Internet Engineering Task Force (IETF) K. Hoeper, Ed.
Request for Comments: 5749 M. Nakhjiri
Category: Standards Track Motorola
ISSN: 2070-1721 Y. Ohba, Ed.
Toshiba
March 2010
Distribution of EAP-Based Keys for Handover and Re-Authentication
Abstract
This document describes an abstract mechanism for delivering root
keys from an Extensible Authentication Protocol (EAP) server to
another network server that requires the keys for offering security
protected services, such as re-authentication, to an EAP peer. The
distributed root key can be either a usage-specific root key (USRK),
a domain-specific root key (DSRK), or a domain-specific usage-
specific root key (DSUSRK) that has been derived from an Extended
Master Session Key (EMSK) hierarchy previously established between
the EAP server and an EAP peer. This document defines a template for
a key distribution exchange (KDE) protocol that can distribute these
different types of root keys using a AAA (Authentication,
Authorization, and Accounting) protocol and discusses its security
requirements. The described protocol template does not specify
message formats, data encoding, or other implementation details. It
thus needs to be instantiated with a specific protocol (e.g., RADIUS
or Diameter) before it can be used.
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/rfc5749.
Hoeper, et al. Standards Track [Page 1]
RFC 5749 HOKEY Key Distribution March 2010
Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Key Delivery Architecture . . . . . . . . . . . . . . . . . . 5
4. Key Distribution Exchange (KDE) . . . . . . . . . . . . . . . 6
4.1. Context and Scope for Distributed Keys . . . . . . . . . . 7
4.2. Key Distribution Exchange Scenarios . . . . . . . . . . . 8
5. KDE Used in the EAP Re-Authentication Protocol (ERP) . . . . . 8
6. Security Considerations . . . . . . . . . . . . . . . . . . . 9
6.1. Requirements on AAA Key Transport Protocols . . . . . . . 9
6.2. Distributing RK without Peer Consent . . . . . . . . . . . 10
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10
8. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 10
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
9.1. Normative References . . . . . . . . . . . . . . . . . . . 10
9.2. Informative References . . . . . . . . . . . . . . . . . . 11
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RFC 5749 HOKEY Key Distribution March 2010
1. Introduction
The Extensible Authentication Protocol (EAP) [RFC3748] is an
authentication framework supporting authentication methods that are
specified in EAP methods. By definition, any key-generating EAP
method derives a Master Session Key (MSK) and an Extended Master
Session Key (EMSK). [RFC5295] reserves the EMSK for the sole purpose
of deriving root keys that can be used for specific purposes called
usages. In particular, [RFC5295] defines how to create a usage-
specific root key (USRK) for bootstrapping security in a specific
application, a domain-specific root key (DSRK) for bootstrapping
security of a set of services within a domain, and a usage-specific
DSRK (DSUSRK) for a specific application within a domain. [RFC5296]
defines a re-authentication root key (rRK) that is a USRK designated
for re-authentication.
The MSK and EMSK may be used to derive further keying material for a
variety of security mechanisms [RFC5247]. For example, the MSK has
been widely used for bootstrapping the wireless link security
associations between the peer and the network attachment points.
However, performance as well as security issues arise when using the
MSK and the current bootstrapping methods in mobile scenarios that
require handovers, as described in [RFC5169]. To address handover
latencies and other shortcomings, [RFC5296] specifies an EAP re-
authentication protocol (ERP) that uses keys derived from the EMSK or
DSRK to enable efficient re-authentications in handover scenarios.
Neither [RFC5295] nor [RFC5296] specifies how root keys are delivered
to the network server requiring the key. Such a key delivery
mechanism is essential because the EMSK cannot leave the EAP server
([RFC5295]), but root keys are needed by other network servers
disjoint with the EAP server. For example, in order to enable an EAP
peer to re-authenticate to a network during a handover, certain root
keys need to be made available by the EAP server to the server
carrying out the re-authentication.
