Internet Engineering Task Force (IETF) L. Johansson
Request for Comments: 6880 SUNET
Category: Standards Track March 2013
ISSN: 2070-1721
An Information Model for Kerberos Version 5
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 Key Distribution Center
(KDC). This document describes the services exposed by an
administrative interface to a KDC.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6880.
Johansson Standards Track [Page 1]RFC 6880 KDC Information Model March 2013
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RFC 6880 KDC Information Model March 2013
Table of Contents
1. Introduction ....................................................3
2. Requirements Notation ...........................................4
3. Information Model Demarcation ...................................5
4. Information Model Specification .................................6
4.1. Principal ..................................................6
4.1.1. Principal: Attributes ...............................6
4.1.2. Principal: Associations .............................7
4.2. KeySet .....................................................8
4.2.1. KeySet: Attributes ..................................8
4.2.2. KeySet: Associations ................................8
4.3. Key ........................................................9
4.3.1. Key: Attributes .....................................9
4.3.2. Key: Associations ..................................10
4.3.3. Key: Remarks .......................................10
4.4. Policy ....................................................10
4.4.1. Policy: Attributes .................................10
4.4.2. Mandatory-to-Implement Policy ......................11
5. Implementation Scenarios .......................................11
5.1. LDAP Backend to KDC .......................................12
5.2. LDAP Frontend to KDC ......................................12
5.3. SOAP ......................................................12
5.4. NETCONF ...................................................12
6. Security Considerations ........................................12
7. Acknowledgments ................................................13
8. References .....................................................13
8.1. Normative References ......................................13
8.2. Informative References ....................................14
1. Introduction
The Kerberos version 5 authentication service described in [RFC4120]
describes how a Key Distribution Center (KDC) provides authentication
to clients. RFC 4120 does not stipulate how a KDC is managed, and
several "kadmin" servers have evolved since RFC 4120 was written.
This document describes the services required to administer a KDC and
the underlying information model assumed by a kadmin-type service.
The information model is written in terms of "attributes" and either
"services" or "interfaces", but the use of these particular words
must not be taken to imply any particular modeling paradigm. Neither
an object-oriented model nor a Lightweight Directory Access Protocol
(LDAP) [RFC4510] schema is intended. The author has attempted to
describe, in prose, the intended semantics and syntax of the
components of the model. For instance, an LDAP schema based on this
model will be more precise in the expression of the syntax while
preserving the semantics of this model.
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Implementations of this document MAY decide to change the names used
(e.g., principalName). If so, an implementation MUST provide a name-
to-name mapping to this document. In particular, schema languages
may have different typographical conventions, e.g., the use of an
uppercase letter (as seen in camelCase) versus the use of '_' and '-'
to separate 'words' in a name. Implementations MUST call out such
conventions explicitly.
Implementations of this document MUST be able to support default
values for attributes and have the ability to specify syntax for
attribute values.
2. Requirements Notation
This document uses the standard key words ("MUST", "MUST NOT",
"REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT",
"RECOMMENDED", "MAY", and "OPTIONAL") that are defined in [RFC2119],
but with modifications to those definitions as described below. The
reason for this (which was discussed extensively in the Kerberos
working group) is as follows:
This document describes an information model for Kerberos 5, but it
does not directly describe any mapping onto a particular schema or
modeling language. Hence, an implementation of this model consists
of a mapping to such a language, e.g., an LDAP or SQL schema.
Therefore, the standard normative key words require precise
definition.
The terms "MUST" and "REQUIRED" mean that a schema implementing this
model must have a way to represent a feature (i.e., that it is
mandatory to implement it in the schema), but that, unless otherwise
specified, the feature may represent an optional element in the
chosen schema definition language.
However, "MUST" also means that a KDC or administrative interface
implementing this information model has to provide the feature and
associated behavior consistent with the schema.
For instance, principalName (see Section 4.1.1.1) represents the name
of a principal. In an LDAP schema (for instance), this may be
represented as an optional attribute even though all KDCs
implementing this specification must support this attribute.
The terms "MAY" and "OPTIONAL" mean that it is optional for a KDC or
administrative interface implementing this information model to
implement this feature. These terms also mean that implementing the
feature in the schema is optional.
