GSS-API Naming Extensions

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 .

As described in the GSS-API's naming architecture suffers from certain limitations. This document proposes concrete GSS-API extensions as outlined in . A number of extensions to the GSS-API and its C Bindings are described herein with the goal of making authorization information, and other information that can be modeled as "name attributes" available as such to applications. For example, Kerberos V authorization data elements, both, in their raw forms as well as mapped to more useful value types, can be made available to GSS-API applications through these interfaces. The model is that GSS names have attributes. The attributes of a name may be authenticated by the credential whence the name comes, or may have been set locally on a GSS name for the purpose of "asserting" the attribute during credential acquisition or security context exchange. Name attributes' values are network representations thereof (e.g., the actual value octets of the contents of an X.509 certificate extension, for example) and are intended to be useful for constructing portable access control facilities. Applications may often require language- or platform-specific data types, rather than network representations of name attributes, so a function is provided to obtain objects of such types associated with names and name attributes.

A given GSS name object's name attributes may be authenticated, mapped and/or critical. These flags are explained below. An attribute is 'authenticated' iff there is a secure association between the attribute (and its values) and the trusted source of the peer credential. Examples of authenticated attributes are (any part of) the signed portion of an X.509 certificate or AD-KDCIssued authorization-data elements in Kerberos V Tickets. Note that the fact that an attribute is authenticated does not imply anything about the semantics of the attribute nor that the trusted credential source authorized any one semantic of the attribute. Such interpretations MAY be the result of applying local policy to the attribute. That a given name's given attribute is 'mapped' means that it was obtained through some mapping mechanism applied to another attribute of the name that was not, itself, mapped. For example, such attributes as platform-specific internal identifiers may sometimes be mapped from other name attributes. Name attributes may be "critical," meaning that applications that do not understand them MUST reject security contexts where the peer has such unknown, critical attributes. [NOTE(leifj): The criticality flag seems to have limited applicability in practice. As written the security context should not be established unless all critically marked naming attributes are supported and understood. But what happens if the peer doesn't understand naming extensions at all. It seems more reasonable to state that name attribute extensions MUST only be used to as a basis for authorization decisions.] [NOTE(leifj): The mapped flag also seems to have limited applicability in practice - interpretation of the attribute will be entierly up to the peer anyway which will need to know much more about the attribute than the fact than its value is derived.]

Some name attributes (e.g., numeric user or group identifiers) may be useful as subjects of access control list (ACL) entries, some may not (e.g., time of day login restrictions). The GSS_Inquire_name_attribute() function indicates this. To facilitate the development of portable applications that make use of name attributes to construct and evaluate portable ACLs the GSS-API makes name attribute values available in canonical network encodings thereof. To facilitate the development of platform- or language-specific applications that need access to native types of representations of name attributes an optional facility is provided, GSS_Map_name_to_any().

[NOTE: This entire section should probably be split into one or more separate Internet-Drafts. It is here in the -00 of this I-D to help readers understand how to mechanism-specific name attributes would be accessed through these GSS-API extensions.] Kerberos V and the Simple Public-Key GSS-API Mechanism, SPKM , both support the concept and encoding of containers of "authorization-data" as described in . PKIX supports a number of authorization-data-like features, like Extended Key Usage values (EKUs) and certificate extensions. The authorization data can be accessed through the GSS-API name attributes facility defined herein.

Authorization-data non-container elements asserted in Kerberos V AP-REQ Authenticators MUST be mapped into asserted GSS-API name attributes; if not contained in AD-IF-RELEVANT then they MUST be mapped into critical GSS-API name attributes. AD-AND-OR authorization-data elements MUST be mapped into a single critical attribute, (TBD). Authorization-data included in Kerberos V Tickets that is not contained in AD-KDCIssued (with valid signature) MUST be mapped into asserted GSS-API name attributes. Conversely, authorization-data elements in Kerberos V Tickets contained by AD-KDCIssued MUST be mapped into authenticated GSS-API name attributes As with authorization-data elements in Authenticators, authorization-data elements in Tickets not contained in AD-IF-RELEVANT are to be mapped to critical name attributes, and similarly with AD-AND-OR (see above). The OIDs for authorization-data elements are to be the authorization-data element's 'ad-type' positive integer ID, relative to the base OID <TBD> Negative values are reserved for local experiments. [NOTE: what about negative ad-type's? OID arcs are positive integers... ad-type is an Int32, so clearly something can be done.]

