IPSEC Working Group Dan Harkins
INTERNET-DRAFT Charlie Kaufman
Steve Kent
Tero Kivinen
Radia Perlman
editors
draft-ietf-ipsec-ikev2-02.txt April 2002
Proposal for the IKEv2 Protocol
<draft-ietf-ipsec-ikev2-02.txt>
Status of this Memo
This document is an Internet Draft and is in full conformance with
all provisions of Section 10 of RFC2026 [Bra96]. Internet Drafts are
working documents of the Internet Engineering Task Force (IETF), its
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Abstract
This document describes version 2 of the IKE (Internet Key Exchange)
protocol. IKE performs mutual authentication and establishes an IKE
security association that can be used to efficiently establish SAs
for ESP, AH and/or IPcomp. This version greatly simplifies IKE by
replacing the 8 possible phase 1 exchanges with a single exchange
based on either public signature keys or shared secret keys. The
single exchange provides identity hiding, yet works in 2 round trips
(all the identity hiding exchanges in IKE v1 required 3 round trips).
Latency of setup of an IPsec SA is further reduced from IKEv1 by
allowing setup of an SA for ESP, AH, and/or IPcomp to be piggybacked
on the initial IKE exchange. It also improves security by allowing
the Responder to be stateless until it can be assured that the
Initiator can receive at the claimed IP source address. This version
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also presents the entire protocol in a single self-contained
document, in contrast to IKEv1, in which the protocol was described
in ISAKMP (RFC 2408), IKE (RFC 2409), and the DOI (RFC 2407)
documents.
Table of Contents
1. Introduction..............................................3
1.1 The IKE Protocol.........................................3
1.2 Change History...........................................4
1.3 Requirements Terminology.................................7
2 Protocol Overview..........................................7
2.1 Use of Retransmission Timers.............................8
2.2 Use of Sequence Numbers for Message ID...................8
2.3 Window Size for overlapping requests.....................9
2.4 State Synchronization and Connection Timeouts............9
2.5 Version Numbers and Forward Compatibility................11
2.6 Cookies..................................................12
2.7 Cryptographic Algorithm Negotiation......................16
2.8 Rekeying.................................................17
2.9 Traffic Selector Negotiation.............................18
2.10 Nonces..................................................18
2.11 Address and Port Agility................................19
2.12 Reuse of Diffie-Hellman Exponentials....................19
3 The Phase 1 Exchange.......................................20
3.1 Generating Keying Material for the IKE-SA................21
3.2 Authentication of the IKE-SA.............................22
4 The CREATE-CHILD-SA (Phase 2) Exchange.....................23
4.1 Generating Keying Material for Child-SAs.................24
4.2 Generating Keying Material for IKE-SAs during rollover...25
5 Informational (Phase 2) Exchange...........................26
6 Error Handling.............................................27
7 Header and Payload Formats.................................28
7.1 The IKE Header...........................................28
7.2 Generic Payload Header...................................30
7.3 Security Association Payload.............................32
7.3.1 Proposal Substructure..................................34
7.3.2 Transform Substructure.................................36
7.3.3 Mandatory Transform Types..............................39
7.3.4 Mandatory Transform-IDs................................39
7.3.5 Transform Attributes...................................40
7.3.6 Attribute Negotiation..................................41
7.4 Key Exchange Payload.....................................41
7.5 Identification Payload...................................42
7.6 Certificate Payload......................................44
7.7 Certificate Request Payload..............................45
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7.8 Authentication Payload...................................46
7.9 Nonce Payload............................................47
7.10 Notify Payload..........................................48
7.10.1 Notify Message Types..................................49
7.11 Delete Payload..........................................53
7.12 Vendor ID Payload.......................................54
7.13 Traffic Selector Payload................................55
7.13.1 Traffic Selector Substructure.........................56
7.14 Other Payload types.....................................58
8 Diffie-Hellman Groups......................................58
9 Security Considerations....................................60
10 IANA Considerations.......................................61
10.1 Transform Types and Attribute Values....................61
10.2 Exchange Types..........................................59
10.3 Payload Types...........................................63
11 Acknowledgements..........................................63
12 References................................................63
Appendix A: Attribute Assigned Numbers.......................66
Appendix B: Cryptographic Protection of IKE Data.............68
Authors' Addresses...........................................70
1. Introduction
IP Security (IPsec) provides confidentiality, data integrity, and
data source authentication to IP datagrams. These services are
provided by maintaining shared state between the source and the sink
of an IP datagram. This state defines, among other things, the
specific services provided to the datagram, which cryptographic
algorithms will be used to provide the services, and the keys used as
input to the cryptographic algorithms.
Establishing this shared state in a manual fashion does not scale
well. Therefore a protocol to establish this state dynamically is
needed. This memo describes such a protocol-- the Internet Key
Exchange (IKE). This is version 2 of IKE. Version 1 of IKE was
defined in RFCs 2407, 2408, and 2409. This single document is
intended to replace all three of those RFCs.
1.1 The IKE Protocol
IKE performs mutual authentication between two parties and
establishes an IKE security association that includes shared secret
information that can be used to efficiently establish SAs for ESP
(RFC 2406), AH (RFC 2402) and/or IPcomp (RFC 2393). We call the IKE
SA an "IKE-SA", and the SAs for ESP, AH, and/or IPcomp that get set
up through that IKE-SA we call "child-SA"s.
We call the setup of the IKE-SA "phase 1" and subsequent IKE
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exchanges "phase 2" even though setup of a child-SA can be
piggybacked on the initial phase 1 exchange. The phase 1 exchange is
two request/response pairs. A phase 2 exchange is one
request/response pair, and can be used to create or delete a child-
SA, rekey or delete the IKE-SA, or give information such as error
conditions.
IKE message flow always consists of a request followed by a response.
It is the responsibility of the requester to ensure reliability. If
the response is not received within a timeout interval, the requester
retransmits the request.
The first request/response of a phase 1 exchange, which we'll call
IKE_SA_init, negotiates security parameters for the IKE-SA, and sends
Diffie-Hellman values. We call the response IKE_SA_init_response.
The second request/response, which we'll call IKE_auth, transmits
identities, proves knowledge of the private signature key, and sets
up an SA for the first (and often only) AH and/or ESP and/or IPcomp.
We call the response IKE_auth_response.
If the Responder feels it is under attack, and wishes to use a
stateless cookie (see section on cookies). it can respond to an
IKE_SA_init with an IKE_SA_init_reject with a cookie value that must
be sent with a subsequent IKE_SA_init_request. The Initiator then
sends another IKE_SA_init, but this time including the Responder's
cookie value.
Phase 2 exchanges each consist of a single request/response pair. The
types of exchanges are CREATE_CHILD_SA (creates a child-SA), or an
informational exchange which deletes a child-SA or the IKE-SA or
informs the other side of some error condition. All these messages
require a response, so an informational message with no payloads can
serve as a check for liveness.
1.2 Change History
1.2.1 Changes from IKEv1 to IKEv2-00 November 2001
The goals of this revision to IKE are:
1) To define the entire IKE protocol in a single document, rather
than three that cross reference one another;
2) To simplify IKE by eliminating the Aggressive Mode option and all
but one of the authentication algorithms making phase 1 a single
exchange (based on public signature keys);
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3) To remove the Domain of Interpretation (DOI), Situation (SIT), and
Labeled Domain Identifier fields, and the Commit and Authentication
only bits;
4) To decrease IKE's latency by making the initial exchange be 2
round trips (4 messages), and allowing the ability to piggyback setup
of a Child-SA on that exchange;
5) To replace the cryptographic algorithms for protecting the IKE
messages themselves with one based closely on ESP to simplify
implementation and security analysis;
6) To reduce the number of possible error states by making the
protocol reliable (all messages are acknowledged) and sequenced. This
allows shortening Phase 2 exchanges from 3 messages to 2;
7) To increase robustness by allowing the Responder, if under attack,
to require return of a cookie before the Responder commits any state
to the exchange;
8) To fix bugs such as the hash problem documented in [draft-ietf-
ipsec-ike-hash-revised-02.txt];
9) To specify Traffic Selectors in their own payload type rather then
overloading ID payloads, and making more flexible the Traffic
Selectors that may be specified;
10) To avoid unnecessary exponential explosion of space in attribute
negotiation, by allowing choices when multiple algorithms of one type
(say, encryption) can work with any of a number of acceptable
algorithms of another type (say, integrity protection);
11) To specify required behavior under certain error conditions or
when data that is not understood is received in order to make it
easier to make future revisions in a way that does not break
backwards compatibility;
12) To simplify and clarify how shared state is maintained in the
presence of network failures and Denial of Service attacks; and
13) To maintain existing syntax and magic numbers to the extent
possible to make it likely that implementations of IKEv1 can be
enhanced to support IKEv2 with minimum effort.
1.2.2 Changes from IKEv2-00 to IKEv2-01 February 2002
1) Changed Appendix B to specify the encryption and authentication
processing for IKE rather than referencing ESP. Simplified the format
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by removing idiosyncracies not needed for IKE.
2) Added option for authentication via a shared secret key.
3) Specified different keys in the two directions of IKE messages.
Removed requirement of different cookies in the two directions since
now no longer required.
4) Change the quantities signed by the two ends in AUTH fields to
assure the two parties sign different quantities.
5) Changed reference to AES to AES_128.
6) Removed requirement that Diffie-Hellman be repeated when rekeying
IKE SA.
7) Fixed typos.
8) Clarified requirements around use of port 500 at the remote end in
support of NAT.
9) Clarified required ordering for payloads.
10) Suggested mechanisms for avoiding DoS attacks.
11) Removed claims in some places that the first phase 2 piggybacked
on phase 1 was optional.
1.2.3 Changes from IKEv2-01 to IKEv2-02 April 2002
1) Moved the Initiator CERTREQ payload from message 1 to message 3.
2) Added a second optional ID payload in message 3 for the Initiator
to name a desired Responder to support the case where multiple named
identities are served by a single IP address.
3) Deleted the optimization whereby the Diffie-Hellman group did not
need to be specified in phase 2 if it was the same as in phase 1 (it
complicated the design with no meaningful benefit).
4) Added a section on the implications of reusing Diffie-Hellman
expontentials
5) Changed the specification of sequence numbers to being at 0 in
both directions.
6) Many editorial changes and corrections, the most significant being
a global replace of "byte" with "octet".
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1.3 Requirements Terminology
Keywords "MUST", "MUST NOT", "REQUIRED", "SHOULD", "SHOULD NOT" and
"MAY" that appear in this document are to be interpreted as described
in [Bra97].
2 Protocol Overview
IKE runs over UDP port 500. Since UDP is a datagram (unreliable)
protocol, IKE includes in its definition recovery from transmission
errors, including packet loss, packet replay, and packet forgery. IKE
is designed to function so long as at least one of a series of
retransmitted packets reaches its destination before timing out and
the channel is not so full of forged and replayed packets so as to
exhaust the network or CPU capacities of either endpoint. Even in the
absence of those minimum performance requirements, IKE is designed to
fail cleanly (as though the network were broken).
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2.1 Use of Retransmission Timers
All messages in IKE exist in pairs: a request and a response. The
setup of an IKE SA normally consists of two request/response pairs.
Once the IKE SA is set up, either end of a security association may
initiate requests at any time, and there can be many requests and
responses "in flight" at any given moment. But each message is
labelled as either a request or a response and for each pair one end
of the security association is the Initiator and the other is the
Responder.
For every pair of messages, the Initiator is responsible for
retransmission in the event of a timeout. The Responder will never
retransmit a response unless it receives a retransmission of the
request. In that event, the Responder MUST either ignore the
retransmitted request except insofar as it triggers a retransmission
of the response OR if processing the request a second time has no
adverse effects, the Responder may choose to process the request
again and send a semantically equivalent reply.
IKE is a reliable protocol, in the sense that the Initiator MUST
retransmit a request until either it receives a corresponding reply
OR it deems the IKE security association to have failed and it
discards all state associated with the IKE-SA and any Child-SAs
negotiated using that IKE-SA.
2.2 Use of Sequence Numbers for Message ID
Every IKE 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.
The Message ID is a 32 bit quantity, which is zero for the first IKE
request in each direction. The IKE SA initial setup messages will
always be numbered 0 and 1. Each endpoint in the IKE Security
Association maintains two "current" Message IDs: the next one to be
used for a request it initiates and the next one it expects to see
from the other end. These counters increment as requests are
generated and received. Responses always contain the same message ID
as the corresponding request. That means that after the initial
exchange, each integer n will appear as the message ID in four
distinct messages: The nth request from the original IKE Initiator,
the corresponding response, the nth request from the original IKE
Responder, and the corresponding response. If the two ends make very
different numbers of requests, the Message IDs in the two directions
can be very different. There is no ambiguity in the messages,
however, because each packet contains enough information to determine
which of the four messages a particular one is.
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In the case where the IKE_SA_init is rejected (e.g. in order to
require a cookie), the second IKE_SA_init message will begin the
sequence over with Message #0.
2.3 Window Size for overlapping requests
In order to maximize IKE throughput, an IKE endpoint MAY issue
multiple requests before getting a response to any of them. For
simplicity, an IKE implementation MAY choose to process requests
strictly in order and/or wait for a response to one request before
issuing another. Certain rules must be followed to assure
interoperability between implementations using different strategies.