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Implementers of the schema should be aware that, unless the schema
definition can represent critical but optional elements, language
confusion may arise when optional elements are used but not
understood by all implementations in a particular deployment.
The expression "MUST NOT be OPTIONAL" means that it is mandatory that
a feature be implemented ("MUST" per the definition in [RFC2119]) and
additionally that it must not be marked as optional in the schema
language. In particular, this expression means that the feature is
both mandatory to implement and must be present in all
representations of the object to which it applies.
The terms "SHOULD" and "RECOMMENDED" mean that the consequences of
not implementing the feature as if it were described with the "MUST"
keyword must be carefully weighed before choosing a different course.
In particular, these terms imply that interoperability concerns may
arise from not following the recommended practice in schema that
implement this model.
Context will determine whether the "SHOULD" key word applies to the
schema, to the underlying behavior of the KDC, or both. For
instance, when this document states that principalIsDisabled (see
Section 4.1.1.4) SHOULD default to FALSE, this statement implies both
a recommendation for the behavior of KDCs as well as a recommendation
for the representation of that behavior in schema.
3. Information Model Demarcation
The information model specified in Section 4 describes objects, their
properties, and the relationships between the 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 the ordering specified between the
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 should always use
a normative reference to the relevant RFCs in detailing the change in
KDC function.
This model describes a number of elements related to password policy
management. Not all of the elements in this model are unique to
Kerberos. For example, an LDAP implementation of this model should
incorporate existing LDAP schema where functional overlap exists,
rather than defining additional Kerberos-specific elements.
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4. Information Model Specification
4.1. Principal
The fundamental entity stored in a KDC is the principal. The
principal is associated with keys and generalizes the "user" concept.
The principal MUST be implemented in full and MUST NOT be OPTIONAL in
an implementation
4.1.1. Principal: Attributes
4.1.1.1. principalName
The principalName MUST uniquely identify the principal within the
administrative context of the KDC. The principalName MUST be
equivalent to the string representation of the principal name (see
Section 2.1.1 of [RFC1964]), including, if applicable for the name
type, the realm.
The attribute MAY be multivalued if the implementation supports
aliases, enterprise names, or both. In this case, exactly one of the
principalName values MAY be designated as the canonical
principalName. If the implementation supports encryption types
(enctypes) that require salt, exactly one of the values of
principalName MAY be designated as the canonical salting
principalName.
Implementations (i.e., schema) that support enterprise names,
aliases, or both, SHOULD provide for efficient lookup of principal
objects based on the alias or enterprise name.
4.1.1.2. principalNotUsedBefore
The principal MUST NOT be used before this date. The syntax of the
attribute MUST be Internet date/time format from [RFC3339]. The
attribute MUST be single-valued.
4.1.1.3. principalNotUsedAfter
The principal MUST NOT be used after this date. The syntax of the
attribute MUST be Internet date/time format from [RFC3339]. The
attribute MUST be single-valued.
4.1.1.4. principalIsDisabled
A boolean attribute used to disable a principal. The attribute
SHOULD default to boolean FALSE.
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4.1.1.5. principalLastCredentialChangeTime
This single-valued attribute contains the time of the last successful
change of credentials (e.g., password or private key) associated with
this principal. The syntax of the attribute MUST be Internet
date/time format from [RFC3339].
4.1.1.6. principalCreateTime
This single-valued attribute contains the time and date when this
principal was created. The syntax of the attribute MUST be Internet
date/time format from [RFC3339].
4.1.1.7. principalModifyTime
This single-valued attribute contains the time and date when this
principal was last modified, excluding changes to credentials. The
syntax of the attribute MUST be Internet date/time format from
[RFC3339].
4.1.1.8. principalMaximumTicketLifetime
This single-valued attribute contains the time, in seconds,
representing the maximum lifetime of a ticket issued for this
principal.
4.1.1.9. principalMaximumRenewableTicketLifetime
This single-valued attribute contains the delta time, in seconds,
representing the maximum amount of time a ticket may be renewed for
this principal.
4.1.1.10. principalAllowedEnctype
This OPTIONAL multivalued attribute lists the enctypes allowed for
this principal. If empty or absent, any enctype supported by the
implementation is allowed for this principal.
This attribute is intended as a policy attribute and restricts all
uses of enctypes, including server, client, and session keys. Data
models MAY choose to use policy objects in order to represent more
complex decision mechanisms.
