One document matched: draft-ietf-cat-acc-cntrl-frmw-02.txt

Differences from draft-ietf-cat-acc-cntrl-frmw-01.txt

0. Status Of this Document

This document is an Internet-Draft and is in full conformance
with all provisions of Section 10 of RFC2026.

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1. Abstract

This document describes a unified model to support authorization in a
wide range of applications, including metacomputing, remote printing,
video conference, and any other application which will require
interactions between entities across autonomous security domains.

The document proposes requirements for the support of:

   - flexible and expressive mechanism for representing
     and evaluating security policies

   - uniform authorization service interface for facilitating access
     control decisions for applications and requesting access control
     information about a particular resource.

This specification defines structures and their uses at a level
independent of underlying mechanism and programming language environment.
This document is accompanied by a second one describing the details of
proposed structures and services along with bindings for C language
environments. This document is to be found in
draft-ietf-cat-gaa-cbind-02.txt.

2. Introduction

The variety of services available on the Internet continues to increase
and new classes of applications such as metacomputing, remote printing,
video conference is evolving. These applications will require
interactions between entities across autonomous security domains.

The distributed nature of the system, consisting of mutually suspicious
security domains, requires a mechanism, which provides fine-grained
access control to resources.

For example, access control requirements of a remote printing application
may include:

   - authorized individual users and organizations
   - time availability, e.g. time of the day or day of the week
   - restrictions on resources consumed by the clients, e.g. maximum
     job size, maximum number of pages per job
   - required confidentiality/integrity message protection
   - accounting for consumed resources

Access control requirements of large-scale multicast application [4],
e.g. corporate video conference may include:

   - authorized individual users and organizations
   - host properties; users on slow hosts or hosts running the wrong
     OS will be denied communication
   - accounting for consumed resources

Some of the security requirements are common across different
applications, while others are more individual.

Access control policies can be formulated in many ways. Administrators
Of each domain might use domain-specific policy syntax and
heterogeneous implementations of the policies.

It is necessary to define a particular framework applicable for
a wide range of systems and applications, which will allow to discuss
specific requirements for the representation and evaluation of security
policies.
An application developer should not have to give anything up by using
this framework, therefore the framework should provide support for
integration with existing OS or toolkit provided access control
mechanisms.
High-speed implementations should be provided for operations that are
supported natively by the underlying system.

The focus of this framework is based on the following two abstractions:

1) uniform mechanism for representation and evaluation of security
   policies

It should be capable of implementing a number of different security
policies, based on diverse authorization models, which can coexist in
distributed system. Standardizing the way that applications define their
security requirements provides the means for integration of local and
distributed security policies and translation of security policies
across multiple authorization models.

The mechanism should support the common authorization requirements but
provide the means to defining and integration with application or
organization specific policies as well. Applications should not need to
re-implement the basic authorization functions in an
application-specific manner.

2) Generic Authorization and Access control Application Program
   Interface (GAA API)

A common API will facilitate authorization decisions for applications.
An application invokes the API functions to determine if a requested
operation or set of operation are authorized or if additional checks
are necessary.

The API will support the needs of most applications, thus not forcing
the developers to design their own authorization mechanisms.
The API will allow better integration of multiple mechanisms with
application servers. The GSS API can be used by the GAA API to
obtain principal's identity see section 11.

Section 12 gives an extended example how the GAA API can be used by
applications.

3. Glossary

OBJECT (RESOURCE)
   entity that has to be protected e.g. hosts, files.

SUBJECT
   entity that can initiate requests to an object e.g. individual users,
   hosts, applications and groups.

PRINCIPAL
   identity associated with a subject as a result of some unspecified
   authentication protocol. It can refer to a person, group, host, and
   application. Several principals can be associated with the same
   subject.

SECURITY POLICY
   the set of rules that govern access to objects

ACCESS RIGHT (OPERATION, PERMISSION)
   a particular type of access to a protected object e.g. read, write,
   and execute

RESTRICTION (CONDITION)
   a specific policy allowing an operation to be performed on an
   object

This policy can have two meanings:

    1) descriptive
         An operation is allowed  if certain condition is satisfied.
         For instance, a policy may require concurrence of two
         principals to perform some operation. If participation of
         both principals can be proved then this policy is satisfied.

    2) prescriptive
         An operation will be allowed if certain restrictive policy
         is enforced.

         For example, a process will be authorized to run on a host
         if the memory usage limits that a process can occupy in main
         memory satisfies certain constraints. Continuous evaluation of
         restriction is required.

DELEGATION
   is the ability of a principal to give to another principal limited
   authority to act on its behalf.

CREDENTIAL
   a statement of identity, group membership and non-membership,
   privilege attribute and transfer of privilege encoded in
   certificates.

