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Network Working Group J. Salowey
Request for Comments: 5295 Cisco Systems
Updates: 5247 L. Dondeti
Category: Standards Track V. Narayanan
Qualcomm, Inc
M. Nakhjiri
Motorola
August 2008
Specification for the Derivation of Root Keys
from an Extended Master Session Key (EMSK)
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Abstract
The Extensible Authentication Protocol (EAP) defined the Extended
Master Session Key (EMSK) generation, but reserved it for unspecified
future uses. This memo reserves the EMSK for the sole purpose of
deriving root keys. Root keys are master keys that can be used for
multiple purposes, identified by usage definitions. This document
also specifies a mechanism for avoiding conflicts between root keys
by deriving them in a manner that guarantees cryptographic
separation. Finally, this document also defines one such root key
usage: Domain-Specific Root Keys are root keys made available to and
used within specific key management domains.
Salowey, et al. Standards Track [Page 1]
RFC 5295 EMSK Root Key Derivation August 2008
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Applicable Usages of Keys Derived from the EMSK . . . . . 3
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5
2. Cryptographic Separation and Coordinated Key Derivation . . . 6
3. EMSK Key Root Derivation Framework . . . . . . . . . . . . . . 7
3.1. USRK Derivation . . . . . . . . . . . . . . . . . . . . . 8
3.1.1. On the KDFs . . . . . . . . . . . . . . . . . . . . . 9
3.1.2. Default KDF . . . . . . . . . . . . . . . . . . . . . 9
3.2. EMSK and USRK Name Derivation . . . . . . . . . . . . . . 10
4. Domain-Specific Root Key Derivation . . . . . . . . . . . . . 11
4.1. Applicability of Multi-Domain Usages . . . . . . . . . . . 12
5. Requirements for Usage Definitions . . . . . . . . . . . . . . 13
5.1. Root Key Management Guidelines . . . . . . . . . . . . . . 13
6. Requirements for EAP System . . . . . . . . . . . . . . . . . 14
7. Security Considerations . . . . . . . . . . . . . . . . . . . 15
7.1. Key Strength . . . . . . . . . . . . . . . . . . . . . . . 15
7.2. Cryptographic Separation of Keys . . . . . . . . . . . . . 15
7.3. Implementation . . . . . . . . . . . . . . . . . . . . . . 15
7.4. Key Distribution . . . . . . . . . . . . . . . . . . . . . 16
7.5. Key Lifetime . . . . . . . . . . . . . . . . . . . . . . . 16
7.6. Entropy Consideration . . . . . . . . . . . . . . . . . . 16
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
8.1. Key Labels . . . . . . . . . . . . . . . . . . . . . . . . 17
8.2. PRF Numbers . . . . . . . . . . . . . . . . . . . . . . . 18
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 18
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 19
10.1. Normative References . . . . . . . . . . . . . . . . . . . 19
10.2. Informative References . . . . . . . . . . . . . . . . . . 19
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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 meant for specific purposes called usages; a special usage class
is the Domain-Specific Root Keys made available to and used within
specific key management domains. This document also provides
guidelines for creating usage definitions for the various uses of EAP
key material and for the management of the root keys. In this
document, the terms application and usage (or "usage definition")
refer to a specific use case of the EAP keying material.
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 so-called usage
definition. This document does not define how the derived Usage-
Specific Root Keys (USRK) are used; see the following section for
discussion of applicable usages. 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 or no effect on the
security properties of other usages.
This document does define a special class of USRK, called a Domain-
Specific Root Key (DSRK) for use in deriving keys specific to a key
management domain. Each DSRK is a root key used to derive Domain-
Specific Usage-Specific Root Keys (DSUSRK). The DSUSRKs are USRKs
specific to a particular key management domain.
In order to keep root keys for specific purposes separate from one
another, two requirements are defined in the following sections. One
is coordinated key derivation and another is cryptographic
separation.
1.1. Applicable Usages of Keys Derived from the EMSK
The EMSK is typically established as part of network access
authentication and authorization. It is expected that keys derived
from EMSK will be used in protocols related to network access, such
Salowey, et al. Standards Track [Page 3]
RFC 5295 EMSK Root Key Derivation August 2008
as handover optimizations, and the scope of these protocols is
usually restricted to the endpoints of the lower layers over which
EAP packets are sent.
In particular, it is inappropriate for the security of higher-layer
applications to solely rely on keys derived from network access
authentication. Even when used together with another, independent
security mechanism, the use of these keys needs to be carefully
evaluated with regards to the benefits of the optimization and the
need to support multiple solutions. Performance optimizations may
not warrant the close tie-in that may be required between the layers
in order to use EAP-based keys. Such optimizations may be offset by
the complexities of managing the validity and usage of key materials.
