Internet Engineering Task Force (IETF) T. Kivinen
Request for Comments: 7815 INSIDE Secure
Category: Informational March 2016
ISSN: 2070-1721
Minimal Internet Key Exchange Version 2 (IKEv2) Initiator Implementation
Abstract
This document describes a minimal initiator version of the Internet
Key Exchange version 2 (IKEv2) protocol for constrained nodes. IKEv2
is a component of IPsec used for performing mutual authentication and
establishing and maintaining Security Associations (SAs). IKEv2
includes several optional features, which are not needed in minimal
implementations. This document describes what is required from the
minimal implementation and also describes various optimizations that
can be done. The protocol described here is interoperable with a
full IKEv2 implementation using shared secret authentication (IKEv2
does not require the use of certificate authentication). This
minimal initiator implementation can only talk to a full IKEv2
implementation acting as the responder; thus, two minimal initiator
implementations cannot talk to each other.
This document does not update or modify RFC 7296 but provides a more
compact description of the minimal version of the protocol. If this
document and RFC 7296 conflict, then RFC 7296 is the authoritative
description.
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/rfc7815.
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Copyright Notice
Copyright (c) 2016 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
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Contributions published or made publicly available before November
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Without obtaining an adequate license from the person(s) controlling
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outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . 5
2. Exchanges . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1. Initial Exchange . . . . . . . . . . . . . . . . . . . . 5
2.2. Other Exchanges . . . . . . . . . . . . . . . . . . . . . 12
2.3. Generating Keying Material . . . . . . . . . . . . . . . 12
3. Conformance Requirements . . . . . . . . . . . . . . . . . . 13
4. Implementation Status . . . . . . . . . . . . . . . . . . . . 14
5. Security Considerations . . . . . . . . . . . . . . . . . . . 14
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 15
6.1. Normative References . . . . . . . . . . . . . . . . . . 15
6.2. Informative References . . . . . . . . . . . . . . . . . 15
Appendix A. Header and Payload Formats . . . . . . . . . . . . . 17
A.1. The IKE Header . . . . . . . . . . . . . . . . . . . . . 17
A.2. Generic Payload Header . . . . . . . . . . . . . . . . . 19
A.3. Security Association Payload . . . . . . . . . . . . . . 21
A.3.1. Proposal Substructure . . . . . . . . . . . . . . . . 23
A.3.2. Transform Substructure . . . . . . . . . . . . . . . 24
A.3.3. Valid Transform Types by Protocol . . . . . . . . . . 26
A.3.4. Transform Attributes . . . . . . . . . . . . . . . . 26
A.4. Key Exchange Payload . . . . . . . . . . . . . . . . . . 27
A.5. Identification Payloads . . . . . . . . . . . . . . . . . 27
A.6. Certificate Payload . . . . . . . . . . . . . . . . . . . 29
A.7. Certificate Request Payload . . . . . . . . . . . . . . . 30
A.8. Authentication Payload . . . . . . . . . . . . . . . . . 31
A.9. Nonce Payload . . . . . . . . . . . . . . . . . . . . . . 31
A.10. Notify Payload . . . . . . . . . . . . . . . . . . . . . 32
A.10.1. Notify Message Types . . . . . . . . . . . . . . . . 33
A.11. Traffic Selector Payload . . . . . . . . . . . . . . . . 34
A.11.1. Traffic Selector . . . . . . . . . . . . . . . . . . 36
A.12. Encrypted Payload . . . . . . . . . . . . . . . . . . . . 37
Appendix B. Useful Optional Features . . . . . . . . . . . . . . 39
B.1. IKE SA Delete Notification . . . . . . . . . . . . . . . 39
B.2. Raw Public Keys . . . . . . . . . . . . . . . . . . . . . 40
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 41
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 41
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1. Introduction
The Internet Protocol Suite is increasingly used on small devices
with severe constraints on power, memory, and processing resources.
This document describes a minimal IKEv2 implementation designed for
use on such constrained nodes that is interoperable with "Internet
Key Exchange Protocol Version 2 (IKEv2)" [RFC7296].
A minimal IKEv2 implementation only supports the initiator end of the
protocol. It only supports the initial IKE_SA_INIT and IKE_AUTH
exchanges and does not initiate any other exchanges. It also replies
with an empty (or error) message to all incoming requests.
This means that most of the optional features of IKEv2 are left out:
NAT traversal, IKE SA rekey, Child SA rekey, multiple Child SAs,
deleting Child / IKE SAs, Configuration payloads, Extensible
Authentication Protocol (EAP) authentication, COOKIEs, etc.
Some optimizations can be done because of the limited set of
supported features, and this text should not be considered for
generic IKEv2 implementations (for example, Message IDs can be done
as specified because minimal implementation is only sending out an
IKE_SA_INIT and IKE_AUTH request and not any other request).
This document is intended to be standalone, meaning everything needed
to implement IKEv2 is copied here except the description of the
cryptographic algorithms. The IKEv2 specification has lots of
background information and rationale that has been omitted from this
document.
Numerous additional numeric values from IANA registries have been
omitted from this document; only those which are of interest for a
minimal implementation are listed.
The main body of this document describes how to use the shared secret
authentication in IKEv2, as it is easiest to implement. In some
cases, that is not enough, and Appendix B.2 describes how to use raw
public keys instead of shared secret authentication.
For more information, check the full IKEv2 specification in [RFC7296]
and [IKEV2IANA].
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 [RFC2119]. The term
"Constrained Node" is defined in "Terminology for Constrained-Node
Networks" [RFC7228].
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1.1. Use Cases
One use case for this kind of minimal implementation is in small
devices doing machine-to-machine communication. In such
environments, the node initiating connections can be very small, and
the other end of the communication channel is some kind of larger
device.
An example of the small initiating node could be a remote garage door
opener device, i.e., a device having buttons that open and close a
garage door and that connects to the home area network server over a
wireless link.
Another example of such a device is some kind of sensor device, for
example, a room temperature sensor, which sends periodic temperature
data to some centralized node.
Those devices usually sleep for a long time and only wake up
periodically or because of user interaction. The data transfer is
always initiated from that sleeping node when they wake up; after
they send packets, there might be ACKs or other packets coming back
before they go back to sleep. If some data needs to be transferred
from a server node to the small device, it can be implemented by
polling, i.e., the small node periodically polls for the server to
see if it, for example, has some configuration changes or similar.
While the device is sleeping, it will not maintain the IKEv2 SA.
That is, it will always create the IKEv2 SA again when it wakes up.
This means there is no need to do liveness checks for the server, as
after the device wakes up again, the minimal implementation will
start from the beginning again.
2. Exchanges
2.1. Initial Exchange
All IKEv2 communications consist of pairs of messages: a request and
a response. The pair is called an "exchange" and is sometimes called
a "request/response pair". Every request requires a response.
For every pair of IKEv2 messages, the initiator is responsible for
retransmission in the event of a timeout. The responder MUST never
retransmit a response unless it receives a retransmission of the
request.
IKEv2 is a reliable protocol: the initiator MUST retransmit a request
until it either receives a corresponding response or deems the IKE SA
to have failed. A retransmission from the initiator MUST be bitwise
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identical to the original request. Retransmission times MUST
increase exponentially.
IKEv2 is run over UDP port 500. All IKEv2 implementations MUST be
able to send, receive, and process IKEv2 messages that are up to 1280
octets long. An implementation MUST accept incoming requests even if
the source port is not 500 and MUST respond to the address and port
from which the request was received.
