One document matched: draft-ietf-hokey-emsk-hierarchy-07.xml
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<rfc category="std" docName="draft-ietf-hokey-emsk-hierarchy-07" ipr="full3978" updates="eap-keying (RFC
Ed to replace this with RFC number)">
<front>
<title abbrev="EMSK Root Key Derivation">Specification for the Derivation of Root Keys from an
Extended Master Session Key (EMSK)</title>
<author fullname="Joseph Salowey" initials="J." surname="Salowey">
<organization>Cisco Systems</organization>
<address>
<email>jsalowey@cisco.com</email>
</address>
</author>
<author fullname="Lakshminath Dondeti" initials="L." surname="Dondeti">
<organization>Qualcomm, Inc</organization>
<address>
<email>ldondeti@qualcomm.com</email>
</address>
</author>
<author fullname="Vidya Narayanan" initials="V." surname="Narayanan">
<organization>Qualcomm, Inc</organization>
<address>
<email>vidyan@qualcomm.com</email>
</address>
</author>
<author fullname="Madjid Nakhjiri" initials="M." surname="Nakhjiri">
<organization>Motorola</organization>
<address>
<email>madjid.nakhjiri@motorola.com</email>
</address>
</author>
<date month="June" year="2008"/>
<area>Security Area</area>
<workgroup>Network Working Group</workgroup>
<abstract>
<t>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 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 guarantee 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.</t>
<t/>
<t/>
</abstract>
</front>
<middle>
<section anchor="intro" title="Introduction">
<t>This document deals with keys generated by authenticated key exchange mechanisms defined
within the EAP framework <xref target="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.</t>
<t>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. </t>
<t>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. </t>
<t>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.</t>
<section title="Applicable usages of keys derived from the EMSK" anchor="applicability">
<t>
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 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.
</t>
<t>
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.
</t>
<t>
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.
</t>
<t>
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.
</t>
<t>
Parties using keys derived from EMSK also need trust relationships
with the EAP endpoints, and mechanisms for securely communicating
the keys.
</t>
<t>
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 access independent manner.
</t>
<t>
Implementation considerations may also complicate communication of
keys to an application from the lower layer. For instance, in many
configurations applications may run on a different device than the
one providing EAP-based network access to it.
</t>
<t>
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.
</t>
<t>
Keys derived from EAP are pairwise 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.
</t>
</section>
<section title="Terminology">
<t>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 <xref target="RFC2119"/></t>
<t>The following terms are taken from <xref target="RFC3748"/>: EAP Server, peer,
authenticator, Master Session Key (MSK), Extended Master Session Key (EMSK), Cryptographic
Separation.</t>
<t>Usage Definition<list>
<t>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.</t>
</list></t>
<t>Usage Specific Root Key (USRK)<list>
<t>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.</t>
</list></t>
<t>Key Management Domain<list>
<t>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 it is derived from. In this document the term domain refers to a key management domain unless otherwise
qualified.