After an IKE-SA is set up, either end can initiate one or more
requests. These requests may pass one another over the network. An
IKE endpoint MUST be prepared to accept and process a request while
it has a request outstanding in order to avoid a deadlock in this
situation. An IKE endpoint SHOULD be prepared to accept and process
multiple requests while it has a request outstanding.
An IKE endpoint MUST NOT exceed the peer's stated window size (see
section 7.3.2) for transmitted IKE requests. In other words, if Bob
stated his window size is N, then when Alice needs to make a request
X, she MUST wait until she has received responses to all requests up
through request X-N. An IKE endpoint MUST keep a copy of (or be able
to regenerate exactly) each request it has sent until it receives the
corresponding response. An IKE endpoint MUST keep a copy of (or be
able to regenerate with semantic equivalence) the number of previous
responses equal to its contracted window size in case its response
was lost and the Initiator requests its retransmission by
retransmitting the request.
An IKE endpoint SHOULD be capable of processing incoming requests out
of order to maximize performance in the event of network failures or
packet reordering.
2.4 State Synchronization and Connection Timeouts
An IKE endpoint is allowed to forget all of its state associated with
an IKE-SA and the collection of corresponding child-SAs at any time.
This is the anticipated behavior in the event of an endpoint crash
and restart. It is important when an endpoint either fails or
reinitializes its state that the other endpoint detect those
conditions and not continue to waste network bandwidth by sending
packets over those SAs and having them fall into a black hole.
Since IKE is designed to operate in spite of Denial of Service (DoS)
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attacks from the network, an endpoint MUST NOT conclude that the
other endpoint has failed based on any routing information (e.g. ICMP
messages) or IKE messages that arrive without cryptographic
protection (e.g., notify messages complaining about unknown SPIs). An
endpoint MUST conclude that the other endpoint has failed only when
repeated attempts to contact it have gone unanswered for a timeout
period. An endpoint SHOULD suspect that the other endpoint has failed
based on routing information and initiate a request to see whether
the other endpoint is alive. To check whether the other side is
alive, IKE provides a null query notify message that requires an
acknowledgment. If a cryptographically protected message has been
received from the other side recently, unprotected notifications MAY
be ignored. Implementations MUST limit the rate at which they
generate responses to unprotected messages.
Numbers of retries and lengths of timeouts are not covered in this
specification because they do not affect interoperability. It is
suggested that messages be retransmitted at least a dozen times over
a period of at least several minutes before giving up on an SA, but
different environments may require different rules. An exception to
this rule is that a Responder who has not received a
cryptographically protected message on an IKE-SA MUST eventually time
it out and delete it. Note that consuming state on an IKE Responder
by setting up large numbers of half-open IKE-SAs is a likely denial
of service attack, so the policy for timing these out and limiting
the resources they consume should be considered carefully.
There is a Denial of Service attack on the Initiator of an IKE SA
that can be avoided if the Initiator takes the proper care. Since the
first two messages of an SA setup are not cryptographically
protected, an attacker could respond to the Initiator's message
before the genuine Responder and poison the connection setup attempt.
To prevent this, the Initiator SHOULD be willing to accept multiple
responses to its first message, treat each as potentially legitimate,
respond to it, and then discard all the invalid half open connections
when she receives a valid cryptographically protected response to any
one of her responses.
Note that with these rules, there is no reason to negotiate and agree
upon an SA lifetime. If IKE presumes the partner is dead, based on
repeated lack of acknowledgment to an IKE message, then the IKE SA
and all child-SAs set up through that IKE-SA are deleted.
An IKE endpoint MAY delete inactive Child-SAs to recover resources
used to hold their state. If an IKE endpoint chooses to do so, it
MUST send Delete payloads to the other end notifying it of the
deletion. It MAY similarly time out the IKE-SA. Closing the IKE-SA
implicitly closes all associated Child-SAs. An IKE endpoint SHOULD
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send a Delete payload indicating that it has closed the IKE-SA.
2.5 Version Numbers and Forward Compatibility
This document describes version 2.0 of IKE, meaning the major version
number is 2 and the minor version number is zero. It is likely that
some implementations will want to support both version 1.0 and
version 2.0, and in the future, other versions.
The major version number should only be incremented if the packet
formats or required actions have changed so dramatically that an
older version node would not be able to interoperate with a newer
version node if it simply ignored the fields it did not understand
and took the actions specified in the older specification. The minor
version number indicates new capabilities, and MUST be ignored by a
node with a smaller minor version number, but used for informational
purposes by the node with the larger minor version number. For
example, it might indicate the ability to process a newly defined
notification message. The node with the larger minor version number
would simply note that its correspondent would not be able to
understand that message and therefore would not send it.
If you receive a message with a higher major version number, you MUST
drop the message and SHOULD send an unauthenticated notification
message containing the highest version number you support. If you
support major version n, and major version m, you MUST support all
versions between n and m. If you receive a message with a major
version that you support, you MUST respond with that version number.
In order to prevent two nodes from being tricked into corresponding
with a lower major version number than the maximum that they both
support, IKE has a flag that indicates that the node is capable of
speaking a higher major version number.
Thus the major version number in the IKE header indicates the version
number of the message, not the highest version number that the
transmitter supports. If A is capable of speaking versions n, n+1,
and n+2, and B is capable of speaking versions n and n+1, then they
will negotiate speaking n+1, where A will set the flag indicating
ability to speak a higher version. If they mistakenly (perhaps
through an active attacker sending error messages) negotiate to
version n, then both will notice that the other side can support a
higher version number, and they MUST break the connection and
reconnect using version n+1.
Note that v1 does not follow these rules, because there is no way in
v1 of noting that you are capable of speaking a higher version
number. So an active attacker can trick two v2-capable nodes into
speaking v1. Given the design of v1, there is no way of preventing
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this, but this version number discipline will prevent such problems
in future versions. When a v2-capable node negotiates down to v1, it
SHOULD note that fact in its logs.
ISSUE: The SSLv2 to SSLv3 upgrade handled this issue in a very clever
way, and we could copy it. SSLv3 specified that certain octets that
in v2 were randomly generated values be set to a constant when a v3
capable node negotiated down to v2. We could, for example, choose a
constant value for part of the IKEv1 cookie to indicate IKEv2
capability. Alternatively, we could define a new IKEv1 cipher suite
that no IKEv1 implementation could accept but which could be used as
such a flag.
Also for forward compatibility, all fields marked RESERVED MUST be
set to zero by a version 2.0 implementation and their content MUST be
ignored by a version 2.0 implementation ("Be conservative in what you
send and liberal in what you receive"). In this way, future versions
of the protocol can use those fields in a way that is guaranteed to
be ignored by implementations that do not understand them.
Similarly, payload types that are not defined are reserved for future
use and implementations of version 2.0 MUST skip over those payloads
and ignore their contents.
IKEv2 adds a "critical" flag to each payload header for further
flexibility for forward compatibility. If the critical flag is set
and the payload type is unsupported, the message MUST be rejected and
the response to the IKE request containing that payload MUST include
a notify payload UNSUPPORTED-CRITICAL-PAYLOAD, indicating an
unsupported critical payload was included. If the critical flag is
not set and the payload type is unsupported, that payload is simply
skipped. While new payload types may be added in the future and may
appear interleaved with the fields defined in this specification,
implementations MUST send the payloads defined in this specification
in the stated order and implementations SHOULD reject as invalid a
message with payloads in an unexpected order.
2.6 Cookies
The term "cookies" originates with Karn and Simpson [RFC 2522] in
Photurus, an early proposal for key managment with IPsec. It has
persisted because the IETF has never rejected an offer involving
cookies. In IKEv2, the cookies serve two purposes. First, they are
used as IKE-SA identifiers in the headers of IKE messages. As with
ESP and AH, in IKEv2 the recipient of a message chooses an IKE-SA
identifier that uniquely defines that SA to that recipient. For this
purpose (IKE-SA identifiers), it might be convenient for the cookie
value to be chosen so as to be a table index for fast lookups of SAs.
But this conflicts with the second purpose of the cookies (to be
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explained shortly).
Unlike ESP and AH where only the recipient's SA identifier appears in
the message, in IKE, the sender's IKE SA identifier is also sent in
every message. In IKEv1 the IKE-SA identifier consisted of the pair
(Initiator cookie, Responder cookie), whereas in IKEv2, the SA is
uniquely defined by the recipient's SA identifier even though both
are included in the IKEv2 header.
The second use of cookies in IKEv2 is for a limited protection from
denial of service attacks. Receipt of a request to start an SA can
consume substantial resources. A likely denial of service attack
against IKE is to overwhelm a system with large numbers of SA
requests from forged IP addresses. This can consume CPU resources
doing the crypto, and memory resources remembering the state of the
"half open" connections until they time out. A robust design would
limit the resources it is willing to devote to new connection
establishment, but even so the denial of service attack could
effectively prevent any new connections.
This attack can be rendered more difficult by requiring that the
Responder to an SA request do minimal computation and allocate no
memory until the Initiator has proven that it can receive messages at
the address it claims to be sending from. This is done in a stateless
way by computing the cookie in a way that the Responder can recompute
the same value, but the Initiator can't guess it. A recommended
strategy is to compute the cookie as a cryptographic hash of the
Initiator's IP address, the Initiator's cookie value (its chosen IKE
security identifier), and a secret known only to the Responder. That
secret should be changed periodically to prevent the "cookie jar"
attack where an attacker accumulates lots of cookies from lots of IP
addresses over time and then replays them all at once to overwhelm
the Responder.
In ISAKMP and IKEv1, the term cookie was used for the connection
identifier, but the protocol did not permit their use against this
particular denial of service attack. To avoid the cookie exchange
adding extra messages to the protocol in the common case where the
Responder is not under attack, IKEv2 goes back to the approach in
Oakley (RFC 2412) where the cookie challenge is optional. Upon
receipt of an IKE_SA_init, a Responder may either proceed with
setting up the SA or may tell the Initiator to send another
IKE_SA_init, this time providing a supplied cookie.
It may be convenient for the IKE-SA identifier to be an index into a
table. It is not difficult for the Initiator to choose an IKE-SA
identifier that is convenient as a table identifier, since the
Initiator does not need to use it as an anti-clogging token, and is
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keeping state. IKEv2 allows the Responder to initially choose a
stateless anti-clogging type cookie by responding to an IKE_SA_init
with a cookie request, and then upon receipt of an IKE_SA_init with a
valid cookie, change his cookie value from the computed anti-clogging
token to a more convenient value, by sending a different value for
his cookie in the IKE_SA_init_response. This will not confuse the
Initiator (Alice), because she will have chosen a unique cookie value
A, so if her SA state for the partially set up IKE-SA says that Bob's
cookie for the SA that Alice knows as "A" is B, and she receives a
response from Bob with cookies (A,C), that means that Bob wants to
change his value from B to C for the SA that Alice knows uniquely as
"A".
Another reason why Bob might want to change his cookie value is that
it is possible (though unlikely) that Bob will choose the same cookie
for multiple SAs if the hash of the Initiator cookie, Initiator IP
address, and whatever other information might be included happens to
hash to the same value.
In IKEv2, like IKEv1, both 8-octet cookies appear in the message, but
in IKEv2 (unlike v1), the value chosen by the message recipient
always appears first in the message. This change eliminates a flaw in
IKEv1, as well as having other advantages (allowing the recipient to
look up the SA based on a small, conveniently chosen value rather
than a 16-octet pseudorandom value.)
The flaw in IKEv1 is that it was possible (though unlikely) for two
connections to have the same set of cookies. For instance, if Alice
chose A as the Initiator cookie when initiating a connection to Bob,
she might subsequently receive a connection request from Carol, and
Carol might also have chosen A as the Initiator cookie. Whatever
value Alice responds to Carol, say B, might be selected as the
Responder cookie by Bob for the Alice-Bob SA. Then Alice would be
involved in two IKE sessions, both of which had Initiator cookie=A
and Responder cookie=B. To minimize, but not eliminate, the
probability of this happening, version 1 IKE recommended that cookies
be chosen at random.
The cookies are one of the inputs into the function that computes the
keying material. If the Responder initially sends a stateless cookie
value in its IKE_SA_init_reject, and changes to a different value
when it sends its IKE_SA_init_response, it is the cookie value in the
IKE_SA_init_response that is the input for generating the keying
material.
Note that one of the denial of service attacks that cookies are
designed to thwart is exhaustion of state at the target by creating
half-open connections. This defense would be ineffective if there
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were another equally easy way for an attacker to consume state at the
target. IKE runs over UDP, and may send messages sufficiently large
that they must be fragmented. But accumulating fragments of UDP
packets consumes state at the target, so if an IKE responder were
required to accept and reassemble UDP packets from unknown sources,
another equally easy denial of service attack would be possible.
To thwart the UDP reassembly buffer attack, the IKE responder SHOULD,
when it detects that it is under attack, have a mechanism to inform
IP reassembly to only accept UDP fragments from IP addresses from
which it has received a valid cookie and to refuse to accept UDP
fragments from all other IP addresses. To faccilitate this, the
IKE_SA_init message SHOULD be kept under 500 octets and responders
MAY reject fragmented IKE_SA_init messages.
