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Versions: 00 01 02 03 04 05 06 07 RFC 5387

BTNS WG                                     J. Touch, D. Black, Y. Wang
Internet Draft                                          USC/ISI and EMC
Intended status: Informational                        February 13, 2007
Expires: August 2007



                    Problem and Applicability Statement
                  for Better Than Nothing Security (BTNS)
                  draft-ietf-btns-prob-and-applic-05.txt


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   This Internet-Draft will expire on August 13, 2007.

Copyright Notice

   Copyright (C) The IETF Trust (2007).

Abstract

   The Internet network security protocol suite, IPsec, consisting of
   IKE, ESP, and AH, generally requires authentication of network layer
   entities to bootstrap security. This authentication can be based on
   mechanisms such as pre-shared symmetric keys, certificates and



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   associated asymmetric keys, or the use of Kerberos.  The need to
   deploy authentication information and its associated identities to
   network layer entities can be a significant obstacle to use of
   network security.  This document explains the rationale for extending
   the Internet network security suite to enable use of IPsec security
   mechanisms without authentication. These extensions are intended to
   protect communication in a "better than nothing" (BTNS) fashion. The
   extensions may be used on their own (Stand Alone BTNS, or SAB), or
   may be useful in providing network layer security that can be
   authenticated by higher layers in the protocol stack, called Channel
   Bound BTNS (CBB). This document also explains situations in which use
   of SAB and CBB extensions are appropriate.

Conventions used in this document

   In examples, "C:" and "S:" indicate lines sent by the client and
   server respectively.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC-2119 Error!
   Reference source not found..

Table of Contents


   1. Introduction...................................................3
   2. Problem Statement..............................................5
      2.1. Network Layer.............................................5
         2.1.1. Authentication Identities............................5
         2.1.2. Authentication Methods...............................5
         2.1.3. Current IPsec Limits on Unauthenticated Peers........6
      2.2. Upper Layer...............................................6
         2.2.1. Transport Protection from Packet Spoofing............6
         2.2.2. Authentication at Multiple Layers....................8
   3. BTNS-IPsec Overview and Threat Models..........................9
      3.1. BTNS-IPsec Overview.......................................9
      3.2. BTNS-IPsec Security Services.............................10
      3.3. BTNS-IPsec Modes.........................................11
   4. Applicability Statement.......................................12
      4.1. Benefits.................................................13
      4.2. Vulnerabilities..........................................13
      4.3. Stand-Alone BTNS (SAB)...................................14
         4.3.1. Symmetric SAB.......................................14
         4.3.2. Asymmetric SAB......................................14
      4.4. Channel-Bound BTNS (CBB).................................15
      4.5. Summary of Uses, Vulnerabilities, and Benefits...........16


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   5. Security Considerations.......................................16
      5.1. Threat Models and Evaluation.............................16
      5.2. Interaction with Other Extant Security...................17
      5.3. MITM and Masquerader Attacks.............................17
      5.4. DoS Attacks and Resource Consumptions....................18
      5.5. Exposure to Anonymous Access.............................19
      5.6. ICMP Attacks.............................................19
      5.7. Leap of Faith............................................19
      5.8. Connection Hijacking through Rekeying....................20
      5.9. Configuration Errors.....................................21
   6. Other Issues and Related Efforts..............................21
      6.1. NAT Traversal............................................21
      6.2. Mobility and Multihoming.................................22
      6.3. Related IETF Efforts.....................................22
   7. IANA Considerations...........................................22
   8. Acknowledgments...............................................22
   9. References....................................................23
      9.1. Normative References.....................................23
      9.2. Informative References...................................23
   Author's Addresses...............................................25
   Intellectual Property Statement..................................25
   Disclaimer of Validity...........................................26

1. Introduction

   Network security is provided by a variety of protocols at different
   layers in the stack. At the network layer, the IPsec protocol suite
   is used to secure IP traffic. IPsec and Internet Key Exchange
   protocol (IKE) present an all-or-nothing alternative by providing
   protection from a wide array of possible threats, but requiring
   authentication [4][8][9][10].  In turn authentication requires
   deployment of network-level authentication credentials, and this can
   be an obstacle to IPsec usage. This document discusses the issues
   with regard to this dependency, and introduces "Better Than Nothing
   Security" (BTNS) as one solution. It also describes various modes of
   BTNS, and explores the characteristics of applications that will
   benefit from using each mode. The remainder of this section provides
   an overview of the background and problem scenario.

   The process of establishing secure network communications consists of
   two functions: policy and mechanism. The policy function determines
   "who" gets which types of security services, and the mechanism
   function applies the selected services to the corresponding
   communication traffic. The requirement for authentication credentials
   stems from the need to verify the "who;" i.e., to authenticate the
   identities of the communicating peers.



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   There are two basic approaches to authentication: using pre-deployed
   information, or employing out-of-band communications. Out-of-band
   authentication can be done through a trusted third party, a separate
   channel to the peer, or the same channel but at a higher layer. It
   requires mechanisms and interfaces to bind the authenticated
   identities to the secure channels, and is out-of-scope for this
   document (although it may be possible to extend the channel binding
   mode of BTNS to work with such mechanisms). Pre-deployed information
   includes pre-shared secrets and credentials authenticated by trusted
   authorities. This form of authentication often requires manual
   deployment and coordination among communicating peers. Furthermore,
   authenticated credentials such as certificates signed by
   certification authorities (CA) can be cumbersome and expensive to
   obtain.

   These factors increase the impact of IKE's requirement for successful
   authentication based on pre-deployed information before security
   services are offered. Consequently, users and applications often do
   not use IPsec to protect the network layer, but rely solely on higher
   layer security protocols or no security at all. As the problem
   statement section will describe, higher layer security protocols may
   not be enough to protect against some network layer spoofing attacks.

