Internet DRAFT - draft-kivinen-ipsecme-ikev2-minimal
draft-kivinen-ipsecme-ikev2-minimal
IP Security Maintenance and Extensions T. Kivinen
(ipsecme) AuthenTec
Internet-Draft October 1, 2012
Intended status: Informational
Expires: April 4, 2013
Minimal IKEv2
draft-kivinen-ipsecme-ikev2-minimal-01.txt
Abstract
This document describes minimal version of the Internet Key Exchange
version 2 (IKEv2) protocol. 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 which can be done. The
protocol described here is compliant with full IKEv2 with exception
that this document only describes shared secret authentication (IKEv2
requires support for certificate authentication in addition to shared
secret authentication).
This document does not update or modify RFC 5996, but provides more
compact description of the minimal version of the protocol. If this
document and RFC 5996 conflicts then RFC 5996 is the authoritative
description.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
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time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 4, 2013.
Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Exchanges . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.1. Initial Exchange . . . . . . . . . . . . . . . . . . . . . 6
2.2. Other Exchanges . . . . . . . . . . . . . . . . . . . . . 11
2.3. Generating Keying Material . . . . . . . . . . . . . . . . 12
3. Conformance Requirements . . . . . . . . . . . . . . . . . . . 14
4. Security Considerations . . . . . . . . . . . . . . . . . . . 15
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 17
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 18
7.1. Normative References . . . . . . . . . . . . . . . . . . . 18
7.2. Informative References . . . . . . . . . . . . . . . . . . 18
Appendix A. Header and Payload Formats . . . . . . . . . . . . . 19
A.1. The IKE Header . . . . . . . . . . . . . . . . . . . . . . 19
A.2. Generic Payload Header . . . . . . . . . . . . . . . . . . 21
A.3. Security Association Payload . . . . . . . . . . . . . . . 22
A.3.1. Proposal Substructure . . . . . . . . . . . . . . . . 24
A.3.2. Transform Substructure . . . . . . . . . . . . . . . . 25
A.3.3. Valid Transform Types by Protocol . . . . . . . . . . 27
A.3.4. Transform Attributes . . . . . . . . . . . . . . . . . 28
A.4. Key Exchange Payload . . . . . . . . . . . . . . . . . . . 28
A.5. Identification Payloads . . . . . . . . . . . . . . . . . 29
A.6. Certificate Payload . . . . . . . . . . . . . . . . . . . 30
A.7. Certificate Request Payload . . . . . . . . . . . . . . . 31
A.8. Authentication Payload . . . . . . . . . . . . . . . . . . 32
A.9. Nonce Payload . . . . . . . . . . . . . . . . . . . . . . 33
A.10. Notify Payload . . . . . . . . . . . . . . . . . . . . . . 33
A.10.1. Notify Message Types . . . . . . . . . . . . . . . . . 34
A.11. Traffic Selector Payload . . . . . . . . . . . . . . . . . 35
A.11.1. Traffic Selector . . . . . . . . . . . . . . . . . . . 37
A.12. Encrypted Payload . . . . . . . . . . . . . . . . . . . . 38
Appendix B. Useful Optional Features . . . . . . . . . . . . . . 41
B.1. IKE SA Delete Notification . . . . . . . . . . . . . . . . 41
B.2. Raw RSA keys . . . . . . . . . . . . . . . . . . . . . . . 42
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 44
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1. Introduction
This document tells what minimal IKEv2 implementation could look
like. Minimal IKEv2 implementation only supports 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 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, EAP authentication,
COOKIEs etc.
Some optimizations can be done because of 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 as
implementation is only sending out IKE_SA_INIT and IKE_AUTH request,
and do not ever send any other request).
This document should be stand-alone, 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 which 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
minimal implementation are listed in this document.
For more information check the full IKEv2 specification in RFC 5996
[RFC5996] 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].
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 is usually 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 an remote garage
door opener device. I.e. device having buttons which open and close
garage door, and which connects to the home area network server over
wireless link.
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Another example of the such device is some kind of sensor device, for
example room temperature sensor, which sends periodic temperature
data to some centralized node.
Those devices are usually sleeping long times, and only wakes up
because of user interaction or periodically. The data transfer is
always initiated from the sleeping node and 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 server node to
the small device, it can be implemented by polling, i.e. small node
periodically polls for the server to see if it for example have some
configuration changes or similar.
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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
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 first two exchanges
called IKE_SA_INIT and IKE_AUTH. Those are used to create the IKE SA
and the first child SA. In addition to those messages minimal IKEv2
implementation need to understand CREATE_CHILD_SA request so it can
reply with CREATE_CHILD_SA error response saying NO_ADDITIONAL_SAS to
it, and understand INFORMATIONAL request so much, it can reply with
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 implementation need only support of being initiator, so it
does not ever need to send any other request as one IKE_SA_INIT, and
one IKE_AUTH message. 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.
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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 implementation
will only return error or empty notifications 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 the minimal implementation
can always answer to 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 any kind of track
of Message IDs of the incoming requests.
In the following descriptions, the payloads contained in the message
are indicated by names as 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.
