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Internet Engineering Task Force (IETF) V. Smyslov
Request for Comments: 9242 ELVIS-PLUS
Category: Standards Track May 2022
ISSN: 2070-1721
Intermediate Exchange in the Internet Key Exchange Protocol Version 2
(IKEv2)
Abstract
This document defines a new exchange, called "Intermediate Exchange",
for the Internet Key Exchange Protocol Version 2 (IKEv2). This
exchange can be used for transferring large amounts of data in the
process of IKEv2 Security Association (SA) establishment. An example
of the need to do this is using key exchange methods resistant to
Quantum Computers (QCs) for IKE SA establishment. The Intermediate
Exchange makes it possible to use the existing IKE fragmentation
mechanism (which cannot be used in the initial IKEv2 exchange),
helping to avoid IP fragmentation of large IKE messages if they need
to be sent before IKEv2 SA is established.
Status of This Memo
This is an Internet Standards Track document.
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). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9242.
Copyright Notice
Copyright (c) 2022 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
(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 Revised BSD License text as described in Section 4.e of the
Trust Legal Provisions and are provided without warranty as described
in the Revised BSD License.
Table of Contents
1. Introduction
2. Terminology and Notation
3. Intermediate Exchange Details
3.1. Support for Intermediate Exchange Negotiation
3.2. Using Intermediate Exchange
3.3. The IKE_INTERMEDIATE Exchange Protection and Authentication
3.3.1. Protection of IKE_INTERMEDIATE Messages
3.3.2. Authentication of IKE_INTERMEDIATE Exchanges
3.4. Error Handling in the IKE_INTERMEDIATE Exchange
4. Interaction with Other IKEv2 Extensions
5. Security Considerations
6. IANA Considerations
7. References
7.1. Normative References
7.2. Informative References
Appendix A. Example of IKE_INTERMEDIATE Exchange
Acknowledgements
Author's Address
1. Introduction
The Internet Key Exchange Protocol Version 2 (IKEv2) defined in
[RFC7296] uses UDP as a transport for its messages. If the size of a
message is larger than the Path MTU (PMTU), IP fragmentation takes
place, which has been shown to cause operational challenges in
certain network configurations and devices. The problem is described
in more detail in [RFC7383], which also defines an extension to IKEv2
called "IKE fragmentation". This extension allows IKE messages to be
fragmented at the IKE level, eliminating possible issues caused by IP
fragmentation. However, IKE fragmentation cannot be used in the
initial IKEv2 exchange (IKE_SA_INIT). In most cases, this limitation
is not a problem, since the IKE_SA_INIT messages are usually small
enough not to cause IP fragmentation.
However, the situation has been changing recently. One example of
the need to transfer large amounts of data before an IKE SA is
created is using the QC-resistant key exchange methods in IKEv2.
Recent progress in quantum computing has led to concern that
classical Diffie-Hellman key exchange methods will become insecure in
the relatively near future and should be replaced with QC-resistant
ones. Currently, most QC-resistant key exchange methods have large
public keys. If these keys are exchanged in the IKE_SA_INIT
exchange, then IP fragmentation will probably take place; therefore,
all the problems caused by it will become inevitable.
A possible solution to this problem would be to use TCP as a
transport for IKEv2, as defined in [RFC8229]. However, this approach
has significant drawbacks and is intended to be a last resort when
UDP transport is completely blocked by intermediate network devices.
This specification describes a way to transfer a large amount of data
in IKEv2 using UDP transport. For this purpose, the document defines
a new exchange for IKEv2 called "Intermediate Exchange" or
"IKE_INTERMEDIATE". One or more of these exchanges may take place
right after the IKE_SA_INIT exchange and prior to the IKE_AUTH
exchange. The IKE_INTERMEDIATE exchange messages can be fragmented
using the IKE fragmentation mechanism, so these exchanges may be used
to transfer large amounts of data that don't fit into the IKE_SA_INIT
exchange without causing IP fragmentation.