   To improve the situation, one could either reduce the hurdles to
   obtain and configure authentication information, or remove
   authentication at the network layer. The latter is the core idea of
   BTNS, which provides anonymous (unauthenticated) keying for IPsec to
   create Security Associations (SAs) with peers who do not possess
   valid authentication credentials. This requires extensions to the
   IPsec architecture and possibly extensions or profiles of IKE. As the
   new BTNS modes in IPsec relax the authentication requirement, the
   impacts, tradeoffs, and risks must be thoroughly understood before
   applying BTNS to any communications. More specifically, this document
   will address the issues on whether and when network layer
   authentication can be removed, the risks of using BTNS, and finally,
   the impacts to the existing IPsec architecture.

   The next section discusses the issues that IKE's strict requirements
   for network layer authentication cause for IPsec. Section 3 provides
   a high level overview of BTNS-IPsec, including the security services
   offered. Section 4 explores the applicability of BTNS-IPsec, followed
   by a discussion of the risks and other security considerations in
   Section 5. Section 6 lists other related efforts.






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2. Problem Statement

   This section describes the problems that motivated the development of
   BTNS. The primary concern is that IPsec is not widely utilized
   despite rigorous development effort and emphasis on network security
   by users and organizations. There are also debates on which layer is
   best for securing network communications, and how security protocols
   at different layers should interact. The following discussion roughly
   categorizes these issues by layers: network layer and higher layers.

2.1. Network Layer

   At the network layer, one of the hurdles is to satisfy the
   authentication requirements of IPsec and IKE. This section
   investigates the problems on network layer authentication and the
   result of this requirement.

2.1.1. Authentication Identities

   Current IPsec authentication supports several types of identities in
   the Peer Authorization Database (PAD): IPv4 addresses, IPv6
   addresses, DNS names, Distinguished Names, RFC 822 email addresses,
   and Key IDs [10]. All require either CA-signed certificates or pre-
   shared secrets to authenticate. These can be roughly categorized into
   network layer identifiers and other identifiers.

   The first three, IPv4/IPv6 addresses and DNS names are network layer
   identifiers. The main issue with IP addresses is that they are no
   longer stable identifiers representing the same physical systems
   consistently due to dynamic address assignments (DHCP) and increases
   in system mobility. DNS names are affected because the name to
   address mapping is not always up to date as a result.

   There are two main drawbacks with other, non-network-layer
   identifiers. It is too restrictive because there are likely other
   forms of identifiers not covered by the PAD specification. It could
   also result in multiple authentications on the same identifiers at
   different layers. In addition, the list of identifiers is not
   complete; some higher layer protocols use additional types of
   identifiers that are not supported by IPsec.  These issues are also
   related to channel binding and will be further discussed later.

2.1.2. Authentication Methods

   As described earlier, CA-signed certificates and pre-shared secrets
   are the only methods of authentications accepted by current IPsec and
   IKE specifications. Pre-shared secrets require manual configuration


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   and out-of-band communications. The verification process of CA-signed
   certificates is cumbersome and there may be a monetary cost in
   obtaining the certificates. The combination of these factors is one
   likely reason why IPsec is not widely used except in environments
   with the highest security requirements.

2.1.3. Current IPsec Limits on Unauthenticated Peers

   Pre-configuration only works if the peer identities are known in
   advance. The lack of unauthenticated IPsec modes prevents secure
   communications at the network layer with unauthenticated or unknown
   peers, even when they are subsequently authenticated in a higher
   layer protocol or application. The lack of a channel binding API
   between IPsec and upper layer protocols further forces such
   communications to completely bypass IPsec, leaving network layer
   unprotected.

2.2. Upper Layer

   For upper layers, the following discussion first focuses on whether
   IPsec is necessary if transport layer security is already in use.
   This would further motivate the need to reduce the hurdle of using
   IPsec. Another issue is regarding authentication at both IPsec and
   higher layer protocols for the same connection.

2.2.1. Transport Protection from Packet Spoofing

   Consider the case of transport protocols. Increases in network
   performance and the use of long-lived connections have resulted in
   increased vulnerability of connection-oriented transport protocols to
   attacks. TCP, like many other protocols, is susceptible to off-path
   third-party attacks, such as injection of a TCP RST [20]. The network
   lacks comprehensive ingress filtering to drop such spoofed traffic.
   These attacks can affect BGP sessions between core Internet routers,
   and are thus of significant concern [2]. As a result, a number of
   proposed solutions have been developed; most of these are transport
   layer solutions.

   Some of these solutions augment the transport protocol by improving
   its own security, e.g., TCP/MD5 [5]. Others modify the core TCP
   processing rules to make it harder for off-path attackers to inject
   meaningful packets either during the initial handshake (e.g.
   SYNcookies) or after a connection is established (e.g., TCPsecure)
   [17][19]. Some of these modifications are new to TCP, but have
   already been incorporated into other transport protocols (e.g., SCTP)
   or intermediate (so-called L3.5) protocols (e.g., HIP) [13][18].



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   The TCP-specific modifications are, at best, temporary patches to the
   ubiquitous vulnerability to spoofing attacks. The obvious solution to
   spoofing is to validate the segments of a connection, either at the
   transport layer or the network layer. The IPsec suite already
   provides authentication of a network layer packet and its contents,
   but the infrastructure required for deployment of IPsec can be
   prohibitive.

   Protecting systems from spoofed packets is ultimately an issue of
   authentication, ensuring that a receiver's interpretation of the
   source of a packet is accurate. Authentication validates the identity
   of the source of the packet. The current IPsec suite assumes that
   identity is validated either by a trusted third party - e.g., a
   certification authority - or by a pre-deployed shared secret. Such an
   identity is unique and invariant across associations (pair-wise
   security configuration), and can be used to reject packets that are
   not authentic.

   There is weaker notion of identity, one which is bootstrapped from
   the session association itself. The identity doesn't mean "Bill
   Smith" or "owner of shared secret X2YQ", but means something more
   like "the entity with which I have been communicating on connection
   #23". Such identity is not invariant across associations, but because
   it is invariant within an association it can still be used to provide
   protection during the lifetime of that association. This is the core
   notion of identity used by BTNS.

