Network Working Group K. Hoeper, Ed.
Internet-Draft Motorola
Intended status: Standards Track Y. Ohba, Ed.
Expires: October 5, 2009 Toshiba
April 3, 2009
Distribution of EAP based keys for handover and re-authentication
draft-ietf-hokey-key-mgm-06
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Abstract
This document describes a 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. The document defines a key distribution exchange (KDE)
protocol using Remote Authentication Dial In User Service (RADIUS)
that can distribute these different types of root keys and discusses
its security requirements.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Key Delivery Architecture . . . . . . . . . . . . . . . . . . 5
4. Key Distribution Exchange (KDE) . . . . . . . . . . . . . . . 7
4.1. Context and Scope for Distributed Keys . . . . . . . . . . 8
4.2. Key Distribution Exchange Scenarios . . . . . . . . . . . 8
5. RADIUS KDE Attribute . . . . . . . . . . . . . . . . . . . . . 9
6. KDE used in the EAP Re-authentication Protocol (ERP) . . . . . 10
7. Conflicting Messages . . . . . . . . . . . . . . . . . . . . . 11
8. Security Considerations . . . . . . . . . . . . . . . . . . . 11
8.1. Requirements on RADIUS Key Transport . . . . . . . . . . . 11
8.2. Distributing RK without Peer Consent . . . . . . . . . . . 12
9. IANA consideration . . . . . . . . . . . . . . . . . . . . . . 12
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12
11. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 12
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
12.1. Normative References . . . . . . . . . . . . . . . . . . . 13
12.2. Informative references . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 14
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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 an 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.
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 EMSK or
DSRK to enable efficient re-authentications in handover scenarios.
[RFC5295] and [RFC5296] both do not specify 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 a mechanism for the delivery of 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 Key Delivering Server
(KDS) and a server authorized to request and receive root keys from a
KDS is referred to as Key Requesting Server (KRS). The Key
Distribution Exchange (KDE) protocol defined in this document uses
RADIUS [RFC2865], [RFC3579] and has several variants depending on the
type of key that is requested and delivered (i.e. DRSK, USRK, and
DSUSRK). The document also describes security requirements for the
secure key delivery over RADIUS.
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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].
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 an 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 an 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 an AAA server in a domain, responsible for a
particular service offered within this domain.
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 KRS requesting root keys. The EAP
server and DSR-KH can 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. USR-KH, DSR-KH, and DSUSR-KH can all act as KRS.
3. Key Delivery Architecture
An EAP server carries out the EAP authentications with EAP peers but
is typically not making any, potentially future, service
authorization decisions involving peers. Authorizations as well as
the service provisioning are handled by the respective network server
offering the requested service. These servers can be AAA servers or
other service servers. Whenever EAP-based keying material is used to
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protect a requested service, a network server needs to request the
root key associated with the offered service from the respective KDS.
This kind 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 must be derived and delivered
by the EAP server itself, whereas 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 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).
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 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
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4. Key Distribution Exchange (KDE)
In this section, a generic mechanism for a key distribution exchange
(KDE)over RADIUS 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
RADIUS transport security mechanism (see Section 8). 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 roundtrip, i.e. two
messages between the KRS and the KDS, as illustrated in Figure 2.
First, the KRS sends a KDE-Request consisting of a RADIUS Access-
Request message with a KDE attribute in which the K-flag is cleared.
As a response, the KDS sends a KDE-Response consisting of a RADIUS
Access-Accept message with a KDE attribute in which the K-flag set.
The RADIUS KDE attribute used in this exchange is defined in
Section 5.
KRS KDS
-------- -------
| |
| |
| KDE-Request (KRT) |
| (i.e., RADIUS Access-Request{KDE(K=0)}) |
|----------------------------------------->|
| KDE-Response(KDT) |
| (i.e., RADIUS Access-Accept{KDE(K=1)}) |
|<-----------------------------------------|
Figure 2: KDE Message Flow
KDE-Request: The KRS sends a Key Request Token (KRT) to the KDS.
The contents of KRT are detailed below.
