KERBEROS WORKING GROUP Johansson
Internet-Draft Stockholm university
Intended status: Standards Track March 8, 2009
Expires: September 9, 2009
An information model for Kerberos version 5
draft-ietf-krb-wg-kdc-model-04
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Abstract
This document describes an information model for Kerberos version 5
from the point of view of an administrative service. There is no
standard for administrating a kerberos 5 KDC. This document
describes the services exposed by an administrative interface to a
KDC.
Table of Contents
1. Requirements notation . . . . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. How to interpret RFC2119 terms . . . . . . . . . . . . . . . . 5
4. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 6
5. Information model demarcation . . . . . . . . . . . . . . . . 7
6. Information model specification . . . . . . . . . . . . . . . 8
6.1. Principal . . . . . . . . . . . . . . . . . . . . . . . . 8
6.1.1. Principal: Attributes . . . . . . . . . . . . . . . . 8
6.1.2. Principal: Associations . . . . . . . . . . . . . . . 10
6.1.3. Principal: Remarks . . . . . . . . . . . . . . . . . . 10
6.2. KeySet . . . . . . . . . . . . . . . . . . . . . . . . . . 10
6.2.1. KeySet: Attributes . . . . . . . . . . . . . . . . . . 10
6.2.2. KeySet: Associations . . . . . . . . . . . . . . . . . 10
6.2.3. KeySet: Remarks . . . . . . . . . . . . . . . . . . . 11
6.3. Key . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
6.3.1. Key: Attributes . . . . . . . . . . . . . . . . . . . 11
6.3.2. Key: Associations . . . . . . . . . . . . . . . . . . 12
6.3.3. Key: Remarks . . . . . . . . . . . . . . . . . . . . . 12
6.4. Policy . . . . . . . . . . . . . . . . . . . . . . . . . . 12
6.4.1. Policy: Attributes . . . . . . . . . . . . . . . . . . 12
6.4.2. Mandatory-to-implement Policy . . . . . . . . . . . . 13
7. Implementation Scenarios . . . . . . . . . . . . . . . . . . . 14
7.1. LDAP backend to KDC . . . . . . . . . . . . . . . . . . . 14
7.2. LDAP frontend to KDC . . . . . . . . . . . . . . . . . . . 14
7.3. SOAP . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
7.4. Netconf . . . . . . . . . . . . . . . . . . . . . . . . . 14
8. Security Considerations . . . . . . . . . . . . . . . . . . . 15
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
10.1. Normative References . . . . . . . . . . . . . . . . . . . 17
10.2. Informative References . . . . . . . . . . . . . . . . . . 17
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 18
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1. Requirements notation
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].
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2. Introduction
The Kerberos version 5 authentication service described in [RFC4120]
describes how a Key Distribution Service (KDC) provides
authentication to clients. The standard does not stipulate how a KDC
is managed and several "kadmin" servers have evolved. This document
describes the services required to administrate a KDC and the
underlying information model assumed by a kadmin-type service.
The information model is written in terms of "attributes" and
"services" or "interfaces" but the use of these particular words MUST
NOT be taken to imply any particular modeling paradigm so that
neither an object oriented model or an LDAP schema is intended. The
author has attempted to describe in natural language the intended
semantics and syntax of the components of the model. An LDAP schema
(for instance) based on this model will be more precise in the
expression of the syntax while preserving the semantics of this
model.
Implementations of this document MAY decide to change the names used
(eg principalName). If so an implementation MUST provide a name to
name mapping to this document.
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3. How to interpret RFC2119 terms
This document describes an information model for kerberos 5 but does
not directly describe any mapping onto a particular schema- or
modelling language. Hence an implementation of this model consists
of a mapping to such a language - eg an LDAP or SQL schema. The
precise interpretation of terms from [RFC2119] therefore require some
extra explanation. The terms MUST, MUST NOT, REQUIRED, SHALL, SHALL
NOT mean that an implementation MUST provide a feature but does not
mean that this feature MUST be REQUIRED by the implementation - eg an
attribute is available in an LDAP schema but marked as OPTIONAL. If
a feature must be implemented and REQUIRED this is made explicit in
this model. The term MAY, OPTIONAL and RECOMMENDED means that an
implementation MAY need to REQUIRE the feature due to the particular
nature of the schema/modelling language. In some cases this is
expressly forbidden by this model (feature X MUST NOT be REQUIRED by
an implementation).