   BTNS thus provides a kind of intra-association integrity, a form of
   authentication where the identity is not authenticated across
   separate associations or out-of-band, but does not change during the
   association. This mode of BTNS is called Stand Alone BTNS (SAB),
   because the protection is afforded solely by the use of BTNS
   extensions, without authentication from higher layers in the protocol
   stack.

   With regard to BGP in particular, it has been understood that the use
   of appropriate authentication is the preferred solution [2] to TCP
   spoofing attacks. Supporting authentication, e.g., by using signed
   certificates, at one router does not solve the problem; that router
   is still at the mercy of all routers it peers with, as it depends on
   them to also support authentication. The reality is that few routers
   are configured to support authentication, and the result is the use
   of unsecured TCP for sending BGP packets. BTNS allows an individual
   router to relax the need for authentication, in order to enable the
   use of protected sessions that are not authenticated. The latter is
   "better than nothing" in cases where "nothing" is the alternative.



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2.2.2. Authentication at Multiple Layers

   Some existing protocols used on the Internet provide authentication
   at a layer above the transport, but rely on the IPsec suite for
   packet-by-packet cryptographic integrity and confidentiality
   services.  Examples of such protocols include iSCSI and the CCM mode
   for NFSv4 security [15][16].  With the current IPsec suite, the
   result is two authentications; one at the IPsec layer, using an
   identity for IKE and an associated secret or key, and another by the
   higher layer protocol using a higher layer identity and secret or
   key. This is necessary if the identity and key formats differ between
   IPsec and the higher layer protocol, and because there is no standard
   interface to pass authentication credentials across these layers.
   End-node software is then responsible for making sure that the
   identities used for these two authentications are consistent in some
   fashion, an authorization policy decision.  In principle a single
   authentication should suffice, removing the need for:

   o  the second authentication

   o  configuration and management of the identities and secrets or keys
      for the second authentication

   o  determining in some fashion that the two authenticated identities
      are consistent.  Note that there are significant potential
      vulnerabilities if this is not done.

   IPsec is not always present for these higher layer protocols, and
   even when present, will not always be used.  Hence, if there is a
   choice, the higher layer protocol authentication is preferable as it
   will always be available for use independent of IPsec.

   A "better than nothing" security approach to IPsec can address this
   problem by setting up IPsec without an authentication and then
   extending the higher layer authentication to establish that the
   higher layer protocol session is protected by a single IPsec SA. This
   counters man-in-the-middle (MITM) attacks on BTNS IPsec session
   establishment by terminating the higher layer session when such an
   attack occurs. This approach is based on the fact that an MITM attack
   on a BTNS SA will result in two different BTNS SAs, each connecting
   the MITM to one of the higher layer endpoints. These different SAs
   contribute different cryptographic binding material to the higher
   layer authentication, causing that authentication to fail, which
   should then cause the higher layer protocol session to terminate. In
   contrast to use of IKE authentication, this approach detects the man-
   in-the-middle after the SAs have been set up, and hence does not



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   match IKE's resistance to denial of service attacks at the network
   layer.

   This check is referred in this document as "channel binding", thus
   the name Channel Bound BTNS (CBB) [22]. Channel binding must be done
   in a fashion that prevents a man-in-the-middle attack from changing
   the SA identity when it is checked and from causing two different SAs
   to have the same identity.  Adding the SA identifier to
   authentication mechanisms based on one-way hashes, key exchanges, or
   (public key) cryptographic signatures are three means by which
   channel binding can be accomplished with resistance to man-in-the-
   middle attacks.  This requires that the SA identifier be the same at
   both ends of the SA [22].

   Currently, the IPsec protocol suite does not define the notion of
   channels for channel binding. Such channels can be constructed by
   transport protocols layered above IP through cooperation between
   these protocols and IPsec, to ensure that all packets for a given
   channel are protected by similar SAs, where similar relates to, among
   other things, the IDs of the peers.  Interfaces between applications
   and transport protocols are also needed for communicating channel
   binding data to applications, and for applications to construct their
   own IPsec channels over connection-less, datagram-oriented
   transports.

3. BTNS-IPsec Overview and Threat Models

   This section provides an overview of BTNS-IPsec and the security
   services it offers. It also describes the modes of BTNS-IPsec.

3.1. BTNS-IPsec Overview

   This is an overview of what is needed to enable BTNS-IPsec. The
   detailed specifications of the extensions will be addressed by the
   relevant protocol specifications.

   The main update to IPsec is adding extensions to security policy that
   permit secure communications with un-authenticated peers. These
   extensions are necessary for both IPsec and IKE. For IPsec, the
   extension applies to the PAD, which specifies the forms of
   authentication for each entry ID. In addition to CA-signed X.509
   certificates and pre-shared secrets, the extension adds two more
   categories: un-authenticated (either null or self-signed
   certificates) and channel binding, to support BTNS and BTNS with
   channel binding respectively. For IKE, the AUTH payload should be
   expended to allow either null payload or self-signed certificates to
   match the proposed PAD extensions.


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   The changes to enable channel binding between IPsec and higher layer
   protocols or applications will be more complex than the policy
   extensions above. It will involve specifications of APIs and
   interactions between IPsec and higher layer protocols. This document
   assumes such provisions will eventually be developed, but does not
   address their details.

3.2. BTNS-IPsec Security Services

   The changes and extensions of BTNS primarily affect policy as
   described above. Other parts of IPsec and IKE specifications are
   unchanged. BTNS-IPsec does not establish nor does it require a
   separate IPsec context. It is integrated with any existing IPsec
   context in a system. The scope of BTNS-IPsec applies only to the SAs
   matching the policies that explicitly specify or enable BTNS modes in
   the PAD. All other non-BTNS policy entries, including entries in the
   SPD and the PAD, and any non-BTNS SAs will not be affected by BTNS-
   IPsec in terms of security services and requirements.

