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rfc:rfc4322

Network Working Group M. Richardson Request for Comments: 4322 SSW Category: Informational D.H. Redelmeier

                                                                Mimosa
                                                         December 2005
   Opportunistic Encryption using the Internet Key Exchange (IKE)

Status of This Memo

 This memo provides information for the Internet community.  It does
 not specify an Internet standard of any kind.  Distribution of this
 memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2005).

Abstract

 This document describes opportunistic encryption (OE) as designed and
 implemented by the Linux FreeS/WAN project.  OE uses the Internet Key
 Exchange (IKE) and IPsec protocols.  The objective is to allow
 encryption for secure communication without any pre-arrangement
 specific to the pair of systems involved.  DNS is used to distribute
 the public keys of each system involved.  This is resistant to
 passive attacks.  The use of DNS Security (DNSSEC) secures this
 system against active attackers as well.
 As a result, the administrative overhead is reduced from the square
 of the number of systems to a linear dependence, and it becomes
 possible to make secure communication the default even when the
 partner is not known in advance.

Table of Contents

 1. Introduction ....................................................3
    1.1. Motivation .................................................3
    1.2. Encryption Regimes .........................................4
    1.3. Peer Authentication in Opportunistic Encryption ............4
    1.4. Use of RFC 2119 Terms ......................................5
 2. Overview ........................................................6
    2.1. Reference Diagram ..........................................6
    2.2. Terminology ................................................6
    2.3. Model of Operation .........................................8

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 3. Protocol Specification ..........................................9
    3.1. Forwarding Plane State Machine .............................9
    3.2. Keying Daemon -- Initiator ................................12
    3.3. Keying Daemon -- Responder ................................20
    3.4. Renewal and Teardown ......................................22
 4. Impacts on IKE .................................................24
    4.1. ISAKMP/IKE Protocol .......................................24
    4.2. Gateway Discovery Process .................................24
    4.3. Self Identification .......................................24
    4.4. Public Key Retrieval Process ..............................25
    4.5. Interactions with DNSSEC ..................................25
    4.6. Required Proposal Types ...................................25
 5. DNS Issues .....................................................26
    5.1. Use of KEY Record .........................................26
    5.2. Use of TXT Delegation Record ..............................27
    5.3. Use of FQDN IDs ...........................................29
    5.4. Key Roll-Over .............................................29
 6. Network Address Translation Interaction ........................30
    6.1. Co-Located NAT/NAPT .......................................30
    6.2. Security Gateway behind a NAT/NAPT ........................30
    6.3. End System behind a NAT/NAPT ..............................31
 7. Host Implementations ...........................................31
 8. Multi-Homing ...................................................31
 9. Failure Modes ..................................................33
    9.1. DNS Failures ..............................................33
    9.2. DNS Configured, IKE Failures ..............................33
    9.3. System Reboots ............................................34
 10. Unresolved Issues .............................................34
    10.1. Control of Reverse DNS ...................................34
 11. Examples ......................................................34
    11.1. Clear-Text Usage (Permit Policy) .........................34
    11.2. Opportunistic Encryption .................................36
 12. Security Considerations .......................................39
    12.1. Configured versus Opportunistic Tunnels ..................39
    12.2. Firewalls versus Opportunistic Tunnels ...................40
    12.3. Denial of Service ........................................41
 13. Acknowledgements ..............................................41
 14. References ....................................................41
    14.1. Normative References .....................................41
    14.2. Informative References ...................................42

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1. Introduction

1.1. Motivation

 The objective of opportunistic encryption is to allow encryption
 without any pre-arrangement specific to the pair of systems involved.
 Each system administrator adds public key information to DNS records
 to support opportunistic encryption and then enables this feature in
 the nodes' IPsec stack.  Once this is done, any two such nodes can
 communicate securely.
 This document describes opportunistic encryption as designed and
 implemented by the Linux FreeS/WAN project in revisions up and
 including 2.00.  Note that 2.01 and beyond implements [RFC3445] in a
 backward compatible way.  A future document [IPSECKEY] will describe
 a variation that complies with RFC 3445.  For project information,
 see http://www.freeswan.org.
 The Internet Architecture Board (IAB) and Internet Engineering
 Steering Group (IESG) have taken a strong stand that the Internet
 should use powerful encryption to provide security and privacy
 [RFC1984].  The Linux FreeS/WAN project attempts to provide a
 practical means to implement this policy.
 The project uses the IPsec, ISAKMP/IKE, DNS, and DNSSEC protocols
 because they are standardized, widely available, and can often be
 deployed very easily without changing hardware or software, or
 retraining users.
 The extensions to support opportunistic encryption are simple.  No
 changes to any on-the-wire formats are needed.  The only changes are
 to the policy decision making system.  This means that opportunistic
 encryption can be implemented with very minimal changes to an
 existing IPsec implementation.
 Opportunistic encryption creates a "fax effect".  The proliferation
 of the fax machine was possible because it did not require that
 everyone buy one overnight.  Instead, as each person installed one,
 the value of having one increased because there were more people that
 could receive faxes.  Once opportunistic encryption is installed, it
 automatically recognizes other boxes using opportunistic encryption,
 without any further configuration by the network administrator.  So,
 as opportunistic encryption software is installed on more boxes, its
 value as a tool increases.
 This document describes the infrastructure to permit deployment of
 Opportunistic Encryption.

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 The term S/WAN is a trademark of RSA Data Systems, and is used with
 permission by this project.

1.2. Encryption Regimes

 To aid in understanding the relationship between security processing
 and IPsec, we divide policies controlling network traffic into four
 categories.  The traffic is categorized by destination address using
 longest prefix match.  Therefore, each category is enumerated by a
 set of network prefixes.  The categories are mutually exclusive; a
 particular prefix should only occur in one category.
  • Deny: network prefixes to which traffic is always forbidden.
  • Permit: network prefixes to which traffic in the clear is

permitted.

  • Opportunistic tunnel: network prefixes to which traffic is

encrypted if possible, when it otherwise might be sent in the

   clear.
 * Configured tunnel: network prefixes to which traffic must be
   encrypted, and traffic in the clear is never permitted.  A
   traditionally defined Virtual Private Network (VPN) is a form of
   configured tunnel.
 Traditional firewall devices handle the first two categories.  No
 authentication is required.  The permit policy is currently the
 default on the Internet.
 This document describes the third category: opportunistic tunnel,
 which is proposed as the new default for the Internet.
 Category four's policy is a very strict "encrypt it or drop it"
 policy, which requires authentication of the endpoints.  As the
 number of endpoints is typically bounded and is typically under a
 single authority, arranging for distribution of authentication
 material, while difficult, does not require any new technology.  The
 mechanism described here, however, does provides an additional way to
 distribute the authentication materials; it is a public key method
 that does not require deployment of an X.509 based infrastructure.

1.3. Peer Authentication in Opportunistic Encryption

 Opportunistic encryption creates tunnels between nodes that are
 essentially strangers.  This is done without any prior bilateral
 arrangement.  Therefore, there is the difficult question of how one
 knows to whom one is talking.

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 One possible answer is that since no useful authentication can be
 done, none should be tried.  This mode of operation is named
 "anonymous encryption".  An active man-in-the-middle attack can be
 used to thwart the privacy of this type of communication.  Without
 peer authentication, there is no way to prevent this kind of attack.
 Although it is a useful mode, anonymous encryption is not the goal of
 this project.  Simpler methods are available that can achieve
 anonymous encryption only, but authentication of the peer is a
 desirable goal.  Authentication of the peer is achieved through key
 distribution in DNS, leveraging upon the authentication of the DNS in
 DNSSEC.
 Peers are, therefore, authenticated with DNSSEC when available.
 Local policy determines how much trust to extend when DNSSEC is not
 available.
 An essential premise of building private connections with strangers
 is that datagrams received through opportunistic tunnels are no more
 special than datagrams that arrive in the clear.  Unlike in a VPN,
 these datagrams should not be given any special exceptions when it
 comes to auditing, further authentication, or firewalling.
 When initiating outbound opportunistic encryption, local
 configuration determines what happens if tunnel setup fails.  The
 packet may go out in the clear, or it may be dropped.

1.4. Use of RFC 2119 Terms

 The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
 SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
 document, are to be interpreted as described in [RFC2119]

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2. Overview

2.1. Reference Diagram

 The following network diagram is used in the rest of this document as
 the canonical diagram:
                            [Q]  [R]
                             .    .              AS2
    [A]----+----[SG-A].......+....+.......[SG-B]-------[B]
           |                 ......
       AS1 |                 ..PI..
           |                 ......
    [D]----+----[SG-D].......+....+.......[C] AS3
                  Figure 1: Reference Network Diagram
 In this diagram, there are four end-nodes: A, B, C, and D.  There are
 three security gateways, SG-A, SG-B, SG-D.  A, D, SG-A, and SG-D are
 part of the same administrative authority, AS1.  SG-A and SG-D are on
 two different exit paths from organization 1.  SG-B and B are part of
 an independent organization, AS2.  Nodes Q and R are nodes on the
 Internet.  PI is the Public Internet ("The Wild").

