Network Working Group                                         V. Smyslov
Internet-Draft                                                ELVIS-PLUS
Intended status: Standards Track                              P. Wouters
Expires: August 1, 2015                                          Red Hat
                                                        January 28, 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
   unauthenticated IKE sessions for those use cases where a peer is
   unwilling or unable to authenticate or identify itself.  This ensures
   IKEv2 can be used for Opportunistic Security (also known as
   Opportunistic Encryption) to defend against Pervasive Monitoring
   attacks without the need to sacrifice anonymity.

Status of this Memo

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   This Internet-Draft will expire on August 1, 2015.

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   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Conventions Used in This Document  . . . . . . . . . . . .  3
   2.  Using the NULL Authentication Method . . . . . . . . . . . . .  5
     2.1.  Authentication Payload . . . . . . . . . . . . . . . . . .  5
     2.2.  Identification Payload . . . . . . . . . . . . . . . . . .  5
     2.3.  INITIAL_CONTACT Notification . . . . . . . . . . . . . . .  6
     2.4.  Interaction with Peer Authorization Database (PAD) . . . .  6
     2.5.  Traffic Selectors  . . . . . . . . . . . . . . . . . . . .  7
   3.  Security Considerations  . . . . . . . . . . . . . . . . . . .  8
     3.1.  Audit trail and peer identification  . . . . . . . . . . .  8
     3.2.  Resource management and robustness . . . . . . . . . . . .  8
     3.3.  IKE configuration selection  . . . . . . . . . . . . . . .  9
     3.4.  Networking topology changes  . . . . . . . . . . . . . . .  9
   4.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 10
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 11
   6.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
     6.1.  Normative References . . . . . . . . . . . . . . . . . . . 12
     6.2.  Informative References . . . . . . . . . . . . . . . . . . 12
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13

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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 and anonymous IKE

   In some situations mutual authentication is undesirable, superfluous
   or impossible.  The following three examples illustrate these
   unauthenticated 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 measurement, 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
      unauthenticated encrypted communication is preferred over
      cleartext communication.  The peers want to use IKE to setup an
      unauthenticated 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 unauthenticated key exchange.

   To meet these needs this document introduces the NULL Authentication
   method, and the ID_NULL ID 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",

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   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 authentication 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]).

   NULL Authentication affects how the Authentication and the
   Identification payloads are formed in the IKE_AUTH exchange.

2.1.  Authentication Payload

   NULL Authentication 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 NULL Authentication, 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 IKEv2 Authentication Method value for NULL Authentication is 13.

2.2.  Identification Payload

   When a remote peer is not authenticated, any ID presented in the
   Identification Data field of the ID payload cannot be validated.  To
   avoid the need of sending a bogus ID Type with placeholder data, this
   specification defines a new ID Type, ID_NULL.  The Identification
   Data field of the ID payload for this ID Type MUST be empty.

   If NULL Authentication is in use and an anonymity is a concern then
   ID_NULL SHOULD be used in Identification payload.  In some cases
   there may be good reasons to use non-null identities (and ID Types
   other than ID_NULL) with NULL Authentication.  The identities may be
   used for logging, troubleshooting or in scenarios when authentication
   takes place out of band after the IKE SA is created (like in
   [AUTOVPN]).  In any case, when NULL Authentication is employed, the
   content of Identification payload MUST NOT be used for any trust and

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   policy checking in IKE_AUTH exchange.

   ID_NULL is primarily intended to be used with the NULL
   Authentication, but it MAY also be used in other situations, when the
   content of Identification payload does not matter.  For example,
   ID_NULL can be used when authentication is performed via raw public
   keys and the identities are these keys themselves.  Another example
   is EAP authentication when the client identity in ID payload is not

   The IKEv2 Identification Payload ID Type for ID_NULL is 13.

2.3.  INITIAL_CONTACT Notification

   The identity of a peer using NULL Authentication cannot be used to
   distinguish from IKE SAs created by other peers using the NULL
   Authentication method.  For that reason the INITIAL_CONTACT
   notifications MUST be ignored for IKE SAs using NULL Authentication.

   The standard IKE Liveness Check procedure, decribed in Section 2.4 of
   [RFC7296], can be used to detect stale IKE SAs created by peers using
   NULL Authentication.  Inactive unauthenticated IKE SAs should be
   checked periodically.  Additionally, the event of creating a new
   unauthenticated IKE SA can be used to trigger an out-of-order check
   on existing unauthenticated IKE SAs, possibly limited to identical or
   close-by IP addresses or to identical identities of the just created
   IKE SA.