   In principle, the result of removing authentication at the network
   layer is that BTNS-IPsec can establish secure connections in a
   fashion similar to regular IPsec and IKE, but it cannot verify or
   authenticate the peer identities of these secure connections. The
   following is a list of security services offered by IPsec protocol
   suite. The notes address only the differences when applied to BTNS-
   IPsec.

   1. Access Control

      Because BTNS-IPsec is integrated with any existing IPsec
      contexts, the same access control mechanisms apply to BTNS-IPsec
      entries in all relevant databases except that the entity IDs for
      BTNS in the PAD are not authenticated. Channel bound BTNS can
      authenticate after the secure connection is established at the
      network layer.

   2. Connectionless Integrity

   3. Data Origin Authentication

   4. Anti-Replay Protection

   5. Confidentiality

   6. (Limited) Traffic Flow Confidentiality

      For the remaining security services offered by IPsec, items 2


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      through 6, it is possible to establish secure connections with
      rogue peers in BTNS-IPsec because authentication is not required.
      But once a secure connection is established, the communication is
      afforded the same security services as regular IPsec.

3.3. BTNS-IPsec Modes

   The previous sections have described two ways of using BTNS: Stand-
   alone (SAB) or with Channel Binding (CBB). It can also be used either
   symmetrically, where both parties lack network layer authentication
   information, or asymmetrically, where only one party lacks the
   ability to authenticate at the network layer. There are a number of
   cases to consider, based on the matrix of the endpoint security
   capabilities of SAB, CBB, and conventional authentication (denoted as
   IKE below). The following table shows all the combinations based on
   the capabilities of the two security endpoints:

           |  IKE  |  SAB  |                | CB-IKE |   CBB   |
      -----+-------+-------+         -------+--------+---------+
           |       |       |                |        |         |
      IKE  |  IKE  | A-SAB |         CB-IKE | CB-IKE |  A-CBB  |
           |       |       |                |        |         |
      -----+-------+-------+         -------+--------+---------+
           |       |       |                |        |         |
      SAB  | A-SAB | S-SAB |           CBB  |  A-CBB |  S-CBB  |
           |       |       |                |        |         |
      -----+-------+-------+         -------+--------+---------+

        No Channel Binding               With Channel Binding

   The first three modes consist of network layer authentication schemes
   used without channel binding to higher layer authentication:

   1. IKE: both sides possess conventional, IKE-supported authentication

   2. Symmetric SAB (S-SAB, or just SAB): both sides lack network layer
      authentication information

   3. Asymmetric SAB (A-SAB): one side lacks network layer
      authentication information, but the other possesses it

   The following modes are the same as above at the network layer, but
   used with channel binding to higher layer authentication credentials:

   4. CB-IKE: this is the case where channel binding is used with
      conventional IKE-authenticated IPsec SAs



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   5. Symmetric CBB (S-CBB, or just CBB): both sides lack network layer
      authentication information, but channel binding is used to bind
      the SAs with higher layer authentication credentials

   6. Asymmetric CBB (A-CBB): this is asymmetric SAB (A-SAB) used with
      channel binding; at the network layer, one side lacks network
      layer authentication information and the other possesses IKE-
      supported authentication, and channel binding is used to bind the
      secure channel to higher layer authentication credentials

   There are three security mechanisms involved in BTNS with channel
   binding: BTNS-IPsec at the network layer, higher layer
   authentication, and the channel binding mechanisms that bind the
   higher layer authentication credentials with the secure channel (or
   its corresponding abstract, cryptographic representation) presented
   by BTNS-IPsec. Both BTNS-IPsec and the higher layer authentication
   can be either symmetric or asymmetric, when one side lacks properly
   authenticated credentials at either layer. The channel binding
   mechanisms, however, must be applied at both ends of the
   communication to prevent MITM attacks. Existing channel binding
   mechanisms and APIs for this purpose, such as defined in GSS-API
   [12], mandate the exchange and verification of the channel binding
   values at both ends to ensure that correct, non-spoofed channel
   characteristics are bound to the higher layer authentication.

4. Applicability Statement

   BTNS is intended for services open to the public but for which
   protected associations are desired, or for services that can be
   authenticated at higher layers in the protocol stack. BTNS can also
   provide some level of protection for private services when the
   alternative is no protection at all (as in the case of BGP, for
   instance).

   BTNS-IPsec uses the IPsec protocol suite, therefore should not be
   used in situations where IPsec or IKE are unsuitable. IPsec and IKE
   incur additional computation overhead, and IKE further requires extra
   message exchanges and round-trip times to setup security
   associations. These are generally undesirable in environments with
   limited computational resources and/or high communication latencies.

   This section provides an overview of the types of applications
   suitable for various modes of BTNS. The next two sections describe
   the overall benefits and vulnerabilities, followed by the
   applicability analysis for each BTNS-IPsec mode. The applicability
   statement covers BTNS-specific modes. IKE and CB-IKE are out of scope
   for this discussion.


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4.1. Benefits

   BTNS protects associations once established. It reduces vulnerability
   after associations have been established to attacks from parties not
   participating in the association. BTNS-IPsec protects network and
   transport layers without requiring network layer authentication
   information. It can be deployed without pre-deployment of
   authentication material for IPsec or pre-shared information, and
   protects all transport layer protocols using a single mechanism.

   BTNS also helps protect systems from low-effort attacks on sessions
   or connections involving higher levels of resources. It raises the
   level of effort for many types of network or transport layer attacks.
   Casual, simple packet attacks need to be augmented to full
   associations and connection establishment for SAB, so that an
   attacker must perform as much work as regular host. SAB thus raises
   the effort for a DDoS attack to that of emulating a flash crowd. For
   open services, there may be no way to distinguish such a DDoS attack
   from a legitimate flash crowd anyway.

   BTNS also allows individual associations to be protected without
   requiring pre-deployed authentication credentials. We anticipate that
   it will use the extant, ephemeral Diffie-Hellman exchange employed in
   IKE to establish pairwise secret keys between ends of an association,
   effectively removing the authentication responsibility from IKE.

