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This document describes the Handover Keying (HOKEY) problem statement. The current Extensible Authentication Protocol (EAP) keying framework is not designed to support re-authentication and handovers. This often causes unacceptable latency in various mobile wireless environments. The HOKEY Working Group plans to address these problems by designing a generic mechanism to reuse derived EAP keying material for handover.
3. Problem Statement
4. Design Goals
5. Security Goals
5.1. Key Context and Domino Effect
5.2. Key Freshness
5.5. Channel Binding
5.6. Transport Aspects
6. Use Cases and Related Work
6.1. IEEE 802.11r Applicability
6.2. IEEE 802.21 Applicability
6.3. CAPWAP Applicability
6.4. Inter-Technology Handover
7. Security Considerations
8. IANA Considerations
10.1. Normative References
10.2. Informative References
§ Authors' Addresses
§ Intellectual Property and Copyright Statements
The Extensible Authentication Protocol (EAP), specified in RFC 3748 [RFC3748] (Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H. Levkowetz, “Extensible Authentication Protocol (EAP),” June 2004.) is a generic framework supporting multiple authentication methods. The primary purpose of EAP is network access control. It also supports exporting session keys derived during the authentication. The EAP keying hierarchy defines two keys that are derived at the top level, the Master Session Key (MSK) and the Extended Master Session Key (EMSK).
In many common deployment scenario, an EAP peer and EAP server authenticate each other through a third party known as the pass-through authenticator (hereafter referred to as simply "authenticator"). The authenticator is responsible for translating EAP packets from the layer 2 (L2) or layer 3 (L3) network access technology to the Authentication, Authorization, and Accounting (AAA) protocol. The authenticator does not directly participate in the EAP exchange, and simply acts as a gateway during the EAP method execution.
According to [RFC3748] (Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H. Levkowetz, “Extensible Authentication Protocol (EAP),” June 2004.), after successful authentication, the server to transports the MSK to the authenticator. Note that this is performed using AAA protocols, not EAP itself. The underlying L2 or L3 protocol uses the MSK to derive additional keys, including the transient session keys (TSKs) used for per-packet encryption and authentication.
Note that while the authenticator is one logical device, there can be multiple physical devices involved. For example, the CAPWAP model [RFC3990] (O'Hara, B., Calhoun, P., and J. Kempf, “Configuration and Provisioning for Wireless Access Points (CAPWAP) Problem Statement,” February 2005.) splits authenticators into two logical devices: Wireless Termination Points (WTPs) and Access Controllers (ACs). Depending on the configuration, authenticator features can be split in a variety of ways between physical devices, however from the EAP perspective there is only one logical authenticator.
The current models of EAP authentication and keying are unfortunately not efficient in cases where the peer is a mobile device. When a peer arrives at the new authenticator, the security restraints will require the peer to run an EAP method irrespective of whether it has been authenticated to the network recently and has unexpired keying material. A full EAP method execution may involve several round trips between the EAP peer and the server.
There have been attempts to solve the problem of efficient re-authentication in various ways. However, those solutions are either EAP-method specific, EAP lower-layer specific, or are otherwise limited in scope. Furthermore, these solutions do not deal with scenarios involving handovers to new authenticators, or do not conform to the AAA keying requirements specified in [RFC4962] (Housley, R. and B. Aboba, “Guidance for Authentication, Authorization, and Accounting (AAA) Key Management,” July 2007.).
This document provides a detailed description of efficient EAP-based re-authentication protocol requirements.
In this document, several words are used to signify the requirements of the specification. These words are often capitalized. 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] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).
With respect to EAP, this document follows the terminology that has been defined in [RFC3748] (Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H. Levkowetz, “Extensible Authentication Protocol (EAP),” June 2004.) and [I‑D.ietf‑eap‑keying] (Aboba, B., Simon, D., and P. Eronen, “Extensible Authentication Protocol (EAP) Key Management Framework,” November 2007.).
Under the existing model, any re-authentication requires a full EAP exchange with the EAP server in its home domain [RFC3748] (Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H. Levkowetz, “Extensible Authentication Protocol (EAP),” June 2004.). An EAP conversation with a full EAP method run can take several round trips and significant time to complete, causing delays in re-authentication and handover times. Some methods [RFC4187] (Arkko, J. and H. Haverinen, “Extensible Authentication Protocol Method for 3rd Generation Authentication and Key Agreement (EAP-AKA),” January 2006.) specify the use of keys and state from the initial authentication to finish subsequent authentications in fewer round trips. However, even in those cases, multiple round trips to the EAP server are still involved. Furthermore, many commonly-used EAP methods do not offer such a fast re-authentication feature. In summary, it is undesirable to have to run a full EAP method each time a peer authenticates to a new authenticator or needs to extend its current authentication with the same authenticator. Furthermore, it is desirable to specify a method-independent, efficient, re-authentication protocol. Keying material from the full authentication can be used to enable efficient re-authentication.