The Intermediate Exchange can be used to transfer large public keys
of QC-resistant key exchange methods, but its application is not
limited to this use case. This exchange can also be used whenever
some data needs to be transferred before the IKE_AUTH exchange and
for some reason the IKE_SA_INIT exchange is not suited for this
purpose. This document defines the IKE_INTERMEDIATE exchange without
tying it to any specific use case. It is expected that separate
specifications will define for which purposes and how the
IKE_INTERMEDIATE exchange is used in IKEv2. Some considerations must
be taken into account when designing such specifications:
* The IKE_INTERMEDIATE exchange is not intended for bulk transfer.
This document doesn't set a hard cap on the amount of data that
can be safely transferred using this mechanism, as it depends on
its application. However, in most cases, it is anticipated that
the amount of data will be limited to tens of kilobytes (a few
hundred kilobytes in extreme cases), which is believed to cause no
network problems (see [RFC6928] as an example of experiments with
sending similar amounts of data in the first TCP flight). See
also Section 5 for the discussion of possible DoS attack vectors
when the amount of data sent in the IKE_INTERMEDIATE exchange is
too large.
* It is expected that the IKE_INTERMEDIATE exchange will only be
used for transferring data that is needed to establish IKE SA and
not for data that can be sent later when this SA is established.
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.
It is expected that readers are familiar with the terms used in the
IKEv2 specification [RFC7296]. Notation for the payloads contained
in IKEv2 messages is defined in Section 1.2 of [RFC7296].
3. Intermediate Exchange Details
3.1. Support for Intermediate Exchange Negotiation
The initiator indicates its support for Intermediate Exchange by
including a notification of type INTERMEDIATE_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(INTERMEDIATE_EXCHANGE_SUPPORTED)] -->
<-- HDR, SAr1, KEr, Nr, [CERTREQ],
[N(INTERMEDIATE_EXCHANGE_SUPPORTED)]
The INTERMEDIATE_EXCHANGE_SUPPORTED is a Status Type IKEv2
notification with Notify Message Type 16438. When it is sent, the
Protocol ID and SPI Size fields in the Notify payload are both set to
0. This specification doesn't define any data that this notification
may contain, so the Notification Data is left empty. However, future
enhancements to this specification may override this.
Implementations MUST ignore non-empty Notification Data if they don't
understand its purpose.
3.2. Using Intermediate Exchange
If both peers indicated their support for the Intermediate Exchange,
the initiator may use one or more these exchanges to transfer
additional data. Using the Intermediate Exchange is optional; the
initiator may find it unnecessary even when support for this exchange
has been negotiated.
The Intermediate Exchange is denoted as IKE_INTERMEDIATE; its
Exchange Type is 43.
Initiator Responder
----------- -----------
HDR, ..., SK {...} -->
<-- HDR, ..., SK {...}
The initiator may use several IKE_INTERMEDIATE exchanges if
necessary. Since window size is initially set to 1 for both peers
(Section 2.3 of [RFC7296]), these exchanges MUST be sequential 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 IKE_INTERMEDIATE exchanges MUST be chosen
according to the standard IKEv2 rule, described in Section 2.2 of
[RFC7296], i.e., it is set to 1 for the first IKE_INTERMEDIATE
exchange, 2 for the next (if any), and so on. Implementations MUST
verify that Message IDs in the IKE_INTERMEDIATE messages they receive
actually follow this rule. The Message ID for the first pair of
IKE_AUTH messages is one more than the value used in the last
IKE_INTERMEDIATE exchange.
If the presence of NAT is detected in the IKE_SA_INIT exchange via
NAT_DETECTION_SOURCE_IP and NAT_DETECTION_DESTINATION_IP
notifications, then the peers switch to port 4500 in the first
IKE_INTERMEDIATE exchange and use this port for all subsequent
exchanges, as described in Section 2.23 of [RFC7296].
The content of the IKE_INTERMEDIATE exchange messages depends on the
data being transferred and will be defined by specifications
utilizing this exchange. However, since the main motivation for the
IKE_INTERMEDIATE exchange is to avoid IP fragmentation when large
amounts of data need to be transferred prior to the IKE_AUTH
exchange, the Encrypted payload MUST be present in the
IKE_INTERMEDIATE exchange messages, and payloads containing large
amounts of data MUST be placed inside it. This will allow IKE
fragmentation [RFC7383] to take place, provided it is supported by
the peers and negotiated in the initial exchange.
