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Network Working Group V. Smyslov
Internet-Draft ELVIS-PLUS
Intended status: Standards Track January 25, 2018
Expires: July 29, 2018
Auxiliary Exchange in the IKEv2 Protocol
draft-smyslov-ipsecme-ikev2-aux-00
Abstract
This documents defines a new exchange, called Auxiliary Exchange, for
the Internet Key Exchange protocol Version 2 (IKEv2). This exchange
can be used for transferring large amount of data in the process of
IKEv2 Security Association (SA) establishment. Introducing Auxiliary
Exchange allows to re-use existing IKE Fragmentation mechanism, that
helps to avoid IP fragmentation of large IKE messages, but cannot be
used in the initial IKEv2 exchange.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on July 29, 2018.
Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved.
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology and Notation . . . . . . . . . . . . . . . . . . 3
3. Auxiliary Exchange Details . . . . . . . . . . . . . . . . . 3
3.1. Support for Auxiliary Exchange Negotiation . . . . . . . 3
3.2. Using Auxiliary Exchange . . . . . . . . . . . . . . . . 4
3.3. Keying Material and Authentication . . . . . . . . . . . 4
3.4. IKE Fragmentation . . . . . . . . . . . . . . . . . . . . 5
4. Interaction with other IKEv2 Extensions . . . . . . . . . . . 5
5. Security Considerations . . . . . . . . . . . . . . . . . . . 6
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 6
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
8.1. Normative References . . . . . . . . . . . . . . . . . . 7
8.2. Informative References . . . . . . . . . . . . . . . . . 7
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 7
1. Introduction
The Internet Key Exchange protocol version 2 (IKEv2) defined in
[RFC7296] uses UDP as a transport for its messages. If size of the
messages is large enough, IP fragmentation may take place that may
interfere badly with some network devices. The problem is described
in more detail in [RFC7383], which also defines an extension to the
IKEv2 called IKE Fragmentation. This extension allows IKE messages
to be fragmented at IKE level, which eliminates possible issues
caused by IP fragmentation. However, the IKE Fragmentation cannot be
used in the initial IKEv2 exchange, IKE_SA_INIT. This limitation in
most cases is not a problem, since the IKE_SA_INIT messages used to
be small enough to not cause IP fragmentation.
Recent progress in Quantum Computing has brought a concern that
classical Diffie-Hellman key exchange methods will become insecure in
a relatively near future and should be replaced with Quantum Computer
(QC) resistant ones. Currently most of QC-resistant key exchange
methods have large public keys. If these keys are exchanged in the
IKE_SA_INIT, then most probably IP fragmentation would take place,
therefore all the problems caused by it would become inevitable.
A possible solution would be to use TCP as a transport for IKEv2, as
described in [RFC8229]. However this approach has significant
drawbacks and is intended to be a "last resort" when UDP transport is
blocked by intermediate network devices.
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This document defines a new exchange for the IKEv2 protocol, called
Auxiliary Exchange or IKE_AUX. One or more these exchanges may take
place right after the IKE_SA_INIT exchange and prior to the IKE_AUTH
exchange. These exchanges may be used to exchange large amounts of
data, which don't fit into the IKE_SA_INIT exchange without causing
IP fragmentation. The IKE_AUX messages can be fragmented using IKE
Fragmentation mechanism.
While ability to transfer large public keys of QC-resistant methods
was a primary motivation for the Auxiliary Exchange, its application
is not limited to this use case. This exchange may be used whenever
large messages need to be exchanged before the IKE_AUTH exchange. It
is expected that separate specifications will define how and when the
IKE_AUX exchange is used in the IKEv2.
2. Terminology and Notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. Auxiliary Exchange Details
3.1. Support for Auxiliary Exchange Negotiation
The initiator indicates its support for Auxiliary Exchange by
including a notification of type AUX_EXCHANGE_SUPPORTED in the
IKE_SA_INIT request message. If the responder also supports this
exchange, it includes this notification in the response message.