4.2. Vulnerabilities

   BTNS removes network layer authentication. Hosts connecting to BTNS
   hosts are vulnerable to communicating with a masquerader throughout
   the association for SAB, or until higher layers provide additional
   authentication for CBB. As a result, authentication data (e.g.,
   passwords) sent to a masquerading peer could be disclosed to an
   attacker. This is a deliberate design tradeoff; in BTNS, network and
   transport layer access is no longer gated by the identity presented
   by the other host, so this opens hosts to masquerading and flash
   crowd attacks. Conversely, BTNS secures connections to hosts that
   cannot authenticate at the network layer, so the network and
   transport layers are more protected.

   Lacking network layer authentication information, other means must be
   used to provide access control for local resources. Traffic selectors
   of the BTNS SPD entries can be used to limit which interfaces,
   address ranges, and port ranges can access BTNS-enabled services.
   Rate limiting can further restrict resource usage. For SAB, these
   protections need to be considered throughout associations, whereas
   for CBB they need be present only until higher layer protocols


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   provide the missing authentication. CCB also relies on the
   effectiveness of the binding of higher layer authentication to the
   BTNS network association.

4.3. Stand-Alone BTNS (SAB)

   SAB is intended for applications without IKE-compatible
   authentication credentials and without any higher layer protection.
   It is also suitable when the identities of either party are not
   important, or are deliberately omitted. This section discusses
   symmetric and asymmetric SAB.

4.3.1. Symmetric SAB

   Symmetric SAB (S-SAB) assumes that both parties lack network layer
   authentication information and that authentication is not available
   from higher layer protocols. S-SAB can still protect network and
   transport protocols, but does not provide authentication at all. It
   is useful where large files or long connections would benefit from
   not being interrupted by DoS attacks, but where the particular
   endpoint identities are not important.

   Open services, such as web servers, and peer-to-peer networks could
   utilize S-SAB when their identities need not be authenticated, but
   where the communication would benefit from protection. Such services
   might provide files either not validated or validated by other means
   (e.g., published MD5 hashes). These transmissions present a target
   for off-path attacks, which could be mitigated by the use of S-SAB.
   S-SAB may also be useful for protecting the transport of voice-over-
   IP (VoIP) between peers, such as direct calls between VoIP clients.

   SAB is also useful in protecting any transport protocol when the
   endpoints decide not to deploy authentication, for whatever reason.
   This is the particular case for BGP TCP connections between core
   routers, where the protection afforded by S-SAB is better than no
   protection at all, even though BGP is not intended as an open
   service.

4.3.2. Asymmetric SAB

   Asymmetric SAB (A-SAB) allows one party lacking network layer
   authentication information to establish associations with another
   party that possesses authentication credentials, the latter by any
   applicable IKE authentication mechanisms.

   Asymmetric SAB is useful for protecting transport connections for
   open services on the Internet, e.g., commercial web servers, etc. In


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   these cases, the server is typically authenticated by a widely known
   CA, as is done with TLS at the application layer, but the clients
   need not be authenticated [3]. Although this may result in IPsec and
   TLS being used on the same connection, it is necessary because TLS
   does not protect from certain spoofing attacks as described in the
   problem statement section (e.g., TLS cannot prevent a spoofed TCP
   RST, as the RST is processed by TCP instead of being passed to TLS).

   A-SAB also secures transport for streaming media as would be used to
   view broadcast streaming such as webcasts for remote education and
   entertainment.

4.4. Channel-Bound BTNS (CBB)

   CBB allows hosts without network layer authentication information to
   cryptographically bind the BTNS-IPsec channels with authentication at
   higher layers. It is intended for applications with higher layer
   authentication, but could benefit from additional network layer
   security to enhance protection. CBB decouples authentication from
   network layer security services. With CBB, applications with IKE-
   incompatible authentication credentials can access security services
   provided by the IPsec security suite. CBB allows IPsec to work with
   more authentication mechanisms, and frees higher layer applications
   and protocols from duplicating security services already available in
   IPsec.

   Symmetric CBB integrates channel binding with S-SAB, as does
   asymmetric CBB with A-SAB. Their target applications have similar
   characteristics at the network layer to their non-channel-binding
   counterparts. The only exception is the binding of authentication
   credentials at higher layer to the resulting IPsec channels.

   Although the modes of CBB refer to the authentication at the network
   layer, higher layer authentication can also be either asymmetric
   (one-way) or symmetric (two-way). Asymmetric CBB can be used to
   complement one-way authentication at higher layer by providing one-
   way authentication of the opposite direction at the network layer.
   Consider an application with one-way, client-only authentication. The
   client can utilize A-CBB where the server must present IKE-
   authenticated credentials at the network layer. This form of A-CBB
   achieves mutual authentication albeit at separate layers. Many remote
   file system protocols, such as iSCSI and NFS, fit into this category,
   and can benefit from channel binding with IPsec for better network
   layer protection and to ensure no MITM attacks.

   Mechanisms and interfaces for channel binding with IPsec are
   discussed in further detail in [22].


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4.5. Summary of Uses, Vulnerabilities, and Benefits

   The following is a summary of the properties of each type of BTNS
   based on the previous subsections:

                 SAB                          CBB
     --------------------------------------------------------------
     Uses     Open services                Same as SAB but plus
              Peer-to-peer                 higher layer auth, e.g.
              Zero-config Infrastructure   iSCSI [15], Kerberos [11]

     Vuln.    Masqueraders                 Masqueraders until bound
              Needs data rate limit        Needs data rate limit
              Load on IPsec                Load on IPsec
              Exposure to open access

     Benefit  Protects L3 & L4             Protects L3 & L4
              Avoids all auth. keys        Avoids L3 auth keys
                                           Full auth. once bound


   These issues were mostly noted in previous sections; some of the more
   generic issues, such as the increased load on IPsec processing, are
   addressed in the Security Considerations section of this document.

