The NULL Authentication Method in the Internet Key Exchange Protocol Version 2 (IKEv2)
RFC 7619

Internet Engineering Task Force (IETF)                        V. Smyslov
Request for Comments: 7619                                    ELVIS-PLUS
Updates: 4301                                                 P. Wouters
Category: Standards Track                                        Red Hat
ISSN: 2070-1721                                              August 2015

                     The NULL Authentication Method
        in the Internet Key Exchange Protocol Version 2 (IKEv2)

Abstract

   This document specifies the NULL Authentication method and the
   ID_NULL Identification Payload ID Type for Internet Key Exchange
   Protocol version 2 (IKEv2).  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

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7619.

Smyslov & Wouters            Standards Track                    [Page 1]
RFC 7619                   NULL Auth in IKEv2                August 2015

Copyright Notice

   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Conventions Used in This Document . . . . . . . . . . . .   4
   2.  Using the NULL Authentication Method  . . . . . . . . . . . .   4
     2.1.  Authentication Payload  . . . . . . . . . . . . . . . . .   4
     2.2.  Identification Payload  . . . . . . . . . . . . . . . . .   4
     2.3.  INITIAL_CONTACT Notification  . . . . . . . . . . . . . .   5
     2.4.  Interaction with the Peer Authorization Database (PAD)  .   5
     2.5.  Traffic Selectors . . . . . . . . . . . . . . . . . . . .   6
   3.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
     3.1.  Audit Trail and Peer Identification . . . . . . . . . . .   7
     3.2.  Resource Management and Robustness  . . . . . . . . . . .   8
     3.3.  IKE Configuration Selection . . . . . . . . . . . . . . .   8
     3.4.  Networking Topology Changes . . . . . . . . . . . . . . .   8
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   5.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     5.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     5.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Appendix A.  Update of PAD processing in RFC 4301 . . . . . . . .  11
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

Smyslov & Wouters            Standards Track                    [Page 2]
RFC 7619                   NULL Auth in IKEv2                August 2015

1.  Introduction

   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
   sessions.

   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
      (MITM) attack to obtain a copy of the unencrypted 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
   identity.

Smyslov & Wouters            Standards Track                    [Page 3]
RFC 7619                   NULL Auth in IKEv2                August 2015

1.1.  Conventions Used in This Document

   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 [RFC2119].

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 host's
   local policy requires that the identity of its peer be (non-null)
   authenticated, and if that host receives an AUTH payload containing
   the NULL Authentication method type, it MUST return an
   AUTHENTICATION_FAILED notification.  If an initiator uses the
   Extensible Authentication Protocol (EAP), the responder MUST NOT use
   the NULL Authentication method (in conformance with 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 Security Association (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 value of SK_pi for the
   initiator and SK_pr for the responder is used as the shared secret
   for the content of the AUTH payload.  Implementers should note this
   means that authentication keys used by the two peers are different in
   each direction.  This is identical to how the contents of the two
   last AUTH payloads are 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.

Smyslov & Wouters            Standards Track                    [Page 4]
RFC 7619                   NULL Auth in IKEv2                August 2015

   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 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 the Peer Authorization Database (PAD)

   Section 4.4.3 of [RFC4301] defines the Peer Authorization Database
   (PAD), which provides the link between the Security Policy Database
   (SPD) and IKEv2.  The PAD contains an ordered list of records with

Smyslov & Wouters            Standards Track                    [Page 5]
RFC 7619                   NULL Auth in IKEv2                August 2015

   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 the PAD
   described in Section 4.4.3 of [RFC4301] is updated for NULL
   authentication as follows.

   NULL authentication is added as one of the supported authentication
   methods.  This method does not have any authentication data.  ID_NULL
   is included into the list of allowed ID types.  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 data.

   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.  That is, 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 explicitly have that flag turned on can be used with
   unauthenticated connections.

   The specific updates of text in Section 4.4.3 of [RFC4301] are listed
   in Appendix A.

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 is different from the
   source IP of their IKE packets.  A rogue IKE peer could use malicious
   Traffic Selectors to trick a remote host into giving it IP traffic
   that the remote host never intended to be sent to remote IKE peers.
   For example, if the remote host uses 192.0.2.1 as the DNS server, a
   rogue IKE peer could set its Traffic Selector to 192.0.2.1 in an
   attempt to receive the remote peer's DNS traffic.  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.

Smyslov & Wouters            Standards Track                    [Page 6]
RFC 7619                   NULL Auth in IKEv2                August 2015

   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
   implementations MUST be able to assign a full /64 address block to
   the peer as described in [RFC5739], even if it is not authenticated.

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
   details.

   If both peers use NULL authentication, the entire key exchange
   becomes unauthenticated.  This makes the IKE session vulnerable to
   active MITM 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 that 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.  Any logging of unproven ID payloads that were
   not authenticated should be clearly marked and treated as "untrusted"
   and possibly accompanied by logging the remote IP address of the IKE
   session.  Rate limiting of logging might be required to prevent
   excessive resource consumption causing system damage.

