Postquantum Preshared Keys for IKEv2
draft-ietf-ipsecme-qr-ikev2-00
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 8784.
|
|
|---|---|---|---|
| Authors | Scott Fluhrer , David McGrew , Panos Kampanakis , Valery Smyslov | ||
| Last updated | 2017-10-19 | ||
| Replaces | draft-fluhrer-qr-ikev2 | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Formats | |||
| Reviews | |||
| Additional resources | Mailing list discussion | ||
| Stream | WG state | WG Document | |
| Document shepherd | David Waltermire | ||
| IESG | IESG state | Became RFC 8784 (Proposed Standard) | |
| Consensus boilerplate | Unknown | ||
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | David Waltermire <david.waltermire@nist.gov> |
draft-ietf-ipsecme-qr-ikev2-00
Internet Engineering Task Force S. Fluhrer
Internet-Draft D. McGrew
Intended status: Standards Track P. Kampanakis
Expires: April 18, 2018 Cisco Systems
V. Smyslov
ELVIS-PLUS
October 15, 2017
Postquantum Preshared Keys for IKEv2
draft-ietf-ipsecme-qr-ikev2-00
Abstract
The possibility of Quantum Computers pose a serious challenge to
cryptography algorithms deployed widely today. IKEv2 is one example
of a cryptosystem that could be broken; someone storing VPN
communications today could decrypt them at a later time when a
Quantum Computer is available. It is anticipated that IKEv2 will be
extended to support quantum secure key exchange algorithms; however
that is not likely to happen in the near term. To address this
problem before then, this document describes an extension of IKEv2 to
allow it to be resistant to a Quantum Computer, by using preshared
keys.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 18, 2018.
Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Changes . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Requirements Language . . . . . . . . . . . . . . . . . . 5
2. Assumptions . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Exchanges . . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Upgrade procedure . . . . . . . . . . . . . . . . . . . . . . 9
5. PPK . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
5.1. PPK_ID format . . . . . . . . . . . . . . . . . . . . . . 9
5.2. Operational Considerations . . . . . . . . . . . . . . . 10
5.2.1. PPK Distribution . . . . . . . . . . . . . . . . . . 10
5.2.2. Group PPK . . . . . . . . . . . . . . . . . . . . . . 11
5.2.3. PPK-only Authentication . . . . . . . . . . . . . . . 11
6. Security Considerations . . . . . . . . . . . . . . . . . . . 11
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 14
8.1. Normative References . . . . . . . . . . . . . . . . . . 14
8.2. Informational References . . . . . . . . . . . . . . . . 14
Appendix A. Discussion and Rationale . . . . . . . . . . . . . . 15
Appendix B. Acknowledgements . . . . . . . . . . . . . . . . . . 16
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 16
1. Introduction
It is an open question whether or not it is feasible to build a
Quantum Computer (and if so, when one might 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 in polynomial time [I-D.hoffman-c2pq], and this
would imply that the security of existing IKEv2 [RFC7296] systems
would be compromised. IKEv1 [RFC2409], when used with strong
preshared keys, is not vulnerable to quantum attacks, because those
keys are one of the inputs to the key derivation function. If the
preshared key has sufficient entropy and the PRF, encryption and
authentication transforms are postquantum secure, then the resulting
system is believed to be quantum resistant, that is, invulnerable to
an attacker with a Quantum Computer.
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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 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 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 document 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 in each version iterations.
draft-ietf-ipsecme-qr-ikev2-00
o Migrated from draft-fluhrer-qr-ikev2-05 to draft-ietf-ipsecme-qr-
ikev2-00 that is a WG item.
draft-fluhrer-qr-ikev2-05
o Nits and editorial fixes.
o Made PPK_ID format and PPK Distributions subsection of the PPK
section. Also added an Operational Considerations section.
o Added comment about Child SA rekey in the Security Considerations
section.
o Added NO_PPK_AUTH to solve the cases where a PPK_ID is not
configured for a responder.
o Various text changes and clarifications.