</t></list></t>
<t>Domain Specific Root Key (DSRK)<list>
<t>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 DSRK are
referred to as domain specific usage specific root keys (DSUSRK). DSUSRKs are similar
to the USRK, except in the fact that their scope is restricted to the same domain as
the parent DSRK from which it is derived. </t>
</list>
</t>
</section>
</section>
<section title="Cryptographic Separation and Coordinated Key Derivation">
<t>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.</t>
<t>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 <xref
target="RFC4306"/> on the EMSK and generate a key stream. Keys of various lengths may be
provided as required 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.</t>
<t>If cryptographic separation is not maintained then the security of 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:</t>
<t>
<list style="symbols">
<t>It MUST be computationally infeasible to compute the EMSK from any root key derived
from it.</t>
<t>Any root key MUST be cryptographically separate from any other root key derived from
the same EMSK or DSRK</t>
<t>Derivation of USRKs MUST be coordinated so that two separate cryptographic usages do
not derive the same key.</t>
<t>Derivation of DSRKs MUST be coordinated so that two separate key management domains do
not derive the same key.</t>
<t>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.</t>
</list>
</t>
<t>This document provides guidelines for a key derivation mechanism, which 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.</t>
</section>
<section anchor="keyframe" title="EMSK Key Root Derivation Framework ">
<t>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 specific
set of uses set forth in a usage definition described in <xref target="usage"/>.</t>
<t>The basic EMSK root key hierarchy looks as follows:</t>
<t>
<figure>
<preamble/>
<artwork><![CDATA[ EMSK
/ \
USRK1 USRK2
]]></artwork>
</figure>
</t>
<t>This document defines how to derive usage specific root keys (USRK) from the EMSK and also defines a specific
USRK called a domain specific root key (DSRK). DSRK 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 (DSUSRK). 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:</t>
<t>
<figure>
<preamble/>
<artwork><![CDATA[
EMSK
/ \
USRK DSRK
/ \
DSUSRK1 DSUSRK2
]]></artwork>
</figure>
</t>
<section title="USRK Derivation" anchor="KDF">
<t>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.</t>
<t>
<figure>
<preamble/>
<artwork><![CDATA[
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
]]></artwork>
<postamble/>
</figure>
</t>
<t>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 <xref target="IANA"/>.
</t>
<t>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.</t>
<t>
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 <xref
target="I-D.ietf-eap-keying"/> 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.
</t>
<t>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. </t>
<t>
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.</t>
<t>A usage definition requiring derivation of a Root Key must specify all the inputs (other
than EMSK) to the key derivation function.</t>
<t>USRKs MUST be at least 64 octets in length.</t>
<section title="On the KDFs">
<t>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
<xref target="defprf"/> 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 <xref target="IANA"/>. The rules for negotiating a KDF are as follows:</t>
<t>
<list style="symbols">
<t>If no other KDF is specified the KDF specified in this document MUST be used. This is
the "default" KDF.</t>
<t>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.</t>
</list>
</t>
<t>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
<xref target="keyframe"/>.</t>
</section>
<section anchor="defprf" title="Default KDF">
<t>The default KDF for deriving root keys from an EMSK is taken from the PRF+ key expansion
specified in <xref target="RFC4306"/> based on HMAC-SHA-256 <xref target="SHA256"/>. The PRF+
construction was chosen because of its simplicity and efficiency over other mechanisms such as
those used in <xref target="RFC4346"/>. The motivation for the design of PRF+ is
described in <xref target="SIGMA"/>. The definition of PRF+ from <xref target="RFC4306"
/>is given below:</t>
<figure>
<preamble/>
<artwork><![CDATA[
PRF+ (K,S) = T1 | T2 | T3 | T4 | ...
]]></artwork>
</figure>
<t>Where:</t>
<figure>
<artwork><![CDATA[
T1 = PRF (K, S | 0x01)
T2 = PRF (K, T1 | S | 0x02)
T3 = PRF (K, T2 | S | 0x03)
T4 = PRF (K, T3 | S | 0x04)]]></artwork>
</figure>
<t>continuing as needed to compute the required length of key material. The key, K, is the
EMSK and S is the concatenation of key label, the NULL octet, optional data and length defined in <xref
target="KDF"/>. For this specification the PRF is
taken as HMAC-SHA-256 <xref target="SHA256"/>. Since PRF+ is only defined for 255
iterations it may produce up to 8160 octets of key material.</t>
</section>
</section>
<section anchor="keyname" title="EMSK and USRK Name Derivation">
<t>The EAP keying framework <xref target="I-D.ietf-eap-keying"/> 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:</t>
<t>
<figure>
<preamble/>
<artwork><![CDATA[
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
</artwork> <postamble/>
</figure>
</t>
<t> </t>
<t>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.</t>
<t>USRKs MAY be named explicitly with a name derivation specified as follows:</t>
<t>
<figure>
<preamble/>
<artwork><![CDATA[
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
]]></artwork>
<postamble/>
</figure>
</t>
<t>USRKName derivation and usage is applicable when there is ambiguity in the 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. </t>
</section>
</section>
<section title="Domain Specific Root Key Derivation">
<t>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 keys can be derived
