Network Working Group                                         V. Smyslov
Internet-Draft                                                ELVIS-PLUS
Intended status: Standards Track                              P. Wouters
Expires: September 27, 2015                                      Red Hat
                                                          March 26, 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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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on September 27, 2015.

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   document authors.  All rights reserved.

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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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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 a 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 anonymity is a concern then
   ID_NULL SHOULD be used in the Identification payload.  Some examples
   of cases where a non-null identity type and value with NULL
   Authentication can be used are logging, troubleshooting and in
   scenarios where authentication takes place out of band after the IKE
   SA is created (like in [AUTOVPN]).  The content of the Identification
   payload MUST NOT be used for any trust and policy checking in
   IKE_AUTH exchange when NULL Authentication is employed (see Section

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   2.4 for details).

   ID_NULL is primarily intended to be used with NULL Authentication but
   could be used in other situations where the content of the
   Identification Payload is not used.  For example, ID_NULL could be
   used when authentication is performed via raw public keys and the
   identities are the keys themselves.  These alternative uses of
   ID_NULL should be described in their own respective documents.

   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
   find existing IKE SAs created by the same peer, as the peer identity
   is not authenticated.  For that reason the INITIAL_CONTACT
   notifications MUST NOT be used to delete any other IKE SAs based on
   the same peer identity without additional verification that the
   existing IKE SAs with matching identity are actually stale.

   The standard IKE Liveness Check procedure, described 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 weigh 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 trusted.  If ID_NULL is used with NULL
   Authentication, there is no ID at all.  The processing of PAD

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   described in Section of [RFC4301] must be updated.

   NULL authentication needs to be added as one of supported
   authentication methods.  This method does not have any authentication
   data.  To add support for ID_NULL, it needs to be included into the
   list of ID types, specified in Section of [RFC4301].  The
   matching rule for ID_NULL consists only of whether this type is used,
   i.e. no actual ID matching is done, as ID_NULL contains no identity

   Section of the [RFC4301] describes how the IKE ID is matched
   against the SPD entries.  When using the NULL authentication method
   those matching rules MUST include matching of a new flag in the SPD
   entry specifying whether unauthenticated users are allowed to use
   that entry.  I.e. each SPD entry needs to be augmented to have a flag
   specifying whether it can be used with NULL authentication or not,
   and only those rules that explictly have that flag turned on can be
   used with unauthenticated connections.

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 this 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 for a certain traffic
   selector range between the peers, then unauthenticated IKE SHOULD NOT
   be allowed for that traffic selector range.  When mixing
   authenticated and unauthenticated IKE with the same peer, policy
   rules should ensure the highest level of security will be used to
   protect the communication between the two peers.  See [RFC7435] for

   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 anonymity 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 remote peers are identified.  With NULL Authentication
   these assumptions 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

   With NULL Authentication 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 ensure their implementation does not make any
   assumptions that depend on IKE peers being "friendly", "trusted" or

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

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   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 adding support for
   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 incoming
   plaintext traffic.

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4.  Acknowledgments

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

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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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