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2.7 Cryptographic Algorithm Negotiation
The payload type known as "SA" indicates a proposal for a set of
choices of protocols (e.g., IKE, ESP, AH, and/or IPcomp) for the SA
as well as cryptographic algorithms associated with each protocol. In
IKEv1 it was extremely complex, and required a separate proposal for
each possible combination. If there were n algorithms of one type
(say encryption) that were acceptable and worked with any one of m
algorithms of another type (say integrity protection), then it would
take space proportional to n*m to express all of the possibilities.
IKEv2 has simplified the format of the SA payload somewhat, but in
addition to simplifying the format, solves the exponential explosion
by allowing, within a proposal, multiple algorithms of the same type.
If more than one algorithm of the same type (say encryption) appears
in a proposal, that means that the sender of that SA proposal is
willing to accept the proposal with any of those choices, and the
recipient when it accepts the proposal selects exactly one of each of
the types of algorithms from the choices offered within that
proposal.
An SA consists of one or more proposals. Each proposal has a number
(so that the recipient can specify which proposal has been accepted),
and contains a protocol (IKE, ESP, AH, or IPcomp), a SPI to identify
the SA for ESP or AH or IPcomp, and set of transforms. Each transform
consists of a type (e.g., encryption, integrity protection,
authentication, Diffie-Hellman group, compression) and a transform ID
(e.g., DES, IDEA, HMAC-MD5). To negotiate an SA that does ESP,
IPcomp, and AH, the SA will contain three proposals with the same
proposal number, one proposing ESP, a 4 octet SPI to be used with
ESP, and a set of transforms; one proposing AH, a 4-octet SPI to be
used with AH, and a set of transforms; and one proposing IPcomp, a
2-octet SPI to be used with IPcomp, and a set of transforms. If the
recipient selects that proposal number, it means that SAs will be
created for all of ESP, AH, and IPcomp.
In IKEv2, since the Initiator sends her Diffie-Hellman value in the
IKE_SA_init, she must guess at the Diffie-Hellman group that Bob will
select from her list of supported groups. Her guess MUST be the first
in the list to allow Bob to unambiguously identify which group the
accompanying KE payload is from. If her guess is incorrect then Bob's
response informs her of the group he would choose, and notifies her
that her offer is invalid because the KE payload is not from the
desired group. In this case Alice will send a new IKE_SA_init, with
the same original choices in the list (this is important to prevent
an active attacker from tricking them into using a weaker group than
they would have agreed upon) but with Bob's preferred group first,
and a KE payload containing an exponential from that group.
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If none of Alice's options are acceptable, then Bob notifies her
accordingly.
2.8 Rekeying
Security associations negotiated in both phase 1 and phase 2 contain
secret keys which may only be used for a limited amount of time. This
determines the lifetime of the entire security association. When the
lifetime of a security association expires the security association
MUST NOT be used. If there is demand, new security associations can
be established. Reestablishment of security associations to take the
place of ones which expire is referred to as "rekeying".
To rekey a child-SA, create a new, equivalent SA (see section 4 and
4.1 below), and when the new one is established, delete the old one.
To rekey an IKE-SA, establish a new equivalent IKE-SA (see section 4
and 4.2 below) with the peer to whom the old IKE-SA is shared using a
Phase 2 negotiation within the existing IKE-SA. An IKE-SA so created
inherits all of the original IKE-SA's child SAs. Use the new IKE-SA
for all control messages needed to maintain the child-SAs created by
the old IKE-SA, and delete the old IKE-SA.
SAs SHOULD be rekeyed proactively, i.e., the new SA should be
established before the old one expires and becomes unusable. Enough
time should elapse between the time the new SA is established and the
old one becomes unusable so that traffic can be switched over to the
new SA.
A difference between IKEv1 and IKEv2 is that in IKEv1 SA lifetimes
were negotiated. In IKEv2, each end of the SA is responsible for
enforcing its own lifetime policy on the SA and rekeying the SA when
necessary. If the two ends have different lifetime policies, the end
with the shorter lifetime will end up always being the one to request
the rekeying.
If the two ends have the same lifetime policies, it is possible that
both will initiate a rekeying at the same time (which will result in
redundant SAs). To reduce the probability of this happening, the
timing of rekeying requests should be jittered (delayed by a random
amount of time).
This form of rekeying will temporarily result in multiple similar SAs
between the same pairs of nodes. When there are two SAs eligible to
receive packets, a node MUST accept incoming packets through either
SA. The node that initiated the rekeying SHOULD delete the older SA
after the new one is established.
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2.9 Traffic Selector Negotiation
When an IP packet is received by an RFC2401 compliant IPsec subsystem
and matches a "protect" selector in its SPD, the subsystem MUST
protect that packet with IPsec. When no SA exists yet it is the task
of IKE to create it. Information about the traffic that needs
protection is transmitted to the IKE subsystem in a manner outside
the scope of this document (see [PFKEY] for an example). This
information is negotiated between the two IKE endpoints using TS
(Traffic Selector) payloads.
The TS payload consists of a set of individual traffic selectors.
The selector from the SPD has "source" and "destination" components
and these are represented in IKE as a pair of TS payloads, TSi
(traffic selector-initiator) and TSr (traffic selector-responder).
TSi describes the addresses and ports that the Initiator will send
from over the SA and which it will accept packets for. TSr describes
the addresses and ports that the Initiator will sent to over the SA
and which it will accept packets from.
The Responder is allowed to narrow the choices by selecting a subset
of the traffic, for instance by eliminating one or more members of
the set of traffic selectors provided the set does not become the
NULL set.
Note that the traffic selectors apply to both child-SAs (from the
Initiator to the Responder and from the Responder to the Initiator),
but the Responder does not change the order of the TS payloads. An
address within the selector of TSi would appear as a source address
in the child-SA from the Initiator, and would appear as a destination
address in traffic on the child-SA to the Initiator (from the
Responder).
IKEv2 is more flexible than IKEv1. IKEv2 allows sets of ranges of
both addresses and ports, and allows the Responder to choose a subset
of the requested traffic rather than simply responding "not
acceptable".
2.10 Nonces
The IKE_SA_init_request and the IKE_SA_init_response each contain a
nonce. These nonces are used as inputs to cryptographic functions.
The child-create-request and the child-create-response also contain a
nonce. These nonces are used to add freshness to the key derivation
technique used to obtain keys for child SAs. Nonces used in IKEv2
MUST therefore have strong pseudo-random properties (see RFC1715).
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2.11 Address and Port Agility
IKE runs over UDP port 500, and implicitly sets up ESP, AH, and
IPcomp associations for the same IP addresses it runs over. The IP
addresses and ports in the outer header are, however, not themselves
cryptographically protected, and IKE is designed to work even through
Network Address Translation (NAT) boxes. An implementation MUST
accept incoming connection requests even if not received from UDP
port 500, and should respond to the address and port from which the
request was received. An implementation MUST, however, accept
incoming requests only on UDP port 500 and send all responses from
UDP port 500. IKE functions identically over IPv4 or IPv6.
2.12 Reuse of Diffie-Hellman Exponentials
IKE generates keying material using an ephemeral Diffie-Hellman
exchange in order to gain the property of "perfect forward secrecy".
This means that once a connection is closed and its corresponding
keys are forgotten, even someone who has recorded all of the data
from the connection and gets access to all of the long term keys of
the two endpoints cannot reconstruct the keys used to protect the
conversation.
Achieving perfect forward secrecy requires that when a connection is
closed, each endpoint must forget not only the keys used by the
connection but any information that could be used to recompute those
keys. In particular, it must forget the secrets used in the Diffie-
Hellman calculation and any state that may persist in the state of a
pseudo-random number generater that could be used to recompute the
Diffie-Hellman secrets.
Since the computing of Diffie-Hellman exponentials is computationally
expensive, an endpoint may find it advantageous to reuse those
exponentials for multiple connection setups. There are several
reasonable strategies for doing this. An endpoint could choose a new
exponential periodically though this could result in less-than-
perfect forward secrecy if some connection lasts for less than the
lifetime of the exponential. Or it could keep track of which
exponential was used for each connection and delete the information
associated with the exponential only when some corresponding
connection was closed. This would allow the exponential to be reused
without losing perfect forward secrecy at the cost of maintaining
more state.
Decisions as to whether and when to reuse Diffie-Hellman exponentials
is a private decision in the sense that it will not affect
interoperability. An implementation that reuses exponentials may
choose to remember the exponential used by the other endpoint on past
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exchanges and if one is reused to avoid the second half of the
calculation.
3 The Phase 1 Exchange
The base Phase 1 exchange is a four message exchange (two
request/response pairs). The first pair of messages, the IKE_SA_init
exchange, negotiate cryptographic algorithms, (optionally) indicate
trusted CA names, exchange nonces, and do a Diffie-Hellman exchange.
This pair might be repeated if the response indicates that none of
the cryptographic proposals are acceptable, or the Diffie-Hellman
group chosen by the Initiator for sending her Diffie-Hellman value is
not the group that the Responder would have chosen, of if the
Responder is under attack and will only answer IKE_SA_init requests
containing a valid returned cookie value.
The second pair of messages, the IKE_auth and the IKE_auth_response,
authenticate the previous messages, exchange identities and
certificates, and establish the first child_SA. This pair of messages
is encrypted with a key established through the IKE_SA_init exchange,
so the identities are hidden from eavesdroppers.
In the following description, the payloads contained in the message
are indicated by names such as SA. The details of the contents of
each payload are described later. Payloads which may optionally
appear will be shown in brackets, such as [CERTREQ], would indicate
that optionally a certificate request payload can be included.
The Phase 1 exchange is as follows:
Initiator Responder
----------- -----------
HDR, SAi1, KEi, Ni -->
The SAi1 payload states the cryptographic algorithms the Initiator
supports for the IKE SA. The KE payload sends the Initiator's
Diffie-Hellman value. Ni is the Initiator's nonce.
<-- HDR, SAr1, KEr, Nr, [CERTREQ]
The Responder chooses among the Initiator's cryptographic algorithms
and expresses that choice in the SAr1 payload, completes the Diffie-
Hellman exchange with the KEr payload, and sends its nonce in the Nr
payload.
At this point in time each party generates SKEYSEED and its
derivatives. The following two messages, the SA_auth and
SA_auth_response, are encrypted and integrity protected (as indicated
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by the '*' following the IKE header) and the encryption bit in the
IKE header is set. The keys used for the encryption and integrity
protection are derived from SK_a and SK_e as described below.
HDR*, IDi, [CERT,] [CERTREQ,] [IDr,] AUTH,
SAi2, TSi, TSr -->
The Initiator identifies herself with the IDi payload and
authenticates herself to the Responder with the AUTH payload, and
begins negotiation of a child-SA using the SAi2 payload. The fields
starting with SAi2 are described in the description of Phase 2.
There are optional fields where the Initiator can provide
certificates [CERT] the Responder might find useful in validating
AUTH, her list of preferred root certifiers [CERTREQ], and the name
of the entity with which she is trying to open a connection [IDr]
(for the case where multiple named entities exist at a single IP
address).
<-- HDR*, IDr, [CERT,] AUTH,
SAr2, TSi, TSr
The Responder identifies himself with an ID payload optionally sends
one or more certificates, authenticates himself with the AUTH
payload, and completes negotiation of a child-SA with the additional
fields described below in the phase 2 exchange.
3.1 Generating Keying Material for the IKE-SA
The shared secret information is computed as follows. A quantity
called SKEYSEED is calculated from the nonces exchanged during the
IKE_SA_init exchange, and the Diffie-Hellman shared secret
established during that exchange. SKEYSEED is used to calculate
three other secrets: SK_d used for deriving new keys for the child-
SAs established with this IKE-SA; SK_a used for authenticating the
component messages of subsequent exchanges; and SK_e used for
encrypting (and of course decrypting) all subsequent exchanges.
SKEYSEED and its derivatives are computed as follows:
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SKEYSEED = prf(Ni | Nr, g^ir)
SK_d = prf(SKEYSEED, g^ir | Ni | Nr | CKY-I | CKY-R | 0)
SK_a = prf(SKEYSEED, SK_d | g^ir | Ni | Nr | CKY-I | CKY-R | 1)
SK_e = prf(SKEYSEED, SK_a | g^ir | Ni | Nr | CKY-I | CKY-R | 2)
CKY-I and CKY-R are the Initiator's and Responder's cookies,
respectively, from the IKE header. g^ir is the shared secret from the
ephemeral Diffie-Hellman exchange. Ni and Nr are the nonces,
stripped of any headers. 0, 1, and 2 are represented by a single
octet containing the value 0, 1, or 2 (the values, not the ASCII
representation of the digits). prf is the "pseudo-random"
cryptographic function negotiated in the IKE-SA-init exchange.
The two directions of flow use different keys. Keys used to protect
messages from the original initiator are taken from the first bits of
SK_a and SK_e. Keys used to protect messages in the other direction
are taken from subsequent bits. Each algorithm takes a fixed number
of bits of keying material, which is specified as part of the
algorithm. If the total number of key bits needed is greater than the
size of the output of the prf function, the keying material must be
expanded.