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o Expanded Security Considerations section to describe some security
concerns and how they should be addressed.
draft-fluhrer-qr-ikev2-03
o Modified how we stir the PPK into the IKEv2 secret state.
o Modified how the use of PPKs is negotiated.
draft-fluhrer-qr-ikev2-02
o Simplified the protocol by stirring in the preshared key into the
child SAs; this avoids the problem of having the responder decide
which preshared key to use (as it knows the initiator identity at
that point); it does mean that someone with a Quantum Computer can
recover the initial IKE negotiation.
o Removed positive endorsements of various algorithms. Retained
warnings about algorithms known to be weak against a Quantum
Computer.
draft-fluhrer-qr-ikev2-01
o Added explicit guidance as to what IKE and IPsec algorithms are
quantum resistant.
draft-fluhrer-qr-ikev2-00
o We switched from using vendor ID's to transmit the additional data
to notifications.
o We added a mandatory cookie exchange to allow the server to
communicate to the client before the initial exchange.
o We added algorithm agility by having the server tell the client
what algorithm to use in the cookie exchange.
o We have the server specify the PPK Indicator Input, which allows
the server to make a trade-off between the efficiency for the
search of the clients PPK, and the anonymity of the client.
o We now use the negotiated PRF (rather than a fixed HMAC-SHA256) to
transform the nonces during the KDF.
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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 any specific
responder. In addition, implementations have 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. The PPK specific
configuration that is assumed on each peer consists of the following
tuple:
Peer, PPK, PPK_ID, mandatory_or_not
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 mandatory),
then it will include a notification PPK_SUPPORT 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, 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.
If the responder does not support this specification or does not have
any PPK configured, then it ignores the received notification and
continues with the IKEv2 protocol as normal. Otherwise the responder
checks if it has a PPK configured, and if it does, then the responder
replies with the IKEv2 initial exchange including a PPK_SUPPORT
notification in the response:
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Initiator Responder
------------------------------------------------------------------
<--- HDR, SAr1, KEr, Nr, [CERTREQ], N(PPK_SUPPORT)
When the initiator receives this reply, it checks whether the
responder included the PPK_SUPPORT notification. If the responder
did not and the flag mandatory_or_not indicates that using PPKs is
mandatory for communication with this responder, then the initiator
MUST abort the exchange. This situation may happen in case of
misconfiguration, when the initiator believes it has a mandatory to
use PPK for the responder, while the responder either doesn't support
PPKs at all or doesn't have any PPK configured for the initiator.
See Section 6 for discussion of the possible impacts of this
situation.
If the responder did not include the PPK_SUPPORT notification and
using PPKs for this responder is optional, then the initiator
continues with the IKEv2 protocol as normal, without using PPKs.
If the responder did include the PPK_SUPPORT notification, then the
initiator selects a PPK, along with its identifier PPK_ID. Then, it
computes this modification of the standard IKEv2 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 IKEv2 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(NO_PPK_AUTH)]} --->
PPK_IDENTITY is a status notification with the type [TBA]; it has a
protocol ID of 0, no SPI and a notification data that consists of the
identifier PPK_ID.
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A situation may happen when the responder has some PPKs, but doesn't
have a PPK with the PPK_ID received from the initiator. In this case
the responder cannot continue with PPK (in particular, it cannot
authenticate the initiator), but it could be able to continue with
normal IKEv2 protocol if the initiator provided its authentication
data computed as in normal IKEv2, without using PPKs. For this
purpose, if using PPKs for communication with this responder is
optional for the initiator, then the initiator MAY include a
notification NO_PPK_AUTH in the above message.
NO_PPK_AUTH is a status notification with the type [TBA]; it has a
protocol ID of 0 and no SPI. A notification data consists of the
initiator's authentication data computed using SK_pi' (i.e. the data
that computed without using PPKs and would normally be placed in the
AUTH payload). Authentication Method for computing the
authentication data MUST be the same as indicated in the AUTH payload
and is not included in the notification. Note that if the initiator
decides to include NO_PPK_AUTH notification, then it means that the
initiator needs to perform authentication data computation twice that
may consume substantial computation power (e.g. if digital signatures
are involved).
