Internet Engineering Task Force (IETF) T. Kivinen

Internet Engineering Task Force (IETF) T. Kivinen
Request for Comments: 6467 AuthenTec
Category: Informational December 2011
ISSN: 2070-1721

Secure Password Framework for Internet Key Exchange Version 2 (IKEv2)

Abstract

This document defines a generic way for Internet Key Exchange version
2 (IKEv2) to use any of the symmetric secure password authentication
methods. Multiple methods are already specified in other documents,
and this document does not add any new one. This document specifies
a way to agree on which method is to be used in the current
connection. This document also provides a common way to transmit,
between peers, payloads that are specific to secure password
authentication methods.

Status of This Memo

This document is not an Internet Standards Track specification; it is
published for informational purposes.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6467.

Kivinen Informational [Page 1]

RFC 6467 Secure Password Framework for IKEv2 December 2011

Copyright Notice

Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://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
2. Method Negotiation ..............................................4
3. Generic Secure Password Method Payload ..........................6
4. IKE_AUTH Exchange ...............................................7
5. Security Considerations .........................................9
6. IANA Considerations .............................................9
7. References .....................................................10
7.1. Normative References ......................................10
7.2. Informative References ....................................10

1. Introduction

The IPsecME working group was chartered to provide for IKEv2
([RFC5996]) a symmetric secure password authentication protocol that
supports the use of low-entropy shared secrets, and to protect
against off-line dictionary attacks without requiring the use of
certificates or the Extensible Authentication Protocol (EAP). There
are multiple such methods, and the working group was to pick one.
Unfortunately, the working group failed to pick one protocol, and
there are multiple candidates going forward as separate documents.
As each of those older versions of those documents used a different
technique to negotiate the use of the method and also used different
payload formats, it is very hard to try to make an implementation
where multiple such systems could co-exist.

Current document versions ([SPSK-AUTH], [PACE], and [PAKE]) use the
method described in this document.

This document describes IKEv2 payload formats that can be used for
multiple secure password methods to negotiate and transmit data so
each different method can easily co-exist in the same implementation.

Kivinen Informational [Page 2]

RFC 6467 Secure Password Framework for IKEv2 December 2011

This document consists of two major parts:

o How to negotiate which secure password method negotiation is used.

o How to transmit data, between peers, that is specific to secure
password methods.

The secure password methods are not usually meant to be used in the
normal end user (remote access VPN) cases. In such cases, EAP-based
authentication works fine, and the asymmetric nature of EAP does not
matter. In such scenarios, the authentication is usually backed up
with the back-end Authentication, Authorization, and Accounting (AAA)
servers and other infrastructure. That is, in such scenarios,
neither of the IKEv2 peers really knows the secret, as on one end it
is typed in by the user when it is needed, and on the other end it is
authenticated by the back-end AAA server.

The new secure password methods are meant to be used, for example, in
the authentication between two servers or routers. These scenarios
are usually symmetric: both peers know the shared secret, no back-end
authentication servers are involved, and either end can initiate an
IKEv2 connection. Note that such a model could also be supported by
EAP when an EAP method that can run in symmetric fashion is in use,
and the EAP method is directly implemented on both peers and no AAA
is in use.

In many cases, each implementation will use only one of the proposed
secure password authentication methods but can include support for
multiple methods even when only one of them will be used. For
example, a general-purpose operating system running IPsec and IKEv2
and supporting secure password authentication methods to protect
services provided by the system might need to implement support for
several methods. It is then up to the administrator which one is to
be used. As the server might need to connect to multiple other
servers, each implementing a different set of methods, it may not be
possible to pick one method that would serve all cases.

The secure password methods mostly keep the existing IKEv2
IKE_SA_INIT exchange and modify the IKE_AUTH authentication step. As
those methods do not want to add new round trips, negotiating which
of the secure password methods to use needs to happen during the
IKE_SA_INIT. As the identity of the other end is only provided
inside IKE_AUTH, the responder needs to select the list of supported
methods based only on the IP address of the initiator. This could
lead to problems if only certain methods would be acceptable for
certain identified peers. Fortunately, as the authentication is done
based on the secret shared between both peers, the shared secret
should be usable in all of the methods; thus, a remote peer usually

Kivinen Informational [Page 3]

RFC 6467 Secure Password Framework for IKEv2 December 2011

does not need to restrict selection of the method based on the
initiator's identity but only based on the supported methods and the
administrative policy.

Also, as the initiator already knows which peer it is connecting
with, it can limit which methods it proposes to the other peer. And
as secure password methods are meant to be used in symmetric cases,
both ends should have similar configuration; i.e., they have the same
shared secret and, most likely, also a list of acceptable
authentication methods to be used. This could also be interpreted so
that there is no need to support method negotiation, as both ends can
already see this from the configuration. On the other hand, in most
cases, either end does not really care which method is used but is
willing to use any secure method that the other end supports. In
such cases, the automatic negotiation provides a way to make the
configuration easy, i.e., no need to pick one method to be used
between the peers.