For situations where the amount of keying material desired is greater
than that supplied by the prf, KEYMAT is expanded by feeding the
results of the prf back into itself and concatenating results until
the required keying material has been reached. In other words,
KEYMAT = K1 | K2 | K3 | ...
where:
K1 = prf(SK_x, 0)
K2 = prf(SK_x, K1)
K3 = prf(SK_x, K2)
etc.
where 0 is represented by a single octet containing the value 0 (the
value, not the ASCII representation of the digit), and SK_x is either
SK_e or SK_a depending on which keying material needs expansion.
3.2 Authentication of the IKE-SA
The peers are authenticated by having each sign (or MAC using a
shared secret as the key) the concatenation of their own first
message and the other peer's nonce. The octets to be signed start
with the first octet of the header and end with the last octet of the
last payload. The octets of the nonce are only the content and not
the header.
Note that all of payloads of the peer's own first message are
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included under the signature, including payload types not defined in
this document. It is possible that some other payloads defined in
the future might appropriately be zeroed before signing, but such a
possibility is not supported by this version of IKE.
Optionally, messages 3 and 4 MAY include a certificate, or
certificate chain providing evidence that the key used to compute a
digital signature belongs to the name in the ID payload. The
signature or MAC will be computed using algorithms dictated by the
type of key used by the signer, an RSA-signed PKCS1-padded-SHA1-hash
for an RSA digital signature, a DSS-signed SHA1-hash for a DSA
digital signature, or the negotiated PRF function for a pre-shared
key. There is no requirement that the Initiator and Responder sign
with the same cryptographic algorithms. The choice of cryptographic
algorithms depends on the type of key each has. This type is either
indicated in the certificate supplied or, if the keys were exchanged
out of band, the key types must have been similarly learned. It will
commonly be the case, but it is not required that if a shared secret
is used for authentication that the same key is used in both
directions. In particular, the initiator may be using a shared key
derived from a password while the responder may have a public
signature key and certificate.
4 The CREATE-CHILD-SA (Phase 2) Exchange
A phase 2 exchange is one request/response pair, and can be used to
create or delete a child-SA, delete or rekey the IKE-SA, check the
liveness of the IKE-SA, or deliver information such as error
conditions. It is encrypted and integrity protected using the keys
negotiated during the creation of the IKE-SA.
Messages are cryptographically protected using the cryptographic
algorithms and keys negotiated in the first two messages of the IKE
exchange using a syntax described in Appendix B. Encryption uses
keys derived from SK_e, one in each direction; Integrity uses keys
derived from SK_a, one in each direction.
Either endpoint may initiate a phase 2 exchange, so in this section
the term Initiator refers to the endpoint initiating this exchange.
When relevant, the Initiator of the IKE SA will be referred to as
such.
A child-SA is created by sending a CREATE_CHILD_SA request. If PFS
for the child-SA is desired, the CREATE_CHILD_SA request contains KE
payloads for an additional Diffie-Hellman exchange. The keying
material for the child-SA is a function of SK_d established during
the establishment of the IKE-SA, the nonces exchanged during the
CREATE_CHILD_SA exchange, and the Diffie-Hellman value (if KE
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payloads are included in the CREATE_CHILD_SA exchange).
In the child-SA created as part of the phase 1 exchange, a second KE
payload MUST NOT be used, and the Nonces are not transmitted but are
assumed to be the same as the phase 1 nonces.
The CREATE_CHILD_SA request contains:
Initiator Responder
----------- -----------
HDR*, SA, Ni, [KEi],
TSi, TSr -->
The Initiator sends SA offer(s) in the SA payload(s), a nonce in the
Ni payload, optionally a Diffie-Hellman value in the KE payload, and
the proposed traffic selectors in the TSi and TSr payloads.
The message past the header is encrypted and the message including
the header is integrity protected using the cryptographic algorithms
negotiated in Phase 1.
The CREATE_CHILD_SA response contains:
<-- HDR*, SA, Nr, [KEr],
TSi, TSr
The Responder replies (using the same Message ID to respond) with the
accepted offer in an SA payload, a Diffie-Hellman value in the KE
payload if and only if the Initiator included one, and the traffic
selectors for traffic to be sent on that SA in the TS payloads, which
may be a subset of what the Initiator of the child-SA proposed.
4.1 Generating Keying Material for IPsec SAs
Child-SAs are created either by being piggybacked on the phase 1
exchange, or in a phase 2 CREATE_CHILD_SA exchange. Keying material
for them is generated as follows:
KEYMAT = prf(SK_d, protocol | SPI | Nin | Nout )
For phase 2 exchanges with PFS the keying material is defined as:
KEYMAT = prf(SK_d, g(p2)^ir | protocol | SPI | Nin | Nout )
where g(p2)^ir is the shared secret from the ephemeral Diffie-Hellman
exchange of this phase 2 exchange,
In either case, "protocol", and "SPI", are from the SA payload that
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contained the negotiated (and accepted) proposal, Nin is the body of
the sender's (inbound using thie SPI) nonce payload minus the generic
header, and Nout is the body of the destination's (outbound using
this SPI) nonce payload minus the generic header.
A single child-SA negotiation results in two security associations--
one inbound and one outbound. Different Nonces and SPIs for each SA
(one chosen by the Initiator, the other by the Responder) guarantee a
different key for each direction. The SPI chosen by the destination
of the SA and the Nonces (ordered source followed by destination) are
used to derive KEYMAT for that SA.
This keying material (whether with PFS or without) MUST be used with
the negotiated SA. In the case of an ESP SA needing two keys for
encryption and authentication, the encryption key is taken from the
first octets of KEYMAT and the authentication key is taken from the
next octets. Each cryptographic algorithm takes a fixed number of
bits of keying material specified as part of the algorithm.
For situations where the amount of keying material desired is greater
than that supplied by the prf, KEYMAT is expanded by feeding the
results of the prf back into itself and concatenating results until
the required keying material has been reached. In other words,
KEYMAT = K1 | K2 | K3 | ...
where:
K1 = prf(SK_d, [ g(p2)^ir | ] protocol | SPI | Nin | Nout)
K2 = prf(SK_d, K1 | [ g(p2)^ir | ] protocol | SPI | Nin | Nout)
K3 = prf(SK_d, K2 | [ g(p2)^ir | ] protocol | SPI | Nin | Nout)
etc.
4.2 Generating Keying Material for IKE-SAs from a create-child exchange
The create-child exchange can be used to re-key an existing IKE-SA
(see section 2.8). New Initiator and Responder cookies are supplied
in the SPI fields. The ID and TS payloads are omitted when rekeying
an IKE-SA. SKEYSEED for the new IKE-SA is computed using SK_d from
the existing IKE-SA as follows:
SKEYSEED = prf(SK_d (old), [g(p2)^ir] | 0 | CKY-I | CKY-R | Ni |
Nr)
where g(p2)^ir is the shared secret from the ephemeral Diffie-Hellman
exchange of this phase 2 exchange, CKY-I is the 8-octet "SPI" from
the SA payload in the CREATE_CHILD_SA request, CKY-R is the 8-octet
"SPI" from the SA payload in the CREATE_CHILD_SA response, and Ni and
Nr are the two nonces stripped of any headers. "0" is a single octet
containing the value zero (the protocol ID of IKE).
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The new IKE SA MUST reset its message counters to 1.
SK_d, SK_a, and SK_e are computed from SKEYSEED as specified in
section 3.1.
5 Informational (Phase 2) Exchange
At various points during an IKE-SA, peers may desire to convey
control messages to each other regarding errors or notifications of
certain events. To accomplish this IKE defines a (reliable)
Informational exchange. Usually Informational exchanges happen
during phase 2 and are cryptographically protected with the IKE
exchange.
Control messages that pertain to an IKE-SA MUST be sent under that
IKE-SA. Control messages that pertain to Child-SAs MUST be sent under
the protection of the IKE-SA which generated them (or its successor
if the IKE-SA keys are rolled over).
There are two cases in which there is no IKE-SA to protect the
information. One is in the response to an IKE_SA_init_request to
request a cookie or to refuse the SA proposal. This would be conveyed
in a Notify payload of the IKE_SA_init_response.
The other case in which there is no IKE-SA to protect the information
is when a packet is received with an unknown SPI. In that case the
notification of this condition will be sent in an informational
exchange that is cryptographically unprotected.
Messages in an Informational Exchange contain zero or more
Notification or Delete payloads. The Recipient of an Informational
Exchange request MUST send some response (else the Sender will assume
the message was lost in the network and will retransmit it). That
response can be a message with no payloads. Actually, the request
message in an Informational Exchange can also contain no payloads.
This is the expected way an endpoint can ask the other endpoint to
verify that it is alive.
ESP, AH, and IPcomp SAs always exist in pairs, with one SA in each
direction. When an SA is closed, both members of the pair MUST be
closed. When SAs are nested, as when data is encapsulated first with
IPcomp, then with ESP, and finally with AH between the same pair of
endpoints, all of the SAs (up to six) must be deleted together. To
delete an SA, an Informational Exchange with one or more delete
payloads is sent listing the SPIs (as known to the recipient) of the
SAs to be deleted. The recipient MUST close the designated SAs.
Normally, the reply in the Informational Exchange will contain delete
payloads for the paired SAs going in the other direction. There is
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one exception. If by chance both ends of a set of SAs independently
decide to close them, each may send a delete payload and the two
requests may cross in the network. If a node receives a delete
request for SAs that it has already issued a delete request for, it
MUST delete the incoming SAs while processing the request and the
outgoing SAs while processing the response. In that case, the
responses MUST NOT include delete payloads for the deleted SAs, since
that would result in duplicate deletion and could in theory delete
the wrong SA.
A node SHOULD regard half open connections as anomalous and audit
their existence should they persist. Note that this specification
nowhere specifies time periods, so it is up to individual endpoints
to decide how long to wait. A node MAY refuse to accept incoming data
on half open connections but MUST NOT unilaterally close them and
reuse the SPIs. If connection state becomes sufficiently messed up, a
node MAY close the IKE-SA which will implicitly close all SAs
negotiated under it. It can then rebuild the SA's it needs on a clean
base under a new IKE-SA.
The Informational Exchange is defined as:
Initiator Responder
----------- -----------
HDR*, N, ..., D, ... -->
<-- HDR*, N, ..., D, ...
The processing of an Informational Exchange is determined by its
component payloads.
6 Error Handling
There are many kinds of errors that can occur during IKE processing.
If a request is received that is badly formatted or unacceptable for
reasons of policy (e.g. no matching cryptographic algorithms), the
response MUST contain a Notify payload indicating the error. If an
error occurs outside the context of an IKE request (e.g. the node is
getting ESP messages on a non-existent SPI), the node SHOULD initiate
an Informational Exchange with a Notify payload describing the
problem.
Errors that occur before a cryptographically protected IKE-SA is
established must be handled very carefully. There is a trade-off
between wanting to be helpful in diagnosing a problem and responding
to it and wanting to avoid being a dupe in a denial of service attack
based on forged messages.
If a node receives a message on UDP port 500 outside the context of
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an IKE-SA (and not a request to start one), it may be the result of a
recent crash. If the message is marked as a response, the node MAY
audit the suspicious event but MUST NOT respond. If the message is
marked as a request, the node MAY audit the suspicious event and MAY
send a response. If a response is sent, the response MUST be sent to
the IP address and port from whence it came with the IKE cookies
reversed in the header and the Message ID copied. The response MUST
NOT be cryptographically protected and MUST contain a notify payload
indicating INVALID-COOKIE.
A node receiving such a message MUST NOT respond and MUST NOT change
the state of any existing SAs. The message might be a forgery or
might be a response the genuine correspondent was tricked into
sending. A node SHOULD treat such a message (and also a network
message like ICMP destination unreachable) as a hint that there might
be problems with SAs to that IP address and SHOULD initiate a
liveness test for any such IKE-SA. An implementation SHOULD limit the
frequency of such tests to avoid being tricked into participating in
a denial of service attack.
A node receiving a suspicious message from an IP address with which
it has an IKE-SA MAY send an IKE notify payload in an IKE
Informational exchange over that SA. The recipient MUST NOT change
the state of any SA's as a result but SHOULD audit the event to aid
in diagnosing malfunctions. A node MUST limit the rate at which it
will send messages in response to unprotected messages.
7 Header and Payload Formats
7.1 The IKE Header
IKE messages use UDP port 500, with one IKE message per UDP datagram.
Information from the UDP header is largely ignored except that the IP
addresses and UDP ports from the headers are reversed and used for
return packets. Each IKE message begins with the IKE header, denoted
HDR in this memo. Following the header are one or more IKE payloads
each identified by a "Next Payload" field in the preceding payload.
Payloads are processed in the order in which they appear in an IKE
message by invoking the appropriate processing routine according to
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.
The Recipient SPI in the header identifies an instance of an IKE
security association. It is therefore possible for a single instance
of IKE to multiplex distinct sessions with multiple peers.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Recipient !
! SPI (aka Cookie) !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Sender !
! SPI (aka Cookie) !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Next Payload ! MjVer ! MnVer ! Exchange Type ! Flags !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Message ID !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Length !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Initialization Vector ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: IKE Header Format
o Recipient SPI (aka Cookie) (8 octets) - A value chosen by the
recipient to identify a unique IKE security association.