When the responder receives this encrypted exchange, it first
computes the values:
SKEYSEED = prf(Ni | Nr, g^ir)
{SK_d' | SK_ai | SK_ar | SK_ei | SK_er | SK_pi' | SK_pr' }
= prf+ (SKEYSEED, Ni | Nr | SPIi | SPIr )
It then uses the SK_ei/SK_ai values to decrypt/check the message and
then scans through the payloads for the PPK_ID attached to the
PPK_IDENTITY notification. If no PPK_IDENTITY notification is found
and the peers successfully exchanged PPK_SUPPORT notifications in the
initial exchange, then the responder MUST send back
AUTHENTICATION_FAILED notification and then fail the negotiation.
If the PPK_IDENTITY notification contains PPK_ID that is not known to
the responder or is not configured for use for the identity from IDi
payload, then the responder checks whether using PPKs for this
initiator is mandatory and whether the initiator included NO_PPK_AUTH
notification in the message. If using PPKs is mandatory or no
NO_PPK_AUTH notification found, then then the responder MUST send
back AUTHENTICATION_FAILED notification and then fail the
negotiation. Otherwise (when PPK is optional and the initiator
included NO_PPK_AUTH notification) the responder MAY continue regular
IKEv2 protocol, except that it uses the data from the NO_PPK_AUTH
notification as the authentication data (which usually resides in the
AUTH payload), for the purpose of the initiator authentication.
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Note, that Authentication Method is still indicated in the AUTH
payload.
This table summarizes the above logic by the responder:
Received Received Have PPK
PPK_SUPPORT NO_PPK_AUTH PPK Mandatory Action
------------------------------------------------------------------
No * No * Standard IKEv2 protocol
No * Yes No Standard IKEv2 protocol
No * Yes Yes Abort negotiation
Yes No No * Abort negotiation
Yes Yes No Yes Abort negotiation
Yes Yes No No Standard IKEv2 protocol
Yes * Yes * Use PPK
If PPK is in use, then the responder extracts corresponding PPK and
computes the following values:
SK_d = prf(PPK, SK_d')
SK_pi = prf(PPK, SK_pi')
SK_pr = prf(PPK, SK_pr')
The responder then continues with the exchange (validating the AUTH
payload that the initiator included) as usual and sends back a
response, which includes the PPK_IDENTITY notification with no data
to indicate that the PPK is used in the exchange:
Initiator Responder
------------------------------------------------------------------
<-- HDR, SK {IDr, [CERT,]
AUTH, SAr2,
TSi, TSr, N(PPK_IDENTITY)}
When the initiator receives the response, then it checks for the
presence of the PPK_IDENTITY notification. If it receives one, it
marks the SA as using the configured PPK to generate SK_d, SK_pi,
SK_pr (as shown above); if it does not receive one, it MUST either
fail the IKE SA negotiation sending the AUTHENTICATION_FAILED
notification in the Informational exchange (if the PPK was configured
as mandatory), or continue without using the PPK (if the PPK was not
configured as mandatory and the initiator included the NO_PPK_AUTH
notification in the request).
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4. Upgrade procedure
This algorithm was designed so that someone can introduce PPKs into
an existing IKE network without causing network disruption.
In the initial phase of the network upgrade, the network
administrator would visit each IKE node, and configure:
o The set of PPKs (and corresponding PPK_IDs) that this node would
need to know.
o For each peer that this node would initiate to, which PPK will be
used.
o That the use of PPK is currently not mandatory.
With this configuration, the node will continue to operate with nodes
that have not yet been upgraded. This is due to the PPK_SUPPORT
notify and the NO_PPK_AUTH 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 to not use a PPK). If both peers have
been upgraded, but the responder isn't yet configured with the PPK
for the initiator, then the responder could do standard IKEv2
protocol if the initiator sent NO_PPK_AUTH notification. If the
responder has not been upgraded and properly configured, 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).