The minimal implementation of IKEv2 only uses the first two
exchanges, called IKE_SA_INIT and IKE_AUTH. These are used to create
the IKE SA and the first Child SA. In addition to those messages, a
minimal IKEv2 implementation needs to understand the CREATE_CHILD_SA
request enough to generate a CREATE_CHILD_SA response containing the
NO_ADDITIONAL_SAS error notify. It needs to understand the
INFORMATIONAL request enough to generate an empty INFORMATIONAL
response to it. There is no requirement to be able to respond to any
other requests.
All messages following the IKE_SA_INIT exchange are cryptographically
protected using the cryptographic algorithms and keys negotiated in
the IKE_SA_INIT exchange.
Every IKEv2 message contains a Message ID as part of its fixed
header. This Message ID is used to match up requests and responses
and to identify retransmissions of messages.
Minimal implementations only need to support the role of initiator,
so it typically only sends an IKE_SA_INIT request that, when
answered, is followed by an IKE_AUTH. As those messages have fixed
Message IDs (0 and 1), it does not need to keep track of its own
Message IDs for outgoing requests after that.
Minimal implementations can also optimize Message ID handling of the
incoming requests, as they do not need to protect incoming requests
against replays. This is possible because minimal implementations
will only return error or empty notification replies to incoming
requests. This means that any of those incoming requests do not have
any effect on the minimal implementation, thus processing them again
does not cause any harm. Because of this, a minimal implementation
can always answer a request coming in, with the same Message ID than
what the request had, and then forget the request/response pair
immediately. This means there is no need to keep track of Message
IDs of the incoming requests.
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In the following descriptions, the payloads contained in the message
are indicated by the names listed below.
Notation Payload
-----------------------------------------
AUTH Authentication
CERTREQ Certificate Request
D Delete
HDR IKE header (not a payload)
IDi Identification - Initiator
IDr Identification - Responder
KE Key Exchange
Ni, Nr Nonce
N Notify
SA Security Association
SK Encrypted and Authenticated
TSi Traffic Selector - Initiator
TSr Traffic Selector - Responder
The initial exchanges are as follows:
Initiator Responder
-------------------------------------------------------------------
HDR(SPIi=xxx, SPIr=0, IKE_SA_INIT,
Flags: Initiator, Message ID=0),
SAi1, KEi, Ni -->
<-- HDR(SPIi=xxx, SPIr=yyy, IKE_SA_INIT,
Flags: Response, Message ID=0),
SAr1, KEr, Nr, [CERTREQ]
HDR contains the Security Parameter Indexes (SPIs), version numbers,
and flags of various sorts. Each endpoint chooses one of the two
SPIs and MUST choose them so as to be unique identifiers of an IKE
SA. An SPI value of zero is special: it indicates that the remote
SPI value is not yet known by the sender.
Incoming IKEv2 packets are mapped to an IKE SA using only the
packet's SPI, not using (for example) the source IP address of the
packet.
The SAi1 payload states the cryptographic algorithms the initiator
supports for the IKE SA. The KEi and KEr payloads contain Diffie-
Hellman values, and Ni and Nr are the nonces. The SAr1 contains the
chosen cryptographic suite from the initiator's offered choices. A
minimal implementation using shared secrets will ignore the CERTREQ
payload.
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Minimal implementation will most likely support exactly one set of
cryptographic algorithms, meaning the SAi1 payload will be static.
It needs to check that the SAr1 received matches the proposal it
sent.
At this point in the negotiation, each party can generate SKEYSEED,
from which all keys are derived for that IKE SA.
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 )
prf+ (K,S) = T1 | T2 | T3 | T4 | ...
where:
T1 = prf (K, S | 0x01)
T2 = prf (K, T1 | S | 0x02)
T3 = prf (K, T2 | S | 0x03)
T4 = prf (K, T3 | S | 0x04)
...
(indicating that the quantities SK_d, SK_ai, SK_ar, SK_ei, SK_er,
SK_pi, and SK_pr are taken in order from the generated bits of the
prf+). g^ir is the shared secret from the ephemeral Diffie-Hellman
exchange. g^ir is represented as a string of octets in big endian
order padded with zeros if necessary to make it the length of the
modulus. Ni and Nr are the nonces, stripped of any headers.
The SK_d is used for deriving new keys for the Child SAs. The SK_ai
and SK_ar are used as a key to the integrity protection algorithm for
authenticating the component messages of subsequent exchanges. The
SK_ei and SK_er are used for encrypting (and of course decrypting)
all subsequent exchanges. The SK_pi and SK_pr are used when
generating an AUTH payload. The lengths of SK_d, SK_pi, and SK_pr
MUST be the preferred key length of the Pseudorandom Function (PRF)
agreed upon.
A separate SK_e and SK_a is computed for each direction. The keys
used to protect messages from the original initiator are SK_ai and
SK_ei. The keys used to protect messages in the other direction are
SK_ar and SK_er. The notation SK { ... } indicates that these
payloads are encrypted and integrity protected using that direction's
SK_e and SK_a.
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Initiator Responder
-------------------------------------------------------------------
HDR(SPIi=xxx, SPIr=yyy, IKE_AUTH,
Flags: Initiator, Message ID=1),
SK {IDi, AUTH, SAi2, TSi, TSr,
N(INITIAL_CONTACT)} -->
<-- HDR(SPIi=xxx, SPIr=yyy, IKE_AUTH, Flags:
Response, Message ID=1),
SK {IDr, AUTH, SAr2, TSi, TSr}
The initiator asserts its identity with the IDi payload, proves
knowledge of the secret corresponding to IDi, and integrity protects
the contents of the first message using the AUTH payload. The
responder asserts its identity with the IDr payload, authenticates
its identity, and protects the integrity of the second message with
the AUTH payload.
As minimal implementation usually has only one host where it
connects, that means it has only one shared secret. This means it
does not need to care about the IDr payload that much. If the other
end sends an AUTH payload that the initiator can verify using the
shared secret it has, then it knows the other end is the peer it was
configured to talk to.
In the IKE_AUTH request, the initiator sends the SA offer(s) in the
SAi2 payload and the proposed Traffic Selectors (TSs) for the Child
SA in the TSi and TSr payloads. The responder replies with the
accepted offer in an SAr2 payload and with the selected Traffic
Selectors. The selected Traffic Selectors may be a subset of what
the initiator proposed.
In the minimal implementation, both SA payloads and TS payloads are
going to be mostly static. The SA payload will have the SPI value
used in the Encapsulating Security Payload (ESP), but the algorithms
are most likely going to be the one and only supported set. The TS
payloads on the initiator end will most likely say from any to any,
i.e., full wildcard ranges, or from the local IP to the remote IP.
In the wildcard case, the responder quite often narrows the range
down to the one IP address pair. Using a single IP address pair as
the Traffic Selectors when sending the IKE_AUTH request will simplify
processing as the responder will either accept the IP address pair or
return an error. If wildcard ranges are used, there is a possibility
that the responder will narrow the Traffic Selector range to range
that is not acceptable by the initiator.
The IKE_AUTH (and IKE_SA_INIT) response may contain multiple status
notification payloads that can be ignored by minimal implementations.
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There can also be Vendor ID, Certificate, Certificate Request, or
Configuration payloads, but any payload unknown to minimal
implementations can simply be skipped over (response messages cannot
have critical unsupported payloads).
The exchange above includes N(INITIAL_CONTACT) notification in the
request as that is quite commonly sent by a minimal implementation.
It indicates to the other end that the initiator does not have any
other IKE SAs between it and the responder, and if there is any left
from previous runs, those can be deleted by the responder. As
minimal implementations delete IKE SAs without sending IKE SA delete
requests, this will help the responder to clean up leftover state.