[NOTE: this is a deviation from ISAKMP and IKEv1, where the
cookies were always sent with the Initiator of the IKE-SA's
cookie first and the Responder's second. See section 2.6.]
o Sender SPI (aka Cookie) (8 octets) - A value chosen by the
sender to identify a unique IKE security association.
o Next Payload (1 octet) - Indicates the type of payload that
immediately follows the header. The format and value of each
payload is 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. Implementations based on
previous versions of IKE and ISAKMP MUST set the Major Version
to 1. Implementations based on this version of IKE MUST reject
(or ignore) messages containing a version number greater than
2.
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
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used. This dictates the payloads sent in each message and
message orderings in the exchanges.
Exchange Type Value
RESERVED 0
Reserved for ISAKMP 1 - 31
Reserved for IKEv1 32 - 33
Phase One 34
CREATE-CHILD-SA 35
Informational 36
Reserved for IKEv2+ 37-239
Reserved for private use 240-255
o Flags (1 octet) - indicates specific options that are set for
the message. Presence of options are indicated by the
appropriate bit in the flags field being set. The bits are
defined LSB first, so bit 0 would be the least significant
bit of the Flags octet. In the description below, a bit
being 'set' means its value is '1', while 'cleared' means
its value is '0'.
-- E(ncryption) (bit 0 of Flags) - If set, all payloads
following the header are encrypted and integrity
protected using the algorithms negotiated during
session establishment and a key derived during the key
exchange portion of IKE. If cleared, the payloads are
not protected. All payloads MUST be protected if a key
has been negotiated and any unprotected payload may
only be used to establish a new session or indicate a
problem.
-- C(ommit) (bit 1 of Flags) - This bit is defined by
ISAKMP but not used by IKEv2. Implementations of IKEv2
MUST clear this bit when sending and SHOULD ignore
it in incoming messages.
-- A(uthentication Only) (bit 2 of Flags) - This bit is
defined by ISAKMP but not used by IKEv2. Implementations
of IKEv2 MUST clear this bit when sending and SHOULD
ignore it in incoming messages.
-- I(nitiator) (bit 3 of Flags) - This bit MUST be set in
messages sent by the original Initiator of the IKE
exchange and MUST be cleared in messages sent
by the original Responder. It is
used by the recipient to determine whether the message
number should be interpreted in the context of its
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initiating state or its responding state.
-- V(ersion) (bit 4 of Flags) - 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.
-- R(eserved) (bits 5-7 of Flags) - These bit MUST be
cleared in messages sent and received messages with
these bits set MUST be rejected.
o Message ID (4 octets) - Message identifier used to control
retransmission of lost packets and matching of requests and
responses. See section 2.2. In the first message of a Phase 1
negotiation, the value MUST be set to 0. The response to that
message MUST also have a Message ID of 0.
o Length (4 octets) - Length of total message (header + payloads)
in octets. Session encryption can expand the size of an IKE
message and that is reflected in the total length of the
message.
o Initialization Vector (variable) - random octets used to
provide
initialization to an encryption mode-- e.g.
cipher block chaining (CBC) mode. This field MUST be present
when the encryption bit is set in the flags field (see below)
and MUST NOT be present otherwise. The length of the
Initialization Vector is cipher and mode dependent.
7.2 Generic Payload Header
Each IKE payload defined in sections 7.3 through 7.13 begins with a
generic header, shown in Figure 2. Figures for each payload below
will include the generic payload header but for brevity a repeat of
the description of each field will be omitted. The construction and
processing of the generic payload header is identical for each
payload and will similarly 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
The Generic Payload Header fields are defined as follows:
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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 it to the end of the message
and setting the "Next Payload" field of the preceding payload
to indicate the new payload's type.
o Critical (1 bit) - MUST be set to zero if the sender wants
the recipient to skip this payload if he does not
understand the payload type code. MUST be set to one if the
sender wants the recipient to reject this entire message
if he does not understand this payload type. MUST be ignored
by 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. The reasoning behind not setting
the critical bit for payloads defined in this document is
that all implementations MUST understand all payload types
defined in this document and therefore must ignore its value.
o RESERVED (7 bits) - MUST be sent as zero; MUST be ignored.
o Payload Length (2 octets) - Length in octets of the current
payload, including the generic payload header.
7.3 Security Association Payload
The Security Association Payload, denoted SA in this memo, is used to
negotiate attributes of a security association. Assembly of Security
Association Payloads requires great peace of mind. An SA may contain
multiple proposals. Each proposal may contain multiple protocols
(where a protocol is IKE, ESP, AH, or IPCOMP), each protocol may
contain multiple transforms, and each transform may contain multiple
attributes. 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.
The syntax of Security Associations, Proposals, Transforms, and
Attributes is based on ISAKMP, however the semantics are somewhat
different. The reason for the complexity and the hierarchy is to
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allow for multiple possible combinations of algorithms to be encoded
in a single SA. Sometimes there is a choice of multiple algorithms,
while other times there is a combination of algorithms. For example,
an Initiator might want to propose using (AH w/MD5 and ESP w/3DES) OR
(ESP w/MD5 and 3DES).
One of the reasons the semantics of the SA payload has changed from
ISAKMP and IKEv1 is to make the encodings more compact in common
cases.
The Proposal structure contains within it a Proposal # and a
Protocol-id. Each structure MUST have the same Proposal # as the
previous one or one greater. The first Proposal MUST have a Proposal
# of one. If two successive structures have the same Proposal number,
it means that the proposal consists of the first structure AND the
second. So a proposal of AH AND ESP would have two proposal
structures, one for AH and one for ESP and both would have Proposal
#1. A proposal of AH OR ESP would have two proposal structures, one
for AH with proposal #1 and one for ESP with proposal #2.
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 a single transform: an
integrity check algorithm. ESP generally has two: an encryption
algorithm AND an integrity check algorithm. IKE generally has five
transforms: a Diffie-Hellman group, an authentication algorithm, an
integrity check algorithm, a PRF algorithm, and an encryption
algorithm. For each Protocol, the set of permissible transforms are
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. For example, to propose ESP with (3DES or IDEA)
and (HMAC-MD5 or HMAC-SHA), the ESP proposal would contain two
Transform Type 1 candidates (one for 3DES and one for IDEA) and two
Transform Type 2 candidates (one for HMAC-MD5 and one for HMAC-SHA).
This effectively proposes four combinations of algorithms. If the
Initiator wanted to propose only a subset of those - say (3DES and
HMAC-MD5) or (IDEA and HMAC-SHA), there is no way to encode that as
multiple transforms within a single Proposal/Protocol. Instead, the
Initiator would have to construct two different Proposals, each with
two transforms.
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
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would specify the algorithm and the attribute would specify the key
size. Most transforms do not have attributes.
Note that the semantics of Transforms and Attributes are quite
different than in IKEv1. In IKEv1, a single Transform carried
multiple algorithms for a protocol with one carried in the Transform
and the others carried in the Attributes.
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.
The payload type for the Security Association Payload is one (1).
7.3.1 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 # ! Protocol-Id ! SPI Size !# of 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. This syntax is inherited
from ISAKMP, but is unnecessary because the last Proposal
could be identified from the length of the SA. The value (2)
corresponds to a Payload Type of Proposal, and the first
four octets of the Proposal structure are designed to look
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somewhat like the header of a Payload.
o RESERVED (1 octet) - MUST be sent as zero; MUST be ignored.
o Proposal Length (2 octets) - Length of this proposal,
including all transforms and attributes that follow.
o Proposal # (1 octet) - When a proposal is made, the first
proposal in an SA MUST be #1, and subsequent proposals
MUST either be the same as the previous proposal (indicating
an AND of the two proposals) or one more than the previous
proposal (indicating an OR of the two proposals). When a
proposal is accepted, all of the proposal numbers in the
SA must be the same and must match the number on the
proposal sent that was accepted.
o Protocol-Id (1 octet) - Specifies the protocol identifier
for the current negotiation. During phase 1 negotiation
this field MUST be zero (0). During phase 2 it will be the
protocol of the SA being established as assigned by IANA,
for example, 50 for ESP, 51 for AH, and 108 for IPComp.
o SPI Size (1 octet) - During phase 1 negotiation this field
MUST be zero. During phase 2 negotiation it is equal to the
size, in octets, of the SPI of the corresponding protocol
(8 for IKE, 4 for ESP and AH, 2 for IPcomp).
o # of Transforms (1 octet) - Specifies the number of
transforms in this proposal.
o SPI (variable) - The sending entity's SPI. Even if the SPI
Size is not a multiple of 4 octets, there is no padding
applied to the payload. When the SPI Size field is zero,
this field is not present in the Security Association
payload. This case occurs when negotiating the IKE-SA
(but not during the rekeying of an IKE-SA).
o Transforms (variable) - one or more transform substructures.
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7.3.2 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. This syntax is
inherited from ISAKMP, but is unnecessary because the last
Proposal could be identified from the length of the SA. The
value (3) corresponds to a Payload Type of Transform, and
the first four octets of the Transform structure are designed
to look somewhat like the header of a Payload.
o RESERVED - MUST be sent as zero; MUST be ignored.
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.
o Transform-ID (1 octet) - The specific instance of the transform
type being proposed.
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Transform Type Values
Transform Used In
Type
Encryption Algorithm 1 (IKE and ESP)
Pseudo-random Function 2 (IKE)
Authentication Method 3 (IKE)
Integrity Algorithm 4 (IKE, AH, and optional in ESP)
Diffie-Hellman Group 5 (IKE and optional in AH and ESP)
Compression 6 (IPcomp)
Window Size 7 (IKE)
values 8-240 are reserved to IANA. Values 241-255 are for
private use among mutually consenting parties.
For Transform Type 1 (Encryption Algorithm), defined Transform-IDs
are:
Name Number Defined In
RESERVED 0
ENCR_DES_IV64 1 (RFC1827)
ENCR_DES 2 (RFC2405)
ENCR_3DES 3 (RFC2451)
ENCR_RC5 4 (RFC2451)
ENCR_IDEA 5 (RFC2451)
ENCR_CAST 6 (RFC2451)
ENCR_BLOWFISH 7 (RFC2451)
ENCR_3IDEA 8 (RFC2451)
ENCR_DES_IV32 9
ENCR_RC4 10
ENCR_NULL 11 (RFC2410)
ENCR_AES_128 12
values 12-240 are reserved to IANA. Values 241-255 are for
private use among mutually consenting parties.
For Transform Type 2 (Pseudo-random Function), defined Transform-IDs
are:
Name Number Defined In
RESERVED 0
PRF_HMAC_MD5 1 (RFC2104)
PRF_HMAC_SHA 2 (RFC2104)
PRF_HMAC_TIGER 3 (RFC2104)
values 3-240 are reserved to IANA. Values 241-255 are for
private use among mutually consenting parties.
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For Transform Type 3 (Authentication Method), defined Transform-IDs
are:
Name Number Defined In
RESERVED 0
RESERVED for IKEv1 1 - 5 (RFC2409)
Authenticated Diffie-Hellman 6 (this memo)
values 7-240 are reserved to IANA. Values 241-255 are for
private use among mutually consenting parties.
For Transform Type 4 (Integrity Algorithm), defined Transform-IDs
are:
Name Number Defined In
RESERVED 0
AUTH_HMAC_MD5 1 (RFC2403)
AUTH_HMAC_SHA 2 (RFC2404)
AUTH_DES_MAC 3
AUTH_KPDK_MD5 4 (RFC1826)
For Transform Type 5 (Diffie-Hellman Group), defined Transform-IDs
are:
Name Number
RESERVED 0
Pre-defined (see section 8) 1 - 5
RESERVED 6 - 200
MODP (exponentiation) 201 (w/attributes)
ECP (elliptic curve over GF[P] 202 (w/attributes)
EC2N (elliptic curve over GF[2^N]) 203 (w/attributes)
values 6-200 are reserved to IANA for new MODP, ECP or EC2N
groups. Values 204-255 are for private use among mutually
consenting parties. Specification of values 201, 202 or 203
allow peers to define a new Diffie-Hellman group in-line as
part of the exchange. Private use of values 204-255 may entail
complete definition of a group or may require attributes to
accompany them. Attributes MUST NOT accompany groups using
values between 6 and 200.
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For Transform Type 6 (Compression), defined Transform-IDs are:
Name Number Defined In
RESERVED 0
IPCOMP_OUI 1 (w/attributes)
IPCOMP_DEFLATE 2
(RFC2394)
IPCOMP_LZS 3
(RFC2395)
values 4-240 are reserved to IANA. Values 241-255 are for
private use among mutually consenting parties.
For Transform Type 7 (Window Size), the Transform-ID specifies the
window size a peer is contracting to support to handle overlapping
requests (see section 2.3).
7.3.3 Mandatory Transform Types
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 support all
mandatory and optional types for each protocol it supports. Whether
the optional types are present in a particular proposal depends
solely on the discretion of the sender.
Protocol Mandatory Types Optional Types
IKE 1, 2, 3, 5, 7
ESP 1 4, 5
AH 4 5
IPCOMP 6
7.3.4 Mandatory Transform-IDs
Each transform type has corresponding transform IDs to specify the
specific transform. Some transforms are mandatory to support and
others are optional to support. The mandatory transform IDs for AH,
ESP, and IPCOMP are left to their respective RFCs, RFC2402, RFC2406,
and RFC2393. The transform IDs that are mandatory to support for
IKEv2 are:
Name TransType Mandatory Transform-ID
Encryption Algorithm 1 12 (ENCR_AES_128)
Pseudo-Random Function 2 2 (PRF_HMAC_SHA)
Authentication Method 3 6 (signed D-H)
Diffie-Hellman Group 5 5 (1536 bit MODP)
Window Size 7 1
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All other transform-IDs for a given transform type are optional to
support. While implementations MUST support a window size of 1, they
SHOULD support a window size of at least 10 and MAY support larger
window sizes.