5. PPK
5.1. PPK_ID format
This standard requires that both the initiator and the responder have
a secret PPK value, with the responder selecting the PPK based on the
PPK_ID that the initiator sends. In this standard, both the
initiator and the responder are configured with fixed PPK and PPK_ID
values, and do the look up based on PPK_ID value. It is anticipated
that later standards will extend this technique to allow dynamically
changing PPK values. To facilitate such an extension, we specify
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that the PPK_ID the initiator sends will have its first octet be the
PPK_ID Type value. This document defines two values for PPK_ID Type:
o PPK_ID_OPAQUE (1) - for this type the format of the PPK_ID (and
the PPK itself) is not specified by this document; it is assumed
to be mutually intelligible by both by initiator and the
responder. This PPK_ID type is intended for those implementations
that choose not to disclose the type of PPK to active attackers.
o PPK_ID_FIXED (2) - in this case the format of the PPK_ID and the
PPK are fixed octet strings; the remaining bytes of the PPK_ID are
a configured value. We assume that there is a fixed mapping
between PPK_ID and PPK, which is configured locally to both the
initiator and the responder. The responder can use 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 value 0 is reserved; values 3-127 are reserved for
IANA; values 128-255 are for private use among mutually consenting
parties.
5.2. Operational Considerations
The need to maintain several independent sets of security credentials
can significantly complicate security administrators job, and can
potentially slow down widespread adoption of this solution. It is
anticipated, that administrators will try to simplify their job by
decreasing the number of credentials they need to maintain. This
section describes some of the considerations for PPK management.
5.2.1. PPK Distribution
PPK_IDs of the type PPK_ID_FIXED (and the corresponding PPKs) are
assumed to be configured within the IKE device in an out-of-band
fashion. While the method of distribution is a local matter and out
of scope of this document or IKEv2, [RFC6030] describes a format for
symmetric key exchange. That format could be reused with the Key Id
field being the PPK_ID (without the PPK_ID Type octet for a
PPK_ID_FIXED), the PPK being the secret, and the algorithm
("Algorithm=urn:ietf:params:xml:ns:keyprov:pskc:pin") as PIN.
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5.2.2. Group PPK
This document doesn't explicitly require that PPK is unique for each
pair of peers. If it is the case, then this solution provides full
peer authentication, but it also means that each host must have that
many independent PPKs, how many peers it is going to communicate
with. As the number of hosts grows this will scale badly.
Even though it is NOT RECOMMENDED, it is possible to use a single PPK
for a group of users. Since each peer uses classical public key
cryptography in addition to PPK for key exchange and authentication,
members of the group can neither impersonate each other nor read
other's traffic, unless they use Quantum Computers to break public
key operations.
Although it's probably safe to use group PPK in short term, the fact,
that the PPK is known to a (potentially large) group of users makes
it more susceptible to theft. If an attacker equipped with a Quantum
Computer got access to a group PPK, then all the communications
inside the group are revealed.
5.2.3. PPK-only Authentication
If Quantum Computers become a reality, classical public key
cryptography will provide little security, so administrators may find
it attractive not to use it at all for authentication. This will
reduce the number of credentials they need to maintain to PPKs only.
Combining group PPK and PPK-only authentication is NOT RECOMMENDED,
since in this case any member of the group can impersonate any other
member even without help of Quantum Computers.
PPK-only authentication can be achieved in IKEv2 if NULL
Authentication method [RFC7619] is employed. Without PPK the NULL
Authentication method provides no authentication of the peers,
however since a PPK is stirred into the SK_pi and the SK_pr, the
peers become authenticated if a PPK is in use. Using PPKs MUST be
mandatory for the peers if they advertise support for PPK in initial
exchange and use NULL Authentication. Addtionally, since the peers
are authenticated via PPK, the ID Type in the IDi/IDr payloads SHOULD
NOT be ID_NULL, despite using NULL Authentication method.