2.2. Terminology

 Note: The network numbers used in this document are for illustrative
 purposes only.  This document could not use the reserved example
 network numbers of [RFC3330] because multiple address ranges were
 needed.
 The following terminology is used in this document:
 Security gateway (or simply gateway): a system that performs IPsec
    tunnel mode encapsulation/decapsulation.  [SG-x] in the diagram.
 Alice: node [A] in the diagram.  When an IP address is needed, this
    is 192.1.0.65.
 Bob: node [B] in the diagram.  When an IP address is needed, this is
    192.2.0.66.
 Carol: node [C] in the diagram.  When an IP address is needed, this
    is 192.1.1.67.
 Dave: node [D] in the diagram.  When an IP address is needed, this is
    192.3.0.68.

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 SG-A: Alice's security gateway.  Internally it is 192.1.0.1,
    externally it is 192.1.1.4.
 SG-B: Bob's security gateway.  Internally it is 192.2.0.1, externally
    it is 192.1.1.5.
 SG-D: Dave's security gateway.  Also Alice's backup security gateway.
    Internally it is 192.3.0.1, externally it is 192.1.1.6.
 Configured tunnel: a tunnel that is directly and deliberately hand-
    configured on participating gateways.  Configured tunnels are
    typically given a higher level of trust than opportunistic
    tunnels.
 Road warrior tunnel: a configured tunnel connecting one node with a
    fixed IP address and one node with a variable IP address.  A road
    warrior (RW) connection must be initiated by the variable node,
    since the fixed node cannot know the current address for the road
    warrior.
 Anonymous encryption: the process of encrypting a session without any
    knowledge of who the other parties are.  No authentication of
    identities is done.
 Opportunistic encryption: the process of encrypting a session with
    authenticated knowledge of who the other party is without
    prearrangement.
 Lifetime: the period in seconds (bytes or datagrams) for which a
    security association will remain alive before rekeying is needed.
 Lifespan: the effective time for which a security association remains
    useful.  A security association with a lifespan shorter than its
    lifetime would be removed when no longer needed.  A security
    association with a lifespan longer than its lifetime would need to
    be re-keyed one or more times.
 Phase 1 SA: an ISAKMP/IKE security association sometimes referred to
    as a keying channel.
 Phase 2 SA: an IPsec security association.
 Tunnel: another term for a set of phase 2 SA (one in each direction).
 NAT: Network Address Translation (see [RFC2663]).
 NAPT: Network Address and Port Translation (see [RFC2663]).

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 AS: an autonomous system.
 FQDN: Fully-Qualified Domain Name
 Default-free zone: a set of routers that maintain a complete set of
    routes to all currently reachable destinations.  Having such a
    list, these routers never make use of a default route.  A datagram
    with a destination address not matching any route will be dropped
    by such a router.

2.3. Model of Operation

 The opportunistic encryption security gateway (OE gateway) is a
 regular gateway node, as described in [RFC0791] section 2.4 and
 [RFC1812], with the additional capabilities described here and in
 [RFC2401].  The algorithm described here provides a way to determine,
 for each datagram, whether or not to encrypt and tunnel the datagram.
 Two important things that must be determined are whether or not to
 encrypt and tunnel and, if so, the destination address or name of the
 tunnel endpoint that should be used.

2.3.1. Tunnel Authorization

 The OE gateway determines whether or not to create a tunnel based on
 the destination address of each packet.  Upon receiving a packet with
 a destination address not recently seen, the OE gateway performs a
 lookup in DNS for an authorization resource record (see Section 5.2).
 The record is located using the IP address to perform a search in the
 in-addr.arpa (IPv4) or ip6.arpa (IPv6) maps.  If an authorization
 record is found, the OE gateway interprets this as a request for a
 tunnel to be formed.

2.3.2. Tunnel Endpoint Discovery

 The authorization resource record also provides the address or name
 of the tunnel endpoint that should be used.
 The record may also provide the public RSA key of the tunnel end
 point itself.  This is provided for efficiency only.  If the public
 RSA key is not present, the OE gateway performs a second lookup to
 find a KEY resource record for the endpoint address or name.
 Origin and integrity protection of the resource records is provided
 by DNSSEC (see [RFC4033]).  Section 3.2.4.1 documents an optional
 restriction on the tunnel endpoint if DNSSEC signatures are not
 available for the relevant records.

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2.3.3. Caching of Authorization Results

 The OE gateway maintains a cache, in the forwarding plane, of
 source/destination pairs for which opportunistic encryption has been
 attempted.  This cache maintains a record of whether or not OE was
 successful so that subsequent datagrams can be forwarded properly
 without additional delay.
 Successful negotiation of OE instantiates a new security association.
 Failure to negotiate OE results in creation of a forwarding policy
 entry either to deny or permit transmission in the clear future
 datagrams.  This negative cache is necessary to avoid the possibly
 lengthy process of repeatedly looking up the same information.
 The cache is timed out periodically, as described in Section 3.4.
 This removes entries that are no longer being used and permits the
 discovery of changes in authorization policy.

3. Protocol Specification

 The OE gateway is modeled to have a forwarding plane and a control
 plane.  A control channel, such as PF_KEY [RFC2367], connects the two
 planes.
 The forwarding plane performs per-datagram operations.  The control
 plane contains a keying daemon, such as ISAKMP/IKE, and performs all
 authorization, peer authentication, and key derivation functions.

3.1. Forwarding Plane State Machine

 Let the OE gateway maintain a collection of objects -- a superset of
 the security policy database (SPD) specified in [RFC2401].  For each
 combination of source and destination address, an SPD object exists
 in one of five following states.  Prior to forwarding each datagram,
 the responder uses the source and destination addresses to pick an
 entry from the SPD.  The SPD then determines if and how the packet is
 forwarded.

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       .--------------.
       | nonexistent  |
       |    policy    |
       `--------------'
              |
              | PF_ACQUIRE
              |
              |<---------.
              V          | new packet
       .--------------.  | (maybe resend PF_ACQUIRE)
       |  hold policy |--'
       |              |--.
       `--------------'   \  pass
          |        |       \ msg    .---------.
          |        |        \       V         | forward
          |        |         .-------------.  | packet
   create |        |         | pass policy |--'
   IPsec  |        |         `-------------'
   SA     |        |
          |         \
          |          \
          V           \ deny
    .---------.        \ msg
    | encrypt |         \
    | policy  |          \         ,---------.
    `---------'           \        |         | discard
                           \       V         | packet
                            .-------------.  |
                            | deny policy |--'
                            `-------------'

3.1.1. Nonexistent Policy

 If the gateway does not find an entry, then this policy applies.  The
 gateway creates an entry with an initial state of "hold policy" and
 requests keying material from the keying daemon.  The gateway does
 not forward the datagram; rather, it SHOULD attach the datagram to
 the SPD entry as the "first" datagram and retain it for eventual
 transmission in a new state.

3.1.2. Hold Policy

 The gateway requests keying material.  If the interface to the keying
 system is lossy (PF_KEY, for instance, can be), the implementation
 SHOULD include a mechanism to retransmit the keying request at a rate
 limited to less than 1 request per second.  The gateway does not
 forward the datagram.  The gateway SHOULD attach the datagram to the
 SPD entry as the "last" datagram, where it is retained for eventual

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 transmission.  If there is a datagram already stored in this way,
 then that already-stored datagram is discarded.
 The rationale behind saving the "first" and "last" datagrams are as
 follows: The "first" datagram is probably a TCP SYN packet.  Once
 there is keying established, the gateway will release this datagram,
 avoiding the need for the endpoint to retransmit the datagram.  In
 the case where the connection was not a TCP connection, but was
 instead a streaming protocol or a DNS request, the "last" datagram
 that was retained is likely the most recent data.  The difference
 between "first" and "last" may also help the endpoints determine
 which data was dropped while negotiation took place.

3.1.3. Pass-Through Policy

 The gateway forwards the datagram using the normal forwarding table.
 The gateway enters this state only by command from the keying daemon,
 and upon entering this state, also forwards the "first" and "last"
 datagrams.

3.1.4. Deny Policy

 The gateway discards the datagram.  The gateway enters this state
 only by command from the keying daemon, and upon entering this state,
 discards the "first" and "last" datagrams.  An implementation MAY
 provide the administrator with a control to determine if further
 datagrams cause ICMP messages to be generated (i.e., ICMP Destination
 Unreachable, Communication Administratively Prohibited.  type=3,
 code=13).

3.1.5. Encrypt Policy

 The gateway encrypts the datagram using the indicated security
 association database (SAD) entry.  The gateway enters this state only
 by command from the keying daemon, and upon entering this state,
 releases and forwards the "first" and "last" datagrams using the new
 encrypt policy.
 If the associated SAD entry expires because of byte, packet or time
 limits, then the entry returns to the Hold policy, and an expire
 message is sent to the keying daemon.
 All states may be created directly by the keying daemon while acting
 as a gateway.