7.3.5 Transform Attributes
Each transform in a Security Association payload may include
attributes that modify or complete the specification of the
transform. These attributes are type/value pairs and are defined in
Appendix A. For example, if an encryption algorithm has a variable
length key, the key length to be used may be specified as an
attribute. Attributes can have a value with a fixed two octet length
or a variable length value. For the latter the attribute is the form
of type/length/value.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
!A! Attribute Type ! AF=0 Attribute Length !
!F! ! AF=1 Attribute Value !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! AF=0 Attribute Value !
! AF=1 Not Transmitted !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6: Data Attributes
o Attribute Type (2 octets) - Unique identifier for each type of
attribute. The identifiers for IKE are defined in Appendix A.
The most significant bit of this field is the Attribute Format
bit (AF). It indicates whether the data attributes follow the
Type/Length/Value (TLV) format or a shortened Type/Value (TV)
format. If the AF bit is zero (0), then the Data Attributes
are of the Type/Length/Value (TLV) form. If the AF bit is a
one (1), then the Data Attributes are of the Type/Value form.
o Attribute Length (2 octets) - Length in octets of the Attribute
Value. When the AF bit is a one (1), the Attribute Value is
only 2 octets and the Attribute Length field is not present.
o Attribute Value (variable length) - Value of the Attribute
associated with the Attribute Type. If the AF bit is a
zero (0), this field has a variable length defined by the
Attribute Length field. If the AF bit is a one (1), the
Attribute Value has a length of 2 octets.
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7.3.6 Attribute Negotiation
During security association negotiation Initiators present offers to
Responders. Responders MUST select a single complete set of
parameters from the offers (or reject all offers if none are
acceptable). If there are multiple proposals, the Responder MUST
choose a single proposal number and return all of the Proposal
substructures with that Proposal number. If there are multiple
Transforms with the same type the Responder MUST choose a single one.
Any attributes of a selected transform MUST be returned unmodified.
The Initiator of an exchange MUST check that the accepted offer is
consistent with one of its proposals, and if not that response MUST
be rejected.
Negotiating Diffie-Hellman groups presents some special challenges.
Diffie-Hellman groups are specified either using a defined group
description (section 5) or by defining all attributes of a group (see
Appendix A) in an IKE policy offer. Group attributes, such as group
type or prime number MUST NOT be offered in conjunction with a
previously defined group. SA offers include proposed attributes and a
Diffie-Hellman public number (KE) in the same message. If the
Initiator offers to use one of several Diffie-Hellman groups, it
SHOULD pick the one the Responder is most likely to accept and
include a KE corresponding to that group. If the guess turns out to
be wrong, the Responder will indicate the correct group in the
response and the Initiator SHOULD start over this time using a
different group (see section 2.7).
Implementation Note:
Certain negotiable attributes can have ranges or could have
multiple acceptable values. These are the Diffie-Hellman group and
the key length of a variable key length symmetric cipher. To
further interoperability and to support upgrading endpoints
independently, implementers of this protocol SHOULD accept values
which they deem to supply greater security. For instance if a peer
is configured to accept a variable lengthed cipher with a key
length of X bits and is offered that cipher with a larger key
length an implementation SHOULD accept the offer.
Support of this capability allows an implementation to express a
concept of "at least" a certain level of security-- "a key length
of _at least_ X bits for cipher foo".
7.4 Key Exchange Payload
The Key Exchange Payload, denoted KE in this memo, is used to
exchange Diffie-Hellman public numbers as part of a Diffie-Hellman
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key exchange. The Key Exchange Payload consists of the IKE generic
header followed by the Diffie-Hellman public value itself.
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 !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! !
~ 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 MUST be equal to the
length of the prime modulus over which the exponentiation was
performed, prepending zero bits to the value if necessary.
A key exchange payload is processed by first checking whether the
length of the key exchange data (the "Payload Length" from the
generic header minus the size of the generic header) is equal to the
length of the prime modulus over which the exponentiation was
performed.
The payload type for the Key Exchange payload is four (4).
7.5 Identification Payload
The Identification Payload, denoted ID in this memo, allows peers to
identify themselves to each other. In Phase 1, the ID Payload names
the identity to be authenticated with the signature. In Phase 2, the
ID Payload is optional and if present names an identity asserted to
be responsible for this SA. An example use would be a shared computer
opening an IKE-SA to a server and asserting the name of its logged in
user for the Phase 2 SA. If missing, this defaults to the Phase 1
identity.
NOTE: In IKEv1, two ID payloads were used in each direction in Phase
2 to hold Traffic Selector information for data passing over the SA.
In IKEv2, this information is carried in Traffic Selector (TS)
payloads (see section 7.13).
The Identification Payload consists of the IKE generic header
followed by identification fields as follows:
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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 !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! ID Type ! RESERVED |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! !
~ Identification Data ~
! !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 8: Identification Payload Format
o ID Type (1 octet) - Specifies the type of Identification being
used.
o RESERVED - MUST be sent as zero; MUST be ignored.
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 payload type for the Identification Payload is five (5).
The following table lists the assigned values for the Identification
Type field, followed by a description of the Identification Data
which follows:
ID Type Value
------- -----
RESERVED 0
ID_IPV4_ADDR 1
A single four (4) octet IPv4 address.
ID_FQDN 2
A fully-qualified domain name string. An example of a
ID_FQDN is, "lounge.org". The string MUST not contain any
terminators (e.g. NULL, CR, etc.).
ID_RFC822_ADDR 3
A fully-qualified RFC822 email address string, An example of
a ID_RFC822_ADDR is, "lizard@lounge.org". The string MUST
not contain any terminators.
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ID_IPV6_ADDR 5
A single sixteen (16) octet IPv6 address.
ID_DER_ASN1_DN 9
The binary DER encoding of an ASN.1 X.500 Distinguished Name
[X.501].
ID_DER_ASN1_GN 10
The binary DER encoding of an ASN.1 X.500 GeneralName
[X.509].
ID_KEY_ID 11
An opaque octet stream which may be used to pass vendor-
specific information necessary to do certain proprietary
forms of identification.
7.6 Certificate Payload
The Certificate Payload, denoted CERT in this memo, provides a means
to transport certificates or other certificate-related information
via IKE. Certificate payloads SHOULD be included in an exchange if
certificates are available to the sender.
The Certificate Payload is defined as follows:
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.
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Certificate Encoding Value
-------------------- -----
NONE 0
PKCS #7 wrapped X.509 certificate 1
PGP Certificate 2
DNS Signed Key 3
X.509 Certificate - Signature 4
Kerberos Token 6
Certificate Revocation List (CRL) 7
Authority Revocation List (ARL) 8
SPKI Certificate 9
X.509 Certificate - Attribute 10
RESERVED 11 - 255
o Certificate Data (variable length) - Actual encoding of
certificate data. The type of certificate is indicated
by the Certificate Encoding field.
The payload type for the Certificate Payload is six (6).
7.7 Certificate Request Payload
The Certificate Request Payload, denoted CERTREQ in this memo,
provides a means to request preferred certificates via IKE and can
appear in the first, second, or third message of Phase 1.
Certificate Request payloads SHOULD be included in an exchange
whenever the peer may have multiple certificates, some of which might
be trusted while others are not. If multiple root CA's are trusted,
then multiple Certificate Request payloads SHOULD be transmitted.
Empty (zero length) CA names MUST NOT be generated and SHOULD be
ignored.
The Certificate Request Payload is defined as follows:
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 ~
! !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 10: Certificate Request Payload Format
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o Certificate Encoding (1 octet) - Contains an encoding of the type
of certificate requested. Acceptable values are listed in
section 7.6.
o Certification Authority (variable length) - Contains an encoding
of an acceptable certification authority for the type of
certificate requested.
The payload type for the Certificate Request Payload is seven (7).
The Certificate Request Payload is constructed by setting the "Cert
Encoding" field to be the type of certificate being desired and the
"Certification Authority" field to a proper encoding of a
certification authority for the specified certificate. For example,
for an X.509 certificate this field would contain the Distinguished
Name encoding of the Issuer Name of an X.509 certification authority
acceptable to the sender of this payload.
The Certificate Request Payload is processed by inspecting the "Cert
Encoding" field to determine whether the processor has any
certificates of this type. If so the "Certification Authority" field
is inspected to determine if the processor has any certificates which
can be validated up to the specified certification authority. This
can be a chain of certificates. If a certificate exists which
satisfies the criteria specified in the Certificate Request Payload
it MUST be sent back to the certificate requestor; if a certificate
chain exists which goes back to the certification authority specified
in the request the entire chain SHOULD be sent back to the
certificate requestor. If no certificates exist then no further
processing is performed-- this is not an error condition of the
protocol. There may be cases where there is a preferred CA, but an
alternate might be acceptable (perhaps after prompting a human
operator).
7.8 Authentication Payload
The Authentication Payload, denoted AUTH in this memo, contains data
used for authentication purposes. The only authentication method
defined in this memo is digital signatures and therefore the contents
of this payload when used with this memo will be the output generated
by a digital signature function.
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The Authentication Payload is defined as follows:
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 !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! !
~ Authentication Data ~
! !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 11: Authentication Payload Format
o Authentication Data (variable length) - Data that results from
applying the digital signature function to the IKE state
(see section 3).
The payload type for the Authentication Payload is nine (9).
The Authentication Payload is constructed by computing a digital
signature (or secret key MAC) over the concatenation of the sender's
first IKE message and the other peer's nonce. The result is placed
in the "Authentication Data" portion of the payload. The encoding
depends on the type of key being used to authenticate (see section
3.2). The payload length is the size of the generic header plus the
size of the "Authentication Data" portion of the payload which
depends on the specific authentication method being used.
The Authentication Payload is processed by extracting the
"Authentication Data" from the payload and verifying it according to
the specific authentication method being used. If authentication
fails a NOTIFY Error message of AUTHENTICATION-FAILED MUST be sent
back to the peer and the connection closed.
7.9 Nonce Payload
The Nonce Payload, denoted Ni and Nr in this memo for the Initiator's
and Responder's nonce respectively, contains random data used to
guarantee liveness during an exchange and protect against replay
attacks.
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The Nonce Payload is defined as follows:
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 payload type for the Nonce Payload is ten (10).
The Nonce Payload is constructed by computing a pseudo-random value
and copying it into the "Nonce Data" field. The size of a Nonce MUST
be between 8 and 256 octets inclusive.
7.10 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), or in an
Informational Exchange (to report an error not in an IKE request).
The Notify Payload is defined as follows:
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 ~
! !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Figure 13: Notification Payload Format
o Protocol-Id (1 octet) - Specifies the protocol about which
this notification is being sent. For phase 1 notifications,
this field MUST be zero (0). For phase 2 notifications
concerning IPsec SAs this field will contain an IPsec
protocol (either ESP, AH, or IPcomp). For notifications
for which no protocol ID is relevant, this field MUST be
sent as zero and MUST be ignored.
o SPI Size (1 octet) - Length in octets of the SPI as defined by
the Protocol-Id or zero if no SPI is applicable. For phase 1
notification concerning the IKE-SA, the SPI Size MUST be zero.
o Notify Message Type (2 octets) - Specifies the type of
notification message.
o SPI (variable length) - Security Parameter Index.
o Notification Data (variable length) - Informational or error data
transmitted in addition to the Notify Message Type. Values for
this field are message specific, see below.
The payload type for the Notification Payload is eleven (11).
7.10.1 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.
For example, a secure front end or security gateway may use the
Notify message to synchronize SA communication. The table below
lists the Notification messages and their corresponding values.
NOTIFY MESSAGES - ERROR TYPES Value
----------------------------- -----
UNSUPPORTED-CRITICAL-PAYLOAD 1
Sent if the payload has the "critical" bit set and the
payload type is not recognised. Notification Data contains
the one octet payload type.
INVALID-COOKIE 4
Indicates an IKE message was received with an unrecognized
destination cookie. This usually indicates that the
recipient has rebooted and forgotten the existence of an
IKE-SA.
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INVALID-MAJOR-VERSION 5
Indicates the recipient cannot handle the version of IKE
specified in the header. The closest version number that the
recipient can support will be in the reply header.
INVALID-EXCHANGE-TYPE 7
Notification Data contains the one octet Exchange Type.
INVALID-FLAGS 8
Notification Data contains one octet with the unacceptable
flag bits set.
INVALID-MESSAGE-ID 9
Sent when an IKE MESSAGE-ID outside the negotiated window is
received. This Notify MUST NOT be sent in a response; the
invalid request MUST NOT be acknowledged. Instead, inform
the other side by initiating an Informational exchange with
Notification data containing the four octet invalid MESSAGE-
ID.
INVALID-PROTOCOL-ID 10
Notification Data contains the one octet invalid protocol
ID.
INVALID-SPI 11
MAY be sent in an IKE Informational Exchange when a node
receives an ESP or AH packet with an invalid SPI. address
as the source address in the invalid packet. This usually
indicates a node has rebooted and forgotten an SA. This
Informational Message is sent outside the context of an IKE-
SA, and therefore should only be used by the recipient as a
"hint" that something might be wrong (because it could
easily be forged).