5. Security Considerations

   This section presents the threat models for BTNS, and discusses other
   security issues based on the threat models for different modes of
   BTNS. Some of the issues were mentioned previously in the document,
   but are listed again for completeness.

5.1. Threat Models and Evaluation

   BTNS is intended to protect sessions from a variety of threats,
   including on-path, man-in-the-middle attacks after key exchange,
   other on-path attacks after key exchange, and off-path attacks. It is
   intended to protect the contents of a session once established using
   a "leap of faith" authentication during session establishment, but
   does not protect session establishment itself.

   BTNS is not intended to protect the key exchange itself, so this
   presents an opportunity for a man-in-the-middle attack or a well-
   timed attack from other sources. Furthermore, Stand-alone BTNS is not
   intended to protect the endpoint from nodes masquerading as
   legitimate clients. Channel-Bound BTNS can protect from such



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   masquerading, though at a later point after the security association
   is established.

   BTNS is also not intended to protect from DoS attacks that seek to
   overload a CPU performing authentication and other security
   computations, nor is it intended to protect from configuration
   mistakes. These final assumptions are the same as that of the IP
   network protocol security suite. Finally, manual keying is not
   considered in because it is unsafe for protocols that exchange large
   amounts of traffic such as IP Storage (e.g., RFC-3723 forbids use of
   manual keying with the IP Storage protocols) [1].

   The following sections discuss the implications of the threat models
   in more details.

5.2. Interaction with Other Extant Security

   As with any aspect of network security, the use of BTNS must not
   interfere with extant security services. Within an IPsec context, the
   scope of BTNS must be limited to the SPD and PAD entries that
   explicitly specify BTNS, and to the resulting SAD entries. It is
   incumbent on system administrators to deploy BTNS only where safe,
   preferably as a substitute to the use of "bypass" which exempts
   specified traffic from IPsec cryptograph protection. In other words,
   BTNS should be used only as a substitute for no security, rather than
   as a substitute for stronger security. This is particularly relevant
   for the use of BTNS for BGP. Full authentication is preferred for
   BGP. When that is not available, other methods, such as IP address
   filtering, can help reduce the vulnerability of SAB to exposure to
   anonymous access.

5.3. MITM and Masquerader Attacks

   Previous sections have described how CBB can counter MITM and
   masquerader attacks, even though BTNS does not protect key exchange
   nor does it authenticate peer identities at the network layer.
   Nonetheless, there are some security issues regarding CBB that must
   be carefully evaluated before deploying BTNS.

   For regular IPsec/IKE, a man in the middle cannot subvert IKE
   authentication, and hence an attempt to attack it via use of two
   separate security associations will cause an IKE authentication
   failure. On the other hand, a man-in-the-middle attack on IPsec with
   CBB is discovered later than if IKE authentication were used. With
   CBB, the BTNS-IKE step will succeed because it is unauthenticated,
   and the security association will be set up. The man in the middle
   will not be discovered until the higher layer authentication fails.


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   There are two security concerns with this approach: possible exposure
   of sensitive authentication information to the attackers, and
   resource consumption before attacks are detected.

   The exposure of information depends on the higher layer
   authentication protocols used in applications. If the higher layer
   authentication requires exchange of sensitive information (e.g.,
   password-derived materials) that can be attacked offline, the
   attackers can gain such information even though they will be
   detected. Therefore, CBB must not be used with higher layer protocols
   that may expose sensitive information during authentication exchange.
   For example, Kerberos V AP exchanges would leak little other than the
   target's krb5 principal name, while Kerberos V AS exchanges using PA-
   ENC-TIMESTAMP pre-authentication would leak material that can then be
   attacked offline. The latter should not be used with BTNS, even with
   Channel Binding. Further, the ways in which BTNS is integrated with
   the higher layer protocol must take into consideration
   vulnerabilities that could be introduced in the APIs between these
   two systems or in the information that they share.

   The resource consumption issue is addressed in the next section on
   DoS attacks.

5.4. DoS Attacks and Resource Consumptions

   BTNS deployment means that more traffic will require cryptographic
   operations, which increase the load on those receiving protected
   traffic and/or verifying incoming traffic. The additional computation
   raises vulnerability to overloading, which can be the result of
   legitimate flash crowds or from zombies utilized in DoS attacks.
   Although this may itself present a substantial impediment to
   deployment, it is a challenge to all cryptographically protected
   communication systems, and BTNS does not create or amplify that
   aspect per se. This document does not address the impact BTNS has on
   such load.

   The effects of the increased resource consumption are twofold. It
   raises the level of effort for attackers such as MITM, but it also
   consumes more resources to detect such attacks or to reject spoofed
   traffic. At the network layer, proper limits or access controls for
   resources should be setup for all BTNS sessions. CBB sessions can be
   granted with better access once the higher layer authentications
   succeed. The same principles apply to the higher layer protocols in
   the CBB sessions. Special care must be taken to avoid undue resource
   usage before the authentication is established in the applications.




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5.5. Exposure to Anonymous Access

   The use of SAB necessarily implies that a service is being offered
   for open access, since network layer authentication information is
   not available. SAB must not be used with services that are not
   intended to be openly available.

5.6. ICMP Attacks

   This document does not consider ICMP attacks because the use of BTNS-
   IPsec does not change the existing guideline [9] on how ICMP traffic
   is handled. BTNS-IPsec focuses on authentication part of establishing
   security associations. It does not alter the IPsec traffic processing
   model and protection boundary. As a result, the entire IPsec packet
   processing guidelines, including ICMP processing, remain the same for
   BTNS-IPsec.

5.7. Leap of Faith

   BTNS allows systems to accept and establish security associations
   with peers without authenticating their identities. This can enable
   functionality similar to "Leap of Faith" utilized in other security
   protocols and applications such as SSH [23].

