Network Working Group                                         V. Smyslov
Internet-Draft                                                ELVIS-PLUS
Intended status: Standards Track                              P. Wouters
Expires: July 17, 2015                                           Red Hat
                                                        January 13, 2015

            The NULL Authentication Method in IKEv2 Protocol


   This document specifies the NULL Authentication Method and the
   ID_NULL Identification Payload ID Type for the IKEv2 Protocol.  This
   allows two IKE peers to establish single-side authenticated or mutual
   un-authenticated IKE sessions for those use cases where a peer is
   unwilling or unable to authenticate itself.  This ensures IKEv2 can
   be used for Opportunistic Security (also known as Opportunsitic
   Encryption) to defend against Pervasive Monitoring attacks without
   the need to sacrifice anonimity.

Status of this Memo

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   This Internet-Draft will expire on July 17, 2015.

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   to this document.  Code Components extracted from this document must
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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Conventions Used in This Document  . . . . . . . . . . . .  3
   2.  Using the NULL Authentication Method . . . . . . . . . . . . .  4
     2.1.  Authentication Payload . . . . . . . . . . . . . . . . . .  4
     2.2.  Identity Payload . . . . . . . . . . . . . . . . . . . . .  4
     2.3.  INITIAL_CONTACT Notification . . . . . . . . . . . . . . .  5
   3.  Security Considerations  . . . . . . . . . . . . . . . . . . .  6
     3.1.  Audit trail and peer identification  . . . . . . . . . . .  6
     3.2.  Resource management and robustness . . . . . . . . . . . .  6
     3.3.  IKE configuration selection  . . . . . . . . . . . . . . .  7
     3.4.  Networking topology changes  . . . . . . . . . . . . . . .  7
     3.5.  Priviledged IKE operations . . . . . . . . . . . . . . . .  8
   4.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . .  9
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 10
   6.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
     6.1.  Normative References . . . . . . . . . . . . . . . . . . . 11
     6.2.  Informative References . . . . . . . . . . . . . . . . . . 11
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12

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1.  Introduction

   The Internet Key Exchange Protocol version 2 (IKEv2), specified in
   [RFC7296], provides a way for two parties to perform an authenticated
   key exchange.  While the authentication methods used by the peers can
   be different, there is no method for one or both parties to remain
   unauthenticated and anonymous.  This document extends the
   authentication methods to support unauthenticated key exchanges.

   In some situations mutual authentication is undesirable, superfluous
   or impossible.  The following three examples illustratate these un-
   authenticated use cases:

   o  A user wants to establish an anonymous secure connection to a
      server.  In this situation the user should be able to authenticate
      the server without presenting or authenticating to the server with
      their own identity.  This case uses a single-sided authentication
      of the responder.

   o  A sensor that periodically wakes up from a suspended state wants
      to send a measurement (e.g. temperature) to a collecting server.
      The sensor must be authenticated by the server to ensure
      authenticity of the measurment, but the sensor does not need to
      authenticate the server.  This case uses a single-sided
      authentication of the initiator.

   o  Two peers without any trust relationship wish to defend against
      widespread pervasive monitoring attacks as described in [RFC7258].
      Without a trust relationship, the peers cannot authenticate each
      other.  Opportunistic Security [RFC7435] states that un-
      authenticated encrypted communication is prefered over cleartext
      communication.  The peers want to use IKE to setup an un-
      authenticated encrypted connection, that gives them protection
      against pervasive monitoring attacks.  An attacker that is able
      and willing to send packets can still launch an Man-in-the-Middle
      attack to obtain access to the decrypted communication.  This case
      uses a fully anonymous un-authenticated key exchange.

   To meet these needs this document introduces the NULL authentication
   method, and the ID_NULL identity type.  This allows an IKE peer to
   explicitly indicate that it is unwilling or unable to certify its

1.1.  Conventions Used in This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

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2.  Using the NULL Authentication Method

   In IKEv2, each peer independently selects the method to authenticate
   itself to the other side.  A peer may choose to refrain from
   authentication by using the NULL Authentication Method.  If a peer
   that requires authentiation receives an AUTH payload containing the
   NULL Authentication Method type, it MUST return an
   AUTHENTICATION_FAILED notification.  If an initiator uses EAP, the
   responder MUST NOT use the NULL Authentication Method (in conformance
   with the section 2.16 of [RFC7296]).

