wdiff rfc6880.original rfc6880.txt

KERBEROS WORKING GROUP
Internet Engineering Task Force (IETF) L. Johansson
Internet-Draft
Request for Comments: 6880 SUNET
Intended status:
Category: Standards Track January 14, 2013
Expires: July 18, March 2013
ISSN: 2070-1721

An information model Information Model for Kerberos version Version 5
draft-ietf-krb-wg-kdc-model-16

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 Kerberos 5 KDC. Key Distribution Center
(KDC). This document describes the services exposed by an
administrative interface to a KDC.

Status of this This Memo

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 ....................................................3
2. Requirements notation . . . . . . . . . . . . . . . . . . . . 4 Notation ...........................................4
3. Information model demarcation . . . . . . . . . . . . . . . . 6 Model Demarcation ...................................5
4. Information model specification . . . . . . . . . . . . . . . 7 Model Specification .................................6
4.1. Principal . . . . . . . . . . . . . . . . . . . . . . . . 7 ..................................................6
4.1.1. Principal: Attributes . . . . . . . . . . . . . . . . 7 ...............................6
4.1.2. Principal: Associations . . . . . . . . . . . . . . . 8 .............................7
4.2. KeySet . . . . . . . . . . . . . . . . . . . . . . . . . . 9 .....................................................8
4.2.1. KeySet: Attributes . . . . . . . . . . . . . . . . . . 9 ..................................8
4.2.2. KeySet: Associations . . . . . . . . . . . . . . . . . 9 ................................8
4.3. Key . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 ........................................................9
4.3.1. Key: Attributes . . . . . . . . . . . . . . . . . . . 10 .....................................9
4.3.2. Key: Associations . . . . . . . . . . . . . . . . . . 10 ..................................10
4.3.3. Key: Remarks . . . . . . . . . . . . . . . . . . . . . 11 .......................................10
4.4. Policy . . . . . . . . . . . . . . . . . . . . . . . . . . 11 ....................................................10
4.4.1. Policy: Attributes . . . . . . . . . . . . . . . . . . 11 .................................10
4.4.2. Mandatory-to-implement Mandatory-to-Implement Policy . . . . . . . . . . . . 12 ......................11
5. Implementation Scenarios . . . . . . . . . . . . . . . . . . . 13 .......................................11
5.1. LDAP backend Backend to KDC . . . . . . . . . . . . . . . . . . . 13 .......................................12
5.2. LDAP frontend Frontend to KDC . . . . . . . . . . . . . . . . . . . 13 ......................................12
5.3. SOAP . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 ......................................................12
5.4. Netconf . . . . . . . . . . . . . . . . . . . . . . . . . 13 NETCONF ...................................................12
6. Security Considerations . . . . . . . . . . . . . . . . . . . 14 ........................................12
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 16
9. ................................................13
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
9.1. .....................................................13
8.1. Normative References . . . . . . . . . . . . . . . . . . . 17
9.2. ......................................13
8.2. Informative References . . . . . . . . . . . . . . . . . . 17
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 18 ....................................14

1. Introduction

The Kerberos version 5 authentication service described in [RFC4120]
describes how a Key Distribution Center (KDC) provides authentication
to clients. The standard RFC 4120 does not stipulate how a KDC is managed managed, and
several "kadmin" servers have evolved. 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" "interfaces", but the use of these particular words
must not be taken to imply any particular modeling paradigm. Neither
an
object oriented object-oriented model nor an LDAP a Lightweight Directory Access Protocol
(LDAP) [RFC4510] schema is intended. The author has attempted to describe
describe, in natural language prose, the intended semantics and syntax of the
components of the model. An For instance, 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
(e.g.
(e.g., principalName). If so so, an implementation MUST provide a name to
name name-
to-name mapping to this document. In particular particular, schema languages
may have different conventions for caseing, eg camelCase vs 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 as well as and have the ability to specify syntax for
attribute values.