This document specifies an abstract mechanism for the delivery of the
EMSK child keys from the server holding the EMSK or a root key to
another network server that requests a root key for providing
protected services (such as re-authentication and other usage and
domain-specific services) to EAP peers. In the remainder of this
document, a server delivering root keys is referred to as a Key
Delivering Server (KDS), and a server authorized to request and
receive root keys from a KDS is referred to as a Key Requesting
Server (KRS). The Key Distribution Exchange (KDE) mechanism defined
in this document runs over a AAA (Authentication, Authorization, and
Accounting) protocol, e.g., RADIUS ([RFC2865], [RFC3579]) or Diameter
[RFC3588], and has several variants depending on the type of key that
is requested and delivered (i.e., DRSK, USRK, or DSUSRK). The
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presented KDE mechanism is a protocol template that must be
instantiated for a particular protocol, such as RADIUS or Diameter,
to specify the format and encoding of the abstract protocol messages.
Only after such an instantiation can the KDE mechanism described in
this document be implemented. This document also describes security
requirements for the secure key delivery over AAA.
2. Terminology
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].
The following acronyms are used.
AAA
Authentication, Authorization and Accounting. AAA protocols with
EAP support include RADIUS ([RFC2865], [RFC3579]) and Diameter
[RFC3588].
USRK
Usage-Specific Root Key. A root key that is derived from the
EMSK; see [RFC5295].
USR-KH
USRK Holder. A network server that is authorized to request and
receive a USRK from the EAP server. The USR-KH can be a AAA
server or dedicated service server.
DSRK
Domain-Specific Root Key. A root key that is derived from the
EMSK; see [RFC5295].
DSR-KH
DSRK Holder. A network server that is authorized to request and
receive a DSRK from the EAP server. The most likely
implementation of a DSR-KH is a AAA server in a domain, enforcing
the policies for the usage of the DSRK within this domain.
DSUSRK
Domain-Specific Usage-Specific Root Key. A root key that is
derived from the DSRK; see [RFC5295].
DSUSR-KH
DSUSRK holder. A network server authorized to request and receive
a DSUSRK from the DSR-KH. The most likely implementation of a
DSUSR-KH is a AAA server in a domain, responsible for a particular
service offered within this domain.
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RFC 5749 HOKEY Key Distribution March 2010
RK
Root Key. An EMSK child key, i.e., a USRK, DSRK, or DSUSRK.
KDS
Key Delivering Server. A network server that holds an EMSK or
DSRK and delivers root keys to a KRS requesting root keys. The
EAP server (together with the AAA server to which it exports the
keys for delivery) and the DSR-KH can both act as KDS.
KRS
Key Requesting Server. A network server that shares an interface
with a KDS and is authorized to request root keys from the KDS. A
USR-KH, DSR-KH, and DSUSR-KH can all act as a KRS.
HOKEY
Handover Keying.
3. Key Delivery Architecture
An EAP server carries out normal EAP authentications with EAP peers
but is typically not involved in potential handovers and re-
authentication attempts by the same EAP peer. Other servers are
typically in place to offer these requested services. These servers
can be AAA servers or other service network servers. Whenever EAP-
based keying material is used to protect a requested service, the
respective keying material has to be available to the server
providing the requested service. For example, the first time a peer
requests a service from a network server, this server acts as a KRS.
The KRS requests the root keys needed to derive the keys for
protecting the requested service from the respective KDS. In
subsequent requests from the same peer and as long as the root key
has not expired, the KRS can use the same root keys to derive fresh
keying material to protect the requested service. These kinds of key
requests and distributions are necessary because an EMSK cannot leave
the EAP server ([RFC5295]). Hence, any root key that is directly
derived from an EMSK can only be derived by the EAP server itself.