   SSH implementations may accept unknown peer credentials (host public
   keys) without authentication, and the applications are further
   allowed to cache these unauthenticated credentials in local databases
   for future authentication of the same peers. Similar to BTNS, such
   measures are allowed due to the lack of 'widely deployed key
   infrastructure' [23] and to improve ease of use and end-user
   acceptance. There are still subtle differences. The following table
   compares the behaviors of SSH and BTNS regarding Leap of Faith.

                                     |   SSH   |  BTNS   |
      -------------------------------+---------+---------+
       Accept unauthenticated        |   Yes   |   Yes   |
       Credentials                   |         |         |
      -------------------------------+---------+---------+
       Options/Warnings to reject    |   Yes   |   No    |
       unauthenticated credentials   |         |         |
      -------------------------------+---------+---------+
       Cache unauthenticated         |   Yes   |   No    |
       credential for future refs    |         |         |
      -------------------------------+---------+---------+

   SSH requires proper warnings and options in the applications to
   reject unauthenticated credentials, while BTNS will accept those


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   automatically if they match the corresponding policy entries. Once
   SSH accepts a credential for the first time, it should be cached, and
   can be reused automatically without further warnings.

   On the other hand, there are two key issues with BTNS-IPsec: whether
   to cache credentials and if so, how to treat cached credentials. The
   main reason to cache a credential is to treat it differently the next
   time it appears. For SAB without Channel Binding, the credentials
   should not be cached because they remain unauthenticated, and BTNS-
   IPsec does not require IPsec to reuse credentials in a manner similar
   to SSH. For CBB, credential caching and verification are usually done
   at the higher layer protocols or applications, as well be discussed
   in the next section. Caching credentials at the BTNS-IPsec is not as
   important because the channel binding will bind whatever credentials
   are presented (new or cached) to the higher layer protocol identity.
   SSH-style credential caching for reuse with SAB can be added as a
   future extension to BTNS-IPsec; such work would need to provide
   warnings and checks on unauthenticated credentials in order to
   establish a level of assurance of authentication compared to SSH's
   "Leap of Faith."

5.8. Connection Hijacking through Rekeying

   Each IPsec SA has a limited lifetime (defined as a time and/or byte
   count), and it must be rekeyed or terminated when the lifetime
   expires. Rekeying SA provides a small window of opportunity where an
   on-path attacker can step in and hijack the connection by spoofing
   the victim during rekeying. This vulnerability affects both regular
   IPsec and BTNS, although BTNS, more specifically SAB, makes it easier
   to spoof without authentication. CBB, on the other hand, can detect
   such attacks by detect the changes in the secure channel properties
   as will be described later.

   To hijack an existing SA (ESP or AH) between Alice and Bob (victim),
   Charles (attacker) must posses credentials that match to the same
   entry in Alice's PAD as Bob. It is possible because of wildcards in
   PAD entry IDs, though the authentication requirements of the regular
   IPsec do provide more of a hurdle to the attackers than BTNS-IPsec.
   The attacker, Charles, must initiate the attack when the IKE SA
   between Alice and Bob expires; or the existing IKE SA would protect
   the rekeying from spoofing attacks. After the IKE SA has expired,
   Charles can spoof Bob to create a new IKE SA and subsequent CHILD SAs
   with Alice through the same PAD entry. It requires precise timing,
   and the attacker must be able to block the IKE rekeying requests
   between Alice and Bob.




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   The problem is the lack of inter-session binding or latching of IKE
   SAs with the corresponding credentials of the two peers. Connection
   latching [21], together with channel binding, enables such binding,
   but requires upper layer protocols or applications to verify the
   consistency across IKE sessions. Connection latching defines a set of
   SA parameters, along with corresponding peer identities and
   authentication data, as a representation of a secure channel. It
   provides this data to the upper layer protocols that wish to "latch"
   on to the channel. Channel binding binds this secure channel (or
   "latch") to higher layer authentication. It is the upper layer
   protocols or applications that determine whether to cache and verify
   the consistency of the peer identities across sessions. If the upper
   layer session is still active, channel binding will lock down the
   channel and prevent the spoofing attack. If the upper layer session
   has also expired, it will require re-authentication at the higher
   layer. The later re-authentication and binding should prevent the
   spoofing whether or not the BTNS-IPsec credentials are cached.
   Without the additional session information from higher layer
   protocols, it is very difficult for network layer protocols such as
   IPsec to predict the lengths of connections and to distinguish
   between legitimate changes of peers vs. spoofing.

   In summary, connection latching defines the notion of a secure
   channel, and channel binding enables higher layer protocol to bind
   its authentication to this secure channel. Caching of this "latch"
   across session is necessary to counter inter-session spoofing
   attacks, and can be done at either the BTNS-IPsec layer or at the
   higher layer.

5.9. Configuration Errors

   BTNS does not address errors of configuration that could result in
   increased vulnerability; such vulnerability is already possible using
   "bypass". This work presumes that associations using BTNS will
   consist of SPD entries, just as "bypass," therefore separated from
   associations with more conventional, stronger security.

6. Other Issues and Related Efforts

   This section discusses other issues not included in any previous
   categories, and lists the related work.

6.1. NAT Traversal

   The issues regarding NAT traversal are mostly orthogonal to BTNS
   because BTNS focuses on relaxing peer authentication in IKE and IPsec
   policy. BTNS with Channel Binding may cause problems with NAT if the


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   IDs are tied to addresses at the application layer. Note that this
   problem is not specific to BTNS, but rather to the design of generic
   IPsec Channel Binding APIs. Therefore, this document does not
   consider the impact of NAT or NAPT on the capabilities it intends to
   provide, except as are already addressed by the current IPsec
   specifications.

6.2. Mobility and Multihoming

   BTNS does not consider the impact of mobility or multihoming on the
   capabilities it intends to provide.

6.3. Related IETF Efforts

   There are a number of related efforts in the IETF and elsewhere to
   reduce the configuration effort of deploying the Internet security
   suite.