Significant network latency between the peer and EAP server is another source of delay during re-authentication. It is desirable to have a local server with low-latency connectivity to the peer that can facilitate re-authentication.
Lastly, a re-authentication protocol should also be capable of supporting handover keying. Handover keying allows an EAP server to pass authentication information to a remote re-authentication server, allowing a peer to re-authenticate to that re-authentication server without having to communicate with its home re-authentication server.
These problems are the primary issues to be resolved. In solving them, there are a number of constraints to conform to and those result in some additional work to be done in the area of EAP keying.
The following are the goals and constraints in designing the EAP re-authentication and key management protocol:
- Lower latency operation:
- The protocol MUST be responsive to handover and re-authentication latency performance objectives within a mobile access network. A solution that reduces latency as compared to a full EAP authentication will be most favorable.
- EAP lower-layer independence:
- Any keying hierarchy and protocol defined MUST be lower layer independent in order to provide the capability over heterogeneous technologies. The defined protocols MAY require some additional support from the lower layers that use it. Any keying hierarchy and protocol defined MUST accommodate inter-technology heterogeneous handover.
- EAP method independence:
- Changes to existing EAP methods MUST NOT be required as a result of the re-authentication protocol. There MUST be no requirements imposed on future EAP methods. Note that the only EAP methods for which independence is required are those that conform to the specifications of [I‑D.ietf‑eap‑keying] (Aboba, B., Simon, D., and P. Eronen, “Extensible Authentication Protocol (EAP) Key Management Framework,” November 2007.) and [RFC4017] (Stanley, D., Walker, J., and B. Aboba, “Extensible Authentication Protocol (EAP) Method Requirements for Wireless LANs,” March 2005.).
- AAA protocol compatibility and keying:
- Any modifications to EAP and EAP keying MUST be compatible with RADIUS and Diameter. Extensions to both RADIUS and Diameter to support these EAP modifications are acceptable. The designs and protocols must satisfy the AAA key management requirements specified in RFC 4962 [RFC4962] (Housley, R. and B. Aboba, “Guidance for Authentication, Authorization, and Accounting (AAA) Key Management,” July 2007.).
- Compatibility and co-existence with compliant ([RFC3748] (Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H. Levkowetz, “Extensible Authentication Protocol (EAP),” June 2004.) [I‑D.ietf‑eap‑keying] (Aboba, B., Simon, D., and P. Eronen, “Extensible Authentication Protocol (EAP) Key Management Framework,” November 2007.)) EAP deployments SHOULD be provided. The keying hierarchy or protocol extensions MUST NOT preclude the use of CAPWAP or IEEE 802.11r.
The section draws from the guidance provided in [RFC4962] (Housley, R. and B. Aboba, “Guidance for Authentication, Authorization, and Accounting (AAA) Key Management,” July 2007.) to further define the security goals to be achieved by a complete re-authentication keying solution.
Any key MUST have a well-defined scope and MUST be used in a specific context and for the intended use. This specifically means the lifetime and scope of each key MUST be defined clearly so that all entities that are authorized to have access to the key have the same context during the validity period. In a hierarchical key structure, the lifetime of lower level keys MUST NOT exceed the lifetime of higher level keys. This requirement MAY imply that the context and the scope parameters have to be exchanged. Furthermore, the semantics of these parameters MUST be defined to provide proper channel binding specifications. The definition of exact parameter syntax definition is part of the design of the transport protocol used for the parameter exchange and that may be outside scope of this protocol.
If a key hierarchy is deployed, compromising lower level keys MUST NOT result in a compromise of higher level keys which they were used to derive the lower level keys. The compromise of keys at each level MUST NOT result in compromise of other keys at the same level. The same principle applies to entities that hold and manage a particular key defined in the key hierarchy. Compromising keys on one authenticator MUST NOT reveal the keys of another authenticator. Note that the compromise of higher-level keys has security implications on lower levels.
Guidance on parameters required, caching, storage and deletion procedures to ensure adequate security and authorization provisioning for keying procedures MUST be defined in a solution document.