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3.2. Keying Daemon – Initiator

 Let the keying daemon maintain a collection of objects.  Let them be
 called "connections" or "conn"s.  There are two categories of
 connection objects: classes and instances.  A class represents an
 abstract policy (i.e., what could be).  An instance represents an
 actual connection (i.e., what is running at the time).
 Let there be two further subtypes of connections: keying channels
 (Phase 1 SAs) and data channels (Phase 2 SAs).  Each data channel
 object may have a corresponding SPD and SAD entry maintained by the
 datagram state machine.
 For the purposes of opportunistic encryption, there MUST, at least,
 be connection classes known as "deny", "always-clear-text", "OE-
 permissive", and "OE-paranoid".  The latter two connection classes
 define a set of destination prefixes for which opportunistic
 encryption will be attempted.  The administrator MAY set policy
 options in a number of additional places.  An implementation MAY
 create additional connection classes to further refine these
 policies.
 The simplest system may need only the "OE-permissive" connection, and
 would list its own (single) IP address as the source address of this
 policy and the wild-card address 0.0.0.0/0 as the destination IPv4
 address.  That is, the simplest policy is to try opportunistic
 encryption with all destinations.
 This simplest policy SHOULD be offered as a preconfigured default.
 The distinction between permissive and paranoid Opportunistic
 Encryption ("OE-paranoid" below) use will become clear in the state
 transition differences.
 In brief, an OE-permissive policy means to permit traffic to flow in
 the clear when there is a failure to find and/or use the encryption
 keys.  OE-permissive permits the network to function, even if in an
 insecure manner.
 On failure, a paranoid OE ("OE-paranoid") will install a drop policy.
 OE-paranoid permits traffic to flow only when appropriate security is
 available.
 In this description of the keying machine's state transitions, the
 states associated with the keying system itself are omitted because
 they are best documented in the keying system ([RFC2407], [RFC2408],
 and [RFC2409] for ISAKMP/IKE), and the details are keying system
 specific.  Opportunistic encryption is not dependent upon any

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 specific keying protocol, but this document does provide requirements
 for those using ISAKMP/IKE to assure that implementations inter-
 operate.
 The state transitions that may be involved in communicating with the
 forwarding plane are omitted.  PF_KEY and similar protocols have
 their own set of states required for message sends and completion
 notifications.
 Finally, the retransmits and recursive lookups that are normal for
 DNS are not included in this description of the state machine.
                       |
                       | PF_ACQUIRE
                       |
                       V
               .---------------.
               |  nonexistent  |
               |  connection   |
               `---------------'
                |      |      |
         send   ,      |      \

expired pass / | \ send conn. msg / | \ deny

 ^           /         |         \ msg
 |          V          | do       \

.—————. | DNS \ .—————. | clear-text | | lookup `→| deny |—>expired | connection | | for | connection | connection `—————' | destination `—————'

  ^ ^                  |                   ^
  | | no record        |                   |
  | | OE-permissive    V                   | no record
  | |            .---------------.         | OE-paranoid
  | `------------|  potential OE |---------'
  |              |  connection   |         ^
  |              `---------------'         |
  |                    |                   |
  |                    | got TXT record    | DNSSEC failure
  |                    | reply             |
  |                    V                   | wrong
  |              .---------------.         | failure
  |              |  authenticate |---------'
  |              | & parse TXT RR|         ^
  | repeated     `---------------'         |
  | ICMP               |                   |
  | failures           | initiate IKE to   |
  | (short timeout)    | responder         |

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  |                    V                   |
  | phase-2      .---------------.         | failure
  | failure      |   pending     |---------'
  | (normal      |     OE        |         ^
  |  timeout)    |               |invalid  | phase-2 fail (normal
  |              |               |<--.SPI  |               timeout)
  |              |               |   |     | ICMP failures (short
  |              | +=======+     |---'     |                timeout)
  |              | |  IKE  |     |   ^     |
  `----------------| states|---------------'
                 | +=======+     |   |
                 `---------------'   |
                       | IPsec SA    | invalid SPI
                       | established |
                       V             | rekey time
                 .--------------.    |
                 |   keyed      |<---|------------------------------.
                 |  connection  |----'                              |
                 `--------------'                                   |
                       | timer                                      |
                       |                                            |
                       V                                            |
                 .--------------.     connection still active       |
 clear-text----->|   expired    |-----------------------------------'
       deny----->|  connection  |
                 `--------------'
                       | dead connection - deleted
                       V

3.2.1. Nonexistent Connection

 There is no connection instance for a given source/destination
 address pair.  Upon receipt of a request for keying material for this
 source/destination pair, the initiator searches through the
 connection classes to determine the most appropriate policy.  Upon
 determining an appropriate connection class, an instance object is
 created of that type.  Both of the OE types result in a potential OE
 connection.
 Failure to find an appropriate connection class results in an
 administrator-defined default.
 In each case, when the initiator finds an appropriate class for the
 new flow, an instance connection is made of the class that matched.

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3.2.2. Clear-Text Connection

 The nonexistent connection makes a transition to this state when an
 always-clear-text class is instantiated, or when an OE-permissive
 connection fails.  During the transition, the initiator creates a
 pass-through policy object in the forwarding plane for the
 appropriate flow.
 Timing out is the only way to leave this state (see Section 3.2.7).

3.2.3. Deny Connection

 The empty connection makes a transition to this state when a deny
 class is instantiated, or when an OE-paranoid connection fails.
 During the transition, the initiator creates a deny policy object in
 the forwarding plane for the appropriate flow.
 Timing out is the only way to leave this state (see Section 3.2.7).

3.2.4. Potential OE Connection

 The empty connection makes a transition to this state when one of
 either OE class is instantiated.  During the transition to this
 state, the initiator creates a hold policy object in the forwarding
 plane for the appropriate flow.
 In addition, when making a transition into this state, DNS lookup is
 done in the reverse-map for a TXT delegation resource record (see
 Section 5.2).  The lookup key is the destination address of the flow.
 There are three ways to exit this state:
 1.  DNS lookup finds a TXT delegation resource record.
 2.  DNS lookup does not find a TXT delegation resource record.
 3.  DNS lookup times out.
 Based upon the results of the DNS lookup, the potential OE connection
 makes a transition to the pending OE connection state.  The
 conditions for a successful DNS look are:
 1.  DNS finds an appropriate resource record.
 2.  It is properly formatted according to Section 5.2.
 3.  If DNSSEC is enabled, then the signature has been vouched for.

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 Note that if the initiator does not find the public key present in
 the TXT delegation record, then the public key must be looked up as a
 sub-state.  Only successful completion of all the DNS lookups is
 considered a success.
 If DNS lookup does not find a resource record or if DNS times out,
 then the initiator considers the receiver not OE capable.  If this is
 an OE-paranoid instance, then the potential OE connection makes a
 transition to the deny connection state.  If this is an OE-permissive
 instance, then the potential OE connection makes a transition to the
 clear-text connection state.
 If the initiator finds a resource record, but it is not properly
 formatted, or if DNSSEC is enabled and reports a failure to
 authenticate, then the potential OE connection makes a transition to
 the deny connection state.  This action SHOULD be logged.  If the
 administrator wishes to override this transition between states, then
 an always-clear class can be installed for this flow.  An
 implementation MAY make this situation a new class.

3.2.4.1. Restriction on Unauthenticated TXT Delegation Records

 An implementation SHOULD also provide an additional administrative
 control on delegation records and DNSSEC.  This control would apply
 to delegation records (the TXT records in the reverse-map) that are
 not protected by DNSSEC.  Records of this type are only permitted to
 delegate to their own address as a gateway.  When this option is
 enabled, an active attack on DNS will be unable to redirect packets
 to other than the original destination.

3.2.5. Pending OE Connection

 The potential OE connection makes a transition to this state when the
 initiator determines that all the information required from the DNS
 lookup is present.  Upon entering this state, the initiator attempts
 to initiate keying to the gateway provided.
 Exit from this state occurs with either a successfully created IPsec
 SA or a failure of some kind.  Successful SA creation results in a
 transition to the key connection state.
 Three failures have caused significant problems.  They are clearly
 not the only possible failures from keying.

Richardson & Redelmeier Informational [Page 16] RFC 4322 Opportunistic Encryption using IKE December 2005

 Note that if there are multiple gateways available in the TXT
 delegation records, then a failure can only be declared after all of
 them have been tried.  Further, creation of a phase 1 SA does not
 constitute success.  A set of phase 2 SAs (a tunnel) is considered
 success.
 The first failure occurs when an ICMP port unreachable is
 consistently received without any other communication, or when there
 is silence from the remote end.  This usually means that either the
 gateway is not alive, or the keying daemon is not functional.  For an
 OE-permissive connection, the initiator makes a transition to the
 clear-text connection, but with a low lifespan.  For an OE-
 pessimistic connection, the initiator makes a transition to the deny
 connection again with a low lifespan.  The lifespan in both cases is
 kept low because the remote gateway may be in the process of
 rebooting or be otherwise temporarily unavailable.
 The length of time to wait for the remote keying daemon to wake up is
 a matter of some debate.  If there is a routing failure, 5 minutes is
 usually long enough for the network to re-converge.  Many systems can
 reboot in that amount of time as well.  However, 5 minutes is far too
 long for most users to wait to hear that they can not connect using
 OE.  Implementations SHOULD make this a tunable parameter.
 The second failure occurs after a phase 1 SA has been created, but
 there is either no response to the phase 2 proposal, or the initiator
 receives a negative notify (the notify must be authenticated).  The
 remote gateway is not prepared to do OE at this time.  As before, the
 initiator makes a transition to the clear-text or the deny connection
 based upon connection class, but this time with a normal lifespan.
 The third failure occurs when there is signature failure while
 authenticating the remote gateway.  This can occur when there has
 been a key roll-over, but DNS has not caught up.  In this case again,
 the initiator makes a transition to the clear-text or the deny
 connection based upon the connection class.  However, the lifespan
 depends upon the remaining time to live in the DNS.  (Note that
 DNSSEC signed resource records have a different expiry time from
 non-signed records.)