When using shared secret authentication, the peers are authenticated
by having each calculating a Message Authentication Code (MAC) over a
block of data:
For the initiator:
AUTH = prf( prf(Shared Secret, "Key Pad for IKEv2"),
<InitiatorSignedOctets>)
For the responder:
AUTH = prf( prf(Shared Secret, "Key Pad for IKEv2"),
<ResponderSignedOctets>)
The string "Key Pad for IKEv2" is 17 ASCII characters without null
termination. The implementation can precalculate the inner prf and
only store the output of it. This is possible because a minimal
IKEv2 implementation usually only supports one PRF.
In the following calculations, IDi' and IDr' are the entire ID
payloads excluding the fixed header, and the Ni and Nr are only the
values, not the payloads containing it. Note that neither the nonce
Ni/Nr nor the value prf(SK_pr, IDr')/prf(SK_pi, IDi') are
transmitted.
The initiator signs the first message (IKE_SA_INIT request), starting
with the first octet of the first SPI in the header and ending with
the last octet of the last payload in that first message. Appended
to this (for purposes of computing the signature) are the responder's
nonce Nr and the value prf(SK_pi, IDi').
For the responder, the octets to be signed start with the first octet
of the first SPI in the header of the second message (IKE_SA_INIT
response) and end with the last octet of the last payload in that
second message. Appended to this are the initiator's nonce Ni and
the value prf(SK_pr, IDr').
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The initiator's signed octets can be described as:
InitiatorSignedOctets = RealMessage1 | NonceRData | MACedIDForI
RealIKEHDR = SPIi | SPIr | . . . | Length
RealMessage1 = RealIKEHDR | RestOfMessage1
NonceRPayload = PayloadHeader | NonceRData
InitiatorIDPayload = PayloadHeader | RestOfInitIDPayload
RestOfInitIDPayload = IDType | RESERVED | InitIDData
MACedIDForI = prf(SK_pi, RestOfInitIDPayload)
The responder's signed octets can be described as:
ResponderSignedOctets = RealMessage2 | NonceIData | MACedIDForR
RealIKEHDR = SPIi | SPIr | . . . | Length
RealMessage2 = RealIKEHDR | RestOfMessage2
NonceIPayload = PayloadHeader | NonceIData
ResponderIDPayload = PayloadHeader | RestOfRespIDPayload
RestOfRespIDPayload = IDType | RESERVED | RespIDData
MACedIDForR = prf(SK_pr, RestOfRespIDPayload)
Note that all of the payloads inside the RestOfMessageX are included
under the signature, including any payload types not listed in this
document.
The initiator might also get an unauthenticated response back that
has a notification payload with an error code inside. As that error
code will be unauthenticated and may be faked, there is no need to do
anything for those. A minimal implementation can simply ignore those
errors and retransmit its request until it times out, and if that
happens, then the IKE SA (and Child SA) creation failed.
The responder might also reply with an IKE_AUTH response packet that
does not contain the payloads needed to set up a Child SA (SAr2, TSi,
and TSr) but instead contain AUTH payload and an error. Minimal
implementation that does not support the CREATE_CHILD_SA exchange
cannot recover from this scenario. It can delete the IKE SA and
start over from the beginning (which might fail again if this is a
configuration error, or it might succeed if this was temporal
failure).
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2.2. Other Exchanges
Minimal implementations MUST be able to reply to INFORMATIONAL
requests by sending back an empty INFORMATIONAL response:
Minimal implementation Other end
-------------------------------------------------------------------
<-- HDR(SPIi=xxx, SPIr=yyy, INFORMATIONAL,
Flags: none, Message ID=m),
SK {...}
HDR(SPIi=xxx, SPIr=yyy, INFORMATIONAL,
Flags: Initiator | Response,
Message ID=m),
SK {} -->
Minimal implementations MUST be able to reply to incoming
CREATE_CHILD_SA requests. A typical implementation will reject the
CREATE_CHILD_SA exchanges by sending a NO_ADDITIONAL_SAS error notify
back:
Minimal implementation Other end
-------------------------------------------------------------------
<-- HDR(SPIi=xxx, SPIy=yyy, CREATE_CHILD_SA,
Flags: none, Message ID=m),
SK {...}
HDR(SPIi=xxx, SPIr=yyy, CREATE_CHILD_SA,
Flags: Initiator | Response, Message ID=m),
SK {N(NO_ADDITIONAL_SAS)} -->
Note that INFORMATIONAL and CREATE_CHILD_SA requests might contain
unsupported critical payloads, in which case a compliant
implementation MUST ignore the request and send a response message
back that has the UNSUPPORTED_CRITICAL_PAYLOAD notification. That
notification payload data contains a 1-octet payload type of the
unsupported critical payload.
2.3. Generating Keying Material
The keying material for the Child SA created by the IKE_AUTH exchange
is generated as follows:
KEYMAT = prf+(SK_d, Ni | Nr)
Where Ni and Nr are the nonces from the IKE_SA_INIT exchange.
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A single CHILD_SA negotiation may result in multiple Security
Associations. ESP and Authentication Header (AH) SAs exist in pairs
(one in each direction), so two SAs are created in a single Child SA
negotiation for them. The keying material for each Child SA MUST be
taken from the expanded KEYMAT using the following rules:
o All keys for SAs carrying data from the initiator to the responder
are taken before SAs going from the responder to the initiator.
o If an IPsec protocol requires multiple keys, the order in which
they are taken from the SA's keying material needs to be described
in the protocol's specification. For ESP and AH, [IPSECARCH]
defines the order, namely: the encryption key (if any) MUST be
taken from the first bits, and the integrity key (if any) MUST be
taken from the remaining bits.
Each cryptographic algorithm takes a fixed number of bits of keying
material specified as part of the algorithm or negotiated in SA
payloads.
3. Conformance Requirements
For an implementation to be called conforming to the RFC 7296
specification, it MUST be possible to configure it to accept the
following:
o Public Key Infrastructure using X.509 (PKIX) Certificates
containing and signed by RSA keys of size 1024 or 2048 bits, where
the ID passed is any of ID_KEY_ID, ID_FQDN, ID_RFC822_ADDR, or
ID_DER_ASN1_DN.
o Shared key authentication where the ID passed is any of ID_KEY_ID,
ID_FQDN, or ID_RFC822_ADDR.
o Authentication where the responder is authenticated using PKIX
Certificates, and the initiator is authenticated using shared key
authentication.
This document only supports the second bullet; it does not support
PKIX Certificates at all. As full RFC 7296 responders must also
support that shared key authentication, this allows a minimal
implementation to be able to interoperate with all implementations
that are compliant with RFC 7296.
PKIX Certificates are left out from the minimal implementation as
those would add quite a lot of complexity to the implementation. The
actual code changes needed in the IKEv2 protocol are small, but the
certificate validation code would be more complex than the whole
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minimal IKEv2 implementation itself. If public-key-based
authentication is needed for scalability reasons, then raw public
keys would probably be the best compromise (see Appendix B.2).
4. Implementation Status
This document describes a minimal implementation written by the
author of this document. The minimal implementation supported the
base IKE_SA_INIT and IKE_AUTH exchanges and successfully
interoperated with a full IKEv2 server. This minimal implementation
was presented in the Interconnecting Smart Objects with Internet
Workshop in Prague in March 2011 [Kiv11]. This implementation was
written as proof of concept in perl.
There was another proof-of-concept implementation written in python,
which also interoperated with a full IKEv2 server.
Both implementations were written just for demonstration purposes and
included fixed configuration built into the code, and both also
implemented ESP, ICMP, and IP layers to the level that was needed to
send and receive one ICMP echo packet. Both implementations were
about 1000 lines of code excluding cryptographic libraries but
including ESP, ICMP, and IP layers.
5. Security Considerations
As this implements the same protocol as RFC 7296, this means all
security considerations from it also apply to this document.
