . . . . . . . . . . . 7 6. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
4. Security Considerations . . . . . . . . . . . . . . . . . . . 7 6.1. Normative References . . . . . . . . . . . . . . . . . . 7
5. References . . . . . . . . . . . . . . . . . . . . . . . . . 8 6.2. Informational References . . . . . . . . . . . . . . . . 7
5.1. Normative References . . . . . . . . . . . . . . . . . . 8 Appendix A. Discussion and Rationale . . . . . . . . . . . . . . 7
5.2. Informational References . . . . . . . . . . . . . . . . 9 Appendix B. Acknowledgement . . . . . . . . . . . . . . . . . . 9
Appendix A. Discussion and Rationale . . . . . . . . . . . . . . 9 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction 1. Introduction
It is an open question whether or not it is feasible to build a 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 quantum computer, but if it is, many of the cryptographic algorithms
and protocols currently in use would be insecure. A quantum computer and protocols currently in use would be insecure. A quantum computer
would be able to solve DH and ECDH problems, and this would imply would be able to solve DH and ECDH problems, and this would imply
that the security of existing IKEv2 systems would be compromised. that the security of existing IKEv2 systems would be compromised.
IKEv1 when used with preshared keys does not share this IKEv1 when used with preshared keys does not share this
vulnerability, because those keys are one of the inputs to the key vulnerability, because those keys are one of the inputs to the key
skipping to change at page 2, line 47 skipping to change at page 2, line 46
This document describes a way to extend IKEv2 to have a similar This document describes a way to extend IKEv2 to have a similar
property; assuming that the two end systems share a long secret key, property; assuming that the two end systems share a long secret key,
then the resulting exchange is quantum resistant. By bringing then the resulting exchange is quantum resistant. By bringing
postquantum security to IKEv2, this note removes the need to use an postquantum security to IKEv2, this note removes the need to use an
obsolete version of the Internet Key Exchange in order to achieve obsolete version of the Internet Key Exchange in order to achieve
that security goal. that security goal.
The general idea is that we add an additional secret that is shared The general idea is that we add an additional secret that is shared
between the initiator and the responder; this secret is in addition between the initiator and the responder; this secret is in addition
to the authentication method that is already provided within IKEv2. to the authentication method that is already provided within IKEv2.
We stir in this secret when generating the IKE keys (along with the We stir in this secret when generating the key material (KEYMAT) keys
parameters that IKEv2 normally uses); this secret adds quantum for the child SAs (along with the parameters that IKEv2 normally
resistance to the exchange. uses); this secret provides quantum resistance to the IPsec SAs.
It was considered important to minimize the changes to IKEv2. The It was considered important to minimize the changes to IKEv2. The
existing mechanisms to do authentication and key exchange remain in existing mechanisms to do authentication and key exchange remain in
place (that is, we continue to do (EC)DH, and potentially a PKI place (that is, we continue to do (EC)DH, and potentially a PKI
authentication if configured). This does not replace the authentication if configured). This does not replace the
authentication checks that the protocol does; instead, it is done as authentication checks that the protocol does; instead, it is done as
a parallel check. a parallel check.
1.1. Changes 1.1. Changes
Changes in this draft from the previous versions 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 draft-01
- Added explicit guidance as to what IKE and IPsec algorithms are - Added explicit guidance as to what IKE and IPsec algorithms are
Quantum Resistant Quantum Resistant
draft-00 draft-00
- We switched from using vendor ID's to transmit the additional data - We switched from using vendor ID's to transmit the additional data
to notifications to notifications
skipping to change at page 3, line 46 skipping to change at page 4, line 9
1.2. Requirements Language 1.2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119]. document are to be interpreted as described in RFC 2119 [RFC2119].
2. Assumptions 2. Assumptions
We assume that each IKE peer (both the initiator and the responder) We assume that each IKE peer (both the initiator and the responder)
has an optional Postquantum Preshared Key (PPK) (potentially on a has an optional Postquantum Preshared Key (PPK) (potentially on a
per-peer basis), and also has a configurable flag that determines per-peer basis, selected by peer identity), and also has a
whether this postquantum preshared key is mandatory. This preshared configurable flag that determines whether this postquantum preshared
key is independent of the preshared key (if any) that the IKEv2 key is mandatory. This preshared key is independent of the preshared
protocol uses to perform authentication. key (if any that the IKEv2 protocol uses to perform authentication.
