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)
AbstractThis 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.
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. 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
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.
RFC2119]. Section 2.16 of [RFC7296]). NULL authentication affects how the Authentication and the Identification payloads are formed in the IKE_AUTH exchange. 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.
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. 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. 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
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.
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. 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.
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.
[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>. [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.
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 220.127.116.11 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 18.104.22.168 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 22.214.171.124 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
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.