   The NULL Authentication Method affects how the Authentication and the
   Identity payloads are formed in the IKE_AUTH exchange.

2.1.  Authentication Payload

   The NULL Authentication Method still requires a properly formed AUTH
   payload to be present in the IKE_AUTH exchange messages, as the AUTH
   payload cryptographically links the IKE_SA_INIT exchange messages
   with the other messages sent over this IKE SA.

   When using the NULL Authentication Method, the content of the AUTH
   payload is computed using the syntax of pre-shared secret
   authentication, described in Section 2.15 of [RFC7296].  The values
   SK_pi and SK_pr are used as shared secrets for the content of the
   AUTH payloads generated by the initiator and the responder
   respectively.  Note that this is identical to how the content of the
   two last AUTH payloads is generated for the non-key-generating EAP
   methods (see Section 2.16 of [RFC7296] for details).

   The KEv2 Authentication Method value for the NULL Authentication
   Method is 13.

2.2.  Identity Payload

   When a remote peer is not authenticated, any ID presented in the
   Identification Data field of the Identification Payload cannot be
   validated and MUST be ignored.  A new Identification Payload ID Type
   is introduced to avoid the need of sending a bogus ID Type with
   placeholder data.  Furthermore, sending a traditional ID field might
   unwittingly compromise the anonimity of the peer.

   This specification defines a new ID Type of ID_NULL, which SHOULD
   only be used with the NULL Authentication Method.  The Identification
   Data field of the Identification Payload MUST be empty.

   The IKEv2 Identification Payload ID Type for ID_NULL is 13.

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2.3.  INITIAL_CONTACT Notification

   The identity of the peer which uses the NULL Authentication Method
   cannot be used to distinguish between IKE SAs created by different
   peers.  For that reason the INITIAL_CONTACT notifications MUST be
   ignored for IKE SAs using the NULL Authentication Method.

   When a new IKE SA is established using the NULL Authentication
   Method, implementations MAY perform a Liveness Check on all other IKE
   SAs that were established using the NULL Authentication Method.  To
   mitigate the potential impact of sending Liveness Check messages on a
   large number of IKE SAs, implementations are advised not to blindly
   perform Liveness Check on every such SA, but to take into
   considerations additional information, that may indicate that the
   particular SA is alive.  This information may include the recent
   receipt of cryptographically protected message on the IKE SA or any
   of its Child SAs, or a successfull Liveness Check that was performed

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3.  Security Considerations

   If both peers use the NULL Authentication Method, the entire key
   exchange becomes unauthenticated.  This makes the IKE session
   vulnerable to active Man-in-the-Middle Attacks.  Un-authenticated IKE
   sessions MUST only attempted when authenticated IKE sessions are not
   possible for the remote host and the only alternative would be to
   send plaintext.  See [RFC7435] for details.

   Implementations SHOULD use the ID_NULL Identity Type with the NULL
   Authenticated Method.  If an un-authenticated remote IKE peer
   presents an Identity Type different from ID_NULL, the Identification
   Payload data MUST NOT be used for anything except logging.

   Using an ID Type other than ID_NULL with the NULL Authentication
   Method compromises the client's anonimity.  This should be avoided
   for regular operation but could be temporarilly enabled, for example
   to aid with troubleshooting diagnostics.  Sending an unverifiable
   identification for any other purpose is strongly discouraged as it
   leads to a false sense of security,

   IKE implementations without the NULL Authentication Method have
   always performed mutual authentication and were not designed for use
   with un-authenticated IKE peers.  Implementations might have made
   assumptions that are no longer valid.  Furthermore, the host itself
   might have made trust assumptions or may not be aware of the network
   topology changes that resulted from IPsec SAs from un-authenticated
   IKE peers.

3.1.  Audit trail and peer identification

   An established IKE session is no longer guaranteed to provide a
   verifiable (authenticated) entity known to the system or network.
   Implementations that add the NULL Authentication Method should audit
   their implementation for any assumptions that depend on IKE peers
   being "friendly", "trusted" or "identifiable".

3.2.  Resource management and robustness

   Section 2.6 of [RFC7296] provides guidance for mitigation of "Denial
   of Service" attacks by issuing COOKIES in response to resource
   consumption of half-open IKE SAs.  Furthermore, [DDOS-PROTECTION]
   offers additional counter-meassures in an attempt to distinguish
   attacking IKE packets from legitimate IKE peers.