PKI certificate extensions MAY/SHOULD/MUST (see comment above) be represented as authenticated GSS-API name attributes with the same OIDs, and if they be marked critical in the certificate then they MUST be mapped as critical GSS-API name attributes. SubjectAltNames and EKUs, specifically, MUST be represented as authenticated GSS-API name attributes; see below. Certificate extensions MUST be represented as GSS-API name attributes whose OIDs are the same as the extensions'

Extended Key Usage extensions, specifically, MUST be mapped as described above, except that GSS-API name attributes for EKUs MUST have NULL values (i.e., zero-length OCTET STRINGs). PKI certificate key usages (KUs, but not EKUs), MUST NOT be represented as GSS-API name attributes.

PKI certificate subjectAltNames MUST be mapped as authenticated, non-critical GSS-API name attributes. PKI certificate extensions MUST be represented as authenticated GSS-API name attributes with the same OIDs, and if they be marked critical in the certificate then they MUST be mapped as critical GSS-API name attributes. Extended Key Usage extensions, specifically, MUST be mapped as described above, except that GSS-API name attributes for EKUs MUST have NULL values (i.e., zero-length OCTET STRINGs).

Any X.509 certificate extension not covered above SHOULD be represented as GSS-AOI name attributes with the OID of the X.509 extension and with OCTET STRING values containing the encoded value of the extension.

Attributes contained in SAML attribute assertions are mapped to GSS-API name attributes with OIDs derived from the SAML attributes: If the SAML attribute is an OID the same OID is used. If the SAML attribute is a URN or a URI then the name MUST be mapped to a corresponding OID by means of an IANA registry.

Inputs: name NAME, display_as_name_type OBJECT IDENTIFIER Outputs: major_status INTEGER, minor_status INTEGER, display_name STRING Return major_status codes: GSS_S_COMPLETE indicates no error. GSS_S_UNAVAILABLE indicates that the given name could not be displayed using the syntax of the given name type. GSS_S_FAILURE indicates a general error. This function displays a given name using the given name syntax, if possible. This operation may require mapping MNs to generic name syntaxes or generic name syntaxes to mechanism-specific name syntaxes; such mappings may not always be feasible and MAY be inexact or lossy, therefore this function may fail.

Inputs: name NAME Outputs: major_status INTEGER, minor_status INTEGER, name_is_MN BOOLEAN, mn_mech OBJECT IDENTIFIER, asserted_attrs SET OF OBJECT IDENTIFIER, authenticated_attrs SET OF OBJECT IDENTIFIER, critical_attrs SET OF OBJECT IDENTIFIER, all_attrs SET OF OBJECT IDENTIFIER, Return major_status codes: GSS_S_COMPLETE indicates no error. GSS_S_FAILURE indicates a general error. This function outputs the sets of attributes of a name, that are authenticated, asserted or critical. It also indicates if a given NAME is an MN or not and, if it is, what mechanism it's an MN of.

Inputs: name NAME, attr OBJECT IDENTIFIER Outputs: major_status INTEGER, minor_status INTEGER, authenticated BOOLEAN, -- TRUE iff authenticated by the trusted peer credential source. negative BOOLEAN, mapped BOOLEAN, critical BOOLEAN, values SET OF OCTET STRING, display_values SET OF STRING Return major_status codes: GSS_S_COMPLETE indicates no error. GSS_S_UNAVAILABLE indicates that the given attribute OID is not known or set. GSS_S_FAILURE indicates a general error. This function outputs the value(s) associated with a given GSS name object for a given name attribute. NOTE: This function relies on the GSS-API notion of "SET OF" allowing for order preservation; this has been discussed on the KITTEN WG mailing list and the consensus seems to be that, indeed, that was always the intention. It should be noted however that the order presented does not always reflect an underlying order of the mechanism specific source of the attribute values.

The C-bindings of GSS_Get_name_attribute() requires one function call per-attribute value, for multi-valued name attributes. This is done by using a single gss_buffer_t for each value and an input/output integer parameter to distinguish initial and subsequent calls and to indicate when all values have been obtained. The 'more' input/output parameter should point to an integer variable whose value, on first call to gss_name_attribute_get() MUST be -1, and whose value upon function call return will be non-zero to indicate that additional values remain, or zero to indicate that no values remain. The caller should not modify this parameter after the initial call.

Inputs: name NAME, critical BOOLEAN, negative BOOLEAN, attr OBJECT IDENTIFIER, values SET OF OCTET STRING Outputs: major_status INTEGER, minor_status INTEGER Return major_status codes: GSS_S_COMPLETE indicates no error. GSS_S_UNAVAILABLE indicates that the given attribute OID is not known or could not be set. GSS_S_FAILURE indicates a general error. NOTE: This function relies on the GSS-API notion of "SET OF" allowing for order preservation; this has been discussed on the KITTEN WG mailing list and the consensus seems to be that, indeed, that was always the intention. It should be noted that underlying mechanisms may not respect the given order.

