Network Working Group J. Salowey
Internet-Draft Cisco Systems
Expires: July 15, 2007 L. Dondeti
V. Narayanan
Qualcomm, Inc
M. Nakhjiri
Huawei USA
January 11, 2007
Specification for the Derivation of Usage Specific Root Keys (USRK) from
an Extended Master Session Key (EMSK)
draft-ietf-hokey-emsk-hierarchy-00.txt
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Copyright (C) The Internet Society (2007).
Abstract
An Extended Master Session Key (EMSK) is a cryptographic key
generated from an Extensible Authentication Protocol (EAP) exchange
reserved solely for the purpose of deriving master keys for one or
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more purposes identified as usage definitions. This document
specifies a mechanism for deriving cryptographically separate root
keys from the EMSK, called usage specific root Keys (USRK). The
document provides a set of requirements for avoiding conflicts
between usage definitions to ensure this cryptographic separation.
The USRK is used according to the usage definition defined for a
specific purpose.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1 Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Cryptographic Separation and Coordinated Key Derivation . . . 4
3. USRK Key Derivation Framework . . . . . . . . . . . . . . . . 5
3.1 The USRK Key Derivation Function . . . . . . . . . . . . 6
3.2 Default PRF . . . . . . . . . . . . . . . . . . . . . . . 7
3.3 Key Naming and Usage Data . . . . . . . . . . . . . . . . 7
4. Requirements for Usage Definitions . . . . . . . . . . . . . . 8
4.1 USRK Management Guidelines . . . . . . . . . . . . . . . . 8
5. Requirements for EAP System . . . . . . . . . . . . . . . . . 9
6. Security Considerations . . . . . . . . . . . . . . . . . . . 10
6.1 Key strength . . . . . . . . . . . . . . . . . . . . . . . 10
6.2 Cryptographic separation of keys . . . . . . . . . . . . . 10
6.3 Implementation . . . . . . . . . . . . . . . . . . . . . . 10
6.4 Key Distribution . . . . . . . . . . . . . . . . . . . . . 10
6.5 Key Lifetime . . . . . . . . . . . . . . . . . . . . . . . 11
6.6 Entropy consideration . . . . . . . . . . . . . . . . . . 11
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
7.1 USRK Key Labels . . . . . . . . . . . . . . . . . . . . . 11
7.2 PRF numbers . . . . . . . . . . . . . . . . . . . . . . . 12
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 13
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
9.1 Normative References . . . . . . . . . . . . . . . . . . . 13
9.2 Informative References . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 14
Intellectual Property and Copyright Statements . . . . . . . . 15
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1. Introduction
This document deals with keys generated by authenticated key exchange
mechanisms defined within the EAP framework [RFC3748]. EAP defines
two types of keying material; a Master Session Key (MSK) and an
Extended Master Session Key (EMSK). The EAP specification implicitly
assumes that the MSK produced by EAP will be used for a single
purpose at a single device, however it does reserve the EMSK for
future use. This document defines the EMSK to be used solely for
deriving root keys using the key derivation specified. The root keys
are called usage specific root keys (USRK). This document also
provides guidelines for creating usage definitions for the various
uses of EAP key material and for the management of the USRKs. Note
that previously USRKs were referred to as application master session
keys (AMSKs); however this term proved to be confusing as it suggests
a particular class of usages dealing with higher layer applications
that it was not limited to serve. In this document, the terms
application and usage (or "usage definition") refer to a specific use
case of the EAP keying material for which a USRK is derived.
<Editor's note: the terminology is not clear yet. We need to work
this out. Context has been suggested as a replacement for usage and
application.>
Different uses for keys derived from the EMSK have been proposed.
Some examples include hand off across access points in various mobile
technologies, mobile IP authentication and higher layer application
authentication. In order for a particular usage of EAP key material
to make use of this specification it must specify a usage definition.