2. Requirements notation 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] [RFC2119],
but with modifications to those definitions as described below. The
reason for this (which was discussed extensively in the kerberos WG) Kerberos
working group) is as follows:

This document describes an information model for kerberos 5 Kerberos 5, but it
does not directly describe any mapping onto a particular schema- schema or
modelling
modeling language. Hence Hence, an implementation of this model consists
of a mapping to such a language - e.g. language, e.g., an LDAP or SQL schema. The
Therefore, the standard normative key word therefore words require precise definition:
definition.

The terms MUST or REQUIRED means "MUST" and "REQUIRED" mean that a schema implementing this
model must have a way to represent a feature (i.e (i.e., that it is
mandatory to implement it in schema) the schema), but that that, unless otherwise specified
specified, the feature may represent an optional element in the
chosen schema definition language.

However MUST

However, "MUST" also means that a KDC or administrative interface
implementing this information model MUST has to provide the feature and
associated behavior consistent with the schema.

For instance, principalLastFailedAuthentication (cf below) principalName (see Section 4.1.1.1) represents the last time an authentication failed for name
of a principal. In an LDAP schema (for instance) instance), this may be
represented as an optional attribute even though all KDCs
implementing this specification must support this attribute.

The terms MAY or OPTIONAL means "MAY" and "OPTIONAL" mean that the feature it is optional to
implement by for a KDC or
administrative interface implementing this information model. It model to
implement this feature. These terms also means mean that implementing the
feature is optional to
implement in schema.

Implementors the schema is optional.

Implementers of the schema should be aware that that, unless there is a way to the schema
definition can represent critical but optional elements in the schema definition 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 a feature it is mandatory to implement that
a feature be implemented ("MUST" cf above) per the definition in [RFC2119]) and that
additionally that it must not be marked as optional in the schema
language. In particular 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 term SHOULD or RECOMMENDED means terms "SHOULD" and "RECOMMENDED" mean that the consequences of
not implementing the feature as if it was were described with the "MUST"
keyword must be carefully weighed before choosing a different course.
In particular this implies particular, these terms imply that interoperability concerns may
arise from not following the recommended practice in schema that implements
implement this model.

The context

Context will determine if whether the "SHOULD" key word applies to the
schema, or to the underlying behaviour behavior of the KDC KDC, or both. For
instance, when this document states that principalIsDisabled (cf below) (see
Section 4.1.1.4) SHOULD default to FALSE FALSE, this statement implies both
a recommendation for the behaviour behavior of KDCs aswell as well as a rekommendation recommendation
for the representation of that behaviour behavior in schema.

3. Information model demarcation Model Demarcation

The information model specified in the next chapter Section 4 describes objects, properties of those objects their
properties, and relations the relationships between those 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 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 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;
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.

4. Information model specification Model Specification

4.1. Principal

The fundamental entity stored in a KDC is the principal. The
Principal
principal is associated to with keys and generalizes the "user" concept.
The Principal 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 principal within the
administrative context of the KDC. The principalName MUST be
equivalent to the string representation of the Principal principal name
(section (see
Section 2.1.1 of [RFC1964]) [RFC1964]), including, if applicable for the name
type, the realm.

The attribute MAY be multi-valued multivalued if the implementation supports
aliases and/or
aliases, enterprise names. names, or both. In that case this case, exactly one of the
principalName values MAY be designated as the canonical principalName
and if
principalName. If the implementation supports enctypes which encryption types
(enctypes) that require salt then salt, exactly one of the values of
principalName MAY be designated as the canonical salting
principalName.

Implementations (i.e. (i.e., schema) that support enterprise names and/or
aliases names,
aliases, or both, SHOULD provide for efficient lookup of Principal principal
objects based on alias/enterprise the alias or enterprise name.

4.1.1.2. principalNotUsedBefore

The Principal principal MUST NOT be used before this date. The syntax of the
attribute MUST be Internet Date/Time Format date/time format from [RFC3339]. The
attribute MUST be single-valued.

4.1.1.3. principalNotUsedAfter

The Principal principal MUST NOT be used after this date. The syntax of the
attribute MUST be Internet Date/Time Format date/time format from [RFC3339]. The
attribute MUST be single-valued.