INVALID-TRANSFORM-ID 12
Notification Data contains the one octet invalid transform
ID.
ATTRIBUTES-NOT-SUPPORTED 13
The "Notification Data" for this type are the attribute or
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attributes that are not supported.
NO-PROPOSAL-CHOSEN 14
BAD-PROPOSAL-SYNTAX 15
PAYLOAD-MALFORMED 16
INVALID-KEY-INFORMATION 17
The KE field is the wrong length.
INVALID-ID-INFORMATION 18
INVALID-CERT-ENCODING 19
The "Notification Data" for this type are the "Cert
Encoding" field from a Certificate Payload or Certificate
Request Payload.
INVALID-CERTIFICATE 20
The "Notification Data" for this type are the "Certificate
Data" field from a Certificate Payload.
INVALID-CERT-AUTHORITY 22
The "Notification Data" for this type are the "Cert
Encoding" field from a Certificate Payload or Certificate
Request Payload.
AUTHENTICATION-FAILED 24
INVALID-SIGNATURE 25
UNSUPPORTED-EXCHANGE-TYPE 29
The "Notification Data" for this type are the Exchange Type
field from the IKE header.
UNEQUAL-PAYLOAD-LENGTHS 30
The "Notification Data" for this type are the entire message
in which the unequal lengths were observed.
UNSUPPORTED-NOTIFY-TYPE 31
The "Notification Data" for this type is the two octet
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Notify Type that was not supported.
IKE-SA-INIT-REJECT 32
This notification is sent in an IKE-SA-RESPONSE to request
that the Initiator retry the request with the supplied
cookie (and optionally the supplied Diffie-Hellman group).
This is not really an error, but is processed like one in
that it indicates that the connection request was rejected.
The Notification Data, if present, contains the Transform
Substructure describing the preferred Diffie-Hellman group.
SINGLE-PAIR-REQUIRED 34
This error indicates that a Phase 2 SA request is
unacceptable because the Responder requires a separate SA
for each source / destination address pair. The Initiator is
expected to respond by requesting an SA for only the
specific traffic he is trying to forward.
RESERVED - Errors 35 - 8191
Private Use - Errors 8192 - 16383
NOTIFY MESSAGES - STATUS TYPES Value
------------------------------ -----
RESERVED 16384 - 24577
INITIAL-CONTACT 24578
This notification asserts that this IKE-SA is the only IKE-
SA currently active between the authenticated identities. It
MAY be sent when an IKE-SA is established after a crash, and
the recipient MAY use this information to delete any other
IKE-SAs it has to the same authenticated identity without
waiting for a timeout if those IKE-SAs reside at the IP
address from which this notification arrived. This
notification MUST NOT be sent by an entity that may be
replicated (e.g. a roaming user's credentials where the user
is allowed to connect to the corporate firewall from two
remote systems at the same time).
RESERVED 24578 - 40959
Private Use - STATUS 40960 - 65535
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7.11 Delete Payload
The Delete Payload, denoted D in this memo, contains a protocol-
specific security association identifier that the sender has removed
from its security association database and is, therefore, no longer
valid. Figure 14 shows the format of the Delete Payload. It is
possible to send multiple SPIs in a Delete payload, however, each SPI
MUST be for the same protocol. Mixing of Protocol Identifiers MUST
NOT be performed with the Delete payload. It is permitted, however,
to include multiple Delete payloads in a single Informational
Exchange where each Delete payload lists SPIs for a different
protocol.
Deletion of the IKE-SA is indicated by a Protocol-Id of 0 (IKE) but
no SPIs. Deletion of a Child-SA, such as ESP or AH, will contain the
Protocol-Id of that protocol (e.g. ESP, AH) and the SPI is the
receiving entity's SPI(s).
NOTE: What's the deal with IPcomp SAs. This mechanism is probably not
appropriate for deleting them!!
The Delete Payload is defined as follows:
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 ! # of SPIs !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! !
~ Security Parameter Index(es) (SPI) ~
! !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 14: Delete Payload Format
o Protocol-Id (1 octet) - Must be zero for an IKE-SA, 50 for
ESP, 51 for AH, and 108 for IPcomp.
o SPI Size (1 octet) - Length in octets of the SPI as defined by
the Protocol-Id. Zero for IKE (SPI is in message header),
four for AH and ESP, two for IPcomp.
o # of SPIs (2 octets) - The number of SPIs contained in the Delete
payload. The size of each SPI is defined by the SPI Size field.
o Security Parameter Index(es) (variable length) - Identifies the
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specific security association(s) to delete.
The length of this field is
determined by the SPI Size and # of SPIs fields.
The payload type for the Delete Payload is twelve (12).
7.12 Vendor ID Payload
The Vendor ID Payload contains a vendor defined constant. The
constant is used by vendors to identify and recognize remote
instances of their implementations. This mechanism allows a vendor
to experiment with new features while maintaining backwards
compatibility.
The Vendor ID payload is not an announcement from the sender that it
will send private payload types but rather an announcement of the
sort of private payloads it is willing to accept. The implementation
sending the Vendor ID MUST not make any assumptions about private
payloads that it may send unless a Vendor ID of like stature is
received as well. Multiple Vendor ID payloads MAY be sent. An
implementation is NOT REQUIRED to send any Vendor ID payload at all.
A Vendor ID payload may be sent as part of any message. Reception of
a familiar Vendor ID payload allows an implementation to make use of
Private USE numbers described throughout this memo-- private
payloads, private exchanges, private notifications, etc. Unfamiliar
Vendor ID's MUST be ignored.
Writers of Internet-Drafts who wish to extend this protocol MUST
define a Vendor ID payload to announce the ability to implement the
extension in the Internet-Draft. It is expected that Internet-Drafts
which gain acceptance and are standardized will be given "magic
numbers" out of the Future Use range by IANA and the requirement to
use a Vendor ID will go away.
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The Vendor ID Payload fields are defined as follows:
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 !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! !
~ Vendor ID (VID) ~
! !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 15: Vendor ID Payload Format
o Vendor ID (variable length) - It is the responsibility of
the person choosing the Vendor ID to assure its uniqueness
in spite of the absence of any central registry for IDs.
Good practice is to include a company name, a person name
or some such. If you want to show off, you might include
the latitude and longitude and time where you were when
you chose the ID and some random input. A message digest
of a long unique string is preferable to the long unique
string itself.
The payload type for the Vendor ID Payload is thirteen (13).
7.13 Traffic Selector Payload
The Traffic Selector Payload, denoted TS in this memo, allows peers
to identify packet flows for processing by IPsec security services.
The Traffic Selector Payload consists of the IKE generic header
followed by selector information fields as follows:
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 16: Traffic Selectors Payload Format
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o Number of TSs (1 octet) - Number of traffic selectors
being provided.
o RESERVED - This field MUST be sent as zero and MUST be ignored.
o Traffic Selectors (variable length) - one or more traffic
selector substructures.
The length of the Traffic Selector payload includes the TS header and
all the traffic selector substructures.
The payload type for the Traffic Selector payload is fourteen (14).
7.13.1 Traffic Selector 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! TS Type ! Protocol ID | Selector Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Start-Port | End-Port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! !
~ Address Selector Data ~
! !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 17: Traffic Selector Substructure
o TS Type (one octet) - Specifies the type of traffic selector.
o Protocol ID (1 octet) - Value specifying an associated IP
protocol ID (e.g. UDP/TCP). 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) - Value specifying the smallest port
number allowed by this Traffic Selector. For protocols for
which port is undefined, or if all ports are allowed by
this Traffic Selector, this field MUST be zero.
o End-Port (2 octets) - Value specifying the largest port
number allowed by this Traffic Selector. For protocols for
which port is undefined, or it all ports are allowed by
this Traffic Selector, this field MUST be 65535.
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o Address Selector Data - a specification of one or more
addresses included in this Traffic Selector with format
determined by TS type.
The following table lists the assigned values for the Traffic
Selector Type field and the corresponding Address Selector Data.
TS Type Value
------- -----
RESERVED 0
TS_IPV4_ADDR 1
A four (4) octet IPv4 address
TS_IPV4_ADDR_SUBNET 4
An IPv4 subnet represented by a pair of four (4) octet
values. The first value is an IPv4 address. The second is
an IPv4 network mask. Note that ones (1s) in the network
mask indicate that the corresponding bit in the address is
fixed, while zeros (0s) indicate a "wildcard" bit.
TS_IPV6_ADDR 5
A sixteen (16) octet IPv6 address
TS_IPV6_ADDR_SUBNET 6
An IPv6 subnet represented by a pair sixteen (16) octet
values. The first value is an IPv6 address. The second is
an IPv6 network mask. Note that ones (1s) in the network
mask indicate that the corresponding bit in the address is
fixed, while zeros (0s) indicate a "wildcard" bit.
TS_IPV4_ADDR_RANGE 7
A range of IPv4 addresses, represented by two four (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 sixteen (16)
octet values. The first value is the beginning IPv6 address
(inclusive) and the second value is the ending IPv6 address
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(inclusive). All addresses falling between the two specified
addresses are considered to be within the list.
7.14 Other Payload Types
Payload type values 15-127 are reserved to IANA for future assignment
in IKEv2 (see section 10). Payload type values 128-255 are for
private use among mutually consenting parties.
8 Diffie-Hellman Groups
There are 5 groups different Diffie-Hellman groups defined for use in
IKE. These groups were generated by Richard Schroeppel at the
University of Arizona. Properties of these primes are described in
[Orm96].
The strength supplied by group one may not be sufficient for the
mandatory-to-implement encryption algorithm and is here for historic
reasons.
8.1 Group 1 - 768 Bit MODP
IKE implementations MAY support a MODP group with the following prime
and generator. This group is assigned id 1 (one).
The prime is: 2^768 - 2 ^704 - 1 + 2^64 * { [2^638 pi] + 149686 }
Its hexadecimal value is:
FFFFFFFF FFFFFFFF C90FDAA2 2168C234 C4C6628B 80DC1CD1 29024E08
8A67CC74 020BBEA6 3B139B22 514A0879 8E3404DD EF9519B3 CD3A431B
302B0A6D F25F1437 4FE1356D 6D51C245 E485B576 625E7EC6 F44C42E9
A63A3620 FFFFFFFF FFFFFFFF
The generator is 2.
8.2 Group 2 - 1024 Bit MODP
IKE implementations SHOULD support a MODP group with the following
prime and generator. This group is assigned id 2 (two).
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The prime is 2^1024 - 2^960 - 1 + 2^64 * { [2^894 pi] + 129093 }.
Its hexadecimal value is:
FFFFFFFF FFFFFFFF C90FDAA2 2168C234 C4C6628B 80DC1CD1 29024E08
8A67CC74 020BBEA6 3B139B22 514A0879 8E3404DD EF9519B3 CD3A431B
302B0A6D F25F1437 4FE1356D 6D51C245 E485B576 625E7EC6 F44C42E9
A637ED6B 0BFF5CB6 F406B7ED EE386BFB 5A899FA5 AE9F2411 7C4B1FE6
49286651 ECE65381 FFFFFFFF FFFFFFFF
The generator is 2.
8.3 Group 3 - 155 Bit EC2N
IKE implementations MAY support a EC2N group with the following
characteristics. This group is assigned id 3 (three). The curve is
based on the Galois Field GF[2^155]. The field size is 155. The
irreducible polynomial for the field is:
u^155 + u^62 + 1.
The equation for the elliptic curve is:
y^2 + xy = x^3 + ax^2 + b.
Field Size: 155
Group Prime/Irreducible Polynomial:
0x0800000000000000000000004000000000000001
Group Generator One: 0x7b
Group Curve A: 0x0
Group Curve B: 0x07338f
Group Order: 0x0800000000000000000057db5698537193aef944
The data in the KE payload when using this group is the value x from
the solution (x,y), the point on the curve chosen by taking the
randomly chosen secret Ka and computing Ka*P, where * is the
repetition of the group addition and double operations, P is the
curve point with x coordinate equal to generator 1 and the y
coordinate determined from the defining equation. The equation of
curve is implicitly known by the Group Type and the A and B
coefficients. There are two possible values for the y coordinate;
either one can be used successfully (the two parties need not agree
on the selection).
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8.4 Group 4 - 185 Bit EC2N
IKE implementations MAY support a EC2N group with the following
characteristics. This group is assigned id 4 (four). The curve is
based on the Galois Field GF[2^185]. The field size is 185. The
irreducible polynomial for the field is:
u^185 + u^69 + 1.
The equation for the elliptic curve is:
y^2 + xy = x^3 + ax^2 + b.
Field Size: 185
Group Prime/Irreducible Polynomial:
0x020000000000000000000000000000200000000000000001
Group Generator One: 0x18
Group Curve A: 0x0
Group Curve B: 0x1ee9
Group Order: 0x01ffffffffffffffffffffffdbf2f889b73e484175f94ebc
The data in the KE payload when using this group will be identical to
that as when using Oakley Group 3 (three).
8.5 Group 5 - 1536 Bit MODP
IKE implementations MUST support a MODP group with the following
prime and generator. This group is assigned id 5 (five).