This document does not define how the derived USRKs should be used or
discuss what types of use cases are valid. It does define a
framework for the derivation of USRKs for different purposes such
that different usages can be developed independently from one
another. The goal is to have security properties of one usage have
minimal affect on the security properties of other usages.
In order to keep specific uses separate from one another two
requirements are defined in the following sections. One is
coordinated key derivation and another is cryptographic separation.
1.1 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 terms are taken from [RFC3748]: EAP Server, peer,
authenticator, Master Session Key (MSK), Extended Master Session Key
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(EMSK), Cryptographic Separation.
Usage Definition
An application of cryptographic key material to provide one or
more security functions such as authentication, authorization,
encryption or integrity protection for related applications or
services. This document provides guidelines and recommendations
for what should be included in usage definitions. This document
does not place any constrains on the types of use cases or
services that create usage definitions.
Usage Specific Root Key (USRK)
Keying material derived from the EMSK for a particular usage
definition as specified in this document. It is used to derive
child keys in a way defined by its usage definition.
<Editor's note: USRK have been previously called AMSK for
application master session key.>
2. Cryptographic Separation and Coordinated Key Derivation
The EMSK is used to derive keys for multiple use cases, and thus it
is required that the derived keys are cryptographically separate.
Cryptographic separation means that when multiple keys are derived
from an EMSK, given any derived key it is computationally infeasible
to derive any of the other derived keys. Note that deriving the EMSK
from any combinations of the derived keys must also be
computationally infeasible. In practice this means that derivation
of an EMSK from a derived key or derivation of one child key from
another must require an amount of computation equivalent to that
required to, say, reversing a cryptographic hash function.
Cryptographic separation of keys derived from the same key can be
achieved in many ways. Two obvious methods are as follows: it is
plausible to use the IKEv2 PRF [RFC4306] on the EMSK and generate a
key stream. Keys of various lengths as required may be provided from
the key stream for various uses. The other option is to derive keys
from EMSK by providing different inputs to the PRF. However, it is
desirable that derivation of one child key from the EMSK is
independent of derivation of another child key. This allows child
keys to be derived in any order, independent of other keys. Thus it
is desirable to use the second option from above. That implies the
additional input to the PRF must be different for each child key
derivation. This additional input to the PRF must be coordinated
properly to meet the requirement of cryptographic separation and to
prevent reuse of key material between usages.
If cryptographic separation is not maintained then the security of
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one usage depends upon the security of all other usages that use key
derived from the EMSK. If a system does not have this property then
a usage's security depends upon all other usages deriving keys from
the same EMSK, which is undesirable. In order to prevent security
problems in one usage from interfering with another usage, the
following cryptographic separation is required:
o It MUST be computationally infeasible to compute the EMSK from any
USRK.
o Any USRK must be cryptographically separate from any other USRK
derived from the same EMSK
o Derivation of USRKs must be coordinated so that two separate
cryptographic usages do not derive the same key.
This document provides guidelines for a mechanism, which can be used
with existing and new EAP methods to provide cryptographic separation
between usages of EAP keying material. This allows for the
development of new usages without cumbersome coordination between
different usage definitions.
3. USRK Key Derivation Framework
The USRK key derivation framework provides a coordinated means for
generating multiple USRKs from an EMSK. Further keys may then be
derived from the USRK for various purposes, including encryption,
integrity protection, entity authentication by way of proof of
possession, and subsequent key derivation. The usages of the USRK
are set forth in a usage definition described in Section 4.
The USRK key derivation function (KDF) derives an USRK from the EMSK
described above, a key label, optional data, and output length. The
KDF is expected to give the same output for the same input. The
basic key derivation function is given below.