   These defense mechanisms do not take into account IKE systems that
   allow un-authenticated IKE peers.  An attacker using the NULL
   Authentication Method is a fully legitimate IKE peer that is only

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   distinguished from authenticated IKE peers by the Authenticaion

   While implementations should have been written to account for abusive
   authenticated clients, any omission or error in handling abusive
   clients may have gone unnoticed because abusive clients has been a
   rare or non-existent problem.  When enabling un-authenticated IKE
   peers, these implementation omissions and errors will be found and
   abused by attackers.  For example, an un-authenticated IKE peer could
   send an abusive amount of Liveness probes or Delete requests.

3.3.  IKE configuration selection

   Combining authenticated and un-authenticated IKE peers on a single
   host can be dangerous, assuming the authenticated IKE peer gains more
   or different access from non-authenticated peers (otherwise, why not
   only allow un-authentcated peers).  An un-authenticated IKE peer MUST
   NOT be able to reach resources only meant for authenticated IKE peers
   and MUST NOT be able to replace the IPsec SAs of an authenticated IKE

   If an IKE peer receives an IKE_AUTH exchange requesting a NULL
   Authentication Method from an IP address that matches a configured
   connection for an authenticated IKE session, it MUST reject the
   IKE_AUTH exchange by sending an AUTHENTICATION_FAILED notification.

3.4.  Networking topology changes

   When a host relies on packet filters or firewall software to protect
   itself, establishing an IKE SA and installing an IPsec SA might
   accidentally circument these packet filters and firewall
   restrictions, as the encrypted ESP (protocol 50) or ESPinUDP (UDP
   port 4500) packets do not match the packet filters defined.  IKE
   peers supporting un-authenticated IKE MUST pass all decrypted traffic
   through the same packet filters and security mechanisms as plaintext

   Traffic Selectors and narrowing allow two IKE peers to mutually agree
   on a traffic range for an IPsec SA.  An un-authenticated peer MUST
   NOT be allowed to use this mechanism to steal traffic that an IKE
   peer intended to be for another host.  This is especially problematic
   when supporting anonymous IKE peers behind NAT, as such IKE peers
   build an IPsec SA using their pre-NAT IP address that are different
   from the source IP of their IKE packets.  A rogue IKE peer could use
   malicious Traffic Selectors to obtain access to traffic that the host
   never intended to hand out.  Implementations SHOULD restrict and
   isolate all anonymous IKE peers from each other and itself and only
   allow it access to itself and possibly its intended network ranges.

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   One of the ways to achive that is to always assign internal IP
   addresses to un-authenticated IKE clients, as described in Section
   2.19 of [RFC7296].  Implementations may also use other techniques,
   such as internal NAT and connection tracking.  Implementations MAY
   force un-authenticated IKE peers to single host-to-host IPsec SAs.

3.5.  Priviledged IKE operations

   Some IKE features are not appropriate for un-authenticated IKE peers
   and should be restricted or forbidden.

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4.  Acknowledgments

   The authors would like to thank Yaron Sheffer and Tero Kivinen for
   their reviews and valuable comments.

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5.  IANA Considerations

   This document defines a new entry in the "IKEv2 Authentication
   Method" registry:

     13       NULL Authentication Method

   This document also defines a new entry in the "IKEv2 Identification
   Payload ID Types" registry:

     13       ID_NULL

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6.  References

6.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC7296]  Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T.
              Kivinen, "Internet Key Exchange Protocol Version 2
              (IKEv2)", STD 79, RFC 7296, October 2014.

6.2.  Informative References

   [RFC7258]  Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
              Attack", BCP 188, RFC 7258, May 2014.

   [RFC7435]  Dukhovni, V., "Opportunistic Security: Some Protection
              Most of the Time", RFC 7435, December 2014.

              Nir, Y., "Protecting Internet Key Exchange (IKE)
              Implementations from Distributed Denial of Service
              Attacks", draft-ietf-ipsecme-ddos-protection-00 (work in
              progress), October 2014.

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Authors' Addresses

   Valery Smyslov
   PO Box 81
   Moscow (Zelenograd)  124460
   Russian Federation

   Phone: +7 495 276 0211

   Paul Wouters
   Red Hat


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