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6. References
6.1. 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,
<http://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, <http://www.rfc-editor.org/info/rfc7296>.
6.2. Informative References
[EAI] Yang, A., Steele, S., and N. Freed, "Internationalized
Email Headers", RFC 6532, DOI 10.17487/RFC6532, February
2012, <http://www.rfc-editor.org/info/rfc6532>.
[IDNA] Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Definitions and Document Framework",
RFC 5890, DOI 10.17487/RFC5890, August 2010,
<http://www.rfc-editor.org/info/rfc5890>.
[IKEV2IANA]
IANA, "Internet Key Exchange Version 2 (IKEv2)
Parameters",
<http://www.iana.org/assignments/ikev2-parameters>.
[IPSEARCH] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, DOI 10.17487/RFC4301,
December 2005, <http://www.rfc-editor.org/info/rfc4301>.
[Kiv11] Kivinen, T., "Interconnecting Smart Objects with Internet
Workshop 2011-03025; IKEv2 and Smart Objects", March 2011,
<https://www.iab.org/wp-content/IAB-uploads/2011/04/
Kivinen.pdf>.
[MODES] National Institute of Standards and Technology, U.S.
Department of Commerce, "Recommendation for Block Cipher
Modes of Operation", SP 800-38A, 2001.
[PKCS1] Jonsson, J. and B. Kaliski, "Public-Key Cryptography
Standards (PKCS) #1: RSA Cryptography Specifications
Version 2.1", RFC 3447, DOI 10.17487/RFC3447, February
2003, <http://www.rfc-editor.org/info/rfc3447>.
Kivinen Informational [Page 15]RFC 7815 Minimal IKEv2 Initiator Implementation March 2016
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
<http://www.rfc-editor.org/info/rfc5280>.
[RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322,
DOI 10.17487/RFC5322, October 2008,
<http://www.rfc-editor.org/info/rfc5322>.
[RFC7228] Bormann, C., Ersue, M., and A. Keranen, "Terminology for
Constrained-Node Networks", RFC 7228,
DOI 10.17487/RFC7228, May 2014,
<http://www.rfc-editor.org/info/rfc7228>.
[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,
<http://www.rfc-editor.org/info/rfc7619>.
[RFC7670] Kivinen, T., Wouters, P., and H. Tschofenig, "Generic Raw
Public-Key Support for IKEv2", RFC 7670,
DOI 10.17487/RFC7670, January 2016,
<http://www.rfc-editor.org/info/rfc7670>.
Kivinen Informational [Page 16]RFC 7815 Minimal IKEv2 Initiator Implementation March 2016
Appendix A. Header and Payload Formats
This appendix describes actual packet payload formats. This is
required to make the document self-contained. The descriptions are
mostly copied from RFC 7296, and more information can be found from
there.
Various payloads contain RESERVED fields, and those MUST be sent as
zero and MUST be ignored on receipt.
All multi-octet fields representing integers are laid out in big
endian order (also known as "most significant byte first" or "network
byte order").
A.1. The IKE Header
Each IKEv2 message begins with the IKE header, denoted HDR in this
document. Following the header are one or more IKE payloads each
identified by a Next Payload field in the preceding payload.
Payloads are identified in the order in which they appear in an IKE
message by looking in the Next Payload field in the IKE header and,
subsequently, according to the Next Payload field in the IKE payload
itself until a Next Payload field of zero indicates that no payloads
follow. If a payload of type "Encrypted" is found, that payload is
decrypted and its contents parsed as additional payloads. An
Encrypted payload MUST be the last payload in a packet, and an
Encrypted payload MUST NOT contain another Encrypted payload.
The format of the IKE header is shown in Figure 1.
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 |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IKE SA Responder's SPI |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Payload | MjVer | MnVer | Exchange Type | Flags |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: IKE Header Format
Kivinen Informational [Page 17]RFC 7815 Minimal IKEv2 Initiator Implementation March 2016
o Initiator's SPI (8 octets) - A value chosen by the initiator to
identify a unique IKE Security Association. This value MUST NOT
be zero.
o Responder's SPI (8 octets) - A value chosen by the responder to
identify a unique IKE Security Association. This value MUST be
zero in the first message of an IKE initial exchange.
o Next Payload (1 octet) - Indicates the type of payload that
immediately follows the header. The format and value of each
payload are defined below.
o Major Version (4 bits) - Indicates the major version of the IKE
protocol in use. Implementations based on this version of IKE
MUST set the major version to 2 and MUST drop the messages with a
higher major version number.
o Minor Version (4 bits) - Indicates the minor version of the IKE
protocol in use. Implementations based on this version of IKE
MUST set the minor version to zero. They MUST ignore the minor
version number of received messages.
o Exchange Type (1 octet) - Indicates the type of exchange being
used. This constrains the payloads sent in each message in an
exchange.
Exchange Type Value
----------------------------------
IKE_SA_INIT 34
IKE_AUTH 35
CREATE_CHILD_SA 36
INFORMATIONAL 37
o Flags (1 octet) - Indicates specific options that are set for the
message. Presence of options is indicated by the appropriate bit
in the flags field being set. The bits are as follows:
+-+-+-+-+-+-+-+-+
|X|X|R|V|I|X|X|X|
+-+-+-+-+-+-+-+-+
Kivinen Informational [Page 18]RFC 7815 Minimal IKEv2 Initiator Implementation March 2016
In the description below, a bit being 'set' means its value is
'1', while 'cleared' means its value is '0'. 'X' bits MUST be
cleared when sending and MUST be ignored on receipt.
* R (Response) - This bit indicates that this message is a
response to a message containing the same Message ID. This bit
MUST be cleared in all request messages and MUST be set in all
responses. An IKEv2 endpoint MUST NOT generate a response to a
message that is marked as being a response.
* V (Version) - This bit indicates that the transmitter is
capable of speaking a higher major version number of the
protocol than the one indicated in the Major Version field.
Implementations of IKEv2 MUST clear this bit when sending and
MUST ignore it in incoming messages.
* I (Initiator) - This bit MUST be set in messages sent by the
original initiator of the IKE SA and MUST be cleared in
messages sent by the original responder. It is used by the
recipient to determine which 8 octets of the SPI were generated
by the recipient. This bit changes to reflect who initiated
the last rekey of the IKE SA.
o Message ID (4 octets, unsigned integer) - Message identifier used
to control retransmission of lost packets and matching of requests
and responses. It is essential to the security of the protocol
because it is used to prevent message replay attacks.
o Length (4 octets, unsigned integer) - Length of the total message
(header + payloads) in octets.
A.2. Generic Payload Header
Each IKE payload begins with a generic payload header, as shown in
Figure 2. Figures for each payload below will include the generic
payload header, but for brevity, the description of each field will
be omitted.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: Generic Payload Header
Kivinen Informational [Page 19]RFC 7815 Minimal IKEv2 Initiator Implementation March 2016
The Generic Payload Header fields are defined as follows:
o Next Payload (1 octet) - Identifier for the payload type of the
next payload in the message. If the current payload is the last
in the message, then this field will be zero. This field provides
a "chaining" capability whereby additional payloads can be added
to a message by appending each one to the end of the message and
setting the Next Payload field of the preceding payload to
indicate the new payload's type. An Encrypted payload, which must
always be the last payload of a message, is an exception. It
contains data structures in the format of additional payloads. In
the header of an Encrypted payload, the Next Payload field is set
to the payload type of the first contained payload (instead of
zero); conversely, the Next Payload field of the last contained
payload is set to zero). The payload type values needed for
minimal implementations are listed here.