Appendix A contains an example of using an IKE_INTERMEDIATE exchange
in creating an IKE SA.
3.3. The IKE_INTERMEDIATE Exchange Protection and Authentication
3.3.1. Protection of IKE_INTERMEDIATE Messages
The keys SK_e[i/r] and SK_a[i/r] for the protection of
IKE_INTERMEDIATE exchanges are computed in the standard fashion, as
defined in Section 2.14 of [RFC7296].
Every subsequent IKE_INTERMEDIATE exchange uses the most recently
calculated IKE SA keys before this exchange is started. So, the
first IKE_INTERMEDIATE exchange always uses SK_e[i/r] and SK_a[i/r]
keys that were computed as a result of the IKE_SA_INIT exchange. If
additional key exchange is performed in the first IKE_INTERMEDIATE
exchange, resulting in the update of SK_e[i/r] and SK_a[i/r], then
these updated keys are used for protection of the second
IKE_INTERMEDIATE exchange. Otherwise, the original SK_e[i/r] and
SK_a[i/r] keys are used again, and so on.
Once all the IKE_INTERMEDIATE exchanges are completed, the most
recently calculated SK_e[i/r] and SK_a[i/r] keys are used for
protection of the IKE_AUTH exchange and all subsequent exchanges.
3.3.2. Authentication of IKE_INTERMEDIATE Exchanges
The IKE_INTERMEDIATE messages must be authenticated in the IKE_AUTH
exchange, which is performed by adding their content into the AUTH
payload calculation. It is anticipated that in many use cases,
IKE_INTERMEDIATE messages will be fragmented using the IKE
fragmentation [RFC7383] mechanism. According to [RFC7383], when IKE
fragmentation is negotiated, the initiator may first send a request
message in unfragmented form, but later turn on IKE fragmentation and
resend it fragmented if no response is received after a few
retransmissions. In addition, peers may resend a fragmented message
using different fragment sizes to perform simple PMTU discovery.
The requirement to support this behavior makes authentication
challenging: it is not appropriate to add on-the-wire content of the
IKE_INTERMEDIATE messages into the AUTH payload calculation, because
implementations are generally unaware of which form these messages
are received by peers. Instead, a more complex scheme is used;
authentication is performed by adding the content of these messages
before their encryption and possible fragmentation, so that the data
to be authenticated doesn't depend on the form the messages are
delivered in.
If one or more IKE_INTERMEDIATE exchanges took place, the definition
of the blob to be signed (or MACed) from Section 2.15 of [RFC7296] is
modified as follows:
InitiatorSignedOctets = RealMsg1 | NonceRData | MACedIDForI | IntAuth
ResponderSignedOctets = RealMsg2 | NonceIData | MACedIDForR | IntAuth
IntAuth = IntAuth_iN | IntAuth_rN | IKE_AUTH_MID
IntAuth_i1 = prf(SK_pi1, IntAuth_i1A [| IntAuth_i1P])
IntAuth_i2 = prf(SK_pi2, IntAuth_i1 | IntAuth_i2A [| IntAuth_i2P])
IntAuth_i3 = prf(SK_pi3, IntAuth_i2 | IntAuth_i3A [| IntAuth_i3P])
...
IntAuth_iN = prf(SK_piN, IntAuth_iN-1 | IntAuth_iNA [| IntAuth_iNP])
IntAuth_r1 = prf(SK_pr1, IntAuth_r1A [| IntAuth_r1P])
IntAuth_r2 = prf(SK_pr2, IntAuth_r1 | IntAuth_r2A [| IntAuth_r2P])
IntAuth_r3 = prf(SK_pr3, IntAuth_r2 | IntAuth_r3A [| IntAuth_r3P])
...