6. 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.
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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 was able to recover
the rest of the inputs to the prf function, it would be infeasible to
use Grover's algorithm with a Quantum Computer to recover the SK_d
value. Similarly, 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).
Although this protocol preserves all the security properties of IKEv2
against adversaries with conventional computers, it 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, implementations MAY rekey the initial IKE SA immediately
after negotiating it to generate a new SKEYSEED with from the
postquantum SK_d. This would reduce the amount of data available to
an attacker with a Quantum Computer.
Alternatively, an initial IKE SA (which is used to exchange
identities) can take place, perhaps by using the protocol documented
in [RFC6023]. After the childless IKE SA is created, implementations
would immediately create a new IKE SA (which is used to exchange
everything else) by using a rekey mechanism for IKE SAs. Because the
rekeyed IKE SA keys are a function of SK_d, which is a function of
the PPK (among other things), traffic protected by that IKE SA is
secure against Quantum capable adversaries.
If some sensitive information (like keys) is to be transferred over
IKE SA, then implementations MUST rekey the initial IKE SA before
sending this information to get protection against Quantum Computers.
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), below is a list of defined IKEv2 and
IPsec algorithms that should NOT be used, as they are known not to be
quantum resistant
o Any IKEv2 Encryption algorithm, PRF or Integrity algorithm with
key size less than 256 bits.
o Any ESP Transform with key size less than 256 bits.
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o PRF_AES128_XCBC and PRF_AES128_CBC; even though they are defined
to be able to use an arbitrary key size, they convert it into a
128-bit key internally.
Section 3 requires the initiator to abort the initial exchange if
using PPKs is mandatory for it, but the responder didn't include the
PPK_SUPPORT notification in the response. In this situation when the
initiator aborts negotiation it leaves half-open IKE SA on the
responder (because the initial exchange completes successfully from
responder's point of view). This half-open SA will eventually expire
and be deleted, but if the initiator continues its attempts to create
IKE SA with a high enough rate, then the responder may consider it as
a Denial-of-Service attack and take some measures (see [RFC8019] for
more detail). It is RECOMMENDED that implementations in this
situation cache the negative result of negotiation for some time and
don't make attempts to create it again for some time, because this is
a result of misconfiguration and probably some re-configuration of
the peers is needed.
If using PPKs is optional for both peers and they authenticate
themselves using digital signatures, then an attacker in between,
equipped with a Quantum Computer capable of breaking public key
operations in real time, is able to mount downgrade attack by
removing PPK_SUPPORT notification from the initial exchange and
forging digital signatures in the subsequent exchange. If using PPKs
is mandatory for at least one of the peers or PSK is used for
authentication, then the attack will be detected and the SA won't be
created.
If using PPKs is mandatory for the initiator, then an attacker
capable to eavesdrop and to inject packets into the network can
prevent creating IKE SA by mounting the following attack. The
attacker intercepts the the initial request containing the
PPK_SUPPORT notification and injects the forget response containing
no PPK_SUPPORT. If the attacker manages to inject this packet before
the responder sends a genuine response, then the initiator would
abort the exchange. To thwart this kind of attack it is RECOMMENDED,
that if using PPKs is mandatory for the initiator and the received
response doesn't contain the PPK_SUPPORT notification, then the
initiator doesn't abort exchange immediately, but instead waits some
time for more responses (possibly retransmitting the request). If
all the received responses contain no PPK_SUPPORT, then the exchange
is aborted.
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7. IANA Considerations
This document defines three new Notify Message Types in the "Notify
Message Types - Status Types" registry:
<TBA> PPK_SUPPORT
<TBA> PPK_IDENTITY
<TBA> NO_PPK_AUTH
This document also creates a new IANA registry for the PPK_ID types.