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 3. Exchanges
If the initiator has a configured postquantum preshared key (whether If the initiator has a configured postquantum preshared key (whether
or not it is optional), then it will include a notify payload in its or not it is optional), then it will include a notify payload in its
initial exchange as follows: initial encrypted exchange as follows:
Initiator Responder Initiator Responder
------------------------------------------------------------------ ------------------------------------------------------------------
HDR, SAi1, KEi, Ni, N(PPK_REQUEST) ---> HDR, SK {IDi, [CERT,] [CERTREQ,]
[IDr,] AUTH, SAi2,
TS, TSr, N(PPK_NOTIFY)} --->
N(PPK_REQUEST) is a status notification payload with the type [TBA]; 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 it has a protocol ID of 0, and no SPI and no notification data
associated with it. associated with it.
When the responder recieves the initial exchange with the notify When the responder receives the initial encrypted exchange, it checks
payload, then (if it is configured to support PPK), it responds with: 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
Initiator Responder mandatory. If the notify was received, and the responder does have a
------------------------------------------------------------------ PPK for that identity, then it responds with the standard IKE
<--- HDR, N(COOKIE), N(PPK_ENCODE) response with the PPK_NOTIFY notify message included, namely:
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).
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
(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 Initiator Responder
------------------------------------------------------------------ ------------------------------------------------------------------
<--- HDR, SAr1, Ker, Nr, [CERTREQ], N(PPK_ACK) <--- HDR, SK {IDr, [CERT,] AUTH,
SAr2, TSi, TSr, N(PPK_NOTIFY)}
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 If the responder is not configured to support PPK with that identity,
presence of the notification. If it receives one, it marks the SA as it continues with the standard IKE protocol, not including the
using the configured preshared key; if it does not receive one, it notification.
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 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.
When it comes time to generate the keying material during the initial This table summarizes the above logic by the responder
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) Received Nonce Have PPK PPK Mandatory Action
(SK_d | SK_ai | SK_ar | SK_ei | SK_er | SK_pi | SK_pr) = ------------------------------------------------------------------
prf+(SKEYSEED, prf(PPK, Ni) | prf(PPK, Nr) | No No * Standard IKE protocol
SPIi | SPIr) No Yes No Standard IKE protocol
No Yes Yes Abort negotiation
Yes No * Standard IKE protocol
Yes Yes * Include PPK_NOTIFY Nonce
where PPK is the postquantum preshared key, Ni, Nr are the nonces When the initiator receives the response, then (if it is configured
exchanged in the IKEv2 exchange, and prf is the pseudorandom function to use a PPK with the responder), then it checks for the presense of
that was negotiated for this SA. 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).
We reuse the negotiated PRF to transform the received nonces. We use The protocol continues as standard until it comes time to compute the
this PRF, rather than negotiating a separate one, because this PRF is child SA keying material.
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 4. Creating Child SA Keying Material
Once both the initiator and the responder have exchanged identities, When it comes time to generate the keying material for a child SA,
they both double-check with their policy database to verify that they the implementation (both the initiator and the responder) checks to
were configured to use those preshared keys when negotiating with the see if they agreed to use a PPK. If they did, then they look up
peer. If they are not, they MUST abort the exchange. (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):
3.3. Child SAs Ni&apos; = prf(PPK, Ni)
Nr&apos; = prf(PPK, Nr)
KEYMAT = prf+(SK_d, Ni&apos; | Nr&apos;)
When you create a child SA, the initiator and the responder will or
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)) Ni&apos; = prf(PPK, Ni)
Nr&apos; = prf(PPK, Nr)
KEYMAT = prf+(SK_d, g^ir (new) | Ni&apos; | Nr&apos;)
or 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.