4.1.2. Principal: Associations
Each principal MAY be associated with 0 or more KeySets and MAY be
associated with 0 or more Policies. The KeySet is represented as an
object in this model, because it has attributes associated with it
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(the key version number). In typical situations, the principal is
associated with exactly one KeySet, but implementations MUST NOT
assume this case. That is, an implementation of this standard MUST
be able to handle the general case of multiple KeySets associated
with each principal. Multiple KeySets may, for instance, be useful
when performing a key rollover for a principal.
4.2. KeySet
In Kerberos, principals are associated with zero or more symmetric
secret keys, and each key has a key version number (kvno) and an
enctype. In this model, we group keys by kvno into KeySet objects.
A principal can have zero or more KeySet objects associated with it,
each of which MUST have one or more keys. Each KeySet is associated
with exactly one principal. A schema derived from this model MAY
lack a direct analogue of KeySet, as described in this document.
It is expected that most Kerberos implementations will use a special-
purpose interface for setting and changing principal passwords and
keys.
If a server supports an enctype for a principal, that enctype must be
present in at least one key for the principal in question. For any
given enctype, a KeySet MUST NOT contain more than one key with that
enctype.
The security of Kerberos 5 depends absolutely on the confidentiality
and integrity of the key objects stored in the KDC. Implementations
of this standard MUST facilitate, to the extent possible, an
administrator's ability to place more restrictive access controls on
KeySets than on other principal data, and to arrange for more secure
backup for KeySets.
4.2.1. KeySet: Attributes
4.2.1.1. kvno
The key version number. This is a single-valued attribute containing
a non-negative integer. This number is incremented by one each time
a key in the KeySet is changed.
4.2.2. KeySet: Associations
Each KeySet MUST be associated with a set of one or more Keys.
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4.3. Key
Implementations of this model MUST NOT force keys to be represented.
That is, a schema that REQUIRED a key to be present would not meet
this constraint.
4.3.1. Key: Attributes
4.3.1.1. keyEncryptionType
The enctype SHOULD be represented as an enumeration of the enctypes
supported by the KDC using the string name ("encryption type") of the
enctype from the IANA registry of Kerberos Encryption Type Numbers.
One example is "aes128-cts-hmac-sha1-96".
4.3.1.2. keyValue
The binary representation of the key data. This MUST be a single-
valued octet string.
4.3.1.3. keySaltValue
The binary representation of the key salt. This MUST be a single-
valued octet string.
4.3.1.4. keyStringToKeyParameter
This MUST be a single-valued octet string representing an opaque
parameter associated with the enctype. This parameter is specified
using the string-to-key method defined in Section 3 of [RFC3961].
4.3.1.5. keyNotUsedBefore
The key MUST NOT be used before this date. The syntax of the
attribute MUST be semantically equivalent to the standard ISO date
format ([RFC3339]). This attribute MUST be single-valued.
4.3.1.6. keyNotUsedAfter
The key MUST NOT be used after this date. The syntax of the
attribute MUST be semantically equivalent to the standard ISO date
format ([RFC3339]). This attribute MUST be single-valued.
4.3.1.7. keyIsDisabled
This is a boolean attribute that SHOULD be set to FALSE by default.
If this attribute is TRUE, the key MUST NOT be used. The
keyIsDisabled attributed is used to temporarily disable a key.
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4.3.2. Key: Associations
None
4.3.3. 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 is
REQUIRED of the implementation.
4.4. Policy
Implementations SHOULD implement policies, but MAY allow them to be
OPTIONAL. The policy should be thought of as a "typed hole", i.e.,
as an opaque binary value paired with an identifier of the type of
data contained in the binary value. Both attributes (type and value)
must be present.
4.4.1. Policy: Attributes
4.4.1.1. policyIdentifier
The policyIdentifier MUST be globally unique. Possible types of
identifiers include:
o An Object Identifier (OID) [RFC4517]
o A URI [RFC3986]
o A UUID [RFC4122]
Implementations of this specification are expected to assign globally
unique identifiers to the list of the standard policy below in
accordance with best practices for identifier management for the
schema language used.
4.4.1.2. policyIsCritical
This boolean attribute indicates that the KDC MUST be able to
correctly interpret and apply the policy for the principal to be
used.