IntAuth_rN = prf(SK_prN, IntAuth_rN-1 | IntAuth_rNA [| IntAuth_rNP])
The essence of this modification is that a new chunk called "IntAuth"
is appended to the string of octets that is signed (or MACed) by the
peers. IntAuth consists of three parts: IntAuth_iN, IntAuth_rN, and
IKE_AUTH_MID.
The IKE_AUTH_MID chunk is a value of the Message ID field from the
IKE Header of the first round of the IKE_AUTH exchange. It is
represented as a four-octet integer in network byte order (in other
words, exactly as it appears on the wire).
The IntAuth_iN and IntAuth_rN chunks represent the cumulative result
of applying the negotiated Pseudorandom Function (PRF) to all
IKE_INTERMEDIATE exchange messages sent during IKE SA establishment
by the initiator and the responder, respectively. After the first
IKE_INTERMEDIATE exchange is complete, peers calculate the IntAuth_i1
value by applying the negotiated PRF to the content of the request
message from this exchange and calculate the IntAuth_r1 value by
applying the negotiated PRF to the content of the response message.
For every subsequent IKE_INTERMEDIATE exchange (if any), peers
recalculate these values as follows: after the nth exchange is
complete, they compute IntAuth_[i/r]n by applying the negotiated PRF
to the concatenation of IntAuth_[i/r](n-1) (computed for the previous
IKE_INTERMEDIATE exchange) and the content of the request (for
IntAuth_in) or response (for IntAuth_rn) messages from this exchange.
After all IKE_INTERMEDIATE exchanges are over, the resulted
IntAuth_[i/r]N values (assuming N exchanges took place) are used in
computing the AUTH payload.
For the purpose of calculating the IntAuth_[i/r]* values, the content
of the IKE_INTERMEDIATE messages is represented as two chunks of
data: mandatory IntAuth_[i/r]*A, optionally followed by IntAuth_[i/
r]*P.
The IntAuth_[i/r]*A chunk consists of the sequence of octets from the
first octet of the IKE Header (not including the prepended four
octets of zeros, if UDP encapsulation or TCP encapsulation of ESP
packets is used) to the last octet of the generic header of the
Encrypted payload. The scope of IntAuth_[i/r]*A is identical to the
scope of Associated Data defined for the use of AEAD algorithms in
IKEv2 (see Section 5.1 of [RFC5282]), which is stressed by using the
"A" suffix in its name. Note that calculation of IntAuth_[i/r]*A
doesn't depend on whether an AEAD algorithm or a plain cipher is used
in IKE SA.
The IntAuth_[i/r]*P chunk is present if the Encrypted payload is not
empty. It consists of the content of the Encrypted payload that is
fully formed but not yet encrypted. The Initialization Vector,
Padding, Pad Length, and Integrity Checksum Data fields (see
Section 3.14 of [RFC7296]) are not included into the calculation. In
other words, the IntAuth_[i/r]*P chunk is the inner payloads of the
Encrypted payload in plaintext form, which is stressed by using the
"P" suffix in its name.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ^ ^
| IKE SA Initiator's SPI | | |
| | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ I |
| IKE SA Responder's SPI | K |
| | E |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| Next Payload | MjVer | MnVer | Exchange Type | Flags | H |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ d |
| Message ID | r A
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | |
| Adjusted Length | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ v |
| | |
~ Unencrypted payloads (if any) ~ |
| | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ^ |
| Next Payload |C| RESERVED | Adjusted Payload Length | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | v
| | |
~ Initialization Vector ~ E
| | E
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ c ^
| | r |
~ Inner payloads (not yet encrypted) ~ P
| | P |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ l v
| Padding (0-255 octets) | Pad Length | d
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| | |
~ Integrity Checksum Data ~ |
| | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ v
Figure 1: Data to Authenticate in the IKE_INTERMEDIATE Exchange
Messages
Figure 1 illustrates the layout of the IntAuth_[i/r]*A (denoted as A)
and the IntAuth_[i/r]*P (denoted as P) chunks in case the Encrypted
payload is not empty.
