draft-williams-gssapi-store-deleg-creds-01.txt [plain text]
INTERNET-DRAFT Nicolas Williams
Sun Microsystems
September 2003
GSS-APIv2 Extension for Storing Delegated Credentials
<draft-williams-gssapi-store-deleg-creds-01.txt>
Status of this Memo
This document is an Internet-Draft and is in full conformance with
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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].
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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
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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
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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)
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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().
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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:
/*
* <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.
*/
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
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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
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