Next Payload Type Notation Value
--------------------------------------------------
No Next Payload 0
Security Association SA 33
Key Exchange KE 34
Identification - Initiator IDi 35
Identification - Responder IDr 36
Certificate CERT 37
Certificate Request CERTREQ 38
Authentication AUTH 39
Nonce Ni, Nr 40
Notify N 41
Delete D 42
Traffic Selector - Initiator TSi 44
Traffic Selector - Responder TSr 45
Encrypted and Authenticated SK 46
o Critical (1 bit) - MUST be set to zero if the sender wants the
recipient to skip this payload if it does not understand the
payload type code in the Next Payload field of the previous
payload. MUST be set to 1 if the sender wants the recipient to
reject this entire message if it does not understand the payload
type. MUST be ignored by the recipient if the recipient
understands the payload type code. MUST be set to zero for
payload types defined in this document. Note that the critical
bit applies to the current payload rather than the "next" payload
whose type code appears in the first octet.
o Payload Length (2 octets, unsigned integer) - Length in octets of
the current payload, including the generic payload header.
Kivinen Informational [Page 20]RFC 7815 Minimal IKEv2 Initiator Implementation March 2016
A.3. Security Association Payload
The Security Association payload, denoted SA in this document, is
used to negotiate attributes of a Security Association.
An SA payload consists of one or more proposals. Each proposal
includes one protocol. Each protocol contains one or more transforms
-- each specifying a cryptographic algorithm. Each transform
contains zero or more attributes (attributes are needed only if the
Transform ID does not completely specify the cryptographic algorithm;
currently, the only attribute is the Key Length attribute for
variable-length ciphers, meaning there is exactly zero or one
attribute).
The responder MUST choose a single suite, which may be any subset of
the SA proposal following the rules below.
Each proposal contains one protocol. If a proposal is accepted, the
SA response MUST contain the same protocol. Each IPsec protocol
proposal contains one or more transforms. Each transform contains a
Transform Type. The accepted cryptographic suite MUST contain
exactly one transform of each type included in the proposal. For
example: if an ESP proposal includes transforms ENCR_3DES, ENCR_AES
w/keysize 128, ENCR_AES w/keysize 256, AUTH_HMAC_MD5, and
AUTH_HMAC_SHA, the accepted suite MUST contain one of the ENCR_
transforms and one of the AUTH_ transforms. Thus, six combinations
are acceptable.
Minimal implementation can create very simple SA proposal, i.e.,
include one proposal, which contains exactly one transform for each
Transform Type. It is important to only include one Diffie-Hellman
group in the proposal, so there is no need to do INVALID_KE_PAYLOAD
processing in responses.
When parsing an SA, an implementation MUST check that the total
Payload Length is consistent with the payload's internal lengths and
counts. Proposals, Transforms, and Attributes each have their own
variable-length encodings. They are nested such that the Payload
Length of an SA includes the combined contents of the SA, Proposal,
Transform, and Attribute information. The length of a Proposal
includes the lengths of all Transforms and Attributes it contains.
The length of a Transform includes the lengths of all Attributes it
contains.
Each Proposal/Protocol structure is followed by one or more transform
structures. The number of different transforms is generally
determined by the Protocol. AH generally has two transforms:
Extended Sequence Numbers (ESNs) and an integrity check algorithm.
Kivinen Informational [Page 21]
RFC 7815 Minimal IKEv2 Initiator Implementation March 2016
ESP generally has three: ESN, an encryption algorithm, and an
integrity check algorithm. IKEv2 generally has four transforms: a
Diffie-Hellman group, an integrity check algorithm, a PRF algorithm,
and an encryption algorithm. For each Protocol, the set of
permissible transforms is assigned Transform ID numbers, which appear
in the header of each transform.
If there are multiple transforms with the same Transform Type, the
proposal is an OR of those transforms. If there are multiple
transforms with different Transform Types, the proposal is an AND of
the different groups.
A given transform MAY have one or more Attributes. Attributes are
necessary when the transform can be used in more than one way, as
when an encryption algorithm has a variable key size. The transform
would specify the algorithm, and the attribute would specify the key
size. To propose alternate values for an attribute (for example,
multiple key sizes for the AES encryption algorithm), an
implementation MUST include multiple transforms with the same
Transform Type each with a single Attribute.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ <Proposals> ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: Security Association Payload
o Proposals (variable) - One or more proposal substructures.
Kivinen Informational [Page 22]RFC 7815 Minimal IKEv2 Initiator Implementation March 2016
A.3.1. Proposal Substructure
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0 (last) or 2 | RESERVED | Proposal Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Proposal Num | Protocol ID | SPI Size |Num Transforms|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ SPI (variable) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ <Transforms> ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: Proposal Substructure
o 0 (last) or 2 (more) (1 octet) - Specifies whether this is the
last Proposal Substructure in the SA.
o Proposal Length (2 octets, unsigned integer) - Length of this
proposal, including all transforms and attributes that follow.
o Proposal Num (1 octet) - When a proposal is made, the first
proposal in an SA payload MUST be 1, and subsequent proposals MUST
be one more than the previous proposal. When a proposal is
accepted, the proposal number in the SA payload MUST match the
number on the proposal sent that was accepted.
o Protocol ID (1 octet) - Specifies the IPsec protocol identifier
for the current negotiation.
Protocol Protocol ID
-----------------------------------
IKE 1
AH 2
ESP 3
o SPI Size (1 octet) - For an initial IKE SA negotiation, this field
MUST be zero; the SPI is obtained from the outer header. During
subsequent negotiations, it is equal to the size, in octets, of
the SPI of the corresponding protocol (8 for IKE and 4 for ESP and
AH).
o Num Transforms (1 octet) - Specifies the number of transforms in
this proposal.
Kivinen Informational [Page 23]RFC 7815 Minimal IKEv2 Initiator Implementation March 2016
o SPI (variable) - The sending entity's SPI. When the SPI Size
field is zero, this field is not present in the Security
Association payload.
o Transforms (variable) - One or more transform substructures.
A.3.2. Transform Substructure
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0 (last) or 3 | RESERVED | Transform Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Transform Type | RESERVED | Transform ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Transform Attributes ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: Transform Substructure
o 0 (last) or 3 (more) (1 octet) - Specifies whether this is the
last Transform Substructure in the Proposal.
o Transform Length - The length (in octets) of the Transform
Substructure including Header and Attributes.
o Transform Type (1 octet) - The type of transform being specified
in this transform. Different protocols support different
Transform Types. For some protocols, some of the transforms may
be optional. If a transform is optional and the initiator wishes
to propose that the transform be omitted, no transform of the
given type is included in the proposal. If the initiator wishes
to make use of the transform optional to the responder, it
includes a transform substructure with Transform ID = 0 as one of
the options.
o Transform ID (2 octets) - The specific instance of the Transform
Type being proposed.
Kivinen Informational [Page 24]RFC 7815 Minimal IKEv2 Initiator Implementation March 2016
The relevant Transform Type values are listed below. For more
information see [RFC7296].
Description Trans. Used In
Type
------------------------------------------------------------------
Encryption Algorithm (ENCR) 1 IKE and ESP
Pseudorandom Function (PRF) 2 IKE
Integrity Algorithm (INTEG) 3 IKE, AH, optional in ESP
Diffie-Hellman group (D-H) 4 IKE, optional in AH & ESP
Extended Sequence Numbers (ESN) 5 AH and ESP
For Transform Type 1 (Encryption Algorithm), the relevant Transform
IDs are listed below.
Name Number
---------------------------
ENCR_AES_CBC 12
ENCR_AES-CCM_8 14
For Transform Type 2 (Pseudorandom Function), the relevant Transform
IDs are listed below.
Name Number
----------------------------------
PRF_HMAC_SHA1 2
For Transform Type 3 (Integrity Algorithm), the relevant Transform
IDs are listed below.