Keys generated from subsequent EAP authentications may be beyond the
knowledge and control of applications.
From an architectural point of view, applications should not make
assumptions about the lower-layer technology (such as network access
authentication) used on any particular hop along the path between the
application endpoints.
From a practical point of view, making such assumptions would
complicate using those applications over lower layers that do not use
EAP, and make it more difficult for applications and network access
technologies to evolve independently of each other.
Parties using keys derived from EMSK also need trust relationships
with the EAP endpoints, and mechanisms for securely communicating the
keys.
For most applications, it is not appropriate to assume that all
current and future access networks are trusted to secure the
application function. Instead, applications should implement the
required security mechanisms in an access-independent manner.
Implementation considerations may also complicate communication of
keys to an application from the lower layer. For instance, in many
configurations, application protocol endpoints may be different from
the EAP endpoints.
Given all this, it is NOT RECOMMENDED to use keys derived from the
EMSK as an exclusive security mechanism, when their usage is not
inherently, and by permanent nature, tied to the lower layer where
network access authentication was performed.
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Keys derived from EAP are pair-wise by nature and are not directly
suitable for multicast or other group usages such as those involved
in some routing protocols. It is possible to use keys derived from
EAP in protocols that distribute group keys to group participants.
The definition of these group key distribution protocols is beyond
the scope of this document and would require additional
specification.
1.2. 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 terms are taken from [RFC3748]: EAP Server, peer,
authenticator, Master Session Key (MSK), Extended Master Session Key
(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 constraints 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. It is used to derive child keys in a way defined by
its usage definition.
Key Management Domain
A key management domain is specified by the scope of a given root
key. The scope is the collection of systems authorized to access
key material derived from that key. Systems within a key
management domain may be authorized to (1) derive key materials,
(2) use key materials, or (3) distribute key materials to other
systems in the same domain. A derived key's scope is constrained
to a subset of the scope of the key from which it is derived. In
this document, the term "domain" refers to a key management domain
unless otherwise qualified.
Domain Specific Root Key (DSRK)
Keying material derived from the EMSK that is restricted to use in
a specific key management domain. It is used to derive child keys
for a particular usage definition. The child keys derived from a
Salowey, et al. Standards Track [Page 5]
RFC 5295 EMSK Root Key Derivation August 2008
DSRK are referred to as Domain-Specific Usage-Specific Root Keys
(DSUSRKs). A DSUSRK is similar to the USRK, except in the fact
that its scope is restricted to the same domain as the parent DSRK
from which it is derived.
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:
o Use a Pseudo-Random Function (PRF) on the EMSK and generate a key
stream. Keys of various lengths may be provided as required from
the key stream for various uses.
o 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. Independent
derivation of keys from the EMSK allows child keys to be derived in
any order, independent of other keys. Thus, it is desirable to use
option 2 from above. Using the second option 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
one usage depends upon the security of all other usages that use keys
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
root key derived from it.
Salowey, et al. Standards Track [Page 6]
RFC 5295 EMSK Root Key Derivation August 2008
o Any root key MUST be cryptographically separate from any other
root key derived from the same EMSK or DSRK.
o Derivation of USRKs MUST be coordinated so that two separate
cryptographic usages do not derive the same key.
o Derivation of DSRKs MUST be coordinated so that two separate key
management domains do not derive the same key.
o Derivation of DSRKs and USRKs MUST be specified such that no
domain can obtain a USRK by providing a domain name identical to a
Usage Key Label.
This document provides guidelines for a key derivation mechanism that
can be used with existing and new EAP methods to provide
cryptographic separation between usages of EMSK. This allows for the
development of new usages without cumbersome coordination between
different usage definitions.
3. EMSK Key Root Derivation Framework
The EMSK key derivation framework provides a coordinated means for
generating multiple root keys from an EMSK. Further keys may then be
derived from the root key for various purposes, including encryption,
integrity protection, entity authentication by way of proof of
possession, and subsequent key derivation. A root key is derived
from the EMSK for a specific set of uses set forth in a usage
definition described in Section 5.