KDE-Response: As a response, the KDS sends the requested RK to the
KRS wrapped inside a token called Key Delivery Token (KDT). The
contents of KDT are detailed below.
KRT : (PID, KT, KL)
KRT carries the identifiers of the peer (PID), the key type (KT)
and the key label (KL). See [RFC5295] for the specification of
key labels.
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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), the key label (KL), the key name
(KN_RK) and the lifetime of RK (LT_RK).
4.1. Context and Scope for Distributed Keys
The key lifetime of each distributed key MUST NOT be greater than
that of its parent key.
The key context of each distributed key is determined by the sequence
of KTs in the key hierarchy. When a DSRK is being delivered and the
DSRK applies to only a specific set of services, the service types
may need to be carried as part of context for the key. Carrying such
a specific set of services is outside the scope of this document.
The key scope of each distributed key is determined by the sequence
of (PID, KT, KL)-tuples in the key hierarchy. The KDF used to
generate the requested keys includes context and scope information,
thus, binding the key to the specific channel [RFC5295].
4.2. Key Distribution Exchange Scenarios
Given the three types of KRS, there are three scenarios for the
distribution of 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 server to USR-KH: In this scenario, the EAP server
delivers a USRK to a USR-KH.
Scenario 2: EAP server to DSR-KH: In this scenario, the EAP 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 roundtrip
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.
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DSUSR-KH DSR-KH EAP Server
-------- ------- -----
| KDE-Request'(KRT') | KDE-Request(KRT) |
|------------------------>|-------------------------->|
| KDE-Response'(KDT') | KDE-Response(KDT) |
|<----------------------- |<--------------------------|
| | |
Figure 3: Combined Message Exchange
5. RADIUS KDE Attribute
This section defines the format of the RADIUS KDE attribute. See
Section 8 for security requirements on transporting this RADIUS
attribute.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |K| Reserved | Key Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Key Label |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Key Name (included only when K=1) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Key (included only when K=1) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Key Lifetime (included only when K=1) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type
= X (KDE) [X to be assigned by IANA].
Length
>4
K (Key included)
A flag to indicate whether this attribute contains a Key field.
This flag is set for a KDE-Response. This flag is cleared for a
KDE-Request.
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Reserved
Reserved bits. All reserved bits MUST be set to 0 by the sender
and ignored by the recipient.
Key Type
A field to contain a KT. The following KT values are defined: 0
(DSRK), 1 (USRK) and 2 (DSUSRK).
Key Label
A field to contain a key label (KL). The first octet contains the
length of the rest of this field in octets.
Key Name
A field to contain a KN_RK. The first octet contains the length
of the rest of this field in octets. This field is contained if
and only if K-flag is set.
Key
A field to contain a RK. The first octet contains the length of
the rest of this field in octets. This field is contained if and
only if K-flag is set.
Key Lifetime
A 4-octet unsigned integer to indicate a LT_RK. This field is
contained if and only if K-flag is set.
6. KDE used in the EAP Re-authentication Protocol (ERP)
This section describes how the presented KDE 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 AAA
server as the KDS. In this case, the local ER server requesting the
DSRK MUST include a KDE attribute with the K-flag cleared in the
RADIUS Access-Request message that carries the first EAP-Response
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message from the peer. A value of the RADIUS User-Name attribute is
used as the PID. Upon receiving a valid KDE-Request, the home AAA
server includes a KDE attribute with K-flag set in the RADIUS Access-
Accept message that carries the EAP-Success message.
Explicit bootstrapping is initiated by a peer if it doesn't know the
domain. Here, EAP-Initiate and EAP-Finish messages are exchanged
between the peer and the home AAA server, with the bootstrapping flag
in the EAP-Initiate message set. In this case, the local ER server
(acting as KRS) MUST include a KDE attribute with the K-bit cleared
in a RADIUS Access-Request message that carries an EAP-Initiate
message with the bootstrapping flag turned on. A value of the RADIUS
User-Name attribute is used as the PID. In its response, the home
AAA server (acting as KDS) MUST include a KDE attribute with K-flag
set in a RADIUS Access-Accept message that carries an EAP-Finish
message for which the bootstrapping flag is set.