The EAP server then exports these keys to a server that can
distribute the keys to the KRS. In the remainder of this document,
the KDS consisting of the EAP server that derives the root keys
together with the AAA server that distributes these keys is denoted
EAP/AAA server. Root keys derived from EMSK child keys, such as a
DSUSRK, can be requested from the respective root key holder. Hence,
a KDS can be either the EAP/AAA server or a DSRK holder (DSR-KH),
whereas a KRS can be either a USRK holder (USR-KH), a DSR-KH, or a
DSUSRK holder (DSUSR-KH).
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The KRS needs to share an interface with the KDS to be able to send
all necessary input data to derive the requested key and to receive
the requested key. The provided data includes the Key Derivation
Function (KDF) that should be used to derive the requested key. The
KRS uses the received root key to derive further keying material in
order to secure its offered services. Every KDS is responsible for
storing and protecting the received root key as well as the
derivation and distribution of any child key derived from the root
key. An example of a key delivery architecture is illustrated in
Figure 1 showing the different types of KRS and their interfaces to
the KDS.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| EAP/AAA server |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ | | \
/ | | \
/ | | \
+-+-+-+-+-+-+-+ +-+-+-+-+-+-+ +-+-+-+-+-+ +-+-+-+-+-+
| USR-KH1 | | USR-KH2 | | DSR-KH1 | | DSR-KH2 |
| HOKEY server| | XYZ server| |Domain 1 | | Domain 2|
+-+-+-+-+-+-+-+ +-+-+-+-+-+-+ +-+-+-+-+-+ +-+-+-+-+-+
/ |
/ |
/ |
+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
| DSUSR-KH | | DSUSR-KH2 |
| Domain 1 | | Domain 2 |
|Home domain | |Visited domain |
|HOKEY server | |HOKEY server |
+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
Figure 1: Example Key Delivery Architecture for the Different KRS and
KDS
4. Key Distribution Exchange (KDE)
In this section, a generic mechanism for a key distribution exchange
(KDE) over AAA is described in which a root key (RK) is distributed
from a KDS to a KRS. It is required that the communication path
between the KDS and the KRS is protected by the use of an appropriate
AAA transport security mechanism (see Section 6 for security
requirements). Here, it is assumed that the KRS and the KDS are
separate entities, logically if not physically, and the delivery of
the requested RK is specified accordingly.
The key distribution exchange consists of one round-trip, i.e., two
messages between the KRS and the KDS, as illustrated in Figure 2.
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RFC 5749 HOKEY Key Distribution March 2010
First, the KRS sends a KDE-Request carrying a Key Request Token
(KRT). As a response, the KDS sends a KDE-Response carrying a Key
Delivery Token (KDT). Both tokens are encapsulated in AAA messages.
The definition of the AAA attributes depends on the implemented AAA
protocol and is out of scope of this document. However, the security
requirements for AAA messages carrying KDE messages are discussed in
Section 6. The contents of KRT and KDT are defined in the following.
KRS KDS
-------- -------
| |
| KDE-Request: AAA{KRT} |
|----------------------------------------->|
| KDE-Response: AAA{KDT} |
|<-----------------------------------------|
Figure 2: KDE Message Flow
KRT : (PID, KT, KL)
KRT carries the identifiers of the peer (PID), the key type (KT)
and the key label (KL). The key type specifies which type of root
key is requested, e.g., DSRK, USRK and DSUSRK. The encoding rules
for each key type are left to the protocol developers who define
the instantiation of the KDE mechanism for a particular protocol.
For the specification of key labels and the associated IANA
registries, please refer to [RFC5295], which specifies key labels
for USRKs and establishes an IANA registry for them. The same
specifications can be applied to other root keys.
KDT : (KT, KL, RK, KN_RK, LT_RK)
KDT carries the root key (RK) to be distributed to the KRS, as
well as the key type (KT) of the key, the key label (KL), the key
name (KN_RK), and the lifetime of RK (LT_RK). The key lifetime of
each distributed key MUST NOT be greater than that of its parent
key.