KEYMAT = prf+(SK_d, g^ir (new) | This is the standard IKE KEYMAT generation, except that the nonces
prf(PPK, Ni) | prf(PPK, Nr)) are transformed (via the negotiated PRF function) using the preshared
PPK value
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 When you rekey an IKE SA (generating a fresh SKEYSEED), the initiator
and the responder will transform the nonces using the same PPK as and the responder will transform the nonces using the same PPK as
they used during the original IKE SA negotiation. That is, they will they used during the original IKE SA negotiation. That is, they will
use the alternate derivation: use the alternate derivation:
SKEYSEED = prf( SK_d (old), g^ir (new) | Ni&apos; = prf(PPK, Ni)
prf(PPK, Ni) | prf(PPK, Nr)) Nr&apos; = prf(PPK, Nr)
SKEYSEED = prf( SK_d (old), g^ir (new) | Ni&apos; | Nr&apos; )
(SK_d | SK_ai | SK_ar | SK_ei | SK_er | SK_pi | SK_pr) = (SK_d | SK_ai | SK_ar | SK_ei | SK_er | SK_pi | SK_pr) =
prf+(SKEYSEED, prf(PPK, Ni) | prf(PPK, Nr) | prf+(SKEYSEED, Ni&apos; | Nr&apos; | SPIi | SPIr)
SPIi | SPIr)
4. Security Considerations 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
The PPK Indicator Input within the PPK_ENCODE notification are there 5. Security Considerations
to prevent anyone from deducing whether two different exchanges use
the same PPK values. To prevent such a leakage, servers are
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 Quantum computers are able to perform Grover's algorithm; that
effectively halves the size of a symmetric key. Because of this, the effectively halves the size of a symmetric key. Because of this, the
user SHOULD ensure that the postquantum preshared key used has at user SHOULD ensure that the postquantum preshared key used has at
least 256 bits of entropy, in order to provide a 128 bit security least 256 bits of entropy, in order to provide a 128 bit security
level. level.
In addition, the policy SHOULD be set to negotiate only quantum- Although this protocol preserves all the security properties of IKE
resistant symmetric algorithms; here is a list of defined IKEv2 (and against adversaries with conventional computers, this protocol allows
IPsec) algorithms which are believed to be Quantum Resistant an adversary with a Quantum Computer to decrypt all traffic encrypted
with the initial IKE SA. In particular, it allows the adversary to
IKE Encryption algorithm: assuming that the negotiated keysize is >= recover the identities of both sides. If there is IKE traffic other
256, then all of: ENCR_AES_CBC, ENCR_AES_CTR, ENCR_AES_CCM_*, than the identities that need to be protected against such an
ENCR_AES-GCM, ENCR_CHACHA20_POLY1305, ENCR_CAMELLIA, ENCR_RC5, adversary, one suggestion would be to form an initial IKE SA (which
ENCR_BLOWFISH is used to exchange identities), perhaps by using the protocol
documented in RFC6023. Then, you would immediately create a child
IKE PRF: PRF_HMAC_SHA2_256, PRF_HMAC_SHA2_384, PRF_SHA2_512. Note IKE SA (which is used to exchange everything else). Because the
that PRF_AES128_XCBC and PRF_AES128_CBC are not on this list, even child IKE SA keys are a function of the PPK (among other things),
though they can use larger keys, because they use a 128 bit key traffic protected by that SA is secure against Quantum capable
internally adversaries.
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 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
ESP Transforms: assuming that the negotiated keysize is >= 256, then Any IKE Encryption algorithm, PRF or Integrity algorithm with key
all of: ESP_AES-CBC, ESP_AES-CR, ESP_AES-CCM, ESP_AES-GCM, size <256 bits
ESP_CAMELLIA, ESP_RC5, ESP_BLOWFISH, ESP_NULL_AUTH_AES-GMAC
ESP Authentication algorithms: HMAC-SHA2-256, HMAC-SHA2-384, HMAC- Any ESP Transform with key size <256 bits
SHA2-512, AES-256-GMAC
5. References 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
5.1. Normative References 6. References
[AES] National Institute of Technology, "Specification for the 6.1. Normative References
Advanced Encryption Standard (AES)", 2001, <FIPS 197>.
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed- [RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104, Hashing for Message Authentication", RFC 2104,
DOI 10.17487/RFC2104, February 1997, DOI 10.17487/RFC2104, February 1997,
<http://www.rfc-editor.org/info/rfc2104>. <http://www.rfc-editor.org/info/rfc2104>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>. <http://www.rfc-editor.org/info/rfc2119>.
[RFC7296] Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T. [RFC7296] Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T.
Kivinen, "Internet Key Exchange Protocol Version 2 Kivinen, "Internet Key Exchange Protocol Version 2
(IKEv2)", STD 79, RFC 7296, DOI 10.17487/RFC7296, October (IKEv2)", STD 79, RFC 7296, DOI 10.17487/RFC7296, October
2014, <http://www.rfc-editor.org/info/rfc7296>. 2014, <http://www.rfc-editor.org/info/rfc7296>.
5.2. Informational References 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)", [SPDP] McGrew, D., "A Secure Peer Discovery Protocol (SPDP)",
2001, <http://www.mindspring.com/~dmcgrew/spdp.txt>. 2001, <http://www.mindspring.com/~dmcgrew/spdp.txt>.