Name Number
---------------------------
AUTH_HMAC_SHA1_96 2
AUTH_AES_XCBC_96 5
For Transform Type 4 (Diffie-Hellman group), the relevant Transform
IDs are listed below.
Name Number
-------------------------
1536-bit MODP 5
2048-bit MODP 14
Kivinen Informational [Page 25]RFC 7815 Minimal IKEv2 Initiator Implementation March 2016
For Transform Type 5 (Extended Sequence Numbers), the relevant
Transform IDs are listed below.
Name Number
--------------------------------------------
No Extended Sequence Numbers 0
Extended Sequence Numbers 1
Note that an initiator who supports ESNs will usually include two ESN
transforms, with values "0" and "1", in its proposals. A proposal
containing a single ESN transform with value "1" means that using
normal (non-extended) sequence numbers is not acceptable.
A.3.3. Valid Transform Types by Protocol
The number and type of transforms that accompany an SA payload are
dependent on the protocol in the SA itself. An SA payload proposing
the establishment of an SA has the following mandatory and optional
Transform Types. A compliant implementation MUST understand all
mandatory and optional types for each protocol it supports (though it
need not accept proposals with unacceptable suites). A proposal MAY
omit the optional types if the only value for them it will accept is
NONE.
Protocol Mandatory Types Optional Types
---------------------------------------------------
IKE ENCR, PRF, INTEG, D-H
ESP ENCR, ESN INTEG, D-H
AH INTEG, ESN D-H
A.3.4. Transform Attributes
Transform Type 1 (Encryption Algorithm) transforms might include one
transform attribute: Key Length.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1| Attribute Type | Attribute Value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6: Data Attributes
o Attribute Type (15 bits) - Unique identifier for each type of
attribute (see below).
o Attribute Value - Value of the attribute associated with the
attribute type.
Kivinen Informational [Page 26]RFC 7815 Minimal IKEv2 Initiator Implementation March 2016
Attribute Type Value
----------------------------
Key Length (in bits) 14
The Key Length attribute specifies the key length in bits (MUST use
network byte order) for certain transforms as follows:
o The Key Length attribute MUST NOT be used with transforms that use
a fixed-length key.
o Some transforms specify that the Key Length attribute MUST be
always included. For example, ENCR_AES_CBC.
A.4. Key Exchange Payload
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Diffie-Hellman Group Num | RESERVED |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Key Exchange Data ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 7: Key Exchange Payload Format
A Key Exchange payload is constructed by copying one's Diffie-Hellman
public value into the "Key Exchange Data" portion of the payload.
The length of the Diffie-Hellman public value for modular
exponentiation groups (MODPs) MUST be equal to the length of the
prime modulus over which the exponentiation was performed, prepending
zero bits to the value if necessary.
The Diffie-Hellman Group Num identifies the Diffie-Hellman group in
which the Key Exchange Data was computed. This Diffie-Hellman Group
Num MUST match a Diffie-Hellman group specified in a proposal in the
SA payload that is sent in the same message.
A.5. Identification Payloads
The Identification payloads, denoted IDi and IDr in this document,
allow peers to assert an identity to one another. When using the
ID_IPV4_ADDR/ID_IPV6_ADDR identity types in IDi/IDr payloads, IKEv2
does not require this address to match the address in the IP header
of IKEv2 packets or anything in the TSi/TSr payloads. The contents
Kivinen Informational [Page 27]RFC 7815 Minimal IKEv2 Initiator Implementation March 2016
of IDi/IDr are used purely to fetch the policy and authentication
data related to the other party. In minimal implementation, it might
be easiest to always use KEY_ID type. This allows the ID payload to
be static. Using an IP address has problems in environments where IP
addresses are dynamically allocated.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ID Type | RESERVED |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Identification Data ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 8: Identification Payload Format
o ID Type (1 octet) - Specifies the type of Identification being
used.
o Identification Data (variable length) - Value, as indicated by the
Identification Type. The length of the Identification Data is
computed from the size in the ID payload header.
The following table lists the assigned semantics for the
Identification Type field.
ID Type Value
-------------------------------------------------------------------
ID_IPV4_ADDR 1
A single four (4) octet IPv4 address.
ID_FQDN 2
A fully qualified domain name string. An example of an ID_FQDN
is "example.com". The string MUST NOT contain any terminators
(e.g., NULL, CR, etc.). All characters in the ID_FQDN are ASCII;
for an "internationalized domain name", the syntax is as defined
in [IDNA], for example, "xn--tmonesimerkki-bfbb.example.net".
ID_RFC822_ADDR 3
A fully qualified RFC 822 email address string based [RFC5322].
An example of an ID_RFC822_ADDR is "jsmith@example.com". The
string MUST NOT contain any terminators. Because of [EAI],
implementations would be wise to treat this field as
UTF-8-encoded text, not as pure ASCII.
Kivinen Informational [Page 28]RFC 7815 Minimal IKEv2 Initiator Implementation March 2016
ID_IPV6_ADDR 5
A single sixteen (16) octet IPv6 address.
ID_KEY_ID 11
An opaque octet stream that may be used to pass vendor-
specific information necessary to do certain proprietary
types of identification. Minimal implementation might use
this type to send out a serial number or similar device-specific
unique static Identification Data for the device.
A.6. Certificate Payload
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Cert Encoding | |
+-+-+-+-+-+-+-+-+ |
~ Certificate Data ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 9: Certificate Payload Format
o Certificate Encoding (1 octet) - This field indicates the type of
certificate or certificate-related information contained in the
Certificate Data field.
Certificate Encoding Value
----------------------------------------------------
X.509 Certificate - Signature 4
Raw Public Key 15
o Certificate Data (variable length) - Actual encoding of
certificate data. The type of certificate is indicated by the
Certificate Encoding field.
The syntax of the types above are:
o "X.509 Certificate - Signature" contains a DER-encoded X.509
certificate whose public key is used to validate the sender's AUTH
payload. Note that with this encoding, if a chain of certificates
needs to be sent, multiple CERT payloads are used, only the first
of which holds the public key used to validate the sender's AUTH
payload.
Kivinen Informational [Page 29]RFC 7815 Minimal IKEv2 Initiator Implementation March 2016
o "Raw Public Key" contains a raw public key. In essence, the
Certificate Payload contains the SubjectPublicKeyInfo part of the
PKIX Certificate (see Section 4.1.2.7 of [RFC5280]). This is a
quite simple ASN.1 object that contains mostly static parts before
the actual public key values. See [RFC7670] for more information.
A.7. Certificate Request Payload
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Cert Encoding | |
+-+-+-+-+-+-+-+-+ |
~ Certification Authority (CA) ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 10: Certificate Request Payload Format
o Certificate Encoding (1 octet) - Contains an encoding of the type
or format of certificate requested.
o Certification Authority (variable length) - Contains an encoding
of an acceptable certification authority for the type of
certificate requested.
The Certificate Encoding field has the same values as those defined
by the certificate payload. The Certification Authority field
contains an indicator of trusted authorities for this certificate
type. The Certification Authority value is a concatenated list of
SHA-1 hashes of the public keys of trusted Certification Authorities.
Each is encoded as the SHA-1 hash of the Subject Public Key Info
element (see Section 4.1.2.7 of [RFC5280]) from each Trust Anchor
certificate. The 20-octet hashes are concatenated and included with
no other formatting.
Kivinen Informational [Page 30]RFC 7815 Minimal IKEv2 Initiator Implementation March 2016
A.8. Authentication Payload
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Auth Method | RESERVED |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Authentication Data ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 11: Authentication Payload Format
o Auth Method (1 octet) - Specifies the method of authentication
used.
