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Network Working Group
Request for Comments: 5588
Category: Standards Track
N. Williams
Sun
July 2009

Generic Security Service Application Program Interface (GSS-API)

Extension for Storing Delegated Credentials

Abstract

This document defines a new function for the Generic Security Service Application Program Interface (GSS-API), which allows applications to store delegated (and other) credentials in the implicit GSS-API credential store. This is needed for GSS-API applications to use delegated credentials as they would use other credentials.

Status of This Memo

This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited.

Copyright Notice

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Table of Contents

   1. Introduction ....................................................2
   2. Conventions Used in This Document ...............................3
   3. GSS_Store_cred() ................................................3
   4. C-Bindings ......................................................5
   5. Examples ........................................................6
   6. Security Considerations .........................................6
   7. Normative References ............................................7

1. Introduction

The GSS-API [RFC2743] clearly assumes that credentials exist in an implicit store whence they can be acquired using GSS_Acquire_cred() and GSS_Add_cred() or through use of the default credential. Multiple credential stores may exist on a given host, but only one store may be accessed by GSS_Acquire_cred() and GSS_Add_cred() at any given time.

This assumption can be seen in Sections 1.1.1.2 and 1.1.1.3 of [RFC2743] as well as in Section 3.5 of [RFC2744].

Applications may be able to change the credential store from which credentials can be acquired, either by changing user contexts (where the applications have the privilege to do so) or by other means (where a user may have multiple credential stores).

Some GSS-API acceptor applications always change user contexts, after accepting a GSS-API security context and making appropriate authorization checks, to the user context corresponding to the initiator principal name or to a context requested by the initiator. The means by which credential stores are managed are generally beyond the scope of the GSS-API.

In the case of delegated credential handles however, such credentials do not exist in the acceptor's credential store or in the credential stores of the user contexts to which the acceptor application might change. The GSS-API provides no mechanism by which delegated credential handles can be made available for acquisition through GSS_Acquire_cred()/GSS_Add_cred(). The GSS-API also does not provide any credential import/export interfaces like the GSS-API context import/export interfaces.

Thus, acceptors are limited to making only direct use of delegated credential handles and only with GSS_Init_sec_context(), GSS_Inquire_cred*(), and GSS_Release_cred(). This limitation is particularly onerous on Unix systems where a call to exec() to replace the process image obliterates any delegated credentials handle that may exist in that process.

In order to make delegated credentials generally as useful as credentials that can be acquired with GSS_Acquire_cred() and GSS_Add_cred(), a primitive is needed that allows storing of credentials in the implicit credential store. We call this primitive "GSS_Store_cred()".

2. Conventions Used in This Document

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].

3. GSS_Store_cred()

Inputs: 

   o  input_cred_handle CREDENTIAL HANDLE, -- credential to store; MUST
      NOT be GSS_C_NO_CREDENTIAL.
   
   o  cred_usage INTEGER -- 0=INITIATE-AND-ACCEPT, 1=INITIATE-ONLY,
      2=ACCEPT-ONLY.
   
   o  desired_mech_element OBJECT IDENTIFIER, -- if GSS_C_NULL_OID, then
      store all the elements of the input_cred_handle, otherwise, store
      only the element of the corresponding mechanism.
  • overwrite_cred BOOLEAN, -- if TRUE, replace any credential for the same principal in the credential store.
  • default_cred BOOLEAN -- advisory input; if TRUE, make the stored credential available as the default credential (for acquisition with GSS_C_NO_NAME as the desired name or for use as GSS_C_NO_CREDENTIAL).

Outputs:

  • major_status INTEGER.
  • minor_status INTEGER.
  • mech_elements_stored SET OF OBJECT IDENTIFIER, -- the set of mechanism OIDs for which credential elements were successfully stored.
  • cred_usage_stored INTEGER -- like cred_usage, but indicates what kind of credential was stored (useful when the cred_usage input parameter is set to INITIATE-AND-ACCEPT).