The reason for using the common IKEv2 payload to transmit, between
peers, data that is specific to the secure password method is that
the payload type field in the IKEv2 is only an 8-bit field, and 62.5%
of the range is already reserved (50% to the private use numbers, and
12.5% to the IKEv1 payload numbers). This leaves 95 usable numbers,
out of which 16 are already in use. Initially, it was proposed that
five payload type numbers be consumed. Those five new payload types
would already represent a 31% increase in the number of currently
allocated payload types.

2. Method Negotiation

Because all of the methods modify the IKE_AUTH exchange, the
negotiation of the secure password method to be used needs to happen
during the IKE_SA_INIT exchange. The secure password negotiation
exchange would be:

Initiator Responder
-------------------------------------------------------------------
HDR(SPIi=xxx, SPIr=0, IKE_SA_INIT,
Flags: Initiator, Message ID=0),
SAi1, KEi, Ni, [N(SECURE_PASSWORD_METHODS)] -->

<-- HDR(SPIi=xxx, SPIr=yyy, IKE_SA_INIT,
Flags: Response, Message ID=0),
SAr1, KEr, Nr, [CERTREQ],
[N(SECURE_PASSWORD_METHODS)]

Kivinen Informational [Page 4]

RFC 6467 Secure Password Framework for IKEv2 December 2011

If the N(SECURE_PASSWORD_METHODS) Notify payload is missing, then
normal IKEv2 authentication methods are used. If the Notify payloads
are included, then the negotiation of the secure password methods
happens inside those payloads.

As it might be possible that future secure password methods will
modify the IKE_AUTH payload in a more substantial way, it is better
that as an end result of the negotiation we have exactly one secure
password method that will be used. The initiator will know which
methods are usable when talking to that responder, so the initiator
will send a list of acceptable methods in its IKE_SA_INIT request.
The responder will pick exactly one method and put that in its
response.

The secure password methods are identified by the 16-bit IANA-
allocated numbers stored in the Notify payload notification data
field. If a method supports multiple different password
preprocessing methods, each of those may be allocated a separate
number from this space, or the method might do its own negotiation of
the preprocessing method later. As the initiator has already
selected the shared secret it will be using, it will also know which
preprocessing it might need, so it should propose only those
preprocessing methods suitable for the selected shared secret. This
means that it is recommended that separate IANA numbers be allocated
for different preprocessing methods.

The actual Notify payload will look like this:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Payload |C| RESERVED | Payload Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Protocol ID | SPI Size | Notify Message Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Security Parameter Index (SPI) ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Notification Data ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The Protocol ID will be zero, and the SPI Size will also be zero,
meaning that the SPI field will be empty. The Notify Message Type
will be 16424.

Kivinen Informational [Page 5]

RFC 6467 Secure Password Framework for IKEv2 December 2011

The Notification Data contains the list of the 16-bit secure password
method numbers:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Secure Password Method #1 | Secure Password Method #2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Secure Password Method #3 | ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The response Notify payload contains exactly one 16-bit secure
password method number inside the Notification Data field.

3. Generic Secure Password Method Payload

This payload will contain the data that is specific to the secure
password payload. The IKE_AUTH exchanges might have a number of
these inside, depending on what is required and specified by the
secure password method. As the secure password method is already
selected during IKE_SA_INIT, there is no need to repeat the
information of the selected secure password method; thus, this
payload only contains the method-specific data. As some secure
password methods require multiple different payloads, they are
assumed to include their method-specific payload type inside the
payload -- for example, inside the first octet of the data. However,
this is method-specific, and a method is free to format the payload
data as it wants.

The Generic Secure Password Method (GSPM) payload will look
like this:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Payload |C| RESERVED | Payload Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Data Specific to the Secure Password Method ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The Payload Type for this payload is 49, and the name used in this
document is "GSPM payload."

Kivinen Informational [Page 6]

RFC 6467 Secure Password Framework for IKEv2 December 2011

If the method uses payload subtypes (which are specific to the secure
password method) inside the GSPM payload, the format will be
like this:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Payload |C| RESERVED | Payload Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Subtype* | |
+-+-+-+-+-+-+-+-+ +
| |
~ Data Specific to the Secure Password Method ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

* method-specific subtype field

This representation is here only for illustrative purposes; the
secure password method will define the exact format of the payload
contents.