Initiator Responder
----------- -----------
HDR, SAi1, KEi, Ni,
[N(AUX_EXCHANGE_SUPPORTED)] -->
<-- HDR, SAr1, KEr, Nr, [CERTREQ],
[N(AUX_EXCHANGE_SUPPORTED)]
The AUX_EXCHANGE_SUPPORTED is a Status Type IKEv2 notification. Its
Notify Message Type is <TBA by IANA>. Protocol ID and SPI Size are
both set to 0. This specification doesn't define any data this
notification may contain, so the Notification Data is left empty.
However, other specifications may override this. Implementations
MUST ignore the non-empty Notification Data if they don't understand
its purpose.
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3.2. Using Auxiliary Exchange
If both peers indicated their support for Auxiliary Exchange, the
initiator may use one or more these exchanges to transfer additional
data, which are not fit into the IKE_SA_INIT exchange. Using the
IKE_AUX exchange is optional, the initiator may find it unnecessary
after completing the IKE_SA_INIT exchange.
The Auxiliary Exchange is denoted as IKE_AUX, its Exchange Type is
<TBA by IANA>.
Initiator Responder
----------- -----------
HDR, ..., SK {...} -->
<-- HDR, ..., SK {...}
The initiator may use several IKE_AUX exchanges if necessary. Since
initiator's Window Size is initially set to one (Section 2.3 of
[RFC7296]), These exchanges MUST follow each other and MUST all be
completed before the IKE_AUTH exchange is initiated. The IKE SA MUST
NOT be considered as established until the IKE_AUTH exchange is
successfully completed.
The Message IDs for the IKE_AUX exchanges MUST be chosen by the
standard IKEv2 rule, described in the Section 2.2. of [RFC7296], i.e.
it is set to 1 for the first IKE_AUX exchange, 2 for the next (if
any) and so on. The message ID for the first pair of the IKE_AUTH
messages is one more than the last IKE_AUX Message ID.
The content of the IKE_AUX messages depends on the data being
transferred and will be defined by specifications utilizing this
exchange. However, since the main motivation for IKE_AUX is to avoid
IP fragmentation when large amount of data need to be transferred
prior to IKE_AUTH, the Encrypted payload SHOULD be present in the
IKE_AUX messages and payloads containing large data SHOULD be placed
inside. This will allow IKE Fragmentation [RFC7383] to take place,
provided it is supported by the peers and negotiated in the initial
exchange.
3.3. Keying Material and Authentication
The keys SK_e and SK_a for the Encrypted payload in the IKE_AUX
exchanges are computed in a standard fashion, as defined in the
Section 2.14 of [RFC7296]. Note that this may be redefined by other
specifications utilizing the IKE_AUX exchange (e.g. in case the
IKE_AUX is used to exchange additional keys which must later be
stirred into the SKEYSEED).
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The data transferred in the IKE_AUX exchanges must be authenticated
in the IKE_AUTH exchange. For this purpose the definition of the
blob to be signed (or MAC'ed) from the Section 2.15 of [RFC7296] is
modified as follows in case of at least one IKE_AUX exchange takes
place:
InitiatorSignedOctets = RealMessage1 | AUX_I | NonceRData | MACedIDForI
AUX_I = ICV_INIT_1 | ICV_INIT_2 | ICV_INIT_3 ...
ResponderSignedOctets = RealMessage2 | AUX_R | NonceIData | MACedIDForR
AUX_R = ICV_RESP_1 | ICV_RESP_2 | ICV_RESP_3 ...
ICV_INIT_1, ICV_INIT_2, ICV_INIT_3, etc. represent the content of the
Integrity Checksum Data field from the Encrypted payloads (or
Encrypted Fragment payloads) from all the IKE_AUX messages sent by
the initiator in an order of increasing MessageIDs (and increasing
Fragment Numbers for the same Message ID). ICV_RESP_1 | ICV_RESP_2 |
ICV_RESP_3 etc. are defined similarly for the messages sent by the
responder.