USRK = KDF(EMSK, key label, optional data, length)
The key labels are printable ASCII strings unique for each usage
definition and are a maximum of 255 bytes. In general they are of
the form label-string@domain where domain is the organization that
controls the specification of the usage definition of the USRK. The
key label is intended to provide global uniqueness. Rules for the
allocation of these labels are given in Section 7. For the optional
data the KDF MUST be capable of processing at least 2048 opaque
octets. The optional data must be constant during the execution of
the KDF. The length is a 2 byte unsigned integer in network byte
order of the output key length in octets. An implementation of the
KDF MUST be capable of producing at least 2048 octets of output,
however it is RECOMMENDED that USRKs be 64 octets long.
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A usage definition requiring derivation of an USRK must specify all
the inputs (other than EMSK) to the key derivation function.
3.1 The USRK Key Derivation Function
The EMSK key derivation function is based on a pseudo random function
(PRF) that has the following function prototype:
KDF = PRF(key, data)
where:
key = EMSK
data = label + "\0" + op-data + length
label = ASCII key label
op-data = optional data
length = 2 byte unsigned integer in network byte order
'\0' = is a NUL byte (0x00 in hex)
+ denotes concatenation
The NUL byte after the key label is used to avoid collisions if one
key label is a prefix of another label (e.g. "foobar" and
"foobarExtendedV2"). This is considered a simpler solution than
requiring a key label assignment policy that prevents prefixes from
occurring.
This specification allows for the use of different PRFs. However, in
order to have a coordinated key derivation function the same PRF
function MUST be used for all key derivations for a given EMSK. If
no PRF is specified, then the default PRF specified in Section 3.2
MUST be used. A system may provide the capability to negotiate
additional PRFs. PRFs are assigned numbers through IANA following
the policy set in section Section 7. The rules for negotiating a PRF
are as follows:
o If no other PRF is specified the PRF specified in this document
MUST be used. This is the "default" PRF.
o The initial authenticated key exchange MAY specify a favored PRF.
For example an EAP method may define a preferred PRF to use in its
specification. If the initial authenticated key exchange
specifies a PRF then this MUST override the default PRF.
o If the authenticated EAP key exchange is carried within another
lower layer protocol that has negotiation capabilities then this
protocol MAY attempt to negotiate a PRF to use. < editor's note:
Is this a good idea, the concern is that it may lead to usage
definitions trying to control what the PRF which may be difficult
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to manage. >
o If the system allows a lower layer protocol to negotiates a PRF
then the negotiated PRF MUST be used. It SHOULD take into account
any hints that are provide by the authenticated key exchange.
Note that this capability MUST protect against bidding down
attacks.
o A system MAY specify a separate default PRF if all participants
within the system have the knowledge of which PRF to use. If
specified this MUST take precedence over key exchange defined PRF.
Note that usage definitions MUST not concern themselves with the
details of the PRF construction or the PRF selection, they only need
to worry about the inputs specified in Section 3.
3.2 Default PRF
The default PRF for deriving USRKs from an EMSK is taken from the
PRF+ key expansion PRF from [RFC4306] based on HMAC-SHA-256 [SHA256].
The prf+ construction was chosen because of its simplicity and
efficiency over other PRFs such as those used in [RFC2246]. The
motivation for the design of this PRF is described in [SIGMA]. The
definition of PRF+ from [RFC4306]is given below:
prf+ (K,S) = T1 | T2 | T3 | T4 | ...
Where:
T1 = prf (K, S | 0x01)
T2 = prf (K, T1 | S | 0x02)
T3 = prf (K, T2 | S | 0x03)
T4 = prf (K, T3 | S | 0x04)
continuing as needed to compute the required length of key material.
The key, K, is the EMSK and S is the data defined in Section 3.1.
For this specification the PRF is taken as HMAC-SHA-256 [SHA256].
Since PRF+ is only defined for 255 iterations it may produce up to
8160 bytes of key material.
3.3 Key Naming and Usage Data
It is RECOMMENDED that the authenticated key exchange export a value,
an EAP Session-ID, that is known to both sides to provide a way to
identify the exchange and the keys derived by the exchange. The EAP
keying framework [I-D.ietf-eap-keying] defines this value and
provides an example of how to name an EMSK. The use of names based
on the Session-ID in [I-D.ietf-eap-keying] is RECOMMENDED.