within the key management domain for those usages. DSRK based usages will follow a key
hierarchy similar to the following:</t>
<t>
<figure>
<preamble/>
<artwork><![CDATA[
EMSK
/ \
/ \
/ \
/ \
DSRK1 DSRK2
/ \ / \
/ \ / \
DSUSRK11 DSUSRK12 DSUSRK21 DSUSRK22
]]></artwork>
<postamble/>
</figure>
</t>
<t> 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 that the root key is being
derived for. The DSRK MUST be at least 64 octets long. </t>
<t>Domain Specific Usage Specific Root Keys (DSUSRK) 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.</t>
<t>
<figure>
<preamble/>
<artwork><![CDATA[
DSUSRK = KDF(DSRK, key label | "\0" | optional data | length)]]></artwork>
<postamble/>
</figure>
</t>
<t>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 <xref target="IANA"/>. 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 least 2048 octets of output, however it is RECOMMENDED that DSUSRKs be at least 64 octets long.</t>
<t>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.</t>
<t>
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.
</t>
<t>DSUSRKs MAY be named explicitly with a name derivation specified as follows:</t>
<t>
<figure>
<preamble/>
<artwork><![CDATA[
DSUSRKName =
KDF(EMSKName,key label | "\0" | optional data | length)]]></artwork>
<postamble/>
</figure>
</t>
<t>where length is the 2 octet unsigned integer 8 in network byte order.</t>
<section title="Applicability of Multi-Domain usages">
<t>When a DSRK is distributed to a domain the domain can generate any DSUSRKs it wishes. This keys 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.</t>
<t>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 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.</t>
</section>
</section>
<section anchor="usage" title="Requirements for Usage Definitions">
<t>In order for a usage definition to meet the guidelines for USRK usage it must meet the
following recommendations:</t>
<t>
<list style="symbols">
<t>The usage must define if it is a domain enabled usage. </t>
<t>The usage definition MUST NOT use the EMSK in any other way except to derive Root Keys
using the key derivation specified in <xref target="keyframe"/> of this document. They
MUST NOT use the EMSK directly.</t>
<t>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.</t>
<t>Usage definition MUST define distinct key labels and optional data used in the key
derivation described in <xref target="keyframe"/>. Usage definitions are encouraged to
use the key name described in <xref target="keyname"/> and include
additional data in the optional data to provide additional entropy. </t>
<t>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).</t>
<t>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.</t>
<t/>
</list>
</t>
<section title="Root Key Management Guidelines">
<t>This section makes recommendations for various aspects of key management of the Root Key
including lifetime, child key derivation, caching and transport.</t>
<t>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.</t>
<t>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.</t>
<t>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 <xref
target="keyframe"/> 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.</t>
<t>
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 <xref target="seckeydist">the security considerations
</xref>.</t>
<t>Usage definition 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 <xref target="RFC3748"
/>.</t>
</section>
</section>
<section anchor="reqeap" title="Requirements for EAP System">
<t>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:</t>
<t>
<list style="symbols">
<t>The EMSK MUST NOT be used for any other purpose than the key derivation described in
this document.</t>
<t>The EMSK MUST be secret and not known to someone observing the authentication mechanism
protocol exchange.</t>
<t>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.</t>
<t>The EMSK MUST be unique for each EAP session</t>
<t>The EAP method MUST provide an identifier for the EAP transaction that generated the
key</t>
<t>The system MUST define which usage definitions are used and how they are invoked.</t>
<t>The system may define ways to select an alternate PRF for key derivation as defined in
<xref target="KDF"/>.</t>
</list>
</t>
<t>The system MAY use the MSK transmitted to the
NAS in any way it chooses in accordance with <xref target="RFC3748"></xref> <xref target="I-D.ietf-eap-keying"></xref>
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.</t>
</section>
<section title="Security Considerations">
<section title="Key strength">
<t>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).</t>
</section>
<section title="Cryptographic separation of keys">
<t>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.</t>
</section>
<section title="Implementation">
<t>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 EAP framework has access to
the EMSK. In particular it MUST NOT assume that a lower layer has access to the EMSK.</t>
</section>
<section anchor="seckeydist" title="Key Distribution">
<t>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.</t>
<t>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.</t>
</section>
<section title="Key Lifetime">
<t>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 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.</t>
</section>
<section title="Entropy consideration">
<t>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 Root Key it is
important that Root Key to child key derivation introduce fresh random numbers in deriving
each key.</t>
</section>
</section>
<section anchor="IANA" title="IANA Considerations">
<t>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 <xref target="RFC5226"/>.</t>
<section title="Key Labels">
<t>
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.