4.1. Context and Scope for Distributed Keys
The key context of each distributed key is determined by the sequence
of KTs in the key hierarchy. The key scope of each distributed key
is determined by the sequence of (PID, KT, KL)-tuples in the key
hierarchy and the identifier of the KRS. The KDF used to generate
the requested keys includes context and scope information, thus,
binding the key to the specific channel [RFC5295].
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4.2. Key Distribution Exchange Scenarios
Given the three types of KRS, there are three scenarios for the
distribution of the EMSK child keys. For all scenarios, the trigger
and mechanism for key delivery may involve a specific request from an
EAP peer and/or another intermediary (such as an authenticator). For
simplicity, it is assumed that USR-KHs reside in the same domain as
the EAP server.
Scenario 1: EAP/AAA server to USR-KH: In this scenario, the EAP/AAA
server delivers a USRK to a USR-KH.
Scenario 2: EAP/AAA server to DSR-KH: In this scenario, the EAP/AAA
server delivers a DSRK to a DSR-KH.
Scenario 3: DSR-KH to DSUSR-KH: In this scenario, a DSR-KH in a
specific domain delivers keying material to a DSUSR-KH in the same
domain.
The key distribution exchanges for Scenario 3 can be combined with
the key distribution exchanges for Scenario 2 into a single round-
trip exchange as shown in Figure 3. Here, KDE-Request and KDE-
Response are messages for Scenarios 2, whereas KDE-Request' and KDE-
Response' are messages for Scenarios 3.
DSUSR-KH DSR-KH EAP/AAA Server
-------- ------ ------------
| KDE-Request'(KRT') | KDE-Request(KRT) |
|------------------------>|-------------------------->|
| KDE-Response'(KDT') | KDE-Response(KDT) |
|<----------------------- |<--------------------------|
| | |
Figure 3: Combined Message Exchange
5. KDE Used in the EAP Re-Authentication Protocol (ERP)
This section describes how the presented KDE mechanism should be used
to request and deliver the root keys used for re-authentication in
the EAP Re-authentication Protocol (ERP) defined in [RFC5296]. ERP
supports two forms of bootstrapping, implicit as well as explicit
bootstrapping, and KDE is discussed for both cases in the remainder
of this section.
In implicit bootstrapping, the local EAP Re-authentication (ER)
server requests the DSRK from the home AAA server during the initial
EAP exchange. Here, the local ER server acts as the KRS and the home
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AAA server as the KDS. In this case, the local ER server requesting
the DSRK includes a KDE-Request in the AAA packet encapsulating the
first EAP-Response message from the peer. Here, a AAA User-Name
attribute is used as the PID. If the EAP exchange is successful, the
home AAA server includes a KDE-Response in the AAA message that
carries the EAP-Success message.
Explicit bootstrapping is initiated by peers that do not know the
domain. Here, the peer sends an EAP-Initiate message with the
bootstrapping flag turned on. The local ER server (acting as KRS)
includes a KDE-Request message in the AAA message that carries the
peer's EAP-Initiate message and sends it to the peer's home AAA
server. Here, a AAA User-Name attribute is used as the PID. In its
response, the home AAA server (acting as KDS) includes a KDE-Response
in the AAA message that carries the EAP-Finish message with the
bootstrapping flag set.
6. Security Considerations
This section provides security requirements and a discussion of
distributing RK without peer consent.
6.1. Requirements on AAA Key Transport Protocols
Any KDE attribute that is exchanged as part of a KDE-Request or KDE-
Response MUST be integrity-protected and replay-protected by the
underlying AAA protocol that is used to encapsulate the attributes.
Additionally, a secure key wrap algorithm MUST be used by the AAA
protocol to protect the RK in a KDE-Response. Other confidential
information as part of the KDE messages (e.g., identifiers if privacy
is a requirement) SHOULD be encrypted by the underlying AAA protocol.