4.1.1.4. principalIsDisabled

A boolean attribute used to disable a Principal. principal. The attribute
SHOULD default to boolean FALSE.

4.1.1.5. principalLastCredentialChangeTime

This single-valued attribute contains the time of the last successful
change of credential (e.g. credentials (e.g., password or private key) associated with
this Principal. principal. The syntax of the attribute MUST be Internet Date/
Time Format
date/time format from [RFC3339].

4.1.1.6. principalCreateTime

This single-valued attribute contains the time and date when this
Principal
principal was created. The syntax of the attribute MUST be Internet
Date/Time Format
date/time format from [RFC3339].

4.1.1.7. principalModifyTime

This single-valued attribute contains the time and date when this
Principal
principal was last modified modified, excluding credentials change. changes to credentials. The
syntax of the attribute MUST be Internet Date/Time Format date/time format from
[RFC3339].

4.1.1.8. principalMaximumTicketLifetime

This single-valued attribute contains the time time, in seconds seconds,
representing the maximum lifetime for tickets of a ticket issued for this
Principal.
principal.

4.1.1.9. principalMaximumRenewableTicketLifetime

This single-valued attribute contains the delta time time, in seconds seconds,
representing the maximum amount of time a ticket may be renewed for
this Principal. principal.

4.1.1.10. principalAllowedEnctype

This OPTIONAL multi-valued multivalued attribute lists the enctypes allowed for
this principal. If empty or absent absent, any enctype supported by the
implementation is allowed for this Principal. principal.

This attribute is intended as a policy attribute and restricts all
uses of enctypes 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 principal MAY be associated with 0 or more KeySet KeySets and MAY be
associated with 0 or more Policies. The KeySet is represented as an
object in this model since model, because it has attributes associated with it
(the key version number). In typical situations situations, the Principal principal is
associated with exactly 1 KeySet one KeySet, but implementations MUST NOT
assume this case, i.e. case. That is, an implementation of this standard MUST
be able to handle the general case of multiple KeySet KeySets associated
with each principal. Multiple KeySets may may, for instance instance, be useful
when performing a key rollover for a principal.

4.2. KeySet

In Kerberos 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 model, we group keys by kvno into KeySet objects.
A
Principal 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. Schemas A schema derived from this model MAY
lack a direct analogue of KeySet KeySet, as described in this document.

It is expected that most Kerberos implementations will use a special-
purpose interface for setting and changing Principal principal passwords and
keys.

If a server supports an enctype for a Principal principal, that enctype must be
present in at least one key for the Principal principal in question. For any
given enctype enctype, a KeySet MUST NOT contain more than one Key key with that
enctype.

The security of Kerberos 5 depends absolutely on the confidentiality
and integrity of the Key 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 principal data, and to arrange for more secure
backup for KeySets.

4.2.1. KeySet: Attributes

4.2.1.1. kvno

Also knowns as the

The key version number. This is a single-valued attribute containing
a non-negative integer. This number is
incremembed incremented by one each time
a key in the KeySet is changed.

4.2.2. KeySet: Associations

To each

Each KeySet MUST be associated with a set of 1 one or more Keys.

4.3. Key

Implementations of this model MUST NOT REQUIRE 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'. "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
in
using the "string-to-key" string-to-key method defined in section Section 3 of [RFC3961].

4.3.1.5. keyNotUsedBefore

This

The key MUST NOT be used before this date. The syntax of the
attribute MUST be semantically equivalent with to the standard ISO date
format.
format ([RFC3339]). This attribute MUST be a single-valued attribute. single-valued.

4.3.1.6. keyNotUsedAfter

This

The key MUST NOT be used after this date. The syntax of the
attribute MUST be semantically equivalent with to the standard ISO date
format.
format ([RFC3339]). This attribute MUST be a single-valued attribute. single-valued.

4.3.1.7. keyIsDisabled

This is a boolean attribute which that SHOULD be set to false FALSE by default.
If this attribute is true TRUE, the key MUST NOT be used. This The
keyIsDisabled attributed is used to temporarily disable a key.