The prime is 2^1536 - 2^1472 - 1 + 2^64 * {[2^1406 pi] + 741804}.
Its hexadecimal value is
FFFFFFFF FFFFFFFF C90FDAA2 2168C234 C4C6628B 80DC1CD1 29024E08
8A67CC74 020BBEA6 3B139B22 514A0879 8E3404DD EF9519B3 CD3A431B
302B0A6D F25F1437 4FE1356D 6D51C245 E485B576 625E7EC6 F44C42E9
A637ED6B 0BFF5CB6 F406B7ED EE386BFB 5A899FA5 AE9F2411 7C4B1FE6
49286651 ECE45B3D C2007CB8 A163BF05 98DA4836 1C55D39A 69163FA8
FD24CF5F 83655D23 DCA3AD96 1C62F356 208552BB 9ED52907 7096966D
670C354E 4ABC9804 F1746C08 CA237327 FFFFFFFF FFFFFFFF
The generator is 2.
9 Security Considerations
Repeated re-keying using Phase 2 without PFS can consume the entropy
of the Diffie-Hellman shared secret. Implementers should take note of
this fact and set a limit on Phase 2 Exchanges between
exponentiations. This memo does not prescribe such a limit.
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The strength of a key derived from a Diffie-Hellman exchange using
any of the groups defined here depends on the inherent strength of
the group, the size of the exponent used, and the entropy provided by
the random number generator used. Due to these inputs it is difficult
to determine the strength of a key for any of the defined groups.
Diffie-Hellman group number two when used with a strong random number
generator and an exponent no less than 160 bits is sufficient to use
for 3DES. Groups three through five provide greater security. Group
one is for historic purposes only and does not provide sufficient
strength to the required cipher (although it is sufficient for use
with DES, which is also for historic use only). Implementations
should make note of these conservative estimates when establishing
policy and negotiating security parameters.
Note that these limitations are on the Diffie-Hellman groups
themselves. There is nothing in IKE which prohibits using stronger
groups nor is there anything which will dilute the strength obtained
from stronger groups. In fact, the extensible framework of IKE
encourages the definition of more groups; use of elliptical curve
groups will greatly increase strength using much smaller numbers.
It is assumed that the Diffie-Hellman exponents in this exchange are
erased from memory after use. In particular, these exponents MUST NOT
be derived from long-lived secrets like the seed to a pseudo-random
generator that is not erased after use.
The security of this protocol is critically dependent on the
randomness of the Diffie-Hellman exponents, which should be generated
by a strong random or properly seeded pseudo-random source (see
RFC1715). While the protocol was designed to be secure even if the
Nonces and other values specified as random are not strongly random,
they should similarly be generated from a strong random source as
part of a conservative design.
10 IANA Considerations
This document contains many "magic numbers" to be maintained by the
IANA. This section explains the criteria to be used by the IANA to
assign additional numbers in each of these lists.
10.1 Transform Types and Attribute Values
10.1.1 Attributes
Transform attributes are uses to modify or complete the specification
of a particular transform. Requests for new transform attributes MUST
be accompanied by an RFC which defines the transform which it
modifies or completes and the method in which it does so.
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10.1.2 Encryption Algorithm Transform Type
Values of the Encryption Algorithm define an encryption algorithm to
use when called for in this document. Requests for assignment of new
encryption algorithm values must be accompanied by a reference to an
RFC that describes how to use this algorithm with ESP.
10.1.3 Pseudo-random function Transform Type
Values for the pseudo-random function define which pseudo-random
function is used in IKE for key generation and expansion. Requests
for assignment of a new pseudo-random function MUST be accompanied by
a reference to an RFC describing this function.
10.1.4 Authentication Method Transform Type
The only Authentication method defined in the memo is for digital
signatures. Other methods of authentication are possible and MUST be
accompanied by an RFC which defines the following:
- the cryptographic method of authentication.
- content of the Authentication Data in the Authentication
Payload.
- new payloads, their construction and processing, if needed.
- additions of payloads to any messages, if needed.
10.1.5 Diffie-Hellman Groups
Values of the Diffie-Hellman Group Transform types define a group in
which a Diffie-Hellman key exchange can be completed. Requests for
assignment of a new Diffie-Hellman group type MUST be accompanied by
a reference to an RFC which fully defines the group.
10.2 Exchange Types
This memo defines three exchange types for use with IKEv2. Requests
for assignment of new exchange types MUST be accompanied by an RFC
which defines the following:
- the purpose of and need for the new exchange.
- the payloads (mandatory and optional) that accompany
messages in the exchange.
- the phase of the exchange.
- requirements the new exchange has on existing
exchanges which have assigned numbers.
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10.3 Payload Types
Payloads are defined in this memo to convey information between
peers. New payloads may be required when defining a new
authentication method or exchange. Requests for new payload types
MUST be accompanied by an RFC which defines the physical layout of
the payload and the fields it contains. All payloads MUST use the
same generic header defined in Figure 2.
11 Acknowledgements
We would like to thank the many members of the IPsec working group
that provided helpful and constructive suggestions on improving IKE.
Special thanks go to those of you who've implemented it!
This protocol is built on the shoulders of many designers who came
before. While they have not necessarily reviewed or endorsed this
version and should not be blamed for any defects, they deserve much
of the credit for its design. We would like to acknowledge Oakley,
SKEME and their authors, Hilarie Orman (Oakley), Hugo Krawczyk
(SKEME). Without the hard work of Doug Maughan, Mark Schertler, Mark
Schneider, Jeff Turner, Dave Carrel, and Derrell Piper, this memo
would not exist. Their contributions to the IPsec WG have been
considerable and critical.
12 References
[CAST] Adams, C., "The CAST-128 Encryption Algorithm", RFC 2144,
May 1997.
[BLOW] Schneier, B., "The Blowfish Encryption Algorithm", Dr.
Dobb's Journal, v. 19, n. 4, April 1994.
[Bra96] Bradner, S., "The Internet Standards Process -- Revision 3",
BCP 9, RFC 2026, October 1996.
[Bra97] Bradner, S., "Key Words for use in RFCs to indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[Ble98] Bleichenbacher, D., "Chosen Ciphertext Attacks against
Protocols Based on RSA Encryption Standard PKCS#1", Advances
in Cryptology Eurocrypt '98, Springer-Verlag, 1998.
[BR94] Bellare, M., and Rogaway P., "Optimal Asymmetric
Encryption", Advances in Cryptology Eurocrypt '94,
Springer-Verlag, 1994.
[DES] ANSI X3.106, "American National Standard for Information
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Systems-Data Link Encryption", American National Standards
Institute, 1983.
[DH] Diffie, W., and Hellman M., "New Directions in
Cryptography", IEEE Transactions on Information Theory, V.
IT-22, n. 6, June 1977.
[DSS] NIST, "Digital Signature Standard", FIPS 186, National
Institute of Standards and Technology, U.S. Department of
Commerce, May, 1994.
[IDEA] Lai, X., "On the Design and Security of Block Ciphers," ETH
Series in Information Processing, v. 1, Konstanz: Hartung-
Gorre Verlag, 1992
[Ker01] Keronytis, A., Sommerfeld, B., "The 'Suggested ID' Extension
for IKE", draft-keronytis-ike-id-00.txt, 2001
[KBC96] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104, February
1997.
[SKEME] Krawczyk, H., "SKEME: A Versatile Secure Key Exchange
Mechanism for Internet", from IEEE Proceedings of the 1996
Symposium on Network and Distributed Systems Security.
[MD5] Rivest, R., "The MD5 Message Digest Algorithm", RFC 1321,
April 1992.
[MSST98] Maughhan, D., Schertler, M., Schneider, M., and J. Turner,
"Internet Security Association and Key Management Protocol
(ISAKMP)", RFC 2408, November 1998.
[Orm96] Orman, H., "The Oakley Key Determination Protocol", RFC
2412, November 1998.
[PFKEY] McDonald, D., Metz, C., and Phan, B., "PFKEY Key Management
API, Version 2", RFC2367, July 1998.
[PKCS1] Kaliski, B., and J. Staddon, "PKCS #1: RSA Cryptography
Specifications Version 2", September 1998.
[PK01] Perlman, R., and Kaufman, C., "Analysis of the IPsec key
exchange Standard", WET-ICE Security Conference, MIT, 2001,
http://sec.femto.org/wetice-2001/papers/radia-paper.pdf.
[Pip98] Piper, D., "The Internet IP Security Domain Of
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Interpretation for ISAKMP", RFC 2407, November 1998.
[RC5] Rivest, R., "The RC5 Encryption Algorithm", Dr. Dobb's
Journal, v. 20, n. 1, January 1995.
[RSA] Rivest, R., Shamir, A., and Adleman, L., "A Method for
Obtaining Digital Signatures and Public-Key Cryptosystems",
Communications of the ACM, v. 21, n. 2, February 1978.
[Sch96] Schneier, B., "Applied Cryptography, Protocols, Algorithms,
and Source Code in C", 2nd edition.
[SHA] NIST, "Secure Hash Standard", FIPS 180-1, National Institute
of Standards and Technology, U.S. Department of Commerce,
May 1994.
[TIGER] Anderson, R., and Biham, E., "Fast Software Encryption",
Springer LNCS v. 1039, 1996.
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Appendix A
Attribute Assigned Numbers
Certain transforms negotiated in an SA payload can have associated
attributes. Attribute types can be either Basic (B) or Variable-
length (V). Encoding of these attributes is defined as Type/Value
(Basic) and Type/Length/Value (Variable). See section 7.3.3.
Attributes described as basic MUST NOT be encoded as variable.
Variable length attributes MUST NOT be encoded as basic even if their
value can fit into two octets. NOTE: This is a change from IKEv1,
where increased flexibility may have simplified the composer of
messages but certainly complicated the parser.
Attribute Classes
class value type
--------------------------------------------------------------
RESERVED 0-5
Group Prime/Irreducible Polynomial 6 V
Group Generator One 7 V
Group Generator Two 8 V
Group Curve A 9 V
Group Curve B 10 V
RESERVED 11-13
Key Length 14 B
Field Size 15 B
Group Order 16 V
Block Size 17 B
values 0-5, 11-13, and 18-16383 are reserved to IANA. Values
16384-32767 are for private use among mutually consenting parties.
- Group Prime/Irreducible Polynomial
The prime number of a MODP Diffie-Hellman group or the irreducible
polynomial of an elliptic curve when specifying a private Diffie-
Hellman group.
- Generator One, Generator Two
The X- and Y-coordinate of a point on an elliptic curve. When the
Y-coordinate (generator two) is not given it can be computed with
the X-coordinate and the definition of the curve.
- Curve A, Curve B
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Coefficients from the definition of an elliptic curve:
y^2 + xy = x^3 + (curve A)x^2 + (curve B)
- Key Length
When using an Encryption Algorithm that has a variable length key,
this attribute specifies the key length in bits. (MUST use network
byte order). This attribute MUST NOT be used when the specified
Encryption Algorithm uses a fixed length key.
- Field Size
The field size, in bits, of a Diffie-Hellman group.
- Group Order
The group order of an elliptic curve group. Note the length of
this attribute depends on the field size.
- Block Size
The number of bits per block of a cipher with a variable block
length.
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Appendix B: Cryptographic Protection of IKE Data
With the exception of the IKE-SA-INIT-REQUEST, IKE-SA-INIT-RESPONSE,
and Informational Exchange error notifications when no IKE-SA exists,
all IKE messages are encrypted and integrity protected. The
algorithms for encryption and integrity protection are negotiated
during IKE-SA setup, and the keys are computed as specified in
sections 3 and 4.2.
The encryption and integrity protection algorithms are modelled after
the ESP algorithms described in RFCs 2104, 2406, 2451. This appendix
completely specifies the cryptographic processing of IKE data, but
those documents should be consulted for design rationale. This
appendix assumes a block cipher with a fixed block size and an
integrity check algorithm that computes a fixed length checksum over
a variable size message. The mandatory to implement algorithms are
AES-128 and HMAC-SHA1.
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The format of an IKE message is shown in Figure 18.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Fixed IKE Header - 28 octets !
! (including cookies, message ID, Length) !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Initialization Vector !
! (length is block size for encryption algorithm) !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! IKE Payloads !
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! ! Padding (0-255 octets) !
+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
! ! Pad Length !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Authentication Data ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 18: IKE message with cryptographic protection
o Initialization Vector - A randomly chosen value whose length
is equal to the block length of the underlying encryption
algorithm. Recipients MUST accept any value. Senders SHOULD
either pick this value pseudo-randomly and independently for
each message or use the final ciphertext block of the previous
message sent. Senders MUST NOT use the same value for each
message, use a sequence of values with low hamming distance
(e.g. a sequence number), or use ciphertext from a received
message.
o IKE Payloads are as specified in Section 7. 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 Authentication Data is the cryptographic checksum of the
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entire message starting with the Fixed IKE Header through
the Pad Length. The checksum MUST be computed over the
encrypted message.
Authors' Addresses
Dan Harkins
dharkins@trpz.com
Trapeze Networks
Charlie Kaufman
ckaufman@iris.com
IBM
Steve Kent
kent@bbn.com
BBN Technologies
Tero Kivinen
kivinen@ssh.com
SSH Communications Security
Radia Perlman
radia.perlman@sun.com
Sun Microsystems
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