| KERBEROS WORKING GROUP | Johansson |
| Internet-Draft | SUNET |
| Intended status: Standards Track | July 31, 2012 |
| Expires: January 30, 2013 |
An information model for Kerberos version 5
draft-ietf-krb-wg-kdc-model-14
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.
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The Kerberos version 5 authentication service described in [RFC4120] describes how a Key Distribution Center (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 administer 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. Neither an object oriented model nor an LDAP [RFC4510] 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 (e.g. principalName). If so an implementation MUST provide a name to name mapping to this document. In particular schema languages may have different conventions for caseing, eg camelCase vs 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 the ability to specify syntax for attribute values.
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].
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 - e.g. an LDAP or SQL schema. The precise interpretation of terms from [RFC2119] therefore require some extra explanation.
The terms MUST or REQUIRED means that schema implementing this model must have a way to represent the feature (i.e that it is mandatory to implement) 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 MUST provide the feature and associated behavior consistent with schema.
For instance, principalLastFailedAuthentication (cf below) represents the last time an authentication failed for 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 or OPTIONAL means that the feature is optional to implement by a KDC or administrative interface implementing this information model. MAY also means that the feature is optional to implement in schema.
Implementors of schema should be aware that unless there is a way to represent critical but optional elements in the schema definition language confusion may arise when optional elements are used but not understood by all implementations in a particular deployment.
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 should always use a normative reference to the relevant RFCs 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; an LDAP implementation of this model should incorporate existing LDAP schema where functional overlap exists, rather than defining additional Kerberos-specific elements.
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
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 (section 2.1.1 of [RFC1964]) including, if applicable for the name type, the realm.
The attribute MAY be multi-valued if the implementation supports aliases and/or enterprise names. In that case exactly one of the principalName values MAY be designated the canonical principalName and if the implementation supports enctypes which require salt then exactly one of the values of principalName MAY be designated as the canonical salting principalName.
Implementations (i.e. schema) that support enterprise names and/or aliases SHOULD provide for efficient lookup of Principal objects based on alias/enterprise name.
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.
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.
A boolean attribute used to disable a Principal. The attribute SHOULD default to boolean FALSE.
This single-valued attribute contains the time of the last successful change of credential (e.g. password or private key) associated with this Principal. The syntax of the attribute MUST be Internet Date/Time Format from [RFC3339].
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].
This single-valued attribute contains the time and date when this Principal was last modified excluding credentials change. The syntax of the attribute MUST be Internet Date/Time Format from [RFC3339].
This single-valued attribute contains the time in seconds representing the maximum lifetime for tickets issued for this Principal.
This single-valued attribute contains the delta time in seconds representing the maximum amount of time a ticket may be renewed for this Principal.
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.
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.
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 MUST be able to handle the general case of multiple KeySet associated with each principal. Multiple KeySets may for instance be useful when performing a key rollover for a principal.
In Kerberos principals are associated with zero or more symmetric secret keys, and each key has a key version number (kvno) and 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. Schemas 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.
Also knowns as the key version number. This is a single-valued attribute containing a non-negative integer. This number is incremembed by one each time a key in the KeySet is changed.
To each KeySet MUST be associated a set of 1 or more Keys.
Implementations of this model MUST NOT REQUIRE keys to be represented.
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'.
The binary representation of the key data. This MUST be a single-valued octet string.
The binary representation of the key salt. This MUST be a single-valued octet string.
This MUST be a single-valued octet string representing an opaque parameter associated with the enctype. This parameter is specified in the "string-to-key" method in section 3 of [RFC3961].
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.
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.
This is a boolean attribute which SHOULD 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.
None
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.
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.
The policyIdentifier MUST be globally unique. Possible types of identifiers include:
Implementations of this specification are expected to assign globally unique identifiers to the list of standard policy below in accordance with best-practice for identifier-management for the schema-language used.
This boolean attribute indicates that the KDC MUST be able to correctly interpret and apply this policy for the Principal to be used.
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.
This is an optional single enumerated string value 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 an initial context-based filtering.
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.
Password quality policy controls the requirements placed by the KDC on new passwords.
Password management policy controls how passwords are changed.
A keying policy specifies the association of enctypes with new principals, e.g. when a Principal is created one of the applicable keying policies is used to determine the set of keys to associate with the principal.
A ticket flag policy specifies the ticket flags allowed for tickets issued for a principal.
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.
Given an LDAP schema implementation of this information model it would be possible to build an administrative service by back-ending 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].
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.
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.
Given a YAML 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.
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 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 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 with elevated rights etc.
This document has no IANA actions.
The author wishes to extend his thanks to Love Hörnquist-Åstrand and Sam Hartman for their important contributions to this document.
| [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. |
| [RFC3961] | Raeburn, K., "Encryption and Checksum Specifications for Kerberos 5", RFC 3961, February 2005. |
| [RFC1964] | Linn, J., "The Kerberos Version 5 GSS-API Mechanism", RFC 1964, June 1996. |
| [RFC3339] | Klyne, G. and C. Newman, "Date and Time on the Internet: Timestamps", RFC 3339, July 2002. |
| [RFC4517] | Legg, S., "Lightweight Directory Access Protocol (LDAP): Syntaxes and Matching Rules", RFC 4517, June 2006. |
| [RFC4122] | Leach, P., Mealling, M. and R. Salz, "A Universally Unique IDentifier (UUID) URN Namespace", RFC 4122, July 2005. |
| [RFC3986] | Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986, January 2005. |
| [I-D.ietf-krb-wg-kerberos-set-passwd] | Williams, N, "Kerberos Set/Change Key/Password Protocol Version 2", Internet-Draft draft-ietf-krb-wg-kerberos-set-passwd-08, November 2008. |
| [RFC4510] | Zeilenga, K., "Lightweight Directory Access Protocol (LDAP): Technical Specification Road Map", RFC 4510, June 2006. |