Return major_status codes:

  • GSS_S_COMPLETE indicates that the credentials were successfully stored.
  • GSS_S_CREDENTIALS_EXPIRED indicates that the input credentials had expired or expired before they could be stored.
  • GSS_S_NO_CRED indicates that no input credentials were given.
  • GSS_S_UNAVAILABLE indicates that the credential store is not available.
  • GSS_S_DUPLICATE_ELEMENT indicates that an element of the input credential could not be stored because a credential for the same principal exists in the current credential store and the overwrite_cred input argument was FALSE.
  • GSS_S_FAILURE indicates that the credential could not be stored for some other reason. The minor status code may provide more information if a non-GSS_C_NULL_OID desired_mech_element was given.

GSS_Store_cred() is used to store, in the current credential store, a given credential that has either been acquired from a different credential store or been accepted as a delegated credential.

Specific mechanism elements of a credential can be stored one at a time by specifying a non-GSS_C_NULL_OID mechanism OID as the desired_mech_element input argument; in which case, the minor status output SHOULD have a mechanism-specific value when the major status is not GSS_S_COMPLETE.

The initiator, acceptor, or both usages of the input credential may be stored as per the cred_usage input argument.

The credential elements that were actually stored, when the major status is GSS_S_COMPLETE, are indicated through the cred_usage_stored and mech_elements_stored function outputs.

If credentials already exist in the current store for the principal of the input_cred_handle, then those credentials are not replaced with the input credentials unless the overwrite_cred input argument is TRUE.

In the GSS-API, the default credential can be used by using GSS_C_NO_CREDENTIAL or a CREDENTIAL handle acquired by calling GSS_Acquire_cred() or GSS_Add_cred() with the desired_name input set to GSS_C_NO_NAME.

If the default_cred input argument is TRUE, and the input credential can be successfully stored, then the input credential SHOULD be stored as the default credential (see above).

If the current credential store has no default credential (see above), then the implementation MAY make the stored credentials available as the default credential regardless of the value of the default_cred input argument.

4. C-Bindings

The C-Bindings for GSS_Store_cred() make use of types from and are designed based on the style of the GSS-APIv2 C-Bindings [RFC2744].

OM_uint32 gss_store_cred( 

            OM_uint32         *minor_status,
            gss_cred_id_t     input_cred_handle,
            gss_cred_usage_t  cred_usage,
            const gss_OID     desired_mech,
            OM_uint32         overwrite_cred,
            OM_uint32         default_cred,
            gss_OID_set       *elements_stored,
            gss_cred_usage_t  *cred_usage_stored)
            
                        Figure 1

The two boolean arguments, 'overwrite_cred' and 'default_cred', are typed as OM_uint32; 0 corresponds to FALSE, non-zero values correspond to TRUE.

5. Examples

The intended usage of GSS_Store_cred() is to make delegated credentials available to child processes of GSS-API acceptor applications. Example pseudo-code: 

      /*
       * <GSS_Accept_sec_context() loop resulting in GSS_S_COMPLETE,
       * an initiator name (hereafter, "src_name") and a delegated
       * credential handle (hereafter "deleg_cred").>
       *
       * <"requested_username" is a username derived from the
       * initiator name or explicitly requested by the initiator
       * application.>
       */
      ...
      
      if (authorize_gss_client(src_name, requested_username)) {
         /*
          * For Unix-type platforms this may mean calling setuid() and
          * it may or may not also mean setting/unsetting such
          * environment variables as KRB5CCNAME and what not -- all
          * OS-specific details.
          */
         if (change_user_context(requested_username))
            (void) gss_store_cred(&minor_status, deleg_cred,
                                  GSS_C_INITIATE, actual_mech,
                                  0, 1, NULL, NULL);
         }
         else ...
      }
      else ...

6. Security Considerations

Acceptor applications MUST only store delegated credentials into appropriate credential stores and only after proper authorization of the authenticated initiator principal to the requested service(s).

Acceptor applications that have no use for delegated credentials MUST release them (such acceptor applications that use the GSS-API C- Bindings may simply provide a NULL value for the delegated_cred_handle argument to gss_accept_sec_context()).

7. Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.
   
   [RFC2743]  Linn, J., "Generic Security Service Application Program
              Interface Version 2, Update 1", RFC 2743, January 2000.
   
   [RFC2744]  Wray, J., "Generic Security Service API Version 2 :
              C-bindings", RFC 2744, January 2000.

Author's Address

   Nicolas Williams
   Sun Microsystems
   5300 Riata Trace Ct
   Austin, TX  78727
   US
   
   EMail: Nicolas.Williams@sun.com