3.2.6. Keyed Connection

 The pending OE connection makes a transition to this state when
 session keying material (the phase 2 SAs) is derived.  The initiator
 creates an encrypt policy in the forwarding plane for this flow.

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 There are three ways to exit this state.  The first is by receipt of
 an authenticated delete message (via the keying channel) from the
 peer.  This is normal teardown and results in a transition to the
 expired connection state.
 The second exit is by expiry of the forwarding plane keying material.
 This starts a re-key operation with a transition back to pending OE
 connection.  In general, the soft expiry occurs with sufficient time
 left to continue using the keys.  A re-key can fail, which may result
 in the connection failing to clear-text or deny as appropriate.  In
 the event of a failure, the forwarding plane policy does not change
 until the phase 2 SA (IPsec SA) reaches its hard expiry.
 The third exit is in response to a negotiation from a remote gateway.
 If the forwarding plane signals the control plane that it has
 received an unknown SPI from the remote gateway, or an ICMP is
 received from the remote gateway indicating an unknown SPI, the
 initiator should consider that the remote gateway has rebooted or
 restarted.  Since these indications are easily forged, the
 implementation must exercise care.  The initiator should make a
 cautious (rate-limited) attempt to re-key the connection.

3.2.7. Expiring Connection

 The initiator will periodically place each of the deny, clear-text,
 and keyed connections into this sub-state.  See Section 3.4 for more
 details of how often this occurs.  The initiator queries the
 forwarding plane for last use time of the appropriate policy.  If the
 last use time is relatively recent, then the connection returns to
 the previous deny, clear-text or keyed connection state.  If not,
 then the connection enters the expired connection state.
 The DNS query and answer that lead to the expiring connection state
 are also examined.  The DNS query may become stale.  (A negative,
 i.e., no such record, answer is valid for the period of time given by
 the MINIMUM field in an attached SOA record.  See [RFC1034] section
 4.3.4.)  If the DNS query is stale, then a new query is made.  If the
 results change, then the connection makes a transition to a new state
 as described in potential OE connection state.
 Note that when considering how stale a connection is, both outgoing
 SPD and incoming SAD must be queried as some flows may be
 unidirectional for some time.
 Also note that the policy at the forwarding plane is not updated
 unless there is a conclusion that there should be a change.

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3.2.8. Expired Connection

 Entry to this state occurs when no datagrams have been forwarded
 recently via the appropriate SPD and SAD objects.  The objects in the
 forwarding plane are removed (logging any final byte and packet
 counts, if appropriate) and the connection instance in the keying
 plane is deleted.
 The initiator sends an ISAKMP/IKE delete to clean up the phase 2 SAs
 as described in Section 3.4.
 Whether or not to delete the phase 1 SAs at this time is left as a
 local implementation issue.  Implementations that do delete the phase
 1 SAs MUST send authenticated delete messages to indicate that they
 are doing so.  There is an advantage to keeping the phase 1 SAs until
 they expire: they may prove useful again in the near future.

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3.3. Keying Daemon – Responder

 The responder has a set of objects identical to those of the
 initiator.
 The responder receives an invitation to create a keying channel from
 an initiator.
                 |
                 | IKE main mode
                 |  phase 1
                 V
         .-----------------.
         | unauthenticated |
         |     OE peer     |
         `-----------------'
                 |
                 | lookup KEY RR in in-addr.arpa
                 |             (if ID_IPV4_ADDR)
                 | lookup KEY RR in forward
                 |             (if ID_FQDN)
                 V
         .-----------------.  RR not found
         |   received DNS  |---------------> log failure
         |     reply       |
         `----+--------+---'
           phase 2 |        \      misformatted
          proposal |         `------------------> log failure
                   V
         .----------------.
         |  authenticated |  identical initiator
         |     OE peer    |--------------------> initiator
         `----------------'  connection found    state machine
               |
               | look for TXT record for initiator
               |
               V
         .---------------.
         |  authorized   |---------------------> log failure
         |    OE peer    |
         `---------------'
               |
               |
               V
          potential OE
          connection in
          initiator state
             machine

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3.3.1. Unauthenticated OE Peer

 Upon entering this state, the responder starts a DNS lookup for a KEY
 record for the initiator.  The responder looks in the reverse-map for
 a KEY record for the initiator if the initiator has offered an
 ID_IPV4_ADDR, and in the forward map if the initiator has offered an
 ID_FQDN type.  (See [RFC2407] section 4.6.2.1.)
 The responder exits this state upon successful receipt of a KEY from
 DNS, and use of the key to verify the signature of the initiator.
 Successful authentication of the peer results in a transition to the
 authenticated OE Peer state.
 Note that the unauthenticated OE peer state generally occurs in the
 middle of the key negotiation protocol.  It is really a form of
 pseudo-state.

3.3.2. Authenticated OE Peer

 The peer will eventually propose one or more phase 2 SAs.  The
 responder uses the source and destination address in the proposal to
 finish instantiating the connection state using the connection class
 table.  The responder MUST search for an identical connection object
 at this point.
 If an identical connection is found, then the responder deletes the
 old instance, and the new object makes a transition to the pending OE
 connection state.  This means that new ISAKMP connections with a
 given peer will always use the latest instance, which is the correct
 one if the peer has rebooted in the interim.
 If an identical connection is not found, then the responder makes the
 transition according to the rules given for the initiator: it
 installs appropriate policy: clear, drop, or OE.
 If OE, and the phase 2 ID (source IP) is different than the phase 1
 ID, then additional authorization is required.  A TXT record
 associated with the proposed phase 2 source IP is requested.  This is
 used to confirm authorization for the phase 1 identity to encrypt on
 behalf of the phase 2.  Successful retrieval results in a transition
 to "Authorized OE Peer".
 Note that if the initiator is in OE-paranoid mode and the responder
 is in either always-clear-text or deny, then no communication is
 possible according to policy.  An implementation is permitted to
 create new types of policies such as "accept OE but do not initiate
 it".  This is a local matter.

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3.3.3. Authorized OE Peer

 This state is entered from the Authenticated OE Peer state, upon
 successful retrieval of the TXT record.  The contents of the record
 are confirmed -- any failures lead to errors, as indicated in Section
 3.2.4.

3.4. Renewal and Teardown

3.4.1. Aging

 A potentially unlimited number of tunnels may exist.  In practice,
 only a few tunnels are used during a period of time.  Unused tunnels
 MUST, therefore, be torn down.  Detecting when tunnels are no longer
 in use is the subject of this section.
 There are two methods for removing tunnels: explicit deletion or
 expiry.
 Explicit deletion requires an IKE delete message.  The deletes MUST
 be authenticated, so both ends of the tunnel must maintain the keying
 channel (phase 1 ISAKMP SA).  An implementation that refuses to
 either maintain or recreate the keying channel SA will be unable to
 use this method.
 The tunnel expiry method simply allows the IKE daemon to expire
 normally without attempting to re-key it.
 Regardless of which method is used to remove tunnels, the
 implementation MUST use a method to determine if the tunnel is still
 in use.  The specifics are a local matter, but the FreeS/WAN project
 uses the following criteria.  These criteria are currently
 implemented in the key management daemon, but could also be
 implemented at the SPD layer using an idle timer.
 Set a short initial (soft) lifespan of 1 minute since many net flows
 last only a few seconds.
 At the end of the lifespan, check to see if the tunnel was used by
 traffic in either direction during the last 30 seconds.  If so,
 assign a longer tentative lifespan of 20 minutes, after which, look
 again.  If the tunnel is not in use, then close the tunnel.
 The expiring state in the key management system (see Section 3.2.7)
 implements these timeouts.  The timer above may be in the forwarding
 plane, but then it must be resettable.

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 The tentative lifespan is independent of re-keying; it is just the
 time when the tunnel's future is next considered.  (The term lifespan
 is used here rather than lifetime for this reason.)  Unlike re-
 keying, this tunnel use check is not costly and should happen
 reasonably frequently.
 A multi-step back-off algorithm is not considered worth the effort
 here.
 If the security gateway and the client host are the same, and not a
 Bump-in-the-Stack or Bump-in-the-Wire implementation, tunnel teardown
 decisions MAY pay attention to TCP connection status as reported by
 the local TCP layer.  A still-open TCP connection is almost a
 guarantee that more traffic is expected.  Closing of the only TCP
 connection through a tunnel is a strong hint that no more traffic is
 expected.