4. Architecture

The major components of the architecture are:

Authentication mechanisms (perhaps involving an authentication
server) perform authentication of users and supply them with
initial credentials.

A group server is trusted to maintain and provide group membership
information. A group is a convenient method to associate a name with
a set of principals for access control purposes. A group server issues
group membership and non-membership certificates. When a connection is
established with an application server, these certificates are evaluated
(evaluation may be deferred until needed) the results are placed into
the GAA API security context. They are checked by the GAA API when
making authorization decisions.

The application calls the GAA API routines to check authorization
against the application authorization model.
These routines obtain access control information from local
files, distributed authorization servers, and from credentials
provided by the user, combining local and distributed authorizations
under a single API according to the requirements of the application.

Delegation is supported through inclusion of delegated credentials.
Mechanism for delegation such as those supported by
restricted proxies [1] in the security context, where they are
available for use by authentication and authorization mechanisms
used for subsequent connections from the server (now acting as an
intermediary) to another server.

5. Objects

The purpose of access control is protecting objects from unauthorized
access. The kinds of objects to be protected are specific to the
application to which the authorization model is applied and are not
included into the authorization model.
The objects that need to be protected include files, directories,
network connections, hosts, auxiliary devices, e.g. printers and
faxes, and other entities. An authorization mechanism should support these
different kinds of objects in a uniform manner. Same security attribute
structure should be used to specify access policies for different kinds of
objects.
Object names should be drawn from the application-specific name space
and must be opaque to the authorization mechanism.

6. Security policy representation

One may think about the security policy associated with a protected
resource as a set of operations which a defined set of principals
is allowed to perform on the target resource, and optional constraints
placed on the granted operations. For example, a system administrator
can define the following security policy to govern access to a
printer: "Joe Smith and members of Department1 are allowed to print
documents Monday through Friday, from 9:00AM to 6:00PM".
This policy can be described by an ACL mechanism, where for each
resource, a list of valid entities is granted a set of access rights.
The same policy can be implemented using a capability mechanism.
However, traditional ACL and capability abstractions should be
extended to allow conditional restrictions on access rights.

Therefore, in implementing a policy, it should be possible to
define:
1) access identity
2) grantor identity
3) a set of access rights
4) a set of conditions

Policy is represented by a sequence of tokens. Each token consists of:

Token Type

     Defines the type of the attribute. Tokens of the same type
     have the same authorization semantics.


Defining Authority

     It indicates the authority responsible for defining the value
     within the attribute type.

 Value

     The value of the security attribute. Its syntax is determined by
     the attribute type. The name space for the value is defined by
     the Defining Authority field



6.1. Access identity

The access identity represents an identity to be used for access
control purposes. The authorization framework should support the
following kinds of access identity:

1) USER
      identifies a person, e.g. authenticated user name.

2) HOST
      identifies a subject as a machine from which request to access
      the object was originated, e.g., an IP address or host DNS name
      or host public key.

3) APPLICATION
      identifies a certain program that has its own associated identity
      (principal),e.g., a checksum or certified name.
      This can be useful to grant access to a certain application,
      e.g. payment program, that is trusted to be written correctly and
      perform only its intended purpose.

4) CA
    identifies a Certification Authority responsible for issuing a
    certificate.

5) GROUP
      identifies a group of subjects. The kind of subjects
      (individual user, host or application) composing the group is
      opaque to the authorization mechanism.

6) ANYBODY
      represents any subject regardless of authentication.
      This may be useful for setting the default policies.

The framework should support multiple existing principal naming
methods. Different administrative domains might use different
authentication mechanisms, each having a particular syntax for
specification of principals. Therefore, Defining Authority for access
identity indicates underlying authentication mechanism used to provide
the principal identity. Value represents particular principal identity.

Examples of access identities are:

Token Type:         access_id_ANYBODY
Defining Authority: none
Value:              none

Token Type:         access_id_HOST
Defining Authority: IPaddress
Value:              164.67.21.82

Token Type:         access_id_GROUP
Defining Authority: DCE
Value:              15

Token Type:         access_id_CA
Defining Authority: X.509
Value:              /C=US/O=Globus/CN=Globus CA

Token Type:         access_id_APPLICATION
Defining Authority: checksum
Value:              123x56

6.2. Grantor identity

The grantor identity represents an identity used  to specify the
grantor of a capability or a delegated credential. Its structure is
similar to the one of the access identity described in the previous
subsection.

Example:

Token Type:         grantor_id_USER
Defining Authority: kerberos.V5
Value:              kot@ISI.EDU

6.3. Access rights

It must be possible to specify which principals or groups of principals are
authorized for specific operations, as well as who is explicitly
denied authorizations, therefore we define positive and negative access
rights.