For the purpose of prf calculation, the Length field in the IKE
Header and the Payload Length field in the Encrypted payload header
are adjusted so that they don't count the lengths of Initialization
Vector, Integrity Checksum Data, Padding, and Pad Length fields. In
other words, the Length field in the IKE Header (denoted as Adjusted
Length in Figure 1) is set to the sum of the lengths of IntAuth_[i/
r]*A and IntAuth_[i/r]*P, and the Payload Length field in the
Encrypted payload header (denoted as Adjusted Payload Length in
Figure 1) is set to the length of IntAuth_[i/r]*P plus the size of
the Encrypted payload header (four octets).
The prf calculations MUST be applied to whole messages only, before
possible IKE fragmentation. This ensures that the IntAuth will be
the same regardless of whether or not IKE fragmentation takes place.
If the message was received in fragmented form, it MUST be
reconstructed before calculating the prf as if it were received
unfragmented. While reconstructing, the RESERVED field in the
reconstructed Encrypted payload header MUST be set to the value of
the RESERVED field in the Encrypted Fragment payload header from the
first fragment (with the Fragment Number field set to 1).
Note that it is possible to avoid actual reconstruction of the
message by incrementally calculating prf on decrypted (or ready to be
encrypted) fragments. However, care must be taken to properly
replace the content of the Next Header and the Length fields so that
the result of computing the prf is the same as if it were computed on
the reconstructed message.
Each calculation of IntAuth_[i/r]* uses its own keys SK_p[i/r]*,
which are the most recently updated SK_p[i/r] keys available before
the corresponded IKE_INTERMEDIATE exchange is started. The first
IKE_INTERMEDIATE exchange always uses the SK_p[i/r] keys that were
computed in the IKE_SA_INIT exchange as SK_p[i/r]1. If the first
IKE_INTERMEDIATE exchange performs additional key exchange resulting
in an SK_p[i/r] update, then these updated SK_p[i/r] keys are used as
SK_p[i/r]2; otherwise, the original SK_p[i/r] keys are used, and so
on. Note that if keys are updated, then for any given
IKE_INTERMEDIATE exchange, the keys SK_e[i/r] and SK_a[i/r] used for
protection of its messages (see Section 3.3.1) and the key SK_p[i/r]
for its authentication are always from the same generation.
3.4. Error Handling in the IKE_INTERMEDIATE Exchange
Since messages of the IKE_INTERMEDIATE exchange are not authenticated
until the IKE_AUTH exchange successfully completes, possible errors
need to be handled with care. There is a trade-off between providing
better diagnostics of the problem and risk of becoming part of a DoS
attack. Sections 2.21.1 and 2.21.2 of [RFC7296] describe how errors
are handled in initial IKEv2 exchanges; these considerations are also
applied to the IKE_INTERMEDIATE exchange with the qualification that
not all error notifications may appear in the IKE_INTERMEDIATE
exchange (for example, errors concerning authentication are generally
only applicable to the IKE_AUTH exchange).
4. Interaction with Other IKEv2 Extensions
The IKE_INTERMEDIATE exchanges MAY be used during the IKEv2 Session
Resumption [RFC5723] between the IKE_SESSION_RESUME and the IKE_AUTH
exchanges. To be able to use it, peers MUST negotiate support for
Intermediate Exchange by including INTERMEDIATE_EXCHANGE_SUPPORTED
notifications in the IKE_SESSION_RESUME messages. Note that a flag
denoting whether peers supported the IKE_INTERMEDIATE exchange is not
stored in the resumption ticket and is determined each time from the
IKE_SESSION_RESUME exchange.
5. Security Considerations
The data that is transferred by means of the IKE_INTERMEDIATE
exchanges is not authenticated until the subsequent IKE_AUTH exchange
is complete. 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 receive messages from
the party they performed the IKE_SA_INIT exchange with if they can
successfully verify the Integrity Checksum Data of the Encrypted
payload.
The main application for the Intermediate Exchange is to transfer
large amounts of data before an IKE SA is set up, without causing IP
fragmentation. For that reason, it is expected that IKE
fragmentation will be employed in IKE_INTERMEDIATE exchanges in most
cases. Section 5 of [RFC7383] contains security considerations for
IKE fragmentation.