The C-bindings of GSS_Set_name_attribute() requires one function call per-attribute value, for multi-valued name attributes -- each call adds one value. To replace an attribute's every value delete the attribute's values first with GSS_Delete_name_attribute().

Inputs: name NAME, attr OBJECT IDENTIFIER, Outputs: major_status INTEGER, minor_status INTEGER Return major_status codes: GSS_S_COMPLETE indicates no error. GSS_S_UNAVAILABLE indicates that the given attribute OID is not known. GSS_S_UNAUTHORIZED indicates that a forbidden delete operation was attempted eg deleting a negative attribute. GSS_S_FAILURE indicates a general error. Deletion of negative authenticated attributes from NAME objects MUST NOT be allowed and must result in a GSS_S_UNAUTHORIZED.

Inputs: name NAME Outputs: major_status INTEGER, minor_status INTEGER, exp_composite_name OCTET STRING Return major_status codes: GSS_S_COMPLETE indicates no error. GSS_S_FAILURE indicates a general error. This function outputs a token which can be imported with GSS_Import_name(), using GSS_C_NT_COMPOSITE_EXPORT as the name type and which preserves any name attribute information associated with the input name (which GSS_Export_name() may well not). The token format is no specified here as this facility is intended for inter-process communication only; however, all such tokens MUST start with a two-octet token ID, hex 04 02, in network byte order. The OID for GSS_C_NT_COMPOSITE_EXPORT is <TBD>.

Inputs: name NAME, authenticated BOOLEAN, -- if TRUE only authenticated attributes will be included type_id OBJECT IDENTIFIER Outputs: major_status INTEGER, minor_status INTEGER, output ANY DEFINED BY type_id Return major_status codes: GSS_S_COMPLETE indicates no error. GSS_S_UNAVAILABLE indicates that the mapping or conversion could not be done. The minor status code may provide additional information. GSS_S_FAILURE indicates a general error. The minor status code may provide additional information. Whereas name attribute's values are encoded in some network representation applications often require native, language- and/or platform-specific data types. This function provides access to such types.

Note the new C bindings type, gss_any_t. We define it as a pointer to an incompletely declared struct.

Inputs: name NAME, type_id OBJECT IDENTIFIER, input ANY DEFINED BY type_id Outputs: major_status INTEGER, minor_status INTEGER, Return major_status codes: GSS_S_COMPLETE indicates no error. GSS_S_UNAVAILABLE indicates that the mapping or conversion could not be done. The minor status code may provide additional information. GSS_S_FAILURE indicates a general error. The minor status code may provide additional information. This function releases, if possible, the objects of language- and/or platform-specific types output by GSS_Map_name_to_any(). If such types have native release functions applications MAY use either those or this function to release the given object.

This document creates a namespace of GSS-API name attributes. Attributes are named by OID, so no single authority might be needed for allocation, however, in the interest of providing the community with an authority for name attribute OID allocation and a way to find the existing set of name attributes, the IANA should establish both, a single OID off of which name attributes could be allocated, and a registry of known GSS name attributes. GSS-API name attribute registry entries should contain all the information that GSS_Inquire_name_attribute() may return about the given name attributes and their OIDs: a name attribute OID (this is a unique key) a name attribute symbolic name, starting with "GSS_C_NA_" (this is a unique key) a brief description, in English whether the attribute is useful as the subject of access control list entries whether the attribute is useful as an indicator of trust an optional normative reference to documentation for the given name attribute The allocation and registration policy should be first come, first served. Registry entries' OIDs need not be based on the base OID given above.

This document extends the GSS-API naming model to include support for name attributes. The intention is that name attributes are to be used as a basis for (among other things) authorization decisions or application personalization for applications relying on GSS-API security contexts. The security of the application may be critically dependent on the security of the attributes. This document classifies attributes as asserted or authenticated. Only authenticated attributes MUST be used if the attribute has security implications for the application (eg authorization decisions) since asserted attributes may easily be controlled by the peer directly. It is important to understand the meaning of 'authenticated' in this setting. It does not mean that any semantic of the attribute is claimed to be true. The only implication is that a trusted third party has asserted the attribute as opposed to the attribute being asserte by the peer itself. Any additional semantics is always the result of applying policy. For instance in a given deployment the mail attribute of the subject may be authenticated and sourced from an email system where 'correct' values are kept. In another setting users may be allowed to modify their mail addresses freely. In both cases the 'mail' attribute may be authenticated by virtue of being included in signed SAML attribute assertions lor by other means authenticated by the underlying mechanism. When the underlying security mechanism does not provide a permanent unique identity (eg anonymous kerberos) the GSS-API naming extensions may be used to provide a replacement permanent unique identity attribute which in this case may be unique for each relying party. This is analogous to the Liberty Alliance targetedID attribute and has similar security implications.