When there is an intermediary, such as a AAA proxy, on the path
between the KRS and the KDS, there will be a series of hop-by-hop
security associations along the path. The use of hop-by-hop security
associations implies that the intermediary on each hop can access the
distributed keying material. Hence, the use of hop-by-hop security
SHOULD be limited to an environment where an intermediary is trusted
not to abuse the distributed key material. If such a trusted AAA
infrastructure does not exist, other means must be applied at a
different layer to ensure the end-to-end security (i.e., between KRS
and KDS) of the exchanged KDE messages. The security requirements
for such a protocol are the same as previously outlined for AAA
protocols and MUST hold when encapsulated in AAA messages.
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6.2. Distributing RK without Peer Consent
When a KDE-Request is sent as a result of explicit ERP bootstrapping
[RFC5296], cryptographic verification of peer consent on distributing
an RK is provided by the integrity checksum of the EAP-Initiate
message with the bootstrapping flag turned on.
On the other hand, when a KDE-Request is sent as a result of implicit
ERP bootstrapping [RFC5296], cryptographic verification of peer
consent on distributing an RK is not provided. A peer is not
involved in the process and, thus, not aware of key delivery requests
for root keys derived from its established EAP keying material.
Hence, a peer has no control where keys derived from its established
EAP keying material are distributed. A possible consequence of this
is that a KRS may request and obtain an RK from the home server even
if the peer does not support ERP. EAP-Initiate/Re-auth-Start
messages send to the peer will be silently dropped by the peer
causing further waste of resources.
7. Acknowledgments
The editors would like to thank Dan Harkins, Chunqiang Li, Rafael
Marin Lopez, and Charles Clancy for their valuable comments.
8. Contributors
The following people contributed to this document: Kedar Gaonkar,
Lakshminath Dondeti, Vidya Narayanan, and Glen Zorn.
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3748] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
Levkowetz, "Extensible Authentication Protocol (EAP)",
RFC 3748, June 2004.
[RFC5295] Salowey, J., Dondeti, L., Narayanan, V., and M. Nakhjiri,
"Specification for the Derivation of Root Keys from an
Extended Master Session Key (EMSK)", RFC 5295,
August 2008.
[RFC5296] Narayanan, V. and L. Dondeti, "EAP Extensions for EAP Re-
authentication Protocol (ERP)", RFC 5296, August 2008.
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9.2. Informative References
[RFC2865] Rigney, C., Willens, S., Rubens, A., and W. Simpson,
"Remote Authentication Dial In User Service (RADIUS)",
RFC 2865, June 2000.
[RFC3579] Aboba, B. and P. Calhoun, "RADIUS (Remote Authentication
Dial In User Service) Support For Extensible
Authentication Protocol (EAP)", RFC 3579, September 2003.
[RFC3588] Calhoun, P., Loughney, J., Guttman, E., Zorn, G., and J.
Arkko, "Diameter Base Protocol", RFC 3588, September 2003.
[RFC5169] Clancy, T., Nakhjiri, M., Narayanan, V., and L. Dondeti,
"Handover Key Management and Re-Authentication Problem
Statement", RFC 5169, March 2008.
[RFC5247] Aboba, B., Simon, D., and P. Eronen, "Extensible
Authentication Protocol (EAP) Key Management Framework",
RFC 5247, August 2008.
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Authors' Addresses
Katrin Hoeper (editor)
Motorola, Inc.
1295 E Algonquin Road
Schaumburg, IL 60196
USA
Phone: +1 847 576 4714
EMail: khoeper@motorola.com
Madjid F. Nakhjiri
Motorola, Inc.
6450 Sequence Drive
San Diego, CA 92121
USA
EMail: madjid.nakhjiri@motorola.com
Yoshihiro Ohba (editor)
Toshiba Corporate Research and Development Center
1 Komukai-Toshiba-cho
Saiwai-ku, Kawasaki, Kanagawa 212-8582
Japan
Phone: +81 44 549 2230
EMail: yoshihiro.ohba@toshiba.co.jp
Hoeper, et al. Standards Track [Page 12]
RFC, FYI, BCP