All the keying material MUST be uniquely named so that it can be managed effectively.
As [RFC4962] (Housley, R. and B. Aboba, “Guidance for Authentication, Authorization, and Accounting (AAA) Key Management,” July 2007.) defines, a fresh key is one that is generated for the intended use. This would mean the key hierarchy MUST provide for creation of multiple cryptographically separate child keys from a root key at higher level. Furthermore, the keying solution needs to provide mechanisms for refreshing each of the keys within the key hierarchy.
Each party in the handover keying architecture MUST be authenticated to any other party with whom it communicates, and securely provide its identity to any other entity that may require the identity for defining the key scope. The identity provided MUST be meaningful according to the protocol over which the two parties communicate.
The EAP Key management document [I‑D.ietf‑eap‑keying] (Aboba, B., Simon, D., and P. Eronen, “Extensible Authentication Protocol (EAP) Key Management Framework,” November 2007.) discusses several vulnerabilities that are common to handover mechanisms. One important issue arises from the way the authorization decisions might be handled at the AAA server during network access authentication. For example, if AAA proxies are involved, they may also influence in the authorization decision. Furthermore, the reasons for making a particular authorization decision are not communicated to the authenticator. In fact, the authenticator only knows the final authorization result. The proposed solution MUST make efforts to document and mitigate authorization attacks.
Channel Binding procedures are needed to avoid a compromised intermediate authenticator providing unverified and conflicting service information to each of the peer and the EAP server. In the architecture introduced in this document, there are multiple intermediate entities between the peer and the back-end EAP server. Various keys need to be established and scoped between these parties and some of these keys may be parents to other keys. Hence the channel binding for this architecture will need to consider layering intermediate entities at each level to make sure that an entity with higher level of trust can examine the truthfulness of the claims made by intermediate parties.
Depending on the physical architecture and the functionality of the elements involved, there may be a need for multiple protocols to perform the key transport between entities involved in the handover keying architecture. Thus, a set of requirements for each of these protocols, and the parameters they will carry, MUST be developed. Following the requirement specifications, recommendations will be provided as to whether new protocols or extensions to existing protocols are needed.
As mentioned, the use of existing AAA protocols for carrying EAP messages and keying material between the AAA server and AAA clients that have a role within the architecture considered for the keying problem will be carefully examined. Definition of specific parameters, required for keying procedures and to be transferred over any of the links in the architecture, are part of the scope. The relation of the identities used by the transport protocol and the identities used for keying also needs to be explored.
In order to further clarify the items listed in scope of the proposed work, this section provides some background on related work and the use cases envisioned for the proposed work.
One of the EAP lower layers, IEEE 802.11 [IEEE.802‑11R‑D7.0] (, “Information technology - Telecommunications and information exchange between systems - Local and metropolitan area networks - Specific requirements - Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications - Amendment 2: Fast BSS Transition,” June 2007.), is in the process of specifying a mechanism to avoid the problem of repeated full EAP exchanges in a limited setting, by introducing a two-level key hierarchy. The EAP authenticator is collocated with what is known as an R0 Key Holder (R0-KH), which receives the MSK from the EAP server. A pairwise master key (PMK-R0) is derived from the last 32 octets of the MSK. Subsequently, the R0-KH derives an PMK-R1 to be handed out to the attachment point of the peer. When the peer moves from one R1-KH to another, a new PMK-R1 is generated by the R0-KH and handed out to the new R1-KH. The transport protocol used between the R0-KH and the R1-KH is not specified.
In some cases, a mobile may seldom move beyond the domain of the R0-KH and this model works well. A full EAP authentication will generally be repeated when the PMK-R0 expires. However, in general cases mobiles may roam beyond the domain of R0-KHs (or EAP authenticators), and the latency of full EAP authentication remains an issue.
Another consideration is that there needs to be a key transfer protocol between the R0-KH and the R1-KH; in other words, there is either a star configuration of security associations between the key holder and a centralized entity that serves as the R0-KH, or if the first authenticator is the default R0-KH, there will be a full-mesh of security associations between all authenticators. This is undesirable.
The proposed work on EAP efficient re-authentication protocol aims at addressing re-authentication in a lower layer agnostic manner that also can fill some of the gaps in IEEE 802.11r.
The IEEE 802.21 working group [IEEE.802‑21] (, “Media Independent Handover Working Group,” .) is standardizing mechanisms for media-independent handover. More specifically, they are looking at transitions from one link-layer protocol to another, which is currently beyond the scope of the HOKEY charter.