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It is RECOMMENDED that each root key has a name derived as follows:
USRK key name = prf-64( EAP Session-ID, key-label )
where prf-64 is the first 64 bits from the output
Usage definitions MAY use the EAP session-ID in the specification of
the optional data parameter that go into the KDF function. This
provides the advantage of providing data into the key derivation that
is unique to the session that generated the keys.
4. Requirements for Usage Definitions
In order for a usage definition to meet the guidelines for USRK usage
it must meet the following recommendations:
o The usage definition MUST NOT use the EMSK in any other way except
to derive USRKs using the key derivation specified in Section 3 of
this document. They MUST NOT use the EMSK directly.
o The usage definition SHOULD NOT require caching of the EMSK. It
is RECOMMENDED that the USRK derived specifically for the usage
definition rather than the EMSK should be used as a root key to
derive child keys for specific cryptographic operations.
o Usage definition MUST define distinct key labels and optional data
used in the key derivation described in Section 3. Usage
definitions are encouraged to use the key name described in
Section 3.3 and include additional data in the optional data to
provide additional entropy.
o Usage definitions MUST define the length of their USRK. It is
RECOMMENDED that the USRK be at least as long as the EMSK (64
bytes).
o Usage definitions MUST define how they use their USRK. This
includes aspects of key management covered in the next section on
USRK Management guidelines.
4.1 USRK Management Guidelines
This section makes recommendations for various aspects of key
management of the USRK including lifetime, child key derivation,
caching and transport.
It is RECOMMENDED that the USRK only used as a root key for deriving
child keys. A usage definition must specify how and when the
derivation of child keys should be done. It is RECOMMENDED that
usages following similar considerations for key derivation are as
outlined in this document for the USRK derivation with respect to
cryptographic separation and key reuse. In addition, usages should
take into consideration the number of keys that will be derived from
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the USRK and ensure that enough entropy is introduced in the
derivation to support this usage. It is desirable that the entropy
is provided by the two parties that derive the child key.
USRKs have the same lifetime as the EMSK. Thus, when the EMSK
expires, the USRKs derived from it should be removed from use. If a
new EMSK is derived from a subsequent EAP transaction then a usage
implementation should begin to use the new USRK derived from the new
EMSK as soon as possible. Whether or not child keys associated with
a USRK are replaced depends on the requirements of the usage
definition. It is conceivable that some usage definition forces the
child key to be replaced and others allow child keys to be used based
on the policy of the entities that use the child key.
<editor's note: It seems it may desirable for a USRK lifetimes to
vary with usage definitions as well>
Recall that the EMSK never leaves the EAP peer and server. That also
holds true for some USRKs; however, some USRKs may be provided to
other entities for child key derivation and delivery. Each usage
definition specification will specify delivery caching and/or
delivery procedures. Note that the purpose of the key derivation in
Section 3 is to ensure that USRKs are cryptographically separate from
each other and the EMSK. In other words, given an USRK, it is
computationally infeasible to derive the EMSK, any other USRKs, or
child keys associated with other USRKs. In addition to the USRK,
several other parameters may need to be sent. USRK name should be
derived from the EMSK key name, and thus the key name needs to be
sent along with the key. When USRKs are delivered to another entity,
the lifetime associated with the specific root keys MUST also be
transported to that entity. Recommendations for transporting keys
are discussed in the security considerations (Section 6.4).
Usage definition may also define how keys are bound to particular
usages entities. This can be done through the inclusion of usage
parameters and identities in the child key derivation. Some of this
data is described as "channel bindings" in [RFC3748].
5. Requirements for EAP System
The system that wishes to make use of EAP USRKs must take certain
things into consideration. The following is a list of these
considerations:
o The EMSK MUST NOT be used for any other purpose than the key
derivation described in this document.