3.4.2. Teardown and Cleanup

 Teardown should always be coordinated between the two ends of the
 tunnel by interpreting and sending delete notifications.  There is a
 detailed sub-state in the expired connection state of the key manager
 that relates to retransmits of the delete notifications, but this is
 considered to be a keying system detail.
 On receiving a delete for the outbound SAs of a tunnel (or some
 subset of them), tear down the inbound ones also and notify the
 remote end with a delete.  If the local system receives a delete for
 a tunnel that is no longer in existence, then two delete messages
 have crossed paths.  Ignore the delete.  The operation has already
 been completed.  Do not generate any messages in this situation.
 Tunnels are to be considered as bidirectional entities, even though
 the low-level protocols don't treat them this way.
 When the deletion is initiated locally, rather than as a response to
 a received delete, send a delete for (all) the inbound SAs of a
 tunnel.  If the local system does not receive a responding delete for
 the outbound SAs, try re-sending the original delete.  Three tries
 spaced 10 seconds apart seems a reasonable level of effort.  A
 failure of the other end to respond after 3 attempts indicates that
 the possibility of further communication is unlikely.  Remove the
 outgoing SAs.  (The remote system may be a mobile node that is no
 longer present or powered on.)
 After re-keying, transmission should switch to using the new outgoing
 SAs (ISAKMP or IPsec) immediately, and the old leftover outgoing SAs
 should be cleared out promptly (delete should be sent for the

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 outgoing SAs) rather than waiting for them to expire.  This reduces
 clutter and minimizes confusion for the operator doing diagnostics.

4. Impacts on IKE

4.1. ISAKMP/IKE Protocol

 The IKE wire protocol needs no modifications.  The major changes are
 implementation issues relating to how the proposals are interpreted,
 and from whom they may come.
 As opportunistic encryption is designed to be useful between peers
 without prior operator configuration, an IKE daemon must be prepared
 to negotiate phase 1 SAs with any node.  This may require a large
 amount of resources to maintain cookie state, as well as large
 amounts of entropy for nonces, cookies, and so on.
 The major changes to support opportunistic encryption are at the IKE
 daemon level.  These changes relate to handling of key acquisition
 requests, lookup of public keys and TXT records, and interactions
 with firewalls and other security facilities that may be co-resident
 on the same gateway.

4.2. Gateway Discovery Process

 In a typical configured tunnel, the address of SG-B is provided via
 configuration.  Furthermore, the mapping of an SPD entry to a gateway
 is typically a 1:1 mapping.  When the 0.0.0.0/0 SPD entry technique
 is used, then the mapping to a gateway is determined by the reverse
 DNS records.
 The need to do a DNS lookup and wait for a reply will typically
 introduce a new state and a new event source (DNS replies) to IKE.
 Although a synchronous DNS request can be implemented for proof of
 concept, experience is that it can cause very high latencies when a
 queue of queries must all timeout in series.
 Use of an asynchronous DNS lookup will also permit overlap of DNS
 lookups with some of the protocol steps.

4.3. Self Identification

 SG-A will have to establish its identity.  Use an IPv4 (IPv6) ID in
 phase 1.
 There are many situations where the administrator of SG-A may not be
 able to control the reverse DNS records for SG-A's public IP address.
 Typical situations include dialup connections and most residential-

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 type broadband Internet access (ADSL, cable-modem) connections.  In
 these situations, a fully qualified domain name that is under the
 control of SG-A's administrator may be used when acting as an
 initiator only.  The FQDN ID should be used in phase 1.  See Section
 5.3 for more details and restrictions.

4.4. Public Key Retrieval Process

 Upon receipt of a phase 1 SA proposal with either an IPv4 (IPv6) ID
 or an FQDN ID, an IKE daemon needs to examine local caches and
 configuration files to determine if this is part of a configured
 tunnel.  If no configured tunnels are found, then the implementation
 should attempt to retrieve a KEY record from the reverse DNS in the
 case of an IPv4/IPv6 ID, or from the forward DNS in the case of FQDN
 ID.
 It is reasonable that if other non-local sources of policy are used
 (COPS, LDAP), they be consulted concurrently, but that some clear
 ordering of policy be provided.  Note that due to variances in
 latency, implementations must wait for positive or negative replies
 from all sources of policy before making any decisions.

4.5. Interactions with DNSSEC

 The implementation described (FreeS/WAN 1.98) neither uses DNSSEC
 directly to explicitly verify the authenticity of zone information,
 nor uses the NSEC records to provide authentication of the absence of
 a TXT or KEY record.  Rather, this implementation uses a trusted path
 to a DNSSEC-capable caching resolver.
 To distinguish between an authenticated and an unauthenticated DNS
 resource record, a stub resolver capable of returning DNSSEC
 information MUST be used.

4.6. Required Proposal Types

4.6.1. Phase 1 Parameters

 Main mode MUST be used.
 The initiator MUST offer at least one proposal using some combination
 of: 3DES, HMAC-MD5 or HMAC-SHA1, DH group 2 or 5.  Group 5 SHOULD be
 proposed first.  (See [RFC3526])
 The initiator MAY offer additional proposals, but the cipher MUST not
 be weaker than 3DES.  The initiator SHOULD limit the number of
 proposals such that the IKE datagrams do not need to be fragmented.

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 The responder MUST accept one of the proposals.  If any configuration
 of the responder is required, then the responder is not acting in an
 opportunistic way.
 The initiator SHOULD use an ID_IPV4_ADDR (ID_IPV6_ADDR for IPv6) of
 the external interface of the initiator for phase 1.  (There is an
 exception, see Section 5.3.)  The authentication method MUST be RSA
 public key signatures.  The RSA key for the initiator SHOULD be
 placed into a DNS KEY record in the reverse space of the initiator
 (i.e., using in-addr.arpa or ip6.arpa).

4.6.2. Phase 2 Parameters

 The initiator MUST propose a tunnel between the ultimate sender
 ("Alice" or "A") and ultimate recipient ("Bob" or "B") using 3DES-CBC
 mode, MD5, or SHA1 authentication.  Perfect Forward Secrecy MUST be
 specified.
 Tunnel mode MUST be used.
 Identities MUST be ID_IPV4_ADDR_SUBNET with the mask being /32.
 Authorization for the initiator to act on Alice's behalf is
 determined by looking for a TXT record in the reverse-map at Alice's
 IP address.
 Compression SHOULD NOT be mandatory.  It MAY be offered as an option.

5. DNS Issues

5.1. Use of KEY Record

 In order to establish their own identities, security gateways SHOULD
 publish their public keys in their reverse DNS via DNSSEC's KEY
 record.  See section 3 of RFC 2535 [RFC2535].
 For example:
 KEY 0x4200 4 1 AQNJjkKlIk9...nYyUkKK8
 0x4200: The flag bits, indicating that this key is prohibited for
    confidentiality use (it authenticates the peer only, a separate
    Diffie-Hellman exchange is used for confidentiality), and that
    this key is associated with the non-zone entity whose name is the
    RR owner name.  No other flags are set.
 4: This indicates that this key is for use by IPsec.

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 1: An RSA key is present.
 AQNJjkKlIk9...nYyUkKK8: The public key of the host as described in
    [RFC3110].
 Use of several KEY records allows for key roll-over.  The SIG Payload
 in IKE phase 1 SHOULD be accepted if the public key, given by any KEY
 RR, validates it.

5.2. Use of TXT Delegation Record

 If, for example, machine Alice wishes SG-A to act on her behalf, then
 she publishes a TXT record to provide authorization for SG-A to act
 on Alice's behalf.  This is done similarly for Bob and SG-B.
 These records are located in the reverse DNS (in-addr.arpa or
 ip6.arpa) for their respective IP addresses.  The reverse DNS SHOULD
 be secured by DNSSEC.  DNSSEC is required to defend against active
 attacks.
 If Alice's address is P.Q.R.S, then she can authorize another node to
 act on her behalf by publishing records at:
    S.R.Q.P.in-addr.arpa
 The contents of the resource record are expected to be a string that
 uses the following syntax, as suggested in RFC1464 [RFC1464].  (Note
 that the reply to query may include other TXT resource records used
 by other applications.)
    X-IPsec-Server(P)=A.B.C.D public-key
             Figure 2: Format of reverse delegation record
 P: Specifies a precedence for this record.  This is similar to MX
    record preferences.  Lower numbers have stronger preference.
 A.B.C.D: Specifies the IP address of the Security Gateway for this
    client machine.
 public-key: Is the encoded RSA Public key of the Security Gateway.
    The public-key is provided here to avoid a second DNS lookup.  If
    this field is absent, then a KEY resource record should be looked
    up in the reverse-map of A.B.C.D.  The key is transmitted in
    base64 format.
 The fields of the record MUST be separated by whitespace.  This MAY
 be: space, tab, newline, or carriage return.  A space is preferred.

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 In the case where Alice is located at a public address behind a
 security gateway that has no fixed address (or no control over its
 reverse-map), then Alice may delegate to a public key by domain name.
    X-IPsec-Server(P)=@FQDN public-key
     Figure 3: Format of reverse delegation record (FQDN version)
 P: Is as above.
 FQDN: Specifies the FQDN that the Security Gateway will identify
    itself with.
 public-key: Is the encoded RSA Public key of the Security Gateway.
 If there is more than one such TXT record with strongest (lowest
 numbered) precedence, one Security Gateway is picked arbitrarily from
 those specified in the strongest-preference records.