4. IKE_AUTH Exchange

As the negotiation takes place during IKE_SA_INIT, the secure
password methods may modify the IKE_AUTH exchange if needed. To
easily enable implementing multiple methods, it is recommended that
IKE_AUTH exchange not be modified unnecessarily. Adding zero, one,
or multiple GSPM payloads to each exchange is needed, as is the
modification to how the AUTH payload is calculated, but all other
changes should be kept minimal.

The IKE_AUTH exchange should look similar to when EAP is used,
meaning that the first request includes IDi, SAi2, TSi, TSr, and some
number of GSPM payloads. The response should include IDr and, again,
a number of GSPM payloads. There may be multiple exchanges, each
consisting of some number of GSPM payloads; finally, when
authentication is done, there should be one final exchange where the
request includes the AUTH payload (along with some number of GSPM
payloads) and the response contains AUTH, SAr2, TSi, TSr, and some
number of GSPM payloads. The number of GSPM payloads is up to the
secure password method but usually will be less than 3. However,
depending on the method, it might be more.

The AUTH payload calculation should include all the data that would
normally be included, in addition to the extra data needed by the
secure password method. The secure password method needs to define
how the AUTH payload is calculated.

Kivinen Informational [Page 7]

RFC 6467 Secure Password Framework for IKEv2 December 2011

As the AUTH payload calculation is changed, the secure payload method
should not use any of the existing authentication method numbers in
the AUTH Payload Auth Method field but instead should use the number
allocated in this document. This number is meant to be used by all
secure password authentication methods.

Initiator Responder
-------------------------------------------------------------------
HDR(SPIi=xxx, SPIr=yyy, IKE_AUTH,
Flags: Initiator, Message ID=1),
SK {IDi, [CERTREQ,]
GSPM, [GSPM, ...,]
[IDr,] SAi2,
TSi, TSr} -->

<-- HDR(SPIi=xxx, SPIr=yyy, IKE_AUTH, Flags:
Response, Message ID=1),
SK {IDr, [CERT,]
GSPM, [GSPM, ...]}

HDR(SPIi=xxx, SPIr=yyy, IKE_AUTH,
Flags: Initiator, Message ID=2),
SK {GSPM, [GSPM, ...,]} -->

<-- HDR(SPIi=xxx, SPIr=yyy, IKE_AUTH, Flags:
Response, Message ID=2),
SK {GSPM, [GSPM, ...]}
...

HDR(SPIi=xxx, SPIr=yyy, IKE_AUTH,
Flags: Initiator, Message ID=x),
SK {[GSPM, ...,], AUTH} -->

<-- HDR(SPIi=xxx, SPIr=yyy, IKE_AUTH, Flags:
Response, Message ID=x),
SK {[GSPM, ...,] AUTH, SAr2,
TSi, TSr}

Note that the number of the GSPM payloads and other payloads in each
packet will be defined only by the secure password method
documentation, and representations in this document are only for
illustrative purposes.

Kivinen Informational [Page 8]

RFC 6467 Secure Password Framework for IKEv2 December 2011

5. Security Considerations

As this document does not describe an exact protocol, the security
considerations are not relevant. Any secure password method
documentation using payload types described here needs to also
describe the security properties of the protocol it defines or
discusses.

6. IANA Considerations

This document allocates one new IKEv2 message type in the "Notify
Messages Types - Status Types" registry:

16424 SECURE_PASSWORD_METHODS

This document also allocates one new number in the "IKEv2
Authentication Method" registry:

12 Generic Secure Password Authentication Method

This document also adds one new payload type to the "IKEv2 Payload
Types" registry:

49 Generic Secure Password Method GSPM

This document creates a new IANA registry -- "IKEv2 Secure Password
Methods":

0 Reserved

Values 1-1023 are unassigned. Values 1024-65535 are for private use
among mutually consenting parties. Changes and additions to this
registry are done by Expert Review.

Kivinen Informational [Page 9]

RFC 6467 Secure Password Framework for IKEv2 December 2011

7. References

7.1. Normative References

[RFC5996] Kaufman, C., Hoffman, P., Nir, Y., and P. Eronen,
"Internet Key Exchange Protocol Version 2 (IKEv2)",
RFC 5996, September 2010.

7.2. Informative References

[PACE] Kuegler, D. and Y. Sheffer, "Password Authenticated
Connection Establishment with IKEv2", Work in Progress,
September 2011.

[PAKE] Shin, S. and K. Kobara, "Most Efficient Augmented
Password-Only Authentication and Key Exchange for
IKEv2", Work in Progress, July 2011.

[SPSK-AUTH] Harkins, D., "Secure PSK Authentication for IKE", Work
in Progress, July 2011.

Author's Address

Tero Kivinen
AuthenTec
Eerikinkatu 28
HELSINKI FI-00180
Finland

EMail: [email protected]

Kivinen Informational [Page 10]