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o The EMSK MUST be secret and not known to someone observing the
authentication mechanism protocol exchange.
o The EMSK MUST be maintained within a protected location inside the
entity where it is generated. Only keys (USRKs) derived according
to this specification may be exported from this boundary.
o The EMSK MUST be unique for each session
o The EAP method MUST provide an identifier for the EAP transaction
that generated the key
o The system MUST define which usage definitions are used and how
they are invoked.
o The system may define ways to select an alternate PRF for key
derivation as defined in Section 3.1.
The system MAY use the MSK transmitted to the NAS in any way it
chooses. This is required for backward compatibility. New usage
definitions following this specification MUST NOT use the MSK. If
more than one usage uses the MSK, then the cryptographic separation
is not achieved. Implementations MUST prevent such combinations.
6. Security Considerations
6.1 Key strength
The effective key strength of the derived keys will never be greater
than the strength of the EMSK (or a master key internal to an EAP
mechanism).
6.2 Cryptographic separation of keys
The intent of the KDF is to derive keys that are cryptographically
separate: the compromise of one of the usage specific root keys
(USRKs) should not compromise the security of other USRKs or the
EMSK. It is believed that the KDF chosen provides the desired
separation.
6.3 Implementation
An implementation of an EAP framework should keep the EMSK internally
as close to where it is derived as possible and only provide an
interface for obtaining USRKs. It may also choose to restrict which
callers have access to which keys. A usage definition MUST NOT
assume that any entity outside the EAP server or EAP peer EAP
framework has access to the EMSK. In particular it MUST NOT assume
that a lower layer has access to the EMSK.
6.4 Key Distribution
In some cases it will be necessary or convenient to distribute USRKs
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from where they are generated. Since these are secret keys they MUST
be transported with their integrity and confidentiality maintained.
They MUST be transmitted between authenticated and authorized
parties. It is also important that the context of the key usage be
transmitted along with the key. This includes information to
identify the key and constraints on its usage such as lifetime.
This document does not define a mechanism for key transport. It is
up to usage definitions and the systems that use them to define how
keys are distributed. Usage definition designers may enforce
constraints on key usage by various parties by deriving a key
hierarchy and by providing entities only with the keys in the
hierarchy that they need.
6.5 Key Lifetime
The key lifetime is dependent upon how the key is generated and how
the key is used. Since the USRK is the responsibility of the usage
definition it must determine how long the key is valid for. If key
lifetime or key strength information is available from the
authenticated key exchange then this information SHOULD be used in
determining the lifetime of the key. If possible it is recommended
that key lifetimes be coordinated throughout the system. Setting a
key lifetime shorter that a system lifetime may result is keys
becoming invalid with no convenient way to refresh them. Setting a
key lifetime to longer may result in decreased security since the key
may be used beyond its recommended lifetime.
6.6 Entropy consideration
The number of root keys derived from the EMSK is expected to be low.
Note that there is no randomness required to be introduced into the
EMSK to root key derivation beyond the root key labels. Thus, if
many keys are going to be derived from an USRK it is important that
USRK to child key derivation introduce fresh random numbers in
deriving each key.
7. IANA Considerations
The keywords "PRIVATE USE", "SPECIFICATION REQUIRED" and "IETF
CONSENSUS" that appear in this document when used to describe
namespace allocation are to be interpreted as described in [RFC2434].
7.1 USRK Key Labels
This specification introduces a new name space for "USRK key labels".
Key labels are of one of two formats: "label-string" or
"label-string@domain" (without the double quotes).
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Labels of the form "label-string" registered by the IANA MUST be
printable US-ASCII strings, and MUST NOT contain the characters at-
sign ("@"), comma (","), whitespace, control characters (ASCII codes
32 or less), or the ASCII code 127 (DEL). Labels are case-sensitive,
and MUST NOT be longer than 64 characters. Labels of this form are
assigned based on the IETF CONSENSUS policy.