4.4.1.3. policyContent
This optional single opaque binary value is used to store a
representation of the policy. In general, a policy cannot be fully
expressed using attribute-value pairs. The policyContent is OPTIONAL
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in the sense that an implementation MAY use it to store an opaque
value for the policy types that are not directly representable in
that implementation.
4.4.1.4. policyUse
This optional single enumerated string value is used to describe the
use of the policy. Implementations SHOULD provide this attribute and
MUST (if the attribute is implemented) describe the enumerated set of
possible values. The intent is that this attribute provide an
initial context-based filtering.
4.4.2. Mandatory-to-Implement Policy
All implementations that represent policy objects 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. In general, the expression of policy may require a
Turing-complete language. This specification does not attempt to
model policy-expression language.
4.4.2.1. Password Quality Policy
Password quality policy controls the requirements placed by the KDC
on new passwords.
4.4.2.2. Password Management Policy
Password management policy controls how passwords are changed.
4.4.2.3. Keying Policy
A keying policy specifies the association of enctypes with new
principals. For example, when a principal is created, one of the
applicable keying policies is used to determine the set of keys to
associate with the principal.
4.4.2.4. Ticket Flag Policy
A ticket flag policy specifies the ticket flags allowed for tickets
issued for a principal.
5. 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.
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5.1. 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 [PASSWORD].
5.2. LDAP Frontend to KDC
An alternative way to provide a directory interface to the KDC is to
implement an LDAP frontend to the KDC that exposes all non-key
objects as entries and attributes. As in the example above, all keys
are modified using the set/change password protocol [PASSWORD]. In
this scenario, the implementation would typically not use a
traditional LDAP implementation, but would treat LDAP as a protocol
to access data in the native KDC database.
5.3. SOAP
Given an XML schema implementation of this information model, it
would be possible to build a SOAP interface to the KDC. This
situation demonstrates the value of creating an abstract information
model that is mappable to multiple schema representations.
5.4. NETCONF
Given a YAML (YAML Ain't Markup Language) implementation of this
information model, it would be possible to create a NETCONF-based
interface to the KDC, enabling management of the KDC from standard
network management applications.
6. 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
be treated as write-only attributes that SHALL be modified (using the
administrative interface) only by the change-password protocol
[PASSWORD].
Exposing the object model of a KDC typically implies that objects can
be modified, deleted, or both. In a KDC, not all principals are
created equal. For instance, deleting krbtgt/EXAMPLE.COM@EXAMPLE.COM
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RFC 6880 KDC Information Model March 2013
effectively disables the EXAMPLE.COM realm. Hence, access control is
paramount to the security of any implementation. This document does
not mandate access control. This situation implies only 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 are 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 that
have elevated rights.
7. Acknowledgments
The author wishes to extend his thanks to Love Hoernquist-Aestrand
and Sam Hartman for their important contributions to this document.
8. References
8.1. Normative References
[RFC1964] Linn, J., "The Kerberos Version 5 GSS-API Mechanism", RFC
1964, June 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3339] Klyne, G., Ed. and C. Newman, "Date and Time on the
Internet: Timestamps", RFC 3339, July 2002.
[RFC3961] Raeburn, K., "Encryption and Checksum Specifications for
Kerberos 5", RFC 3961, February 2005.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66, RFC
3986, January 2005.
[RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
Kerberos Network Authentication Service (V5)", RFC 4120,
July 2005.
[RFC4122] Leach, P., Mealling, M., and R. Salz, "A Universally
Unique IDentifier (UUID) URN Namespace", RFC 4122, July
2005.
[RFC4517] Legg, S., "Lightweight Directory Access Protocol (LDAP):
Syntaxes and Matching Rules", RFC 4517, June 2006.
Johansson Standards Track [Page 13]RFC 6880 KDC Information Model March 2013
8.2. Informative References
[PASSWORD] Williams, N., "Kerberos Set/Change Key/Password Protocol
Version 2", Work in Progress, November 2008.
[RFC4510] Zeilenga, K., "Lightweight Directory Access Protocol
(LDAP): Technical Specification Road Map", RFC 4510, June
2006.
Author's Address
Leif Johansson
Swedish University Network
Thulegatan 11
Stockholm
Sweden
EMail: leifj@sunet.se
URI: http://www.sunet.se
Johansson Standards Track [Page 14]