The basic EMSK root key hierarchy looks as follows:
EMSK
/ \
USRK1 USRK2
This document defines how to derive Usage-Specific Root Keys (USRKs)
from the EMSK and also defines a specific USRK called a Domain-
Specific Root Key (DSRK). DSRKs are root keys restricted to use in a
particular key management domain. From the DSRK, Usage-Specific Root
Keys for a particular application may be derived (DSUSRKs). The
DSUSRKs are equivalent to USRKs that are restricted to use in a
particular domain. The details of lower levels of key hierarchy are
outside scope of this document. The key hierarchy looks as follows:
Salowey, et al. Standards Track [Page 7]
RFC 5295 EMSK Root Key Derivation August 2008
EMSK
/ \
USRK DSRK
/ \
DSUSRK1 DSUSRK2
3.1. USRK Derivation
The EMSK Root Key Derivation Function (KDF) derives a USRK from the
EMSK, 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 | "\0" | optional data | length)
where:
| denotes concatenation
"\0" is a NULL octet (0x00 in hex)
length is a 2-octet unsigned integer in network byte order
The key labels are printable ASCII strings unique for each usage
definition and are a maximum of 255 octets. In general, they are of
the form label-string@specorg, where specorg is the organization that
controls the specification of the usage definition of the Root Key.
The key label is intended to provide global uniqueness. Rules for
the allocation of these labels are given in Section 8.
The NULL octet 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.
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. Usage definitions MAY use the EAP Session-ID
[RFC5247] in the specification of the optional data parameter that
goes into the KDF function. In this case, the advantage is that data
provided into the key derivation is unique to the session that
generated the keys.
The KDF must be able to process input keys of up to 256 bytes. It
may do this by providing a mechanism for "hashing" long keys down to
a suitable size that can be consumed by the underlying derivation
algorithm.
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The length is a 2-octet 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 Root Keys be at least 64 octets long.
A usage definition requiring derivation of a Root Key must specify
all the inputs (other than EMSK) to the key derivation function.
USRKs MUST be at least 64 octets in length.
3.1.1. On the KDFs
This specification allows for the use of different KDFs. However, in
order to have a coordinated key derivation function, the same KDF
function MUST be used for all key derivations for a given EMSK. If
no KDF is specified, then the default KDF specified in Section 3.1.2
MUST be used. A system may provide the capability to negotiate
additional KDFs. KDFs are assigned numbers through IANA following
the policy set in Section 8. The rules for negotiating a KDF are as
follows:
o If no other KDF is specified, the KDF specified in this document
MUST be used. This is the "default" KDF.
o The initial authenticated key exchange MAY specify a favored KDF.
For example, an EAP method may define a preferred KDF to use in
its specification. If the initial authenticated key exchange
specifies a KDF, then this MUST override the default KDF.
Note that usage definitions MUST NOT concern themselves with the
details of the KDF construction or the KDF selection, they only need
to worry about the inputs specified in Section 3.
3.1.2. Default KDF
The default KDF for deriving root keys from an EMSK is taken from the
PRF+ key expansion specified in [RFC4306] based on HMAC-SHA-256
[SHA256]. The PRF+ construction was chosen because of its simplicity
and efficiency over other mechanisms such as those used in [RFC4346].
The motivation for the design of PRF+ is described in [SIGMA]. The
definition of PRF+ from [RFC4306] is given below:
Salowey, et al. Standards Track [Page 9]
RFC 5295 EMSK Root Key Derivation August 2008
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 concatenation of the key label,
the NULL octet, optional data, and length 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 octets of key material.
3.2. EMSK and USRK Name Derivation
The EAP keying framework [RFC5247] specifies that the EMSK MUST be
named using the EAP Session-ID and a binary or textual indication.
Following that requirement, the EMSK name SHALL be derived as
follows:
EMSKname = KDF ( EAP Session-ID, "EMSK" | "\0" | length )
where:
| denotes concatenation
"EMSK" consists of the 4 ASCII values for the letters
"\0" = is a NULL octet (0x00 in hex)
length is the 2-octet unsigned integer 8 in network byte order
It is RECOMMENDED that all keys derived from the EMSK are referred to
by the EMSKname and the context of the descendant key usage. This is
the default behavior. Any exceptions SHALL be signaled by individual
usages.
USRKs MAY be named explicitly with a name derivation specified as
follows:
USRKName =
KDF(EAP Session-ID, key label|"\0"|optional data|length)
where:
key label and optional data MUST be the same as those used
in the corresponding USRK derivation
length is the 2-octet unsigned integer 8 in network byte order
Salowey, et al. Standards Track [Page 10]
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USRKName derivation and usage are applicable when there is ambiguity
in referencing the keys using the EMSKname and the associated context
of the USRK usage. The usage SHALL signal such an exception in key
naming, so both parties know the key name used.