   Implementations should weight the resource consumption of sending
   Liveness Checks against the memory usage of possible orphaned IKE
   SAs.  Implementations may choose to handle situations with thousands
   of unauthenticated IKE SAs differently from situations with very few
   such SAs.

2.4.  Interaction with Peer Authorization Database (PAD)

   Section 4.4.3 of [RFC4301] defines the Peer Authorization Database
   (PAD), which provides the link between Security Policy Database (SPD)
   and the IKEv2.  The PAD contains an ordered list of records, with
   peers' identities along with corresponding authentication data and
   Child SA authorization data.  When the IKE SA is being established
   the PAD is consulted to determine how the peer should be
   authenticated and what Child SAs it is authorized to create.

   When using NULL Authentication, the peer identity is not
   authenticated and cannot be used.  If ID_NULL is used with NULL
   Authentication, there is no ID at all.  The processing of PAD
   described in Section of [RFC4301] must be updated.

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   If NULL Authentication is supported and allowed, then a special entry
   MUST be included in the PAD.  This entry MUST contain no
   authentication data.  It MAY contain a set of constraints for
   creating Child SAs as described in Section 4.4.3 of [RFC4301].  When
   a peer uses NULL Authentication, regular matching rules for the PAD
   MUST be ignored and this special entry MUST be selected regardless of
   the peer identity.  Likewise, if a peer uses any other authentication
   method, then this special entry MUST NOT be selected regardless of
   the peer identity and the regular search of the PAD described in
   Section of [RFC4301] MUST be performed.

   Implementations SHOULD allow to be configured so, that when a peer
   requests NULL Authentication, then regular PAD entries are searched
   before selecting the special entry, to ensure that there is no entry,
   containing peer's IP address.  In this case implementations MUST
   reject the IKE_AUTH exchange by sending an AUTHENTICATION_FAILED
   notification if such an entry is found.

2.5.  Traffic Selectors

   Traffic Selectors and narrowing allow two IKE peers to mutually agree
   on a traffic range for an IPsec SA.  An unauthenticated 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.

   One method to achieve this is to always assign internal IP addresses
   to unauthenticated 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 unauthenticated IKE peers to single host-
   to-host IPsec SAs.  When using IPv6 it is not always possible, so in
   this case implementations MUST be able to assign full /64 address
   block to the peer as described in [RFC5739], even if it is not

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3.  Security Considerations

   If authenticated IKE sessions are possible between the peers, then
   unauthenticated IKE SHOULD NOT be used, unless implementations make
   sure to keep authenticated and unauthenticated IKE sessions separate,
   and has policy rules to specify when to use which IKE session.  See
   [RFC7435] for details.

   If both peers use NULL Authentication, the entire key exchange
   becomes unauthenticated.  This makes the IKE session vulnerable to
   active Man-in-the-Middle Attacks.

   Using an ID Type other than ID_NULL with the NULL Authentication
   Method may compromise the client's anonimity in case of an active
   MITM attack.

   IKE implementations without NULL Authentication have always performed
   mutual authentication and were not designed for use with
   unauthenticated 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 unauthenticated
   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.
   Implementers that implement NULL Authentication 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-measures in an attempt to distinguish
   attacking IKE packets from legitimate IKE peers.

   These defense mechanisms do not take into account IKE systems that
   allow unauthenticated IKE peers.  An attacker using NULL
   Authentication is a fully legitimate IKE peer that is only
   distinguished from authenticated IKE peers by having used NULL

   While implementations should have been written to account for abusive
   authenticated clients, any omission or error in handling abusive

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   clients may have gone unnoticed because abusive clients has been a
   rare or non-existent problem.  When enabling unauthenticated IKE
   peers, these implementation omissions and errors will be found and
   abused by attackers.  For example, an unauthenticated IKE peer could
   send an abusive amount of Liveness probes or Delete requests.

3.3.  IKE configuration selection

   Combining authenticated and unauthenticated 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 unauthenticated peers).  An unauthenticated IKE peer MUST
   NOT be able to reach resources only meant for authenticated IKE peers
   and MUST NOT be able to replace the Child SAs of an authenticated IKE

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 circumvent 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 unauthenticated IKE MUST pass all decrypted traffic
   through the same packet filters and security mechanisms as plaintext

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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

   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.

   [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, December 2005.

   [RFC5739]  Eronen, P., Laganier, J., and C. Madson, "IPv6
              Configuration in Internet Key Exchange Protocol Version 2
              (IKEv2)", RFC 5739, February 2010.

   [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.

   [AUTOVPN]  Sheffer, Y. and Y. Nir, "The AutoVPN Architecture", Work
              in Progress, draft-sheffer-autovpn-00, February 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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