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 draft-ietf-ipsecme-ikev2-null-auth-05 Abstract 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 This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on September 27, 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 Smyslov & Wouters Expires September 27, 2015 [Page 1] Internet-Draft NULL Auth in IKEv2 March 2015 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 . . . . . . . . . . . . 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 Smyslov & Wouters Expires September 27, 2015 [Page 2] Internet-Draft NULL Auth in IKEv2 March 2015 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 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 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 identity. 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 Smyslov & Wouters Expires September 27, 2015 [Page 3] Internet-Draft NULL Auth in IKEv2 March 2015 document are to be interpreted as described in [RFC2119]. Smyslov & Wouters Expires September 27, 2015 [Page 4] Internet-Draft NULL Auth in IKEv2 March 2015 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 Smyslov & Wouters Expires September 27, 2015 [Page 5] Internet-Draft NULL Auth in IKEv2 March 2015 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 Smyslov & Wouters Expires September 27, 2015 [Page 6] Internet-Draft NULL Auth in IKEv2 March 2015 described in Section 4.4.3.4 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 4.4.3.1 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 data. Section 4.4.3.3 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 authenticated. Smyslov & Wouters Expires September 27, 2015 [Page 7] Internet-Draft NULL Auth in IKEv2 March 2015 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 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 "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 Smyslov & Wouters Expires September 27, 2015 [Page 8] Internet-Draft NULL Auth in IKEv2 March 2015 Authentication. 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 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 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. Smyslov & Wouters Expires September 27, 2015 [Page 9] Internet-Draft NULL Auth in IKEv2 March 2015 4. Acknowledgments The authors would like to thank Yaron Sheffer and Tero Kivinen for their reviews, valuable comments and contributed text. Smyslov & Wouters Expires September 27, 2015 [Page 10] Internet-Draft NULL Auth in IKEv2 March 2015 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 Smyslov & Wouters Expires September 27, 2015 [Page 11] Internet-Draft NULL Auth in IKEv2 March 2015 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. [DDOS-PROTECTION] 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. Smyslov & Wouters Expires September 27, 2015 [Page 12] Internet-Draft NULL Auth in IKEv2 March 2015 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 Expires September 27, 2015 [Page 13]