The initial values of this registry are:
PPK_ID Type Value
----------- -----
Reserved 0
PPK_ID_OPAQUE 1
PPK_ID_FIXED 2
Unassigned 3-127
Reserved for private use 128-255
Changes and additions to this registry are by Expert Review
[RFC5226].
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC7296] Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T.
Kivinen, "Internet Key Exchange Protocol Version 2
(IKEv2)", STD 79, RFC 7296, DOI 10.17487/RFC7296, October
2014, <https://www.rfc-editor.org/info/rfc7296>.
8.2. Informational References
[I-D.hoffman-c2pq]
Hoffman, P., "The Transition from Classical to Post-
Quantum Cryptography", draft-hoffman-c2pq-01 (work in
progress), July 2017.
[RFC2409] Harkins, D. and D. Carrel, "The Internet Key Exchange
(IKE)", RFC 2409, DOI 10.17487/RFC2409, November 1998,
<https://www.rfc-editor.org/info/rfc2409>.
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Internet-Draft Postquantum Security for IKEv2 October 2017
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", RFC 5226,
DOI 10.17487/RFC5226, May 2008,
<https://www.rfc-editor.org/info/rfc5226>.
[RFC6023] Nir, Y., Tschofenig, H., Deng, H., and R. Singh, "A
Childless Initiation of the Internet Key Exchange Version
2 (IKEv2) Security Association (SA)", RFC 6023,
DOI 10.17487/RFC6023, October 2010,
<https://www.rfc-editor.org/info/rfc6023>.
[RFC6030] Hoyer, P., Pei, M., and S. Machani, "Portable Symmetric
Key Container (PSKC)", RFC 6030, DOI 10.17487/RFC6030,
October 2010, <https://www.rfc-editor.org/info/rfc6030>.
[RFC7619] Smyslov, V. and P. Wouters, "The NULL Authentication
Method in the Internet Key Exchange Protocol Version 2
(IKEv2)", RFC 7619, DOI 10.17487/RFC7619, August 2015,
<https://www.rfc-editor.org/info/rfc7619>.
[RFC8019] Nir, Y. and V. Smyslov, "Protecting Internet Key Exchange
Protocol Version 2 (IKEv2) Implementations from
Distributed Denial-of-Service Attacks", RFC 8019,
DOI 10.17487/RFC8019, November 2016,
<https://www.rfc-editor.org/info/rfc8019>.
Appendix A. Discussion and Rationale
The idea behind this document is that while a Quantum Computer can
easily reconstruct the shared secret of an (EC)DH exchange, they
cannot as easily recover a secret from a symmetric exchange. This
makes the SK_d, and hence the IPsec KEYMAT and any child SA's
SKEYSEED, depend on both the symmetric PPK, and also the Diffie-
Hellman exchange. If we assume that the attacker knows everything
except the PPK during the key exchange, and there are 2^n plausible
PPKs, then a Quantum Computer (using Grover's algorithm) would take
O(2^(n/2)) time to recover the PPK. So, even if the (EC)DH can be
trivially solved, the attacker still can't recover any key material
(except for the SK_ei, SK_er, SK_ai, SK_ar values for the initial IKE
exchange) unless they can find the PPK, which is too difficult if the
PPK has enough entropy (for example, 256 bits). Note that we do
allow an attacker with a Quantum Computer to rederive the keying
material for the initial IKE SA; this was a compromise to allow the
responder to select the correct PPK quickly.
Another goal of this protocol is to minimize the number of changes
within the IKEv2 protocol, and in particular, within the cryptography
of IKEv2. By limiting our changes to notifications, and translating
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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 or quantum resistant IKEv2 is rolled out incrementally.
This is why we specifically try to allow the PPK to be dependent on
the peer, and why we allow the PPK to be configured as optional.
A fourth goal was to avoid violating any of the security goals of
IKEv2.
Appendix B. Acknowledgements
We would like to thank Tero Kivinen, Paul Wouters, Graham Bartlett
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
Valery Smyslov
ELVIS-PLUS
Phone: +7 495 276 0211
Email: svan@elvis.ru
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