   PKI4IPsec is an IETF WG focused on providing an automatic
   infrastructure for the configuration of Internet security services,
   e.g., to assist in deploying signed certificates and CA information
   [7]. The IETF KINK WG is focused on adapting Kerberos for IKE,
   enabling IKE to utilize the Kerberos key distribution infrastructure
   rather than requiring certificates signed by CAs or shared private
   keys [6]. KINK takes advantage of an existing architecture for
   automatic key management in Kerberos. Opportunistic Encryption (OE)
   is a system for automatic discovery of hosts willing to do a BTNS-
   like encryption, with authentication being exchanged by leveraging
   existing use of the DNS [14]. BTNS differs from all three in that
   BTNS is intended to avoid the need for such infrastructure
   altogether, rather than to automate it.

7. IANA Considerations

   There are no IANA issues in this document.

   This section should be removed by the RFC-Editor prior to final
   publication.

8. Acknowledgments

   This document was inspired by discussions on the IETF TCPM WG about
   the recent spoofed RST attacks on BGP routers and various solutions,
   as well as discussions in the nfsv4 and ips WGs about how to better
   integrate with IPsec.  The concept of BTNS was the result of these
   discussions as well as with USC/ISI's T. Faber, A. Falk, and B. Tung,
   and discussions on the IETF SAAG WG and IPsec mailing list. The


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   authors would like to thank the members of those WGs and lists, as
   well as the IETF BTNS BOFs and WG and its associated ANONsec mailing
   list (http://www.postel.org/anonsec) for their feedback, in
   particular, Steve Kent, Sam Hartman, Nicolas Williams, and Pekka
   Savola.

   This document was prepared using 2-Word-v2.0.template.dot.

9. References

9.1. Normative References

   (none)

9.2. Informative References

   [1]   Aboba, B., J. Tseng, J. Walker, V. Rangan, and F. Travostino,
         "Securing Block Storage Protocols over IP," RFC-3723, April
         2004.

   [2]   CERT Vulnerability Note VU#415294, "The Border Gateway Protocol
         relies on persistent TCP sessions without specifying
         authentication requirements," 4/20/2004.

   [3]   Dierks, T. E. Rescorla, "The Transport Layer Security (TLS)
         Protocol Version 1.1," RFC-4346, April 2006.

   [4]   Harkins, D., D. Carrel, "The Internet Key Exchange (IKE),"
         RFC-2409, Nov. 1998.

   [5]   Heffernan, A., "Protection of BGP Sessions via the TCP MD5
         Signature Option," RFC-2385, Aug. 1998.

   [6]   IETF KINK WG web pages,
         http://www.ietf.org/html.charters/kink-charter.html

   [7]   IETF PKI4IPSEC WG web pages,
         http://www.ietf.org/html.charters/pki4ipsec-charter.html

   [8]   Kaufman, C., (ed.), "Internet Key Exchange (IKEv2) Protocol,"
         RFC-4306, Dec. 2005.

   [9]   Kent, S., R. Atkinson, "Security Architecture for the Internet
         Protocol," RFC-2401, Nov. 1998.

   [10]  Kent, S., K. Seo, "Security Architecture for the Internet
         Protocol," RFC-4301, Dec. 2005.


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   [11]  Kohl, J., C. Neuman, "The Kerberos Network Authentication
         Service (V5)," RFC-1510, Sep. 1993.

   [12]  Linn, J, "Generic Security Service Application Program
         Interface Version 2, Update 1," RFC-2743, Jan. 2000.

   [13]  Mostkowitz, R., P. Nikander, P. Jokela (ed.), T. Henderson,
         "Host Identity Protocol," (work in progress),
         draft-ietf-hip-base-06, Jun. 2006.

   [14]  Richardson, M., Redelmeier, D., "Opportunistic Encryption using
         The Internet Key Exchange (IKE)," RFC-4322, Dec. 2005.

   [15]  Satran, J., K. Meth, C. Sapuntzakis, M. Chadalapaka, E.
         Zeidner, "Internet Small Computer Systems Interface (iSCSI)",
         RFC-3720, April 2004.

   [16]  Shepler, S., B. Callaghan, D. Robinson, R. Thurlow, C., Beame,
         M. Eisler, D. Noveck, "Network File System (NFS) version 4
         Protocol," RFC-3530, April, 2003.

   [17]  Steward, R., Dalal, M., "Improving TCP's Robustness to Blind
         In-Window Attacks," (work in progress),
         draft-ietf-tcpm-tcpsecure-05, Jun. 2006.

   [18]  Stewart, R., et al., "Stream Control Transmission Protocol,"
         RFC-2960, Oct. 2000.

   [19]  TCP SYN-cookies, http://cr.yp.to/syncookies.html

   [20]  Touch, J., "Defending TCP Against Spoofing Attacks," (work in
         progress), draft-ietf-tcpm-tcp-antispoof-05.txt, Sept. 2006.

   [21]  Williams, N., "IPsec Channels: Connection Latching," (work in
         progress), draft-ietf-btns-connection-latching-00, Feb. 2006.

   [22]  Williams, N., "On the Use of Channel Bindings to Secure
         Channels," (work in progress),
         draft-williams-on-channel-binding-00, Jun. 2006.

   [23]  Ylonen, T, Lonvick, C. (ed.), "The Secure Shell (SSH) Protocol
         Architecture," RFC-4251, Jan. 2006.







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Author's Addresses

   Joe Touch
   USC/ISI
   4676 Admiralty Way
   Marina del Rey, CA 90292-6695
   U.S.A.

   Phone: +1 (310) 448-9151
   Email: touch@isi.edu


   David Black
   EMC Corporation
   176 South Street
   Hopkinton, MA 01748
   USA

   Phone: +1 (508) 293-7953
   Email: black_david@emc.com


   Yu-Shun Wang
   USC/ISI
   4676 Admiralty Way
   Marina del Rey, CA 90292-6695
   U.S.A.

   Phone: +1 (310) 448-8742
   Email: yushunwa@isi.edu


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   specification can be obtained from the IETF on-line IPR repository at
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Acknowledgment

   Funding for the RFC Editor function is currently provided by the
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