Smyslov & Wouters            Standards Track                    [Page 7]
RFC 7619                   NULL Auth in IKEv2                August 2015

3.2.  Resource Management and Robustness

   Section 2.6 of [RFC7296] provides guidance for mitigation of denial-
   of-service (DoS) attacks by issuing COOKIES in response to resource
   consumption of half-open IKE SAs.  Furthermore, [DDOS-PROTECTION]
   offers additional countermeasures 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
   authentication.

   Implementers that implement NULL authentication should ensure their
   implementation does not make any assumptions that depend on IKE peers
   being "friendly", "trusted", or "identifiable".  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 have been a
   rare or nonexistent 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 unauthenticated 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
   peer.

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 Encapsulating Security Payload (ESP, protocol
   50) or ESPinUDP (UDP port 4500) packets of the encrypted traffic do
   not match the packet filters defined for unencrypted traffic.  IKE
   peers supporting unauthenticated IKE MUST pass all decrypted traffic
   through the same packet filters and security mechanisms as incoming
   plaintext traffic.

Smyslov & Wouters            Standards Track                    [Page 8]
RFC 7619                   NULL Auth in IKEv2                August 2015

4.  IANA Considerations

   Per this document, IANA has added a new entry in the "IKEv2
   Authentication Method" registry:

     13       NULL Authentication

   Per this document, IANA has added a new entry in the "IKEv2
   Identification Payload ID Types" registry:

     13       ID_NULL

5.  References

5.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, DOI 10.17487/RFC4301,
              December 2005, <http://www.rfc-editor.org/info/rfc4301>.

   [RFC5739]  Eronen, P., Laganier, J., and C. Madson, "IPv6
              Configuration in Internet Key Exchange Protocol Version 2
              (IKEv2)", RFC 5739, DOI 10.17487/RFC5739, February 2010,
              <http://www.rfc-editor.org/info/rfc5739>.

   [RFC7296]  Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T.
              Kivinen, "Internet Key Exchange Protocol Version 2
              (IKEv2)", STD 79, RFC 7296, DOI 10.17487/RFC7296, October
              2014, <http://www.rfc-editor.org/info/rfc7296>.

5.2.  Informative References

   [RFC7258]  Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
              Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
              2014, <http://www.rfc-editor.org/info/rfc7258>.

   [RFC7435]  Dukhovni, V., "Opportunistic Security: Some Protection
              Most of the Time", RFC 7435, DOI 10.17487/RFC7435,
              December 2014, <http://www.rfc-editor.org/info/rfc7435>.

   [AUTOVPN]  Sheffer, Y. and Y. Nir, "The AutoVPN Architecture", Work
              in Progress, draft-sheffer-autovpn-00, February 2014.

Smyslov & Wouters            Standards Track                    [Page 9]
RFC 7619                   NULL Auth in IKEv2                August 2015

   [DDOS-PROTECTION]
              Nir, Y. and V. Smyslov, "Protecting Internet Key Exchange
              (IKE) Implementations from Distributed Denial of Service
              Attacks", Work in Progress, draft-ietf-ipsecme-ddos-
              protection-02, July 2015.

Smyslov & Wouters            Standards Track                   [Page 10]
RFC 7619                   NULL Auth in IKEv2                August 2015

Appendix A.  Update of PAD processing in RFC 4301

   This appendix lists the specific updates of the text in Section 4.4.3
   of [RFC4301] that should be followed when implementing NULL
   authentication.

   A new item is added to the list of supported ID types in
   Section 4.4.3.1 of [RFC4301]

   o  NULL ID (matches ID type only)

   and the following text is added at the end of the section:

   Added text:
      The NULL ID type is defined as having no data.  For this name
      type, the matching function is defined as comparing the ID type
      only.

   A new item is added to the list of authentication data types in
   Section 4.4.3.2 of [RFC4301]:

      - NULL authentication

   and the next paragraph is updated as follows:

   Old:
      For authentication based on an X.509 certificate [...] For
      authentication based on a pre-shared secret, the PAD contains the
      pre-shared secret to be used by IKE.

   New:
      For authentication based on an X.509 certificate [...] For
      authentication based on a pre-shared secret, the PAD contains the
      pre-shared secret to be used by IKE.  For NULL authentication the
      PAD contains no data.

   In addition, the following text is added at the end of
   Section 4.4.3.4 of [RFC4301]:

   Added text:
      When using the NULL Authentication method, implementations MUST
      make sure that they do not mix authenticated and unauthenticated
      SPD rules, i.e., implementations need to keep them separately; for
      example, by adding a flag in the SPD to tell whether NULL
      authentication can be used or not for the entry.  That is, each
      SPD entry needs to be augmented to have a flag specifying whether

Smyslov & Wouters            Standards Track                   [Page 11]
RFC 7619                   NULL Auth in IKEv2                August 2015

      it can be used with NULL authentication or not, and only those
      rules that explicitly have that flag set can be used with
      unauthenticated connections.

Acknowledgments

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

Authors' Addresses

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

   Phone: +7 495 276 0211
   Email: svan@elvis.ru

   Paul Wouters
   Red Hat

   Email: pwouters@redhat.com

Smyslov & Wouters            Standards Track                   [Page 12]