INTERNET-DRAFT Nicolas Williams Sun Microsystems September 2003 GSS-APIv2 Extension for Storing Delegated Credentials Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026 [RFC2026]. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet- Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This draft expires on January 30th, 2004. Please send comments to the authors. Copyright Notice Copyright (C) The Internet Society (2003). All Rights Reserved. Abstract This document defines a new function for the 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. 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]. N. Williams [Page 1] DRAFT GSS_Store_cred() Expires March 2004 Table of Contents 1 Introduction 2 GSS_Store_cred() 2.1 C-Bindings for GSS_Store_cred() 3 Examples 4 Security Considerations 5 References 5.1 Normative References 6 Author's Address 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. [NOTE: 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. Note to the RFC editor: please remove this note before publication.] 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 - which is precisely the raison d'etre of credential delegation. But 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 N. Williams [Page 2] DRAFT GSS_Store_cred() Expires March 2004 replace the process image obliterates the delegated credentials handle. [NOTE: Delegated credentials are practically unusable on Unix implementations of Secure Shell (SSHv2) servers, except where there are extended interfaces for dealing with delegated credentials, which to date have always been mechanism-specific interfaces. Note to the RFC editor: please remove this note before publication.] 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 which allows storing of credentials in the implicit credential store. This primitive we call "GSS_Store_cred()." [NOTE: Simon Wilkinson's patches to OpenSSH for GSS-API sport a simple internal interface for storing delegated credentials in users' credential store - this internal interface wraps around two mechanism specific internal interfaces for storing GSI and Kerberos V credentials. Simon's code shows that: a) a generic method is needed for making delegated credentials available for indirect use through acquisition (as opposed to just using the actual delegated cred handle) b) it is possible to design and implement such a generic method for storing delegated credentials. No new concepts are added to the GSS-API by this document, but the implicit existence of a credential store in the background is made explicit, and a deficiency of the GSS-API is corrected. Compare this to the GGF proposal which includes a credential import/export facility (like the existing context import/ export facility), but with an option to export as "environment variables," meaning something like "store these input creds in some new credential store and then tell me the name of that credential store through some output environment variable"[*]. Thus, the GGF export-cred-to-environment- variable proposal adds knowledge of environment variables to the GSS-API, which this proposal does not. Note that a credential import/export facility along the lines of the existing context import/export facility may be useful and complements the GSS_Store_cred() interface; in fact, with GSS_Store_cred() it should be possible to remove the 'option_req' input parameter and export-to-env-var features N. Williams [Page 3] DRAFT GSS_Store_cred() Expires March 2004 of the GGF's GSS_Export_cred() credential export proposal. [*] For the exact semantics see section 1.2, paragraph 6 of draft-engert-ggf-gss-extensions-00.txt One side effect of GSS_Store_cred(), however, is that it allows applications that can switch their current credential store to move credentials from one store to the other; this is a direct result of making it possible to store a credential given a GSS-API credential handle. Perhaps there should be some text allowing, or recommending, that implementations of GSS_Store_cred() allow only the storage of credentials acquired through credential delegation. Note to the RFC editor: please remove this note before publication.] 2 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 o overwrite_cred BOOLEAN, -- if TRUE replace any credential for the -- same principal in the credential store o default_cred BOOLEAN -- 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: o major_status INTEGER, o minor_status INTEGER, o mech_elements_stored SET OF OBJECT IDENTIFIER, -- the set of -- mechanism OIDs for which credential elements were successfully -- stored o 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) N. Williams [Page 4] DRAFT GSS_Store_cred() Expires March 2004 Return major_status codes: o GSS_S_COMPLETE indicates that the credentials were successfully stored. o GSS_S_CREDENTIALS_EXPIRED indicates that the input credentials had expired or expired before they could be stored. o GSS_S_NO_CRED indicates that no input credentials were given. o GSS_S_UNAVAILABLE indicates that the credential store is not available. o 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. o 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. Finally, if the current credential store has no default credential (that is, no credential that could be acquired for GSS_C_NO_NAME) or if the default_cred input argument is TRUE, and the input credential can be successfully stored, then the input credential will be available for acquisition with GSS_C_NO_NAME as the desired name input to GSS_Acquire_cred() or GSS_Add_cred() as well as for use as GSS_C_NO_CREDENTIAL for the cred_handle inputs to GSS_Inquire_cred(), GSS_Inquire_cred_by_mech(), GSS_Init_sec_context() and GSS_Accept_sec_context(). N. Williams [Page 5] DRAFT GSS_Store_cred() Expires March 2004 2.1 C-Bindings for GSS_Store_cred() 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, 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) The two boolean arguments, 'overwrite_cred' and 'default_cred' are typed as OM_uint32; 0 corresponds to FALSE, non-zero values correspond to TRUE. 3 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: /* * * * <"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. */ if (change_user_context(requested_username)) (void) gss_store_creds(&minor_status, deleg_cred, GSS_C_INITIATE, actual_mech, 0, 1, NULL, NULL); } else ... } else ... 4 Security Considerations N. Williams [Page 6] DRAFT GSS_Store_cred() Expires March 2004 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()). 5 References 5.1 Normative References [RFC2026] S. Bradner, RFC2026: "The Internet Standard Process - Revision 3," October 1996, Obsoletes - RFC 1602, Status: Best Current Practice. [RFC2119] S. Bradner, RFC2119 (BCP14): "Key words for use in RFCs to Indicate Requirement Levels," March 1997, Status: Best Current Practice. [RFC2743] J. Linn, RFC2743: "Generic Security Service Application Program Interface Version 2, Update 1," January 2000, Status: Proposed Standard. [RFC2744] J. Wray, RFC2744: "Generic Security Service API Version 2 : C-bindings," January 2000, Status: Proposed Standard. 6 Author's Address Nicolas Williams Sun Microsystems 5300 Riata Trace Ct Austin, TX 78727 Email: Nicolas.Williams@sun.com Full Copyright Statement Copyright (C) The Internet Society (2003). All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of N. Williams [Page 7] DRAFT GSS_Store_cred() Expires March 2004 developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than English. The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns. This document and the information contained herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Acknowledgement Funding for the RFC Editor function is currently provided by the Internet Society. N. Williams [Page 8]