5.2.1. Long TXT Records

 When packed into wire-format, TXT records that are longer than 255
 characters are divided into smaller <character-strings>.  (See
 [RFC1035] section 3.3 and 3.3.14.)  These MUST be reassembled into a
 single string for processing.  Whitespace characters in the base64
 encoding are to be ignored.

5.2.2. Choice of TXT Record

 It has been suggested to use the KEY, OPT, CERT, or KX records
 instead of a TXT record.  None is satisfactory.
 The KEY RR has a protocol field that could be used to indicate a new
 protocol, and an algorithm field that could be used to indicate
 different contents in the key data.  However, the KEY record is
 clearly not intended for storing what are really authorizations, it
 is just for identities.  Other uses have been discouraged.
 OPT resource records, as defined in [RFC2671], are not intended to be
 used for storage of information.  They are not to be loaded, cached
 or forwarded.  They are, therefore, inappropriate for use here.
 CERT records [RFC2538] can encode almost any set of information.  A
 custom type code could be used permitting any suitable encoding to be
 stored, not just X.509.  According to the RFC, the certificate RRs
 are to be signed internally, which may add undesirable and
 unnecessary bulk.  Larger DNS records may require TCP instead of UDP
 transfers.

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 At the time of protocol design, the CERT RR was not widely deployed
 and could not be counted upon.  Use of CERT records will be
 investigated, and may be proposed in a future revision of this
 document.
 KX records are ideally suited for use instead of TXT records, but had
 not been deployed at the time of implementation.

5.3. Use of FQDN IDs

 Unfortunately, not every administrator has control over the contents
 of the reverse-map.  Where the initiator (SG-A) has no suitable
 reverse-map, the authorization record present in the reverse-map of
 Alice may refer to a FQDN instead of an IP address.
 In this case, the client's TXT record gives the fully qualified
 domain name (FQDN) in place of its security gateway's IP address.
 The initiator should use the ID_FQDN ID-payload in phase 1.  A
 forward lookup for a KEY record on the FQDN must yield the
 initiator's public key.
 This method can also be used when the external address of SG-A is
 dynamic.
 If SG-A is acting on behalf of Alice, then Alice must still delegate
 authority for SG-A to do so in her reverse-map.  When Alice and SG-A
 are one and the same (i.e., Alice is acting as an end-node) then
 there is no need for this when initiating only.
 However, Alice must still delegate to herself if she wishes others to
 initiate OE to her.  See Figure 3.

5.4. Key Roll-Over

 Good cryptographic hygiene says that one should replace
 public/private key pairs periodically.  Some administrators may wish
 to do this as often as daily.  Typical DNS propagation delays are
 determined by the SOA Resource Record MINIMUM parameter, which
 controls how long DNS replies may be cached.  For reasonable
 operation of DNS servers, administrators usually want this value to
 be at least several hours, sometimes as a long as a day.  This
 presents a problem: a new key MUST not be used prior to its
 propagation through DNS.
 This problem is dealt with by having the Security Gateway generate a
 new public/private key pair, at least MINIMUM seconds in advance of
 using it.  It then adds this key to the DNS (both as a second KEY

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 record and in additional TXT delegation records) at key generation
 time.  Note: only one key is allowed in each TXT record.
 When authenticating, all gateways MUST have available all public keys
 that are found in DNS for this entity.  This permits the
 authenticating end to check both the key for "today" and the key for
 "tomorrow".  Note that it is the end which is creating the signature
 (possesses the private key) that determines which key is to be used.

6. Network Address Translation Interaction

 There are no fundamentally new issues for implementing opportunistic
 encryption in the presence of network address translation.  Rather,
 there are only the regular IPsec issues with NAT traversal.
 There are several situations to consider for NAT.

6.1. Co-Located NAT/NAPT

 If a security gateway is also performing network address translation
 on behalf of an end-system, then the packet should be translated
 prior to being subjected to opportunistic encryption.  This is in
 contrast to typically configured tunnels, which often exist to bridge
 islands of private network address space.  The security gateway will
 use the translated source address for phase 2, and so the responding
 security gateway will look up that address to confirm SG-A's
 authorization.
 In the case of NAT (1:1), the address space into which the
 translation is done MUST be globally unique, and control over the
 reverse-map is assumed.  Placing of TXT records is possible.
 In the case of NAPT (m:1), the address will be the security gateway
 itself.  The ability to get KEY and TXT records in place will again
 depend upon whether or not there is administrative control over the
 reverse-map.  This is identical to situations involving a single host
 acting on behalf of itself.  For initiators (but not responders), an
 FQDN-style ID can be used to get around a lack of a reverse-map.

6.2. Security Gateway behind a NAT/NAPT

 If there is a NAT or NAPT between the security gateways, then normal
 IPsec NAT traversal problems occur.  In addition to the transport
 problem, which may be solved by other mechanisms, there is the issue
 of what phase 1 and phase 2 IDs to use.  While FQDN could be used
 during phase 1 for the security gateway, there is no appropriate ID
 for phase 2.  Due to the NAT, the end systems live in different IP
 address spaces.

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6.3. End System behind a NAT/NAPT

 If the end system is behind a NAT (perhaps SG-B), then there is, in
 fact, no way for another end system to address a packet to this end
 system.  Not only is opportunistic encryption impossible, but it is
 also impossible for any communication to be initiated to the end
 system.  It may be possible for this end system to initiate such
 communication.  This creates an asymmetry, but this is common for
 NAPT.

7. Host Implementations

 When Alice and SG-A are components of the same system, they are
 considered to be a host implementation.  The packet sequence scenario
 remains unchanged.
 Components marked Alice are the upper layers (TCP, UDP, the
 application), and SG-A is the IP layer.
 Note that tunnel mode is still required.
 As Alice and SG-A are acting on behalf of themselves, no TXT based
 delegation record is necessary for Alice to initiate.  She can rely
 on FQDN in a forward map.  This is particularly attractive to mobile
 nodes such as notebook computers at conferences.  To respond,
 Alice/SG-A will still need an entry in Alice's reverse-map.

8. Multi-Homing

 If there are multiple paths between Alice and Bob (as illustrated in
 the diagram with SG-D), then additional DNS records are required to
 establish authorization.
 In Figure 1, Alice has two ways to exit her network: SG-A and SG-D.
 Previously, SG-D has been ignored.  Postulate that there are routers
 between Alice and her set of security gateways (denoted by the +
 signs and the marking of an autonomous system number for Alice's
 network).  Datagrams may, therefore, travel to either SG-A or SG-D en
 route to Bob.
 As long as all network connections are in good order, it does not
 matter how datagrams exit Alice's network.  When they reach either
 security gateway, the security gateway will find the TXT delegation
 record in Bob's reverse-map, and establish an SA with SG-B.
 SG-B has no problem establishing that either of SG-A or SG-D may
 speak for Alice, because Alice has published two equally weighted TXT
 delegation records:

Richardson & Redelmeier Informational [Page 31] RFC 4322 Opportunistic Encryption using IKE December 2005

    X-IPsec-Server(10)=192.1.1.5 AQMM...3s1Q==
    X-IPsec-Server(10)=192.1.1.6 AAJN...j8r9==
        Figure 4: Multiple gateway delegation example for Alice
 Alice's routers can now do any kind of load sharing needed.  Both
 SG-A and SG-D send datagrams addressed to Bob through their tunnel to
 SG-B.
 Alice's use of non-equal weight delegation records to show preference
 of one gateway over another, has relevance only when SG-B is
 initiating to Alice.
 If the precedences are the same, then SG-B has a more difficult time.
 It must decide which of the two tunnels to use.  SG-B has no
 information about which link is less loaded, nor which security
 gateway has more cryptographic resources available.  SG-B, in fact,
 has no knowledge of whether both gateways are even reachable.
 The Public Internet's default-free zone may well know a good route to
 Alice, but the datagrams that SG-B creates must be addressed to
 either SG-A or SG-D; they can not be addressed to Alice directly.
 SG-B may make a number of choices:
 1.  It can ignore the problem and round robin among the tunnels.
     This causes losses during times when one or the other security
     gateway is unreachable.  If this worries Alice, she can change
     the weights in her TXT delegation records.
 2.  It can send to the gateway from which it most recently received
     datagrams.  This assumes that routing and reachability are
     symmetrical.
 3.  It can listen to BGP information from the Internet to decide
     which system is currently up.  This is clearly much more
     complicated, but if SG-B is already participating in the BGP
     peering system to announce Bob, the results data may already be
     available to it.
 4.  It can refuse to negotiate the second tunnel.  (It is unclear
     whether or not this is even an option.)
 5.  It can silently replace the outgoing portion of the first tunnel
     with the second one while still retaining the incoming portions
     of both.  Thus, SG-B can accept datagrams from either SG-A or
     SG-D, but send only to the gateway that most recently re-keyed
     with it.

Richardson & Redelmeier Informational [Page 32] RFC 4322 Opportunistic Encryption using IKE December 2005

 Local policy determines which choice SG-B makes.  Note that even if
 SG-B has perfect knowledge about the reachability of SG-A and SG-D,
 Alice may not be reachable from either of these security gateways
 because of internal reachability issues.
 FreeS/WAN implements option 5.  Implementing a different option is
 being considered.  The multi-homing aspects of OE are not well
 developed and may be the subject of a future document.