7. Conflicting Messages
In addition to the rules specified in Section 2.6.3. of [RFC3579],
the following combinations SHOULD NOT be sent by a RADIUS Server:
Access-Accept/EAP-Message/EAP-Finish with 'R' flag set to 1
Access-Reject/EAP-Message/EAP-Finish with 'R' flag set to 0
Access-Reject/Keying-Material
Access-Reject/KDE
Access-Challenge/EAP-Message/EAP-Initiate
Access-Challenge/EAP-Message/EAP-Finish
Access-Challenge/KDE
8. Security Considerations
This section provides security requirements and an analysis on
transporting EAP keying material using RADIUS.
8.1. Requirements on RADIUS Key Transport
RADIUS messages that carry a KDE attribute MUST be encrypted,
integrity-protected and replay-protected with a security association
created by a RADIUS transport protocol such as TLS
[I-D.ietf-radext-radsec]. When there is an intermediary such as a
RADIUS 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
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intermediary is trusted not to abuse the distributed key material.
8.2. Distributing RK without Peer Consent
When a KDE-Request message is sent as a result of explicit ERP
bootstrapping [RFC5296], cryptographic verification of peer consent
on distributing a RK is provided by the integrity checksum of the
EAP-Initiate message with the bootstrapping flag turned on.
When a KDE-Request message is sent as a result of implicit ERP
bootstrapping [RFC5296], cryptographic verification of peer consent
on distributing a RK is not provided. As a result, it is possible
for a KRS to request a RK from the home server and obtain the RK even
if the peer does not support ERP, which can lead to an unintended use
of a RK and failed authentication attempts.
9. IANA consideration
This document defines a new namespace for maintaining Key Type used
to identify the type of the root key RK. The range of values 0 - 255
are for permanent, standard message types, allocated by IETF Review
[IANA]. This document defines the values 0 (DSRK), 1 (USRK) and 2
(DSUSRK).
This document defines a new RADIUS Attribute Type for KDE in
Section 5.
10. Acknowledgements
The author would like to thank Dan Harkins, Chunqiang Li, Rafael
Marin Lopez and Charles Clancy for their valuable comments.
11. Contributors
The following people contributed to this document.
Madjid Nakhjiri (madjid.nakhjiri@motorola.com)
Kedar Gaonkar (kgaonkar3@gatech.edu)
Lakshminath Dondeti (ldondeti@qualcomm.com)
Vidya Narayanan (vidyan@qualcomm.com)
Glen Zorn (glenzorn@comcast.net)
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12. References
12.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2865] Rigney, C., Willens, S., Rubens, A., and W. Simpson,
"Remote Authentication Dial In User Service (RADIUS)",
RFC 2865, June 2000.
[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.
[IANA] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
12.2. Informative references
[RFC3579] Aboba, B. and P. Calhoun, "RADIUS (Remote Authentication
Dial In User Service) Support For Extensible
Authentication Protocol (EAP)", RFC 3579, September 2003.
[RFC5247] Aboba, B., Simon, D., and P. Eronen, "Extensible
Authentication Protocol (EAP) Key Management Framework",
RFC 5247, August 2008.
[RFC5169] Clancy, T., Nakhjiri, M., Narayanan, V., and L. Dondeti,
"Handover Key Management and Re-Authentication Problem
Statement", RFC 5169, March 2008.
[I-D.ietf-radext-radsec]
Winter, S., McCauley, M., Venaas, S., and K. Wierenga,
"TLS encryption for RADIUS over TCP (RadSec)",
draft-ietf-radext-radsec-04 (work in progress),
March 2009.
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Authors' Addresses
Katrin Hoeper (editor)
Motorola
1301 E Algonquin Road
Schaumburg, IL 60196
USA
Phone: +1 847 576 4714
Email: khoeper@motorola.com
Yoshihiro Ohba (editor)
Toshiba America Research, Inc.
1 Telcordia Drive
Piscataway, NJ 08854
USA
Phone: +1 732 699 5305
Email: yohba@tari.toshiba.com
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