Mechanism Value
-----------------------------------------------------------------
RSA Digital Signature 1
Using an RSA private key with an RSASSA-PKCS1-v1_5 signature
scheme specified in [PKCS1]; see Section 2.15 of [RFC7296] for
details.
Shared Key Message Integrity Code 2
Computed as specified earlier using the shared key associated
with the identity in the ID payload and the negotiated PRF.
o Authentication Data (variable length) - see Section 2.1.
A.9. Nonce Payload
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Nonce Data ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 12: Nonce Payload Format
o Nonce Data (variable length) - Contains the random data generated
by the transmitting entity.
Kivinen Informational [Page 31]RFC 7815 Minimal IKEv2 Initiator Implementation March 2016
The size of the Nonce Data MUST be between 16 and 256 octets,
inclusive. Nonce values MUST NOT be reused.
A.10. Notify Payload
The Notify payload, denoted N in this document, is used to transmit
informational data, such as error conditions and state transitions,
to an IKE peer. A Notify payload may appear in a response message
(usually specifying why a request was rejected), in an INFORMATIONAL
exchange (to report an error not in an IKE request), or in any other
message to indicate sender capabilities or to modify the meaning of
the request.
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 ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 13: Notify Payload Format
o Protocol ID (1 octet) - If this notification concerns an existing
SA whose SPI is given in the SPI field, this field indicates the
type of that SA. If the SPI field is empty, this field MUST be
sent as zero and MUST be ignored on receipt.
o SPI Size (1 octet) - Length in octets of the SPI as defined by the
IPsec protocol ID or zero if no SPI is applicable. For a
notification concerning the IKE SA, the SPI Size MUST be zero and
the SPI field must be empty.
o Notify Message Type (2 octets) - Specifies the type of
notification message.
o SPI (variable length) - Security Parameter Index.
Kivinen Informational [Page 32]RFC 7815 Minimal IKEv2 Initiator Implementation March 2016
o Notification Data (variable length) - Status or error data
transmitted in addition to the Notify Message Type. Values for
this field are type specific.
A.10.1. Notify Message Types
Notification information can be error messages specifying why an SA
could not be established. It can also be status data that a process
managing an SA database wishes to communicate with a peer process.
Types in the range 0 - 16383 are intended for reporting errors. An
implementation receiving a Notify payload with one of these types
that it does not recognize in a response MUST assume that the
corresponding request has failed entirely. Unrecognized error types
in a request and status types in a request or response MUST be
ignored, and they should be logged.
Notify payloads with status types MAY be added to any message and
MUST be ignored if not recognized. They are intended to indicate
capabilities and, as part of SA negotiation, are used to negotiate
non-cryptographic parameters.
NOTIFY messages: error types Value
-------------------------------------------------------------------
UNSUPPORTED_CRITICAL_PAYLOAD 1
Indicates that the 1-octet payload type included in the
Notification Data field is unknown.
INVALID_SYNTAX 7
Indicates the IKE message that was received was invalid because
some type, length, or value was out of range or because the
request was rejected for policy reasons. To avoid a
Denial-of-Service (DoS) attack using forged messages, this
status may only be returned for and in an encrypted packet if
the Message ID and cryptographic checksum were valid. To avoid
leaking information to someone probing a node, this status MUST
be sent in response to any error not covered by one of the other
status types. To aid debugging, more detailed error information
should be written to a console or log.
NO_PROPOSAL_CHOSEN 14
None of the proposed crypto suites was acceptable. This can be
sent in any case where the offered proposals are not acceptable
for the responder.
NO_ADDITIONAL_SAS 35
Specifies that the node is unwilling to accept any more Child
SAs.
Kivinen Informational [Page 33]RFC 7815 Minimal IKEv2 Initiator Implementation March 2016
NOTIFY messages: status types Value
-------------------------------------------------------------------
INITIAL_CONTACT 16384
Asserts that this IKE SA is the only IKE SA currently active
between the authenticated identities.
A.11. Traffic Selector Payload
Traffic Selector (TS) payloads allow endpoints to communicate some of
the information from their Security Policy Database (SPD) to their
peers. TS payloads specify the selection criteria for packets that
will be forwarded over the newly set up SA.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Number of TSs | RESERVED |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ <Traffic Selectors> ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 14: Traffic Selectors Payload Format
o Number of TSs (1 octet) - Number of Traffic Selectors being
provided.
o Traffic Selectors (variable length) - One or more individual
Traffic Selectors.
The length of the Traffic Selector payload includes the TS header and
all the Traffic Selectors.
There is no requirement that TSi and TSr contain the same number of
individual Traffic Selectors. Thus, they are interpreted as follows:
a packet matches a given TSi/TSr if it matches at least one of the
individual selectors in TSi and at least one of the individual
selectors in TSr.
Two TS payloads appear in each of the messages in the exchange that
creates a Child SA pair. Each TS payload contains one or more
Traffic Selectors. Each Traffic Selector consists of an address
range (IPv4 or IPv6), a port range, and an IP protocol ID.
Kivinen Informational [Page 34]RFC 7815 Minimal IKEv2 Initiator Implementation March 2016
The first of the two TS payloads is known as TSi (Traffic Selector -
initiator). The second is known as TSr (Traffic Selector -
responder). TSi specifies the source address of traffic forwarded
from (or the destination address of traffic forwarded to) the
initiator of the Child SA pair. TSr specifies the destination
address of the traffic forwarded to (or the source address of the
traffic forwarded from) the responder of the Child SA pair.
IKEv2 allows the responder to choose a subset of the traffic proposed
by the initiator.
When the responder chooses a subset of the traffic proposed by the
initiator, it narrows the Traffic Selectors to some subset of the
initiator's proposal (provided the set does not become the null set).
If the type of Traffic Selector proposed is unknown, the responder
ignores that Traffic Selector, so that the unknown type is not
returned in the narrowed set.
To enable the responder to choose the appropriate range, if the
initiator has requested the SA due to a data packet, the initiator
SHOULD include as the first Traffic Selector in each TSi and TSr a
very specific Traffic Selector including the addresses in the packet
triggering the request. If the initiator creates the Child SA pair
not in response to an arriving packet, but rather, say, upon startup,
then there may be no specific addresses the initiator prefers for the
initial tunnel over any other. In that case, the first values in TSi
and TSr can be ranges rather than specific values.
As minimal implementations might only support one SA, the Traffic
Selectors will usually be from the initiator's IP address to the
responder's IP address (i.e., no port or protocol selectors and only
one range).
Kivinen Informational [Page 35]
RFC 7815 Minimal IKEv2 Initiator Implementation March 2016
A.11.1. Traffic Selector
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TS Type |IP Protocol ID | Selector Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Start Port | End Port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Starting Address ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Ending Address ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 15: Traffic Selector
o TS Type (1 octet) - Specifies the type of Traffic Selector.
o IP protocol ID (1 octet) - Value specifying an associated IP
protocol ID (such as UDP, TCP, and ICMP). A value of zero means
that the protocol ID is not relevant to this Traffic Selector --
the SA can carry all protocols.
o Selector Length - Specifies the length of this Traffic Selector
substructure including the header.
o Start Port (2 octets, unsigned integer) - Value specifying the
smallest port number allowed by this Traffic Selector. For
protocols for which port is undefined (including protocol 0), or
if all ports are allowed, this field MUST be zero.
o End Port (2 octets, unsigned integer) - Value specifying the
largest port number allowed by this Traffic Selector. For
protocols for which port is undefined (including protocol 0), or
if all ports are allowed, this field MUST be 65535.
o Starting Address - The smallest address included in this Traffic
Selector (length determined by TS Type).
o Ending Address - The largest address included in this Traffic
Selector (length determined by TS Type).
