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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
   areas, and working groups. Note that other groups may also distribute
   working documents as Internet Drafts.

   Internet Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time. It is inappropriate to use Internet Drafts as reference
   material or to cite them other than as "work in progress."

     The list of current Internet-Drafts can be accessed at
     http://www.ietf.org/1id-abstracts.html

     The list of Internet-Draft Shadow Directories can be accessed at
     http://www.ietf.org/shadow.html

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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INTERNET DRAFT                                                April 2002


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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INTERNET DRAFT                                                April 2002


         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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