4. Domain-Specific Root Key Derivation
A specific USRK called a Domain-Specific Root Key (DSRK) is derived
from the EMSK for a specific set of usages in a particular key
management domain. Usages derive specific keys for specific services
from this DSRK. The DSRK may be distributed to a key management
domain for a specific set of usages so that keys can be derived
within the key management domain for those usages. DSRK-based usages
will follow a key hierarchy similar to the following:
EMSK
/ \
/ \
/ \
/ \
DSRK1 DSRK2
/ \ / \
/ \ / \
DSUSRK11 DSUSRK12 DSUSRK21 DSUSRK22
The DSRK is a USRK with a key label of "dsrk@ietf.org" and the
optional data containing a domain label. The optional data MUST
contain an ASCII string representing the key management domain for
which the root key is being derived. The DSRK MUST be at least 64
octets long.
Domain-Specific Usage-Specific Root Keys (DSUSRKs) are derived from
the DSRK. The KDF is expected to give the same output for the same
input. The basic key derivation function is given below.
DSUSRK = KDF(DSRK, key label | "\0" | optional data | length)
The key labels are printable ASCII strings unique for each usage
definition within a DSRK usage and are a maximum of 255 octets. In
general, they are of the form label-string@specorg where specorg is
the organization that controls the specification of the usage
definition of the DSRK. The key label is intended to provide global
uniqueness. Rules for the allocation of these labels are given in
Section 8. 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-octet
unsigned integer in network byte order of the output key length in
octets. An implementation of the KDF MUST be capable of producing at
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least 2048 octets of output; however, it is RECOMMENDED that DSUSRKs
be at least 64 octets long.
Usages that make use of the DSRK must define how the peer learns the
domain label to use in a particular derivation. A multi-domain usage
must define how both DSRKs and specific DSUSRKs are transported to
different key management domains. Note that usages may define
alternate ways to constrain specific keys to particular key
management domains.
To facilitate the use of EMSKname to refer to keys derived from
DSRKs, EMSKname SHOULD be sent along with the DSRK. The exception is
when a DSRKname is expected to be used. The usage SHALL signal such
an exception in key naming, so both parties know the key name used.
DSUSRKs MAY be named explicitly with a name derivation specified as
follows:
DSUSRKName =
KDF(EMSKName,key label | "\0" | optional data | length)
where length is the 2-octet unsigned integer 8 in network byte order.
4.1. Applicability of Multi-Domain Usages
The DSUSRKs generated by a domain can be used to authorize entities
in a domain to perform specific functions. In cases where it is
appropriate for only a specific domain to be authorized to perform a
function, the usage SHOULD NOT be defined as multi-domain.
In some cases, only certain domains are authorized for a particular
multi-domain usage. In this case, domains that do not have full
authorization should not receive the DSRK and should only receive
DSUSRKs for the usages for which they are authorized. If it is
possible for a peer to know which domains are authorized for a
particular usage without relying on restricting access to the DSRK to
specific domains, then this recommendation may be relaxed.
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5. 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 must define if it is a domain-enabled usage.
o The usage definition MUST NOT use the EMSK in any other way except
to derive Root Keys 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 Root Key derived specifically for the
usage definition (rather than the EMSK) should be used to derive
child keys for specific cryptographic operations.
o Usage definitions 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 3.2 and include additional data in the optional data to
provide additional entropy.
o Usage definitions MUST define the length of their Root Keys. It
is RECOMMENDED that the Root Keys be at least as long as the EMSK
(at least 64 octets).
o Usage definitions MUST define how they use their Root Keys. This
includes aspects of key management covered in the next section on
Root Key management guidelines.
5.1. Root Key Management Guidelines
This section makes recommendations for various aspects of key
management of the Root Key including lifetime, child key derivation,
caching, and transport.
It is RECOMMENDED that the Root Key is only used 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 Root Key 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 the Root
Key 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.
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Root Keys' lifetimes should not be more than that of the EMSK. Thus,
when the EMSK expires, the Root Keys 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
Root Keys derived from the new EMSK as soon as possible. Whether or
not child keys associated with a Root Key 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.
Recall that the EMSK never leaves the EAP peer and server. That also
holds true for some Root Keys; however, some Root Keys 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 Root Keys are
cryptographically separate from each other and the EMSK. In other
words, given a Root Key, it is computationally infeasible to derive
the EMSK, any other Root Keys, or child keys associated with other
Root Keys. In addition to the Root Key, several other parameters may
need to be sent.
Root Key names may be derived using the EAP Session-ID, and thus the
key name may need to be sent along with the key. When Root Keys are
delivered to another entity, the EMSKname and 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 7.4).
Usage definitions may also define how keys are bound to particular
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].
6. Requirements for EAP System
The system that wishes to make use of EAP root keys derived from the
EMSK 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.
o The EMSK MUST be secret and not known to someone observing the
authentication mechanism protocol exchange.