Kivinen Informational [Page 36]RFC 7815 Minimal IKEv2 Initiator Implementation March 2016
The following table lists values for the Traffic Selector Type field
and the corresponding Address Selector Data.
TS Type Value
-------------------------------------------------------------------
TS_IPV4_ADDR_RANGE 7
A range of IPv4 addresses, represented by two 4-octet
values. The first value is the beginning IPv4 address
(inclusive), and the second value is the ending IPv4 address
(inclusive). All addresses falling between the two specified
addresses are considered to be within the list.
TS_IPV6_ADDR_RANGE 8
A range of IPv6 addresses, represented by two 16-octet
values. The first value is the beginning IPv6 address
(inclusive), and the second value is the ending IPv6 address
(inclusive). All addresses falling between the two specified
addresses are considered to be within the list.
A.12. Encrypted Payload
The Encrypted payload, denoted as SK{...} in this document, contains
other payloads in encrypted form.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Initialization Vector |
| (length is block size for the encryption algorithm) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Encrypted IKE Payloads ~
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | Padding (0-255 octets) |
+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
| | Pad Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Integrity Checksum Data ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 16: Encrypted Payload Format
Kivinen Informational [Page 37]RFC 7815 Minimal IKEv2 Initiator Implementation March 2016
o Next Payload - The payload type of the first embedded payload.
Note that this is an exception in the standard header format,
since the Encrypted payload is the last payload in the message;
therefore, the Next Payload field would normally be zero. But
because the content of this payload is embedded payloads and there
was no natural place to put the type of the first one, that type
is placed here.
o Payload Length - Includes the lengths of the header,
initialization vector (IV), Encrypted IKE payloads, Padding, Pad
Length, and Integrity Checksum Data.
o Initialization Vector - For Cipher Block Chaining (CBC) mode
ciphers, the length of the initialization vector (IV) is equal to
the block length of the underlying encryption algorithm. Senders
MUST select a new unpredictable IV for every message; recipients
MUST accept any value. The reader is encouraged to consult
[MODES] for advice on IV generation. In particular, using the
final ciphertext block of the previous message is not considered
unpredictable. For modes other than CBC, the IV format and
processing is specified in the document specifying the encryption
algorithm and mode.
o IKE payloads are as specified earlier in this section. This field
is encrypted with the negotiated cipher.
o Padding MAY contain any value chosen by the sender and MUST have a
length that makes the combination of the payloads, the Padding,
and the Pad Length to be a multiple of the encryption block size.
This field is encrypted with the negotiated cipher.
o Pad Length is the length of the Padding field. The sender SHOULD
set the Pad Length to the minimum value that makes the combination
of the payloads, the Padding, and the Pad Length a multiple of the
block size, but the recipient MUST accept any length that results
in proper alignment. This field is encrypted with the negotiated
cipher.
o Integrity Checksum Data is the cryptographic checksum of the
entire message starting with the Fixed IKE header through the Pad
Length. The checksum MUST be computed over the encrypted message.
Its length is determined by the integrity algorithm negotiated.
Kivinen Informational [Page 38]RFC 7815 Minimal IKEv2 Initiator Implementation March 2016
Appendix B. Useful Optional Features
There are some optional features of IKEv2, which might be useful for
minimal implementations in some scenarios. Such features include raw
public keys authentication and sending an IKE SA delete notification.
B.1. IKE SA Delete Notification
In some scenarios, a minimal implementation device creates an IKE SA,
sends one or few packets, perhaps gets some packets back, and then
the device goes back to sleep, forgetting the IKE SA. In such
scenarios, it would be nice for the minimal implementation to send
the IKE SA delete notification to tell the other end that the IKE SA
is going away, so it can free the resources.
Deleting the IKE SA can be done by sending one packet with a fixed
Message ID and with only one payload inside the Encrypted payload.
The other end will send back an empty response:
Initiator Responder
-------------------------------------------------------------------
HDR(SPIi=xxx, SPIr=yyy, INFORMATIONAL,
Flags: Initiator, Message ID=2),
SK {D} -->
<-- HDR(SPIi=xxx, SPIr=yyy, INFORMATIONAL,
Flags: Response, Message ID=2),
SK {}
The Delete payload format is:
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 | Num of SPIs |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Security Parameter Index(es) (SPI) ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 17: Delete Payload Format
o Protocol ID (1 octet) - Must be 1 for an IKE SA.
Kivinen Informational [Page 39]RFC 7815 Minimal IKEv2 Initiator Implementation March 2016
o SPI Size (1 octet) - Length in octets of the SPI as defined by the
protocol ID. It MUST be zero for IKE (SPI is in the message
header).
o Num of SPIs (2 octets, unsigned integer) - The number of SPIs
contained in the Delete payload. This MUST be zero for IKE.
o Security Parameter Index(es) (variable length) - Identifies the
specific Security Association(s) to delete. The length of this
field is determined by the SPI Size and Num of SPIs fields. This
field is empty for the IKE SA delete.
B.2. Raw Public Keys
In some scenarios, the shared secret authentication is not safe
enough, as anybody who knows the secret can impersonate the server.
If the shared secret is printed on the side of the device, then
anybody who gets physical access to the device can read it. In such
environments, public key authentication allows stronger
authentication with minimal operational overhead. Certificate
support is quite complex, and minimal implementations do not usually
have need for them. Using Raw Public Keys is much simpler, and it
scales similar to certificates. The fingerprint of the raw public
key can still be distributed by, for example, printing it on the side
of the device allowing setup similar to using a shared secret.
Raw public keys can also be used in a "leap of faith" or baby duck
style initial setup, where the device imprints itself to the first
device it sees when it boots up the first time. After that initial
connection, it stores the fingerprint of the Raw Public Key of the
server in its own configuration and verifies that it never changes
(unless a "reset to factory settings" or similar command is issued).
This changes the initial IKE_AUTH payloads as follows:
Initiator Responder
-------------------------------------------------------------------
HDR(SPIi=xxx, SPIr=yyy, IKE_AUTH,
Flags: Initiator, Message ID=1),
SK {IDi, CERT, AUTH, SAi2, TSi, TSr,
N(INITIAL_CONTACT)} -->
<-- HDR(SPIi=xxx, SPIr=yyy, IKE_AUTH, Flags:
Response, Message ID=1),
SK {IDr, CERT, AUTH, SAr2, TSi, TSr}
Kivinen Informational [Page 40]RFC 7815 Minimal IKEv2 Initiator Implementation March 2016
The CERT payloads contain the raw public keys used to sign the hash
of the InitiatorSignedOctects/ResponderSignedOctects when generating
an AUTH payload. Minimal implementations should use SHA-1 as the
hash function as that is the "SHOULD" support algorithm specified in
RFC 7296, so it is the most likely one that is supported by all
devices.
Note that RFC 7296 already obsoleted the old Raw RSA Key method, and
"Generic Raw Public-Key Support for IKEv2" [RFC7670] adds a new
format to allow using any types of raw public keys with IKEv2. This
document only specifies how to use the new format.
In these setups, it might be possible that authenticating the server
is not needed at all. If a minimal device is sending, for example,
sensor information to the server, the server wants to verify that the
sensor is who it claims to be using raw public keys, but the sensor
does not really care who the server is. In such cases, the NULL
authentication method [RFC7619] would be useful, as it allows devices
to do one-way authentication.
Acknowledgements
Most of the content of this document is copied from RFC 7296.
Author's Address
Tero Kivinen
INSIDE Secure
Eerikinkatu 28
HELSINKI FI-00180
FINLAND
Email: kivinen@iki.fi
Kivinen Informational [Page 41]