Note that any implementation of this model SHOULD be published as an
RFC.
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4. Acknowledgments
Love Hoernquist-Aestrand <lha@it.su.se> for important contributions.
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5. Information model demarcation
The information model specified in the next chapter describes
objects, properties of those objects and relations between those
objects. These elements comprise an abstract view of the data
represented in a KDC. It is important to understand that the
information model is not a schema. In particular the way objects are
compared for equality beyond that which is implied by the
specification of a syntax is not part of this specification. Nor is
ordering specified between elements of a particular syntax.
Further work on Kerberos will undoubtedly prompt updates to this
information model to reflect changes in the functions performed by
the KDC. Such extensions to the information model MUST always use a
normative reference to the relevant RFCs detailing the change in KDC
function.
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6. Information model specification
6.1. Principal
The fundamental entity stored in a KDC is the principal. The
principal is associated to keys and generalizes the "user" concept.
The principal MUST be implemented in full and MUST NOT be optional in
an implementation
6.1.1. Principal: Attributes
6.1.1.1. principalName
The principalName MUST uniquely identify the principal within the
administrative context of the KDC. The type of the principalName is
not described in this document. It is a unique identifier and can be
viewed as an opaque byte string which can be compared for equality.
The attribute MAY be multivalued if the implmementation supports
aliases. In that case one of the principalName values MUST be
designated the canonical principalName and if the implementation
supports enctypes which require salt then one of the values of
principalName MUST be designated as the canonical salting
principalName.
6.1.1.2. principalNotUsedBefore
The principal may not be used before this date. The syntax of the
attribute MUST be semantically equivalent with the standard ISO date
format. The attribute MUST be single valued.
6.1.1.3. principalNotUsedAfter
The principal may not be used after this date. The syntax of the
attribute MUST be semantically equivalent with the standard ISO date
format. The attribute MUST be single valued.
6.1.1.4. principalIsDisabled
A boolean attribute used to (temporarily) disable a principal. The
attribute MUST default to false.
6.1.1.5. principalNumberOfFailedAuthenticationAttempts
This single valued integer attribute contains a count of the number
of times an authentication attempt was unsuccessful for this
principal. Implementations SHOULD NOT allow this counter to be
reset.
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6.1.1.6. principalLastFailedAuthentication
This single valued attribute contains the time and date for the last
failed authentication attempt for this principal.
6.1.1.7. principalLastSuccessfulAuthentication
This single valued attribute contains the time and date for the last
successful authentication attempt for this principal.
6.1.1.8. principalLastCredentialChangeTime
This single valued attribute contains the time and date for the last
successful change of credential (eg password or private key)
associated with this principal.
6.1.1.9. principalCreateTime
This single valued attribute contains the time and date when this
principal was created
6.1.1.10. principalModifyTime
This single valued attribute contains the time and date when this
principal was modified excluding credentials change.
6.1.1.11. principalMaximumTicketLifetime
This single valued attribute contains the delta time in seconds
representing the maximum ticket lifetime for tickets issued for this
principal.
6.1.1.12. principalMaximumRenewableTicketLifetime
This single valued attribute contains the delta time in seconds
representing the maximum amount of time a ticket may be renewed for.
6.1.1.13. principalAllowedEnctype
This optional multi valued attribute lists the enctypes allowed for
this principal. If empty or absent any enctype supported by the
implementation is allowed for this principal.
Implementations MAY choose to use policy objects in order to
represent more complex decision mechanisms.
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6.1.2. Principal: Associations
Each principal MAY be associated with 0 or more KeySet and MAY be
associated with 0 or more Policies. The KeySet is represented as an
object in this model since it has attributes associated with it (the
key version number). In typical situations the principal is
associated with exactly 1 KeySet but implementations MUST NOT assume
this case, i.e an implementation of this standard (e.g an LDAP
schema) MUST be able to handle the general case of multiple KeySet
associated with each principal.