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RFC 5295 EMSK Root Key Derivation August 2008
o The EMSK MUST be maintained within a protected location inside the
entity where it is generated. Only root keys derived according to
this specification may be exported from this boundary.
o The EMSK MUST be unique for each EAP 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 Network Access Server
(NAS) in any way it chooses in accordance with [RFC3748], [RFC5247],
and other relevant specifications. 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.
7. Security Considerations
7.1. 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).
7.2. 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.
7.3. 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 Root Keys. 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 has access
to the EMSK. In particular, it MUST NOT assume that a lower layer
has access to the EMSK.
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7.4. Key Distribution
In some cases, it will be necessary or convenient to distribute USRKs
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.
7.5. Key Lifetime
The key lifetime is dependent upon how the key is generated and how
the key is used. Since the Root Key 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 in 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.
7.6. 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 a Root Key, it is important
that Root-Key-to-child-key derivation introduce fresh random numbers
in deriving each key.
8. 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 [RFC5226].
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8.1. Key Labels
This specification introduces a new name space for "USRK Key Labels".
Key labels MUST be printable US-ASCII strings, and MUST NOT contain
the characters at-sign ("@") except as noted below, 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 can be assigned based on Specification Required policy
[RFC5226]. In addition, the labels "experimental1" and
"experimental2" are reserved for experimental use. The following
considerations apply to their use:
Production networks do not necessarily support the use of
experimental code points. The network scope of support for
experimental values should carefully be evaluated before deploying
any experiment across extended network domains, such as the public
Internet. The potential to disrupt the stable operation of EAP
devices is a consideration when planning an experiment using such
code points.
The network administrators should ensure that each code point is used
consistently to avoid interference between experiments. Particular
attention should be given to security vulnerabilities and the freedom
of different domains to employ their own experiments. Cross-domain
usage is NOT RECOMMENDED.
Similarly, labels "private1" and "private2" have been reserved for
Private Use within an organization. Again, cross-domain usage of
these labels is NOT RECOMMENDED.
Labels starting with a string and followed by the "@" and a valid,
fully qualified Internet domain name [RFC1034] can be requested by
the person or organization that is in control of the domain name.
Such labels can be allocated based on Expert Review with
Specification Required. Besides the review needed for Specification
Required (see Section 4.1 of [RFC5226]), the expert needs to review
the proposed usage for conformance to this specification, including
the suitability of the usage according to the applicability statement
outlined in Section 1.1. 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 Root Key
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RFC 5295 EMSK Root Key Derivation August 2008
o If optional data is used, what it is and how it is maintained
o How child keys will be derived from the Root Key and how they will
be used
o How lifetime of the Root Key and its child keys will be managed
o Where the Root Keys or child keys will be used and how they are
communicated if necessary
The following labels are reserved by this document: "EMSK",
"dsrk@ietf.org".
8.2. PRF Numbers
This specification introduces a new number space for "EMSK PRF
numbers". The numbers are in the range 0 to 255. Numbers from 0 to
220 are assigned through the policy IETF Consensus, and numbers in
the range 221 to 255 are left for Private Use. The initial registry
contains the following values:
0 RESERVED
1 HMAC-SHA-256 PRF+ (Default)
9. 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, Russ Housley, Glen
Zorn, Charles Clancy, Dan Harkins, Alan DeKok, Yoshi Ohba, and
members of the EAP and HOKEY working groups.
Thanks to Dan Harkins for the idea of using a single root key name to
refer to all keys.
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10. References
10.1. 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.
[RFC4306] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
RFC 4306, December 2005.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
[RFC5247] Aboba, B., Simon, D., and P. Eronen, "Extensible
Authentication Protocol (EAP) Key Management Framework",
RFC 5247, August 2008.
[SHA256] National Institute of Standards and Technology, "Secure
Hash Standard", FIPS 180-2, With Change Notice 1 dated
February 2004, August 2002.
10.2. Informative References
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987.
[RFC4346] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.1", RFC 4346, April 2006.
[SIGMA] Krawczyk, H., "SIGMA: the 'SIGn-and-MAc' Approach to
Authenticated Diffie-Hellman and its Use in the IKE
Protocols", LNCS 2729, Springer, 2003,
<http://www.informatik.uni-trier.de/~ley/db/conf/
crypto/crypto2003.html>.
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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
Motorola
EMail: madjid.nakhjiri@motorola.com
Salowey, et al. Standards Track [Page 20]
RFC 5295 EMSK Root Key Derivation August 2008
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Salowey, et al. Standards Track [Page 21]