Labels with the at-sign in them of the form "label-string@domain"
where the part preceding the at-sign is the label. The format of the
part preceding the at-sign is not specified; however, these labels
MUST be printable US-ASCII strings, and MUST NOT contain the comma
character (","), whitespace, control characters (ASCII codes 32 or
less), or the ASCII code 127 (DEL). They MUST have only a single at-
sign in them. The part following the at-sign MUST be a valid, fully
qualified internet domain name [RFC1034] controlled by the person or
organization defining the label. Labels are case-sensitive, and MUST
NOT be longer than 64 characters. It is up to each domain how it
manages its local namespace. Note that the total number of octets in
a label is limited to 255. It has been noted that these labels
resemble STD 11 [RFC0822] addresses and network access identifiers
(NAI) defined in [RFC4282]. This is purely coincidental and has
nothing to do with STD 11 [RFC0822] or [RFC4282]. An example of a
locally defined label is "service@example.com" (without the double
quotes).
Labels within the "ietf.org" domain are assigned based on the IETF
CONSENSUS policy with specification recommended. Labels from other
domains may be registered with IANA by the person or organization
controlling the domain with an assignment policy of SPECIFICATION
REQUIRED. It is RECOMMENDED that the specification contain the
following information:
o A description of the usage
o The key label to be used
o Length of the USRK
o If optional data is used, what it is and how it is maintained
o How child keys will be derived from the USRK and how they will be
used
o How lifetime of the USRK and its child keys will be managed
o Where the USRKs or child keys will be used and how they are
communicated if necessary
7.2 PRF numbers
This specification introduces a new number space for "EMSK PRF
numbers". The numbers are int he range 0 to 255 Numbers from 0 to
220 are assigned through the policy IETF CONSENSUS and numbers in the
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range 221 to 255 are left for PRIVATE USE. The initial registry
should contain the following values:
0 RESERVED
1 HMAC-SHA-256 PRF+ (Default)
8. Acknowledgements
This document expands upon previous collaboration with Pasi Eronen.
This document reflects conversations with Bernard Aboba, Jari Arkko,
Avi Lior, David McGrew, Henry Haverinen, Hao Zhou and members of the
EAP working group.
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.
[RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 2434,
October 1998.
[RFC3748] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
Levkowetz, "Extensible Authentication Protocol (EAP)",
RFC 3748, June 2004.
[RFC4306] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
RFC 4306, December 2005.
[SHA256] National Institute of Standards and Technology, "Secure
Hash Standard", FIPS 180-2, August 2002.
With Change Notice 1 dated February 2004
9.2 Informative References
[I-D.ietf-eap-keying]
Aboba, B., "Extensible Authentication Protocol (EAP) Key
Management Framework", draft-ietf-eap-keying-16 (work in
progress), January 2007.
[RFC0822] Crocker, D., "Standard for the format of ARPA Internet
text messages", STD 11, RFC 822, August 1982.
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
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STD 13, RFC 1034, November 1987.
[RFC2246] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
RFC 2246, January 1999.
[RFC4282] Aboba, B., Beadles, M., Arkko, J., and P. Eronen, "The
Network Access Identifier", RFC 4282, December 2005.
[SIGMA] Krawczyk, H., "SIGMA: the 'SIGn-and-MAc' Approach to
Authenticated Diffie-Hellman and its Use in the IKE
Protocols", LNCS 2729, Springer, 2003.
Available at http://www.informatik.uni-trier.de/~ley/db/
conf/crypto/crypto2003.html
Authors' Addresses
Joseph Salowey
Cisco Systems
Email: jsalowey@cisco.com
Lakshminath Dondeti
Qualcomm, Inc
Email: ldondeti@qualcomm.com
Vidya Narayanan
Qualcomm, Inc
Email: vidyan@qualcomm.com
Madjid Nakhjiri
Huawei USA
Email: mnakhjiri@huawei.com
Salowey, et al. Expires July 15, 2007 [Page 14]
Internet-Draft USRK Derivation January 2007
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