6.1.3. Principal: Remarks
Traditionally a principal consists of a local-part and a realm
denoted in string form by local-part@REALM. The realm concept is
used to provide administrative boundaries and together with cross-
realm authentication provides scalability to Kerberos 5. However the
realm is not central to an administrative information model. For
instance the initialization or creation of a realm is equivalent to
creating a specific set of principals (krbtgt@REALM, etc) which is
covered by the model and services described in this document. A
realm is typically associated with policy covering (for instance)
keying and password management. The management of such policy and
their association to realms is beyond the scope of this document.
6.2. KeySet
A KeySet is a set of keys associated with exactly one principal.
This object and its associations MUST NOT be REQUIRED by an
implementation. It is expected that most implementations of this
standard will use the set/change password protocol for all aspects of
key management [I-D.ietf-krb-wg-kerberos-set-passwd]. This
information model only includes these objects for the sake of
completenes.
6.2.1. KeySet: Attributes
6.2.1.1. keySetVersionNumber
This is traditionally called the key version number (kvno). This is
a single valued attribute containing a positive integer.
6.2.2. KeySet: Associations
To each KeySet MUST be associated a set of 1 or more Keys.
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6.2.3. KeySet: Remarks
The reason for separating the KeySet from the Principal is security.
The security of Kerberos 5 depends absolutely on the security of the
keys stored in the KDC. The KeySet type is provided to make this
clear and to make separation of keys from other parts of the model
clear.
Implementations of this standard (eg an LDAP schema) MUST make a
clear separation between the representation of KeySet from other
information objects.
6.3. Key
Implementations of this model MUST NOT REQUIRE keys to be
represented.
6.3.1. Key: Attributes
6.3.1.1. keyEncryptionType
The enctype SHOULD be represented as an enumeration of the enctypes
supported by the KDC.
6.3.1.2. keyValue
The binary representation of the key data. This MUST be a single
valued octet string.
6.3.1.3. keySaltValue
The binary representation of the key salt. This MUST be a single
valued octet string.
6.3.1.4. keyStringToKeyParameter
This MUST be a single valued octet string representing an opaque
parameter associated with the enctype.
6.3.1.5. keyNotUsedAfter
This key MUST NOT be used after this date. The syntax of the
attribute MUST be semantically equivalent with the standard ISO date
format. This MUST be a single-valued attribute.
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6.3.1.6. keyNotUsedBefore
This key MUST NOT be used before this date. The syntax of the
attribute MUST be semantically equivalent with the standard ISO date
format. This MUST be a single-valued attribute.
6.3.1.7. keyIsDisabled
This is a boolean attribute which must be set to false by default.
If this attribute is true the key MUST NOT be used. This is used to
temporarily disable a key.
6.3.2. Key: Associations
None
6.3.3. Key: Remarks
The security of the keys is an absolute requirement for the operation
of Kerberos 5. If keys are implemented adequate protection from
unauthorized modification and disclosure MUST be available and
REQUIRED by the implementation.
6.4. Policy
Implementations SHOULD implement policy but MAY allow them to be
OPTIONAL. The Policy should be thought of as a 'typed hole'. i.e an
opaque binary value paired with an identifier of type of data
contained in the binary value. Both attributes (type and value) must
be present.
6.4.1. Policy: Attributes
6.4.1.1. policyIdentifier
The policyIdentifier MUST be unique within the local administrative
context and MUST be globally unique. Possible types of identifiers
include:
An Object Identifier (OID)
A URN
A UUID
The use of OIDs is recommended for this purpose.
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6.4.1.2. policyIsCritical
This boolean attribute indicates that the KDC MUST be able to
correctly interpret and apply this policy for the key to be used.
6.4.1.3. policyContent
This is an optional single opaque binary value used to store a
representation of the policy. In general a policy cannot be fully
expressed using attribute-value pairs. The policyContent is OPTIONAL
in the sense that an implementation MAY use it to store an opaque
value for those policy-types which are not directly representable in
that implementation.