All operations defined on the object are grouped by type of access to
the object they represent, and named using a tag.
For example, for a file the following operations are defined:

Token Type:         pos_access_rights
Defining Authority: local_manager
Value:              FILE:read,write,execute

However, in a bank application, an object might also be a customer
account, and the following set of operation might be defined:

Token Type:         pos_access_rights
Defining Authority: local_manager
Value:              ACCOUNT:deposit,withdraw,transfer

Internally a tagged bit vector represents access rights.
Each bit in the vector corresponds to an access right.
The tag indicates how the bits in the bit vector are to be interpreted,
in the example above, for the set of rights associated with the tag FILE the
first bit should be interpreted as read, while for the set
associated with tag ACCOUNT, the same bit should be interpreted as
deposit.

6.4. Conditions

The conditions specify the type-specific policies under which an
operation can be performed on an object.

Conditions can be categorized as generic or specific. A condition
is generic if it is evaluated by the access control model. Specific
conditions are application-dependent and usually evaluated by the
application.
Conditions are evaluated recursively. Recursive evaluation of
conditions is done not through recursive GAA API calls, but though a
recursive evaluation within the GAA API.

6.4.1. Generic conditions

The following list of generic conditions should not be considered
exhaustive.

  a) time

     Time periods for which access is granted, e.g. time of day or
     day of the week

  b) location

     Location of the principal. Authorization is granted to the
     principals residing in specific hosts or domains.

  c) connection

    c.1. security of the connection

        1) required confidentiality message protection
           This condition specifies a mechanism, or a set of
           mechanisms to be used in confidentiality message protection.

        2) required integrity message protection
           This condition specifies a mechanism, or a set of
           mechanisms to be used in integrity message protection.

    c.2. bandwidth

    c.3. particular network (SAN vs. LAN for a cluster)

    c.4. dialup

  d) privilege constraints

    In general, a principal may belong to more than one group.
    By default, principal operates with the union of privileges of
    all groups to which it belongs, as well as all of his individual
    privileges.
    In assigning privileges, one can choose to:

      1) have the subject operate with the privilege of only one group
         at a time. This can be used to reduce privileges as a
         protection against accidents.
         E.g. a person is a member of two groups: PROGRAMMERS and
         SYSTEM_MANAGERS. The person may act with the privileges of the
         group PROGRAMMERS most of the time, and enable privileges of
         the SYSTEM_MANAGERS group only on occasion.

      2) have the subject operate with privileges of several specified
         groups at a time. This condition class allows one to express
        "group A and group B"

      3) endorsement
         Concurrence of several principals to perform some operation.

  e) multi-level security constraints

  f) payment

    Specifies currency and amount that must be paid prior
    access to an object will be granted.

  g) quota

   Specifies a currency and a limit. It limits the quantity of a resource
   that can be consumed or obtained.

  h) strength of authentication

   Specifies the authentication mechanism or a set of suitable
   mechanisms, used to authenticate a user.

  i) attributes of subjects

  This class of conditions defines a set of attributes that must be
  possessed by subjects in order to get access to the object, e.g.
  security label.

  j) trust constrains

   This class of conditions specify constraints on legitimacy of
   the received certificate chain and the authenticity of the
   specified keys:

      1) Constraints placed in the certificates by CA when issuing
         a certificate. They are represented as a list of certificate
         extensions and marked as critical/non-critical. For example,
         the extension may contain policy specifying that no further
         delegation of CA authority is allowed.

      2) Specify trustworthiness of CA to sign certificates for limited
         set of users.

      3) Restrictions on certification path:

         a) Permitted and denied subtrees of CA authorities.

         b) Limit the length of certificate chains (depth of the subtree).

      4) Restrictions imposed on cryptographic attributes:
         - accepted signature scheme (e.g. DSA/SHA, RSA/MD2)
         - minimum public key length (768, 1024 bits)

      5) Deciding to trust particular end-entity certificate for
         particular purpose (operation).

6.4.2. Application-specific conditions

If generic conditions are not sufficient for expressing
application-specific security policies, applications specify their
own conditions.
Anything that can be expressed as type : value alphanumeric string can
be a condition. The application must provide a means for evaluation
of the application-specific conditions.

Token Type:        printer_load
Defining Authority: PrinterManager
Value:              10

7. Capabilities and ACLs

Proposed security attributes allow to implement both capabilities
and ACLs. For example, the following sequence of security attributes
implements an ACL, stating that anyone authenticated by Kerberos.V5
has read access to the targeted object and any member of
group 15 connecting from the USC.EDU domain has read and write
access to the object.