3.4. IKE Fragmentation
If both peers indicated their support for IKE Fragmentation, then
some additional restrictions are applied to ensure that the values of
Integrity Checksum Data is unambiguous. These restrictions MUST be
applied to the IKE_AUX exchanges only and MAY be lifted once all
these exchanges are over.
The responder MUST send the IKE_AUX response in the same form
(fragmented or not) as the request message. The initiator MUST NOT
switch from unfragmented to fragmented request in a single IKE_AUX
exchange - either the request is sent unfragmented and retransmitted
until unfragmented response is received (applicable if message size
is small and no IP fragmentation is expected), or the request is
fragmented from the beginning of exchange. The initiator MAY however
send either fragmented or unfragmented messages in different IKE_AUX
exchanges. The initiator SHOULD use IKE Fragmentation if the size of
request (or the expected size of response) is large enough to cause
IP fragmentation. The PMTU discovery for IKE Fragmentation as
defined in Section 2.5.2 of [RFC7383] MUST NOT be used for the
IKE_AUX exchanges.
4. Interaction with other IKEv2 Extensions
The IKE_AUTH exchanges may be used in the IKEv2 Session Resumption
[RFC5723] between the IKE_SESSION_RESUME and the IKE_AUTH exchanges.
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5. Security Considerations
The data that is transferred by means of the IKE_AUX exchanges is not
authenticated until the subsequent IKE_AUTH exchange is completed.
However, if the data is placed inside the Encrypted payload, then it
is protected from passive eavesdroppers. In addition the peers can
be certain that they receives messages from the party he/she
performed the IKE_SA_INIT with if they can successfully verify the
Integrity Checksum Data of the Encrypted payload.
The main application for Auxiliary Exchange is to transfer large
amount of data before IKE SA is set up without causing IP
fragmentation. For that reason it is expected that in most cases IKE
Fragmentation will be employed in the IKE_AUX exchanges. Section 5
of [RFC7383] contains security considerations for IKE Fragmentation.
Note, that if an attacker was able to break key exchange from the
IKE_SA_INIT in real time (e.g. by means of Quantum Computer), then
the security of IKE_AUX would degrage. In particular, such an
attacker would be able both to read data contained in the Encrypted
payload and to forge it. THe forgery would become evident in the
IKE_AUTH exchange (provided the attacker caanot break employed
authentication mechanism), but the ability to inject forged IKE_AUX
messages with valid ICV would allow the attacker to mount Denial-of-
Service attack.
6. IANA Considerations
This document defines a new Exchange Type in the "IKEv2 Exchange
Types" registry:
<TBA> IKE_AUX
This document also defines a new Notify Message Types in the "Notify
Message Types - Status Types" registry:
<TBA> AUX_EXCHANGE_SUPPORTED
7. Acknowledgements
The idea to use an intermediate exchange between IKE_SA_INIT and
IKE_AUTH was first suggested by Tero Kivinen.
8. References
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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>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[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>.
[RFC7383] Smyslov, V., "Internet Key Exchange Protocol Version 2
(IKEv2) Message Fragmentation", RFC 7383,
DOI 10.17487/RFC7383, November 2014, <https://www.rfc-
editor.org/info/rfc7383>.
8.2. Informative References
[RFC8229] Pauly, T., Touati, S., and R. Mantha, "TCP Encapsulation
of IKE and IPsec Packets", RFC 8229, DOI 10.17487/RFC8229,
August 2017, <https://www.rfc-editor.org/info/rfc8229>.
[RFC5723] Sheffer, Y. and H. Tschofenig, "Internet Key Exchange
Protocol Version 2 (IKEv2) Session Resumption", RFC 5723,
DOI 10.17487/RFC5723, January 2010, <https://www.rfc-
editor.org/info/rfc5723>.
Author's Address
Valery Smyslov
ELVIS-PLUS
PO Box 81
Moscow (Zelenograd) 124460
RU
Phone: +7 495 276 0211
Email: svan@elvis.ru
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