6.4.1.4. policyUse
This is an optional single enumerated string value used to describe
the applicability of the policy. Implementations SHOULD provide this
attribute and MUST (if the attribute is implemented) describe the
enumerated set of possible values.
6.4.2. Mandatory-to-implement Policy
All implementations MUST be able to represent the policies listed in
this section. Implementations are not required to use the same
underlying data-representation for the policyContent binary value but
SHOULD use the same OIDs as the policyIdentifier.
6.4.2.1. Password Quality Policy
Password quality policy controls the requirements placed by the KDC
on new passwords. This policy SHOULD be identified by the OID <TBD>.
6.4.2.2. Password Management Policy
Password management policy controls how passwords are changed. This
policy SHOULD be identified by the OID <TBD>.
6.4.2.3. Keying Policy
A keying policy specifies the association of enctypes with new
principals, i.e when a principal is created one of the possibly many
applicable keying policies determine the set of keys to associate
with the principal. In general the expression of a keying policy may
require a Turing-complete language. This policy SHOULD be identified
by the OID <TBD>.
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7. Implementation Scenarios
There are several ways to implement an administrative service for
Kerberos 5 based on this information model. In this section we list
a few of them.
7.1. LDAP backend to KDC
Given an LDAP schema implementation of this information model it
would be possible to build an administrative service by backending
the KDC to a directory server where principals and keys are stored.
Using the security mechanisms available on the directory server keys
are protected from access by anyone apart from the KDC.
Administration of the principals, policy and other non-key data is
done through the directory server while the keys are modified using
the set/change password protocol
[I-D.ietf-krb-wg-kerberos-set-passwd].
7.2. LDAP frontend to KDC
An alternative way to provide a directory interface to the KDC is to
implement an LDAP-frontend to the KDC which exposes all non-key
objects as entries and attributes. As in the example above all keys
are modified using the set/change password protocol
[I-D.ietf-krb-wg-kerberos-set-passwd]. In this scenario the
implementation would typically not use a traditional LDAP
implementation but treat LDAP as an access-protocol to data in the
native KDC database.
7.3. SOAP
Given an XML schema implementation of this information model it would
be possible to build a SOAP-interface to the KDC. This demonstrates
the value of creating an abstract information model which is mappable
to multiple schema representations.
7.4. Netconf
Given a YAML implementation of this information model it would be
possible to create a Netconf-based interface to the KDC in theory
enabling management of the KDC from standard network management
applications
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8. Security Considerations
This document describes an abstract information model for Kerberos 5.
The Kerberos 5 protocol depends on the security of the keys stored in
the KDC. The model described here assumes that keys MUST NOT be
transported in the clear over the network and furthermore that keys
are treated as write-only attributes that SHALL only be modified
(using the administrative interface) by the change-password protocol
[I-D.ietf-krb-wg-kerberos-set-passwd].
Exposing the object model of a KDC typically implies that objects can
be modified and/or deleted. In a KDC not all principals are created
equal, so that for instance deleting krbtgt/EXAMPLE.COM@EXAMPLE.COM
effectively disables the EXAMPLE.COM realm. Hence access control is
paramount to the security of any implementation. This document does
not (at the time of writing - leifj) mandate access control. This
only implies that access control is beyond the scope of the standard
information model, i.e that access control may not be accessible via
any protocol based on this model. If access control objects is
exposed via an extension to this model the presence of access control
may in itself provide points of attack by giving away information
about principals with elevated rights etc. etc.
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9. IANA Considerations
None
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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.
[RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
Kerberos Network Authentication Service (V5)", RFC 4120,
July 2005.
10.2. Informative References
[I-D.ietf-krb-wg-kerberos-set-passwd]
Williams, N., "Kerberos Set/Change Key/Password Protocol
Version 2", draft-ietf-krb-wg-kerberos-set-passwd-07 (work
in progress), September 2007.
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Author's Address
Leif Johansson
Stockholm university
Avdelningen foer IT och Media
Stockholm SE-106 91
Email: leifj@it.su.se
URI: http://people.su.se/~leifj/
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