Token Type:         access_id_ANYBODY
Defining Authority: none
Value: none

Token Type:         pos_access_rights
Defining Authority: local_manager
Value:              FILE:read

Token Type:         authentication_mechanism
Defining Authority: system_manager
Value:              kerberos.V5

Token Type:         access_id_GROUP
Defining Authority: DCE
Value:              15

Token Type:         pos_access_rights
Defining Authority: local_manager
Value:              FILE:read,write

Token Type:         location
Defining Authority: system_manager
Value:              *.USC.EDU

The following sequence of security attributes implements
a capability, stating that the capability granted by the group "admin"
grants read access if the capability is presented during the specified
time period.

Token Type:         grantor_id_GROUP
Defining Authority: kerberos.V5
Value:              admin@USC.EDU

Token Type:         pos_access_rights
Defining Authority: local_manager
Value:              FILE:read

Token Type:         time_window
Defining Authority: eastern_timezone
Value:              8:00AM-5:00PM

8. Ordering Issues

The order in which security attributes are appearing is very important
for correctly interpreting the intended security policy.
It is not clear if interpretation of security attribute ordering should
be included in the draft or left as an implementation issue.

9. Inheritance

Underlying systems may use inheritance, for example the Netware security
model propagates inheritance rights down the directory tree, so that there
is not a single ACL to evaluate. In NT one can get access to an file either
through the directory or through the file itself, so again there are two
ACLs.
Inheritance can be supported in the function one provides to obtain
the ACL (see section 11.1), i.e. it needs to look at the object name and
determine which sources (possibly multiple sources) to draw ACL entries
from.
It might also add entries that refer to other named ACLs that would be
"included" in the primary ACL or interpreted at other points.  How
this should work is something that is open for definition/discussion
with those needing this feature.

10. Security context

The security context is a GAA API data structure, which is passed as
an argument to the GAA API.
It stores information relevant to access control policy, e.g.
authentication and authorization credentials presented or used by the
peer entity (usually the client of the request), connection state
information.

The context consists of:

1) Identity

Verified authentication information, such as principal name for a
particular security mechanism.

2) Authorized credentials
   This type of credentials is used to hold delegated credentials and
   capabilities.

3) Group membership
   This type of credentials specifies that the grantee is a member of
   only the listed groups.

4) Group non-membership
   This type of credentials specifies that the grantee is NOT a member
   of the listed groups.

5) Attributes
   This type of credentials contains  miscellaneous attributes
   attached to the grantee, e.g. age of the grantee, grantee's security
   clearance.

6) Unevaluated Credentials
   Evaluation of the acquired credentials can be deferred till the
   credential is needed to perform the operation.

7) Evaluation and Retrieval Functions for Upcalls
  These functions are called to evaluate application-specific conditions,
  to request additional credentials and verify them.
  The GSS API is an example of how this can be filled in.

8) Connection State Information
  Contains a mechanism-specific representation of per-connection
  context, some of the data stored here include keyblocks, addresses.

11. Generic Authorization and Access API

The GAA API is built into applications through a library.
It is used by applications to decide whether a subject
is authorized to perform particular operations on an object.  In
this section we provide a brief description of the main GAA API
routines.

11.1. GAA API functions

 1) gaa_get_object_policy_info

   The gaa_get_object_policy_info function is called to obtain
   security policy information associated with the object. In the
   ACL-based systems, this information represents object ACLs, in
   the capability-based systems, this information may contain a
   list of authorities allowed to grant capabilities. If no security
   information is attached to the object, then this function can
   be omitted.

   Input:

   o  Reference to the object to be accessed

   The identifier for the object is from an application-dependent name
   space, it can be represented as unique object identifier, or symbolic
   name local to the application.

   o  Pointer to the application-specific authorization database

   o  Upcall function for the retrieval of the object authorization
      information.

      The application maintains authorization information in a form
      understood by the application.  It can be stored in a file,
      database, directory service or in some other way. The upcall
      function provided for the GAA API retrieves this information.

   Output:

   o  Mechanism-specific status code

   o  A handle to the sequence of  security attributes which constitute
      the security policy associated with the targeted object.

 2) gaa_inquire_object_policy_info

   The gaa_inquire_object_policy_info function allows application
   to discover a particular user's rights on an object. It returns a
   list of rights that the principal is authorized for and corresponding
   conditions, if any.

   Input:

   o  A handle to the sequence of security attributes, returned by
      gaa_get_object_policy_info

   o  Principal's security context

   Output:

   o A list of authorized rights and corresponding conditions, if any,
     is returned.

 3) gaa_check_authorization

   The gaa_check_authorization function tells the application
   server whether the requested operation or a set of operations
   is authorized, or if additional checks are required.

   Input:

   o  A handle to the sequence of security attributes, returned by the
      gaa_get_object_policy_info

   o  Principal's security context

   o  Operations for authorization
      It indicates operations to be performed.

   o  GAA API options structure
      This argument describes the behavior of the