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Incoming IKEv2 packets are mapped to an IKE SA only using 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 payload contain Diffie-
Hellman values and Ni and Nr are the nonces. The SAr1 contains
chosen cryptographic suite from initiator's offered choices. Minimal
implementation using shared secrets will ignore the CERTREQ payload.
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 PRF agreed upon.
A separate SK_e and SK_a is computed for each direction. The keys
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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.
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, and that means it has only one shared secret. This means
it does not need to care about IDr payload that much. If the other
end sends AUTH payload which 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 initiator sends SA offer(s) in the SAi2 payload, and
the proposed Traffic Selectors for the proposed Child SA in the TSi
and TSr payloads. The responder replies with the accepted offer in
an SAr2 payload, and 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 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 server
quite often narrow the range down to the one IP address pair. Using
single IP address pair as a traffic selectors when sending IKE_AUTH
will simplify processing as then server will either accept that pair
or return error. If wildcard ranges are used, there is possibility
that server narrows the range to some other range than what was
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intended.
The IKE_AUTH (and IKE_SA_INIT) responses may contain multiple status
notification payloads which can be ignored by minimal implementation.
There can also be Vendor ID, Certificate, Certificate Request or
Configuration payloads, but any payload unknown to minimal
implementation 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 the minimal implementation.
It indicates to the other end that the initiator does not have any
other IKE SAs between them, and if there is any left from previous
runs they can be deleted. As minimal implementation does not delete
IKE SAs by sending IKE SA delete, this will help server to clean up
leftover state.
When using shared secret authentication, the peers are authenticated
by having each calculating a 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 minimal IKEv2
implementation usually only supports one PRF.
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').
In these calculations, IDi' and IDr' are the entire ID payloads
excluding the fixed header and the Ni, and Nr are only the value, not
the payload containing it. Note that neither the nonce Ni/Nr nor the
value prf(SK_pr, IDr')/prf(SK_pi, IDi') are transmitted.
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The initiator's signed octets can be described as:
InitiatorSignedOctets = RealMessage1 | NonceRData | MACedIDForI
GenIKEHDR = [ four octets 0 if using port 4500 ] | RealIKEHDR
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
GenIKEHDR = [ four octets 0 if using port 4500 ] | RealIKEHDR
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 unauthenticated response back having
notification payload with error code inside. As that error code will
be unauthenticated and may be faked, there is no need to do anything
for those. 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.
Responder might also reply with IKE_AUTH response packet which do not
contain payloads needed to set up Child SA (SAr2, TSi and TSr), but
contains AUTH payload and an error. As minimal implementation
probably do not support multiple SAs nor sending the CREATE_CHILD_SA
exchanges the IKE SA is useless for initiator. It can delete the IKE
SA and start over from the beginning (which might fail again if this
is configuration error, or it might succeed if this was temporal
failure).
2.2. Other Exchanges
Minimal implementation MUST be able to reply to INFORMATIONAL request
by sending empty response back:
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Initiator Responder
-------------------------------------------------------------------
<-- 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 implementation also MUST be able to reply to incoming
CREATE_CHILD_SA requests. Typical implementation will reject the
CREATE_CHILD_SA exchanges by sending NO_ADDITIONAL_SAS error notify
back:
Initiator Responder
-------------------------------------------------------------------
<-- 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 complient implementation
MUST ignore the request, and send response message back having the
UNSUPPORTED_CRITICAL_PAYLOAD notification. That notification payload
data contains one-octet payload type of the unsupported critical
payload.
2.3. Generating Keying Material
Keying material for 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.
A single CHILD_SA negotiation may result in multiple Security
Associations. ESP and 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:
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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.
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3. Conformance Requirements
For an implementation to be called conforming to RFC 5996
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 RFC5996 responders must also
support that shared key authentication, this allows minimal
implementation to be able to interoperate with all RFC 5996 compliant
implementations.
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
minimal IKEv2 implementation itself. If public key based
authentication is needed for scalability reasons, then raw RSA keys
would probably be the best compromize (see Appendix B.2).
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4. Security Considerations
As this implements same protocol as RFC 5996 this means all security
considerations from it also apply to this document.
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5. IANA Considerations
There is no new IANA considerations in this document.
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6. Acknowledgements
Most of the contents of this document is copied from the RFC 5996.
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7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5996] Kaufman, C., Hoffman, P., Nir, Y., and P. Eronen,
"Internet Key Exchange Protocol Version 2 (IKEv2)",
RFC 5996, September 2010.
7.2. Informative References
[IKEV2IANA]
"Internet Key Exchange Version 2 (IKEv2) Parameters",
<http://www.iana.org>.
[MODES] National Institute of Standards and Technology, U.S.
Department of Commerce, "Recommendation for Block Cipher
Modes of Operation", SP 800-38A, 2001.
[RFC3447] Jonsson, J. and B. Kaliski, "Public-Key Cryptography
Standards (PKCS) #1: RSA Cryptography Specifications
Version 2.1", RFC 3447, February 2003.
[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, May 2008.