9. Failure Modes

9.1. DNS Failures

 If a DNS server fails to respond, local policy decides whether or not
 to permit communication in the clear as embodied in the connection
 classes in Section 3.2.  It is easy to mount a denial of service
 attack on the DNS server responsible for a particular network's
 reverse-map.  Such an attack may cause all communication with that
 network to go in the clear if the policy is permissive, or fail
 completely if the policy is paranoid.  Please note that this is an
 active attack.
 There are still many networks that do not have properly configured
 reverse-maps.  Further, if the policy is not to communicate, the
 above denial of service attack isolates the target network.
 Therefore, the decision of whether or not to permit communication in
 the clear MUST be a matter of local policy.

9.2. DNS Configured, IKE Failures

 DNS records claim that opportunistic encryption should occur, but the
 target gateway either does not respond on port 500, or refuses the
 proposal.  This may be because of a crash or reboot, a faulty
 configuration, or a firewall filtering port 500.
 The receipt of ICMP port, host or network unreachable messages
 indicates a potential problem, but MUST NOT cause communication to
 fail immediately.  ICMP messages are easily forged by attackers.  If
 such a forgery caused immediate failure, then an active attacker
 could easily prevent any encryption from ever occurring, possibly
 preventing all communication.
 In these situations a log should be produced and local policy should
 dictate if communication is then permitted in the clear.

Richardson & Redelmeier Informational [Page 33] RFC 4322 Opportunistic Encryption using IKE December 2005

9.3. System Reboots

 Tunnels sometimes go down because the remote end crashes,
 disconnects, or has a network link break.  In general there is no
 notification of this.  Even in the event of a crash and successful
 reboot, other SGs don't hear about it unless the rebooted SG has
 specific reason to talk to them immediately.  Over-quick response to
 temporary network outages is undesirable.  Note that a tunnel can be
 torn down and then re-established without any effect visible to the
 user except a pause in traffic.  On the other hand, if one end
 reboots, the other end can't get datagrams to it at all (except via
 IKE) until the situation is noticed.  So a bias toward quick response
 is appropriate, even at the cost of occasional false alarms.
 A mechanism for recovery after reboot is a topic of current research
 and is not specified in this document.
 A deliberate shutdown should include an attempt, using delete
 messages, to notify all other SGs currently connected by phase 1 SAs
 that communication is about to fail.  Again, a remote SG will assume
 this is a teardown.  Attempts by the remote SGs to negotiate new
 tunnels as replacements should be ignored.  When possible, SGs should
 attempt to preserve information about currently-connected SGs in
 non-volatile storage, so that after a crash, an Initial-Contact can
 be sent to previous partners to indicate loss of all previously
 established connections.

10. Unresolved Issues

10.1. Control of Reverse DNS

 The method of obtaining information by reverse DNS lookup causes
 problems for people who cannot control their reverse DNS bindings.
 This is an unresolved problem in this version, and is out of scope.

11. Examples

11.1. Clear-Text Usage (Permit Policy)

 Two example scenarios follow.  In the first example, GW-A (Gateway A)
 and GW-B (Gateway B) have always-clear-text policies, and in the
 second example they have an OE policy.  The clear-text policy serves
 as a reference for what occurs in TCP/IP in the absence of
 Opportunistic Encryption.
 Alice wants to communicate with Bob.  Perhaps she wants to retrieve a
 web page from Bob's web server.  In the absence of opportunistic
 encryptors, the following events occur:

Richardson & Redelmeier Informational [Page 34] RFC 4322 Opportunistic Encryption using IKE December 2005

   Alice         SG-A       DNS       SG-B           Bob
    Human or application
    'clicks' with a name.
    (1)
  1. —–(2)————–>

Application looks up

     name in DNS to get
     IP address.
     <-----(3)---------------
     Resolver returns "A" RR
     to application with IP
     address.
    (4)
    Application starts a TCP session
    or UDP session and OS sends
    first datagram
   Alice         SG-A       DNS       SG-B           Bob
        ----(5)----->
        Datagram is seen at first gateway
        from Alice (SG-A).
  1. ———(6)——>

Datagram traverses

                    network.
  1. —–(7)—–>

Datagram arrives

                                        at Bob, is provided
                                        to TCP.
                                       <------(8)------
                                        A reply is sent.
                    <----------(9)------
                    Datagram traverses
                    network.
     <----(10)-----
     Alice receives
     answer.
   Alice         SG-A       DNS       SG-B           Bob
    (11)----------->
     A second exchange
     occurs.

Richardson & Redelmeier Informational [Page 35] RFC 4322 Opportunistic Encryption using IKE December 2005

  1. ———(12)—–>
    1. ————→

←————–

                    <-------------------
     <-------------
              Figure 5: Timing of regular transaction

11.2. Opportunistic Encryption

 In the presence of properly configured opportunistic encryptors, the
 event list is extended.  Only changes are annotated.
 The following symbols are used in the time-sequence diagram:
  1. A single dash represents clear-text datagrams.

= An equals sign represents phase 2 (IPsec) cipher-text datagrams.

 ~  A single tilde represents clear-text phase 1 datagrams.
 #  A hash sign represents phase 1 (IKE) cipher-text datagrams.

Richardson & Redelmeier Informational [Page 36] RFC 4322 Opportunistic Encryption using IKE December 2005

   Alice          SG-A      DNS       SG-B           Bob
    (1)
     ------(2)-------------->
     <-----(3)---------------
    (4)----(5)----->+
                   ----(5B)->
                   <---(5C)--
                   ~~~~~~~~~~~~~(5D)~~~>
                   <~~~~~~~~~~~~(5E)~~~~
                   ~~~~~~~~~~~~~(5F)~~~>
                   <~~~~~~~~~~~~(5G)~~~~
                   #############(5H)###>
                            <----(5I)---
                            -----(5J)-->
                   <############(5K)####
                   #############(5L)###>
                            <----(5M)---
                            -----(5N)-->
                   <############(5O)####
                   #############(5P)###>
                    ============(6)====>
                                        ------(7)----->
                                       <------(8)------
                   <==========(9)======
     <-----(10)----
    (11)----------->
                    ==========(12)=====>
                                        -------------->
                                       <---------------
                    <===================
     <-------------
       Figure 6: Timing of opportunistic encryption transaction

Richardson & Redelmeier Informational [Page 37] RFC 4322 Opportunistic Encryption using IKE December 2005

 For the purposes of this section, we will describe only the changes
 that occur between Figure 5 and Figure 6.  This corresponds to time
 points 5, 6, 7, 9, and 10 on the list above.
 At point (5), SG-A intercepts the datagram because this
 source/destination pair lacks a policy (the nonexistent policy
 state).  SG-A creates a hold policy, and buffers the datagram.  SG-A
 requests keys from the keying daemon.
 (5B) DNS query for TXT record.
 (5C) DNS response for TXT record.
 (5D) Initial IKE message to responder.
 (5E) Message 2 of phase 1 exchange.
      SG-B receives the message.  A new connection instance is created
      in the unauthenticated OE peer state.
 (5F) Message 3 of phase 1 exchange.
      SG-A sends a Diffie-Hellman exponent.  This is an internal state
      of the keying daemon.
 (5G) Message 4 of phase 1 exchange.
      SG-B responds with a Diffie-Hellman exponent.  This is an
      internal state of the keying protocol.
 (5H) Message 5 of phase 1 exchange.
      SG-A uses the phase 1 SA to send its identity under encryption.
      The choice of identity is discussed in Section 4.6.1.  This is
      an internal state of the keying protocol.
 (5I) Responder lookup of initiator key.  SG-B asks DNS for the public
      key of the initiator.  DNS looks for a KEY record by IP address
      in the reverse-map.  That is, a KEY resource record is queried
      for 4.1.1.192.in-addr.arpa (recall that SG-A's external address
      is 192.1.1.4).  SG-B uses the resulting public key to
      authenticate the initiator.  See Section 5.1 for further
      details.
 (5J) DNS replies with public key of initiator.
      Upon successfully authenticating the peer, the connection
      instance makes a transition to authenticated OE peer on SG-B.
      The format of the TXT record returned is described in
      Section 5.2.
      Responder replies with ID and authentication.
      SG-B sends its ID along with authentication material, completing
      the phase 1 negotiation.
 (5L) IKE phase 2 negotiation.
      Having established mutually agreeable authentications (via KEY)
      and authorizations (via TXT), SG-A proposes to create an IPsec
      tunnel for datagrams transiting from Alice to Bob.  This tunnel
      is established only for the Alice/Bob combination, not for any
      subnets that may be behind SG-A and SG-B.