Since authentication of peers occurs only in the IKE_AUTH exchange, a
malicious initiator may use the Intermediate Exchange to mount a DoS
attack on the responder. In this case, it starts creating an IKE SA,
negotiates using the Intermediate Exchanges, and transfers a lot of
data to the responder that may also require computationally expensive
processing. Then, it aborts the SA establishment before the IKE_AUTH
exchange. Specifications utilizing the Intermediate Exchange MUST
NOT allow an unlimited number of these exchanges to take place at the
initiator's discretion. It is recommended that these specifications
be defined in such a way that the responder would know (possibly via
negotiation with the initiator) the exact number of these exchanges
that need to take place. In other words, after the IKE_SA_INIT
exchange is complete, it is preferred that both the initiator and the
responder know the exact number of IKE_INTERMEDIATE exchanges they
have to perform; it is possible that some IKE_INTERMEDIATE exchanges
are optional and are performed at the initiator's discretion, but if
a specification defines optional use of IKE_INTERMEDIATE, then the
maximum number of these exchanges must be hard capped by the
corresponding specification. In addition, [RFC8019] provides
guidelines for the responder of how to deal with DoS attacks during
IKE SA establishment.
Note that if an attacker was able to break the key exchange in real
time (e.g., by means of a quantum computer), then the security of the
IKE_INTERMEDIATE exchange would degrade. In particular, such an
attacker would be able to both read data contained in the Encrypted
payload and forge it. The forgery would become evident in the
IKE_AUTH exchange (provided the attacker cannot break the employed
authentication mechanism), but the ability to inject forged
IKE_INTERMEDIATE exchange messages with a valid Integrity Check Value
(ICV) would allow the attacker to mount a DoS attack. Moreover, in
this situation, if the negotiated PRF was not secure against a second
preimage attack with known key, then the attacker could forge the
IKE_INTERMEDIATE exchange messages without later being detected in
the IKE_AUTH exchange. To do this, the attacker would find the same
IntAuth_[i/r]* value for the forged message as for the original.
6. IANA Considerations
This document defines a new Exchange Type in the "IKEv2 Exchange
Types" registry:
+=======+==================+===========+
| Value | Exchange Type | Reference |
+=======+==================+===========+
| 43 | IKE_INTERMEDIATE | RFC 9242 |
+-------+------------------+-----------+
Table 1: IKEv2 Exchange Types
This document also defines a new Notify Message Type in the "IKEv2
Notify Message Types - Status Types" registry:
+=======+=================================+===========+
| Value | NOTIFY MESSAGES - STATUS TYPES | Reference |
+=======+=================================+===========+
| 16438 | INTERMEDIATE_EXCHANGE_SUPPORTED | RFC 9242 |
+-------+---------------------------------+-----------+
Table 2: IKEv2 Notify Message Types - Status Types
7. References
7.1. 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>.
[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>.
[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>.
7.2. Informative References
[RFC5282] Black, D. and D. McGrew, "Using Authenticated Encryption
Algorithms with the Encrypted Payload of the Internet Key
Exchange version 2 (IKEv2) Protocol", RFC 5282,
DOI 10.17487/RFC5282, August 2008,
<https://www.rfc-editor.org/info/rfc5282>.
[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>.
[RFC6928] Chu, J., Dukkipati, N., Cheng, Y., and M. Mathis,
"Increasing TCP's Initial Window", RFC 6928,
DOI 10.17487/RFC6928, April 2013,
<https://www.rfc-editor.org/info/rfc6928>.
[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>.
[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>.
Appendix A. Example of IKE_INTERMEDIATE Exchange
This appendix contains an example of the messages using
IKE_INTERMEDIATE exchanges. This appendix is purely informative; if
it disagrees with the body of this document, the other text is
considered correct.
In this example, there is one IKE_SA_INIT exchange and two
IKE_INTERMEDIATE exchanges, followed by the IKE_AUTH exchange to
authenticate all initial exchanges. The xxx in the HDR(xxx,MID=yyy)
indicates the Exchange Type, and yyy indicates the Message ID used
for that exchange. The keys used for each SK {} payload are
indicated in the parenthesis after the SK. Otherwise, the payload
notation is the same as is used in [RFC7296].