Appendix A. Discussion and Rationale Appendix A. Discussion and Rationale
The idea behind this is that while a Quantum Computer can easily The idea behind this is that while a Quantum Computer can easily
reconstruct the shared secret of an (EC)DH exchange, they cannot as reconstruct the shared secret of an (EC)DH exchange, they cannot as
easily recover a secret from a symmetric exchange this makes the easily recover a secret from a symmetric exchange this makes the
SKEYSEED depend on both the symmetric PPK, and also the Diffie- IPsec KEYMAT and any child SA's SKEYSEED depend on both the symmetric
Hellman exchange. If we assume that the attacker knows everything PPK, and also the Diffie-Hellman exchange. If we assume that the
except the PPK during the key exchange, and there are 2**n plausible attacker knows everything except the PPK during the key exchange, and
PPK's, then a Quantum Computer (using Grover's algorithm) would take there are 2**n plausible PPK's, then a Quantum Computer (using
O(2**(n/2)) time to recover the PPK. So, even if the (EC)DH can be Grover's algorithm) would take O(2**(n/2)) time to recover the PPK.
trivially solved, the attacker still can't recover any key material So, even if the (EC)DH can be trivially solved, the attacker still
unless they can find the PPK, and that's too difficult if the PPK has can't recover any key material (except for the SK values for the
enough entropy (say, 256 bits). 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 Another goal of this protocol is to minimize the number of changes
within the IKEv2 protocol, and in particular, within the cryptography within the IKEv2 protocol, and in particular, within the cryptography
of IKEv2. By limiting our changes to notifications, and translating of IKEv2. By limiting our changes to notifications, and translating
the nonces, it is hoped that this would be implementable, even on the nonces, it is hoped that this would be implementable, even on
systems that perform much of the IKEv2 processing is in hardware. systems that perform much of the IKEv2 processing is in hardware.
A third goal was to be friendly to incremental deployment in A third goal was to be friendly to incremental deployment in
operational networks, for which we might not want to have a global operational networks, for which we might not want to have a global
shared key, and also if we're rolling this out incrementally. This 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 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. 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 A fourth goal was to avoid violating any of the security goals of
IKEv2. One such goal is anonymity; that someone listening into the IKEv2.
exchanges cannot easily determine who is negotiating with whom.
The third and fourth goals are in partial conflict. In order to The third and fourth goals are in partial conflict. In order to
achieve postquantum security, we need to stir in the PPK when the achieve postquantum security, we need to stir in the PPK when the
keys are computed, however the keys are computed before we know who 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 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 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 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 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 listening in could easily tell when we're using the same PPK for
different exchanges, and thus deduce that the systems are related. different exchanges, and thus deduce that the systems are related.
The compromise we selected was to allow the responder to make the The compromise we selected was to stir in the PPK in all the derived
trade-off between anonymity and efficiency (by including the PPK keys except the initial IKE SA keys, While this allows an attacker
Indicator Input, which varies how the PPK is encoded, and allowing with a Quantum Computer to recover the identities, a poll on the
the responder to specify it). IPsecME mailing list indicated that the majority of the people on the
list did not think anonymity was an important property within IKE.
A responder who values anonymitity may select a random PPK Indicator We stir in the shared secret within the Child SA keying material;
Input each time; in this case, the responder needs to do a linear this allows an implementation that wants to protect the other IKE-
scan over all PPK's it has been configured with 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
A responder who can't afford a linear scan could precompute a small the rest of the negotiation.
(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
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 In addition, when we stir in the PPK, we always use it to modify a
was in response to requests for algorithm agility. At present, it nonce (using the negotiated PRF). We modify the nonce (rather than,
appears unlikely that there would be a need for an additional say, including the PPK in with the prf or prf+ computation directly)
encoding (as the current one is extremely conservative so that this would be easier to implement on an hardware-based IKE
cryptographically); however the option is there. implementation; the prf computations might be built-in, but the
nonces would be external inputs, and so modifying those would
minimize the changes.
The current draft forces a cookie exchange, and hence adds a round Appendix B. Acknowledgement
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 The idea of stirring in the PPK into the IPsec key generation process
guesses at the PPK Indicator Algorithm, selects a random PPK was originally suggested on the list by Tero Kivinen.
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
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 Authors' Addresses
Scott Fluhrer Scott Fluhrer
Cisco Systems Cisco Systems
Email: sfluhrer@cisco.com Email: sfluhrer@cisco.com
David McGrew David McGrew
Cisco Systems Cisco Systems
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