The techniques developed within HOKEY could be applicable to IEEE 802.21 if the necessary issues with handover between different lower layers can be resolved. In particular, pre-authentication may be more appropriate than re-authentication.
The IETF CAPWAP Working Group [RFC3990] (O'Hara, B., Calhoun, P., and J. Kempf, “Configuration and Provisioning for Wireless Access Points (CAPWAP) Problem Statement,” February 2005.) is developing a protocol between what is termed an Access Controller (AC) and Wireless Termination Points (WTP). The AC and WTP can be mapped to a WLAN switch and Access Point respectively. The CAPWAP model supports both split and integrated MAC architectures, with the authenticator always being implemented at the AC.
The proposed work on EAP efficient re-authentication protocol addresses an inter-authenticator handover problem from an EAP perspective, which applies during handover between ACs. Inter-controller handover is a topic yet to be addressed in great detail and the re-authentication work can potentially address it in an effective manner.
EAP is used for access authentication by several technologies and is under consideration for use over several other technologies going forward. Given that, it should be feasible to support smoother handover across technologies. That is one of the big advantages of using a common authentication protocol. Authentication procedures typically add substantial handover delays.
An EAP peer that has multiple radio technologies (802.11 and GSM, for instance) must perform the full EAP exchange on each interface upon every horizontal or vertical handover. With a method independent EAP efficient re-authentication, it is feasible to support faster handover even in the vertical handover cases, when the peer may be roaming from one technology to another.
This document details the HOKEY problem statement. Since HOKEY is an authentication protocol, there are a myriad of security-related issues surrounding its development and deployment.
In this document, we have detailed a variety of security properties inferred from [RFC4962] (Housley, R. and B. Aboba, “Guidance for Authentication, Authorization, and Accounting (AAA) Key Management,” July 2007.) to which HOKEY must conform, including the management of key context, scope, freshness, and transport; resistance to attacks based on the domino effect; and authentication and authorization. See section Section 5 (Security Goals) for further details.
This document does not introduce any new IANA considerations.
This document represents the synthesis of two problem statement documents. In this section, we acknowledge their contributions, and involvement in the early documents.
- Mohan Parthasarathy
- Email: firstname.lastname@example.org
- Julien Bournelle
- France Telecom R&D
- Email: email@example.com
- Hannes Tschofenig
- Email: Hannes.Tschofenig@siemens.com
- Rafael Marin Lopez
- Universidad de Murcia
- Email: firstname.lastname@example.org
|[RFC2119]||Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” BCP 14, RFC 2119, March 1997 (TXT, HTML, XML).|
|[RFC3748]||Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H. Levkowetz, “Extensible Authentication Protocol (EAP),” RFC 3748, June 2004 (TXT).|
|[RFC4017]||Stanley, D., Walker, J., and B. Aboba, “Extensible Authentication Protocol (EAP) Method Requirements for Wireless LANs,” RFC 4017, March 2005 (TXT).|
|[RFC4962]||Housley, R. and B. Aboba, “Guidance for Authentication, Authorization, and Accounting (AAA) Key Management,” BCP 132, RFC 4962, July 2007 (TXT).|
|[I-D.ietf-eap-keying]||Aboba, B., Simon, D., and P. Eronen, “Extensible Authentication Protocol (EAP) Key Management Framework,” draft-ietf-eap-keying-22 (work in progress), November 2007 (TXT).|
|[RFC3990]||O'Hara, B., Calhoun, P., and J. Kempf, “Configuration and Provisioning for Wireless Access Points (CAPWAP) Problem Statement,” RFC 3990, February 2005 (TXT).|
|[RFC4187]||Arkko, J. and H. Haverinen, “Extensible Authentication Protocol Method for 3rd Generation Authentication and Key Agreement (EAP-AKA),” RFC 4187, January 2006 (TXT).|
|[IEEE.802-11R-D7.0]||“Information technology - Telecommunications and information exchange between systems - Local and metropolitan area networks - Specific requirements - Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications - Amendment 2: Fast BSS Transition,” IEEE Standard 802.11r, June 2007.|
|[IEEE.802-21]||“Media Independent Handover Working Group,” IEEE Working Group 802.21.|
|T. Charles Clancy, Editor|
|Laboratory for Telecommunications Sciences|
|US Department of Defense|
|College Park, MD|
|San Diego, CA|
|San Diego, CA|
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