Initiator Responder
----------- -----------
HDR(IKE_SA_INIT,MID=0),
SAi1, KEi, Ni,
N(INTERMEDIATE_EXCHANGE_SUPPORTED) -->
<-- HDR(IKE_SA_INIT,MID=0),
SAr1, KEr, Nr, [CERTREQ],
N(INTERMEDIATE_EXCHANGE_SUPPORTED)
At this point, peers calculate SK_* and store them as SK_*1. SK_e[i/
r]1 and SK_a[i/r]1 will be used to protect the first IKE_INTERMEDIATE
exchange, and SK_p[i/r]1 will be used for its authentication.
Initiator Responder
----------- -----------
HDR(IKE_INTERMEDIATE,MID=1),
SK(SK_ei1,SK_ai1) {...} -->
<Calculate IntAuth_i1 = prf(SK_pi1, ...)>
<-- HDR(IKE_INTERMEDIATE,MID=1),
SK(SK_er1,SK_ar1) {...}
<Calculate IntAuth_r1 = prf(SK_pr1, ...)>
If the SK_*1 keys are updated (e.g., as a result of a new key
exchange) after completing this IKE_INTERMEDIATE exchange, then the
peers store the updated keys as SK_*2; otherwise, they use SK_*1 as
SK_*2. SK_e[i/r]2 and SK_a[i/r]2 will be used to protect the second
IKE_INTERMEDIATE exchange, and SK_p[i/r]2 will be used for its
authentication.
Initiator Responder
----------- -----------
HDR(IKE_INTERMEDIATE,MID=2),
SK(SK_ei2,SK_ai2) {...} -->
<Calculate IntAuth_i2 = prf(SK_pi2, ...)>
<-- HDR(IKE_INTERMEDIATE,MID=2),
SK(SK_er2,SK_ar2) {...}
<Calculate IntAuth_r2 = prf(SK_pr2, ...)>
If the SK_*2 keys are updated (e.g., as a result of a new key
exchange) after completing the second IKE_INTERMEDIATE exchange, then
the peers store the updated keys as SK_*3; otherwise, they use SK_*2
as SK_*3. SK_e[i/r]3 and SK_a[i/r]3 will be used to protect the
IKE_AUTH exchange, SK_p[i/r]3 will be used for authentication, and
SK_d3 will be used for derivation of other keys (e.g., for Child
SAs).
Initiator Responder
----------- -----------
HDR(IKE_AUTH,MID=3),
SK(SK_ei3,SK_ai3)
{IDi, [CERT,] [CERTREQ,]
[IDr,] AUTH, SAi2, TSi, TSr} -->
<-- HDR(IKE_AUTH,MID=3),
SK(SK_er3,SK_ar3)
{IDr, [CERT,] AUTH, SAr2, TSi, TSr}
In this example, two IKE_INTERMEDIATE exchanges took place;
therefore, SK_*3 keys would be used as SK_* keys for further
cryptographic operations in the context of the created IKE SA, as
defined in [RFC7296].
Acknowledgements
The idea to use an Intermediate Exchange between the IKE_SA_INIT and
IKE_AUTH exchanges was first suggested by Tero Kivinen. He also
helped to write the example IKE_INTERMEDIATE exchange shown in
Appendix A. Scott Fluhrer and Daniel Van Geest identified a possible
problem with authentication of the IKE_INTERMEDIATE exchange and
helped to resolve it. The author is grateful to Tobias Brunner, who
raised good questions concerning authentication of the
IKE_INTERMEDIATE exchange and proposed how to make the size of
authentication chunks constant regardless of the number of exchanges.
The author is also grateful to Paul Wouters and Benjamin Kaduk, who
suggested a lot of text improvements for the document.
Author's Address
Valery Smyslov
ELVIS-PLUS
PO Box 81
Moscow (Zelenograd)
124460
Russian Federation
Phone: +7 495 276 0211
Email: svan@elvis.ru