</t>
<t>
Labels can be assigned based on Specification Required policy <xref target="RFC5226"></xref>.
In addition, the labels "experimental1" and "experimental2" are
reserved for experimental use. The following considerations apply
to their use:
</t>
<t>
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.
</t>
<t>
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.
</t>
<t>
Similarly, labels "private1" and "private2" have been reserved
for Private Use within an organization. Again, cross-domain usage
of these labels is NOT RECOMMENDED.
</t>
<t>
Labels starting with a string and followed by the "@" and a valid, fully
qualified Internet domain name [RFC1034] can be requested by
by the person or organization who are 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 <xref target="RFC5226"></xref>),
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 <xref target="applicability"></xref>.
It is RECOMMENDED that the specification
contain the following information:
</t>
<t>
<list style="symbols">
<t>A description of the usage</t>
<t>The key label to be used</t>
<t>Length of the Root Key</t>
<t>If optional data is used, what it is and how it is maintained</t>
<t>How child keys will be derived from the Root Key and how they will be used</t>
<t>How lifetime of the Root Key and its child keys will be managed</t>
<t>Where the Root Keys or child keys will be used and how they are communicated if
necessary</t>
</list>
</t>
<t>The following labels are reserved by this document: "EMSK", "dsrk@ietf.org".</t>
</section>
<section title="PRF numbers">
<t>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 range 221 to 255 are left for Private Use. The initial registry should
contain the following values:</t>
<t>
<list>
<t>0 RESERVED</t>
<t>1 HMAC-SHA-256 PRF+ (Default)</t>
</list>
</t>
</section>
</section>
<section title="Acknowledgements">
<t>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.</t>
<t>Thanks to Dan Harkins for the idea of using a single root key name to refer to all keys.</t>
</section>
</middle>
<back>
<references title="Normative References"> &rfc2119; &rfc3748;
&rfc4306; &ietf-eap-keying; &rfc5226; <reference anchor="SHA256">
<front>
<title>Secure Hash Standard</title>
<author>
<organization>National Institute of Standards and Technology</organization>
</author>
<date month="August" year="2002"/>
</front>
<seriesInfo name="FIPS" value="180-2"/>
<annotation>With Change Notice 1 dated February 2004</annotation>
</reference>
</references>
<references title="Informative References"> &rfc4346; &rfc1034; &rfc0822;
&rfc4282; <reference anchor="SIGMA">
<front>
<title>SIGMA: the 'SIGn-and-MAc' Approach to Authenticated Diffie-Hellman and its Use in
the IKE Protocols</title>
<author initials="H" surname="Krawczyk">
<organization/>
</author>
<date year="2003"/>
</front>
<seriesInfo name="LNCS" value="2729"/>
<seriesInfo name="" value="Springer"/>
<format target="http://www.informatik.uni-trier.de/~ley/db/conf/crypto/crypto2003.html"
type="HTML"/>
<annotation>Available at
http://www.informatik.uni-trier.de/~ley/db/conf/crypto/crypto2003.html</annotation>
</reference>
</references>
</back>
</rfc>
| PAFTECH AB 2003-2026 | 2026-04-24 05:44:09 |