[RSA] R. Rivest, A. Shamir, and L. Adleman, "A Method for
Obtaining Digital Signatures and Public-Key
Cryptosystems", February 1978.
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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 the RFC5996 and more information can be found from
there.
Various payload contains 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.
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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
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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 0. 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|
+-+-+-+-+-+-+-+-+
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
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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 number
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 eight 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, 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.
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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
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 0. 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
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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 0);
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 one 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.
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
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Transform ID does not completely specify the cryptographic algorithm,
currently only attribute is 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 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.
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
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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.
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.
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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, 4 for ESP and
AH).
o Num Transforms (1 octet) - Specifies the number of transforms in
this proposal.
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
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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
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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.
The relevant Transform Type values are listed below.
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 Defined In
---------------------------------------------------
ENCR_3DES 3 (RFC2451)
ENCR_AES_CBC 12 (RFC3602)
For Transform Type 2 (Pseudorandom Function), the relevant Transform
IDs are listed below.
Name Number Defined In
------------------------------------------------------
PRF_HMAC_MD5 1 (RFC2104), [MD5]
PRF_HMAC_SHA1 2 (RFC2104), [SHA]
For Transform Type 3 (Integrity Algorithm), relevant Transform IDs
are listed below.
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Name Number Defined In
----------------------------------------
NONE 0
AUTH_HMAC_MD5_96 1 (RFC2403)
AUTH_HMAC_SHA1_96 2 (RFC2404)
AUTH_AES_XCBC_96 5 (RFC3566)
For Transform Type 4 (Diffie-Hellman group), relevant Transform IDs
are listed below.
Name Number Defined In
----------------------------------------
NONE 0
768-bit MODP 1 Appendix B
1024-bit MODP 2 Appendix B
1536-bit MODP 5 [ADDGROUP]
2048-bit MODP 14 [ADDGROUP]
For Transform Type 5 (Extended Sequence Numbers), 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
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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.
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
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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
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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 group (MODP) groups 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
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 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.
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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. An example of a
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.
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 serial number or similar device
specific unique static identification data for the device.
A.6. Certificate Payload
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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.
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Certificate Encoding Value
----------------------------------------------------
X.509 Certificate - Signature 4
Raw RSA Key 11
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.
o "Raw RSA Key" contains a PKCS #1 encoded RSA key, that is, a DER-
encoded RSAPublicKey structure (see [RSA] and [RFC3447]).
A.7. Certificate Request Payload
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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 ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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
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 (CAs). Each
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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.
A.8. Authentication Payload
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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 RSASSA-PKCS1-v1_5 signature
scheme specified in [PKCS1], see [RFC5996] section 2.15 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.
DSS Digital Signature 3
Using a DSS private key (see [DSS]) over a SHA-1 hash, see
[RFC5996] Section 2.15 for details.
o Authentication Data (variable length) - see Section 2.1.
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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.
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
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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 field must be empty.
o Notify Message Type (2 octets) - Specifies the type of
notification message.
o SPI (variable length) - Security Parameter Index.
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.
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NOTIFY messages: error types Value
-------------------------------------------------------------------
UNSUPPORTED_CRITICAL_PAYLOAD 1
Indicates that the one-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 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.
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 SPD to their peers. TS payloads specify
the selection criteria for packets that will be forwarded over the
newly set up SA.
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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.
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).
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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 of 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 initiator's IP address to responders
IP address (i.e. no port or protocol selectors and only one range).
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 (one 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.
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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).
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 four-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 sixteen-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 SK{...} in this document, contains
other payloads in encrypted form.
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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 encryption algorithm) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Encrypted IKE Payloads ~
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | Padding (0-255 octets) |
+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
| | Pad Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Integrity Checksum Data ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 16: Encrypted Payload Format
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 and
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 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.
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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.
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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
RSA keys authentication, and sending IKE SA delete notification.
B.1. IKE SA Delete Notification
In some scenarios the minimal implementation device creates IKE SA,
sends one or few packets, perhaps gets some packets back, and then
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 using by sending one packet with
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.
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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 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 RSA keys
In some scenarios the shared secret authentication is not safe
enough, as anybody who knows the secret can impersonate himself of
being 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 RSA keys is much simpler, and it
allows similar scalability than certificates. The fingerprint of the
Raw RSA key can still be distributed by for example printing it on
the side of the device allowing similar setup than using shared
secret.
Raw RSA keys can also be used in leap of faith or baby duck style
initial setup, where the device imprints itself to the first device
it sees when it first time boots up. After that initial connection
it stores the fingerprint of the Raw RSA key of the server to its own
configuration and verifies that it never changes (unless reset to
factory setting 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}
The CERT payloads contains the Raw RSA keys used the sign the hash of
the InitiatorSignedOctects/ResponderSignedOctects when generating
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AUTH payload. Minimal implementations should use SHA-1 as the hash
function as that is the SHOULD support algorithm specified in the
RFC5996, so it is the most likely one that is supported by all
devices.
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Author's Address
Tero Kivinen
AuthenTec
Eerikinkatu 28
HELSINKI FI-00180
FI
Email: kivinen@iki.fi
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