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 implemented, adequate protection from
unauthorized modification and disclosure MUST be available and is
REQUIRED by of the implementation.

4.4. Policy

Implementations SHOULD implement policy policies, but MAY allow them to be
OPTIONAL. The Policy policy should be thought of as a 'typed hole'. i.e. "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-practice best practices for identifier-management identifier management for the schema-language
schema language used.

4.4.1.2. policyIsCritical

This boolean attribute indicates that the KDC MUST be able to
correctly interpret and apply this the policy for the Principal principal to be
used.

4.4.1.3. policyContent

This is an optional single opaque binary value is used to store a
representation of the policy. In general 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 the policy types that are not directly representable in
that implementation.

4.4.1.4. policyUse

This is an 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 be useful
in providing provide an
initial context-based filtering.

4.4.2. Mandatory-to-implement Mandatory-to-Implement Policy

All implementations that represent Policy policy objects MUST be able to
represent the policies listed in this section. Implementations are
not required to use the same underlying data-representation data representation for the
policyContent binary value value, but SHOULD use the same OIDs as the
policyIdentifier. In general general, the expression of policy may require a
Turing-complete language. This specification does not attempt to
model policy expression 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, e.g.
principals. For example, when a Principal principal is created 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 section, we list
a few of them.

5.1. LDAP backend Backend to KDC

Given an LDAP schema implementation of this information model model, it
would be possible to build an administrative service by back-ending backending
the KDC to a directory server where principals and keys are stored.
Using the security mechanisms available on the directory 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 server, while the keys are modified using
the set/change password protocol
[I-D.ietf-krb-wg-kerberos-set-passwd]. [PASSWORD].

5.2. LDAP frontend Frontend to KDC

An alternative way to provide a directory interface to the KDC is to
implement an LDAP-frontend LDAP frontend to the KDC which that exposes all non-key
objects as entries and attributes. As in the example above above, all keys
are modified using the set/change password protocol
[I-D.ietf-krb-wg-kerberos-set-passwd]. [PASSWORD]. In
this scenario scenario, the implementation would typically not use a
traditional LDAP
implementation implementation, but would treat LDAP as an access a protocol
to access data in the native KDC database.

5.3. SOAP

Given an XML schema implementation of this information model model, it
would be possible to build a SOAP interface to the KDC. This
situation demonstrates the value of creating an abstract information
model which that is mappable to multiple schema representations.

5.4. Netconf NETCONF

Given a YAML (YAML Ain't Markup Language) implementation of this
information model model, it would be possible to create a Netconf-based 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
are
be treated as write-only attributes that SHALL only be modified (using the
administrative interface) only by the change-password protocol
[I-D.ietf-krb-wg-kerberos-set-passwd].
[PASSWORD].

Exposing the object model of a KDC typically implies that objects can
be modified and/or deleted. modified, deleted, or both. In a KDC KDC, not all principals are
created
equal, so that for instance equal. For instance, deleting krbtgt/EXAMPLE.COM@EXAMPLE.COM
effectively disables the EXAMPLE.COM realm. Hence Hence, access control is
paramount to the security of any implementation. This document does
not mandate access control. This only situation implies only that access
control is beyond the scope of the standard information model, i.e. 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 model, the presence of access control may in itself provide
points of attack by giving away information about principals with that
have elevated rights etc. rights.

7. IANA Considerations

This document has no IANA actions.

8. Acknowledgments

The author wishes to extend his thanks to Love Hoernquist-Aestrand
and Sam Hartman for their important contributions to this document.

8. References

9.1.

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.

9.2.

8.2. Informative References

[I-D.ietf-krb-wg-kerberos-set-passwd]

[PASSWORD]
Williams, N., "Kerberos Set/Change Key/Password Protocol
Version 2", draft-ietf-krb-wg-kerberos-set-passwd-08 (work Work in progress), 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

Email:
Sweden

EMail: leifj@sunet.se
URI: http://www.sunet.se