Richardson & Redelmeier Informational [Page 38] RFC 4322 Opportunistic Encryption using IKE December 2005

 (5M) Authorization for SG-A to speak for Alice.
      While the identity of SG-A has been established, its authority
      to speak for Alice has not yet been confirmed.  SG-B does a
      reverse lookup on Alice's address for a TXT record.
 (5N) Responder determines initiator's authority.
      A TXT record is returned.  It confirms that SG-A is authorized
      to speak for Alice.
      Upon receiving this specific proposal, SG-B's connection
      instance makes a transition into the potential OE connection
      state.  SG-B may already have an instance, and the check is made
      as described above.
 (5O) Responder agrees to proposal.
      SG-B, satisfied that SG-A is authorized, proceeds with the
      phase 2 exchange.
      The responder MUST setup the inbound IPsec SAs before sending
      its reply.
 (5P) Final acknowledgement from initiator.
      The initiator agrees with the responder's choice of proposal and
      sets up the tunnel.  The initiator sets up the inbound and
      outbound IPsec SAs.
      Upon receipt of this message, the responder may now setup the
      outbound IPsec SAs.
 (6)  IPsec succeeds and sets up a tunnel for communication between
      Alice and Bob.
    SG-A sends the datagram saved at step (5) through the newly
    created tunnel to SG-B, where it gets decrypted and forwarded.
    Bob receives it at (7) and replies at (8).  SG-B already has a
    tunnel up with G1 and uses it.  At (9), SG-B has already
    established an SPD entry mapping Bob->Alice via a tunnel, so this
    tunnel is simply applied.  The datagram is encrypted to SG-A,
    decrypted by SG-A, and passed to Alice at (10).

12. Security Considerations

12.1. Configured versus Opportunistic Tunnels

 Configured tunnels are setup using bilateral mechanisms: exchanging
 public keys (raw RSA, DSA, PKIX), pre-shared secrets, or by
 referencing keys that are in known places (distinguished name from
 LDAP, DNS).  These keys are then used to configure a specific tunnel.
 A pre-configured tunnel may be on all the time, or may be keyed only
 when needed.  The endpoints of the tunnel are not necessarily static;
 many mobile applications (road warrior) are considered to be
 configured tunnels.

Richardson & Redelmeier Informational [Page 39] RFC 4322 Opportunistic Encryption using IKE December 2005

 The primary characteristic is that configured tunnels are assigned
 specific security properties.  They may be trusted in different ways
 relating to exceptions to firewall rules, exceptions to NAT
 processing, and to bandwidth or other quality of service
 restrictions.
 Opportunistic tunnels are not inherently trusted in any strong way.
 They are created without prior arrangement.  As the two parties are
 strangers, there MUST be no confusion of datagrams that arrive from
 opportunistic peers and those that arrive from configured tunnels.  A
 security gateway MUST take care that an opportunistic peer cannot
 impersonate a configured peer.
 Ingress filtering MUST be used to make sure that only datagrams
 authorized by negotiation (and the concomitant authentication and
 authorization) are accepted from a tunnel.  This is to prevent one
 peer from impersonating another.
 An implementation suggestion is to treat opportunistic tunnel
 datagrams as if they arrive on a logical interface distinct from
 other configured tunnels.  As the number of opportunistic tunnels
 that may be created automatically on a system is potentially very
 high, careful attention to scaling should be taken into account.
 As with any IKE negotiation, opportunistic encryption cannot be
 secure without authentication.  Opportunistic encryption relies on
 DNS for its authentication information and, therefore, cannot be
 fully secure without a secure DNS.  Without secure DNS, opportunistic
 encryption can protect against passive eavesdropping but not against
 active man-in-the-middle attacks.

12.2. Firewalls versus Opportunistic Tunnels

 Typical usage of per datagram access control lists is to implement
 various kinds of security gateways.  These are typically called
 "firewalls".
 Typical usage of a virtual private network (VPN) within a firewall is
 to bypass all or part of the access controls between two networks.
 Additional trust (as outlined in the previous section) is given to
 datagrams that arrive in the VPN.
 Datagrams that arrive via opportunistically configured tunnels MUST
 not be trusted.  Any security policy that would apply to a datagram
 arriving in the clear SHOULD also be applied to datagrams arriving
 opportunistically.

Richardson & Redelmeier Informational [Page 40] RFC 4322 Opportunistic Encryption using IKE December 2005

12.3. Denial of Service

 There are several different forms of denial of service that an
 implementor should be concerned with.  Most of these problems are
 shared with security gateways that have large numbers of mobile peers
 (road warriors).
 The design of ISAKMP/IKE, and its use of cookies, defend against many
 kinds of denial of service.  Opportunism changes the assumption that
 if the phase 1 (ISAKMP) SA is authenticated, that it was worthwhile
 creating.  Because the gateway will communicate with any machine, it
 is possible to form phase 1 SAs with any machine on the Internet.

13. Acknowledgements

 Substantive portions of this document are based upon previous work by
 Henry Spencer.  [OEspec]
 Thanks to Tero Kivinen, Sandy Harris, Wes Hardarker, Robert
 Moskowitz, Jakob Schlyter, Bill Sommerfeld, John Gilmore, and John
 Denker for their comments and constructive criticism.
 Sandra Hoffman and Bill Dickie did the detailed proof reading and
 editing.

14. References

14.1. Normative References

 [RFC1035]  Mockapetris, P., "Domain names - implementation and
            specification", STD 13, RFC 1035, November 1987.
 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC2401]  Kent, S. and R. Atkinson, "Security Architecture for the
            Internet Protocol", RFC 2401, November 1998.
 [RFC2407]  Piper, D., "The Internet IP Security Domain of
            Interpretation for ISAKMP", RFC 2407, November 1998.
 [RFC2408]  Maughan, D., Schneider, M., and M. Schertler, "Internet
            Security Association and key Management Protocol
            (ISAKMP)", RFC 2408, November 1998.
 [RFC2409]  Harkins, D. and D. Carrel, "The Internet key Exchange
            (IKE)", RFC 2409, November 1998.

Richardson & Redelmeier Informational [Page 41] RFC 4322 Opportunistic Encryption using IKE December 2005

 [RFC2535]  Eastlake, D., "Domain Name System Security Extensions",
            RFC 2535, March 1999.
 [RFC3110]  Eastlake, D., "RSA/SHA-1 SIGs and RSA KEYs in the Domain
            Name System (DNS)", RFC 3110, May 2001.

14.2. Informative References

 [IPSECKEY] Richardson, M., "A Method for Storing IPsec keying
            Material in DNS", RFC 4025, March 2005.
 [OEspec]   H. Spencer and Redelmeier, D., "Opportunistic Encryption",
            paper, http://www.freeswan.org/
            oeid/opportunism-spec.txt, May 2001.
 [RFC0791]  Postel, J., "Internet Protocol", STD 5, RFC 791, September
            1981.
 [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
            STD 13, RFC 1034, November 1987.
 [RFC1464]  Rosenbaum, R., "Using the Domain Name System To Store
            Arbitrary String Attributes", RFC 1464, May 1993.
 [RFC1812]  Baker, F., "Requirements for IP Version 4 Routers", RFC
            1812, June 1995.
 [RFC1984]  IAB, IESG, Carpenter, B., and F. Baker, "IAB and IESG
            Statement on Cryptographic Technology and the Internet",
            RFC 1984, August 1996.
 [RFC2367]  McDonald, D., Metz, C. and B. Phan, "PF_KEY Key Management
            API, Version 2", RFC 2367, July 1998.
 [RFC2538]  Eastlake, D. and O. Gudmundsson, "Storing Certificates in
            the Domain Name System (DNS)", RFC 2538, March 1999.
 [RFC2663]  Srisuresh, P. and M. Holdrege, "IP Network Address
            Translator (NAT) Terminology and Considerations", RFC
            2663, August 1999.
 [RFC2671]  Vixie, P., "Extension Mechanisms for DNS (EDNS0)", RFC
            2671, August 1999.
 [RFC3330]  IANA, "Special-Use IPv4 Addresses", RFC 3330, September
            2002.

Richardson & Redelmeier Informational [Page 42] RFC 4322 Opportunistic Encryption using IKE December 2005

 [RFC3445]  Massey, D. and S. Rose, "Limiting the Scope of the KEY
            Resource Record (RR)", RFC 3445, December 2002.
 [RFC3526]  Kivinen, T. and M. Kojo, "More Modular Exponential (MODP)
            Diffie-Hellman groups for Internet Key Exchange (IKE)",
            RFC 3526, May 2003.
 [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
            Rose, "DNS Security Introduction and Requirements", RFC
            4033, March 2005.

Authors' Addresses

 Michael C. Richardson
 Sandelman Software Works
 470 Dawson Avenue
 Ottawa, ON  K1Z 5V7
 CA
 EMail: mcr@sandelman.ottawa.on.ca
 URI:   http://www.sandelman.ottawa.on.ca/
 D. Hugh Redelmeier
 Mimosa Systems Inc.
 29 Donino Avenue
 Toronto, ON  M4N 2W6
 CA
 EMail: hugh@mimosa.com

Richardson & Redelmeier Informational [Page 43] RFC 4322 Opportunistic Encryption using IKE December 2005

Full Copyright Statement

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 This document is subject to the rights, licenses and restrictions
 contained in BCP 78 and at www.rfc-editor.org/copyright.html, and
 except as set forth therein, the authors retain all their rights.
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Acknowledgement

 Funding for the RFC Editor function is currently provided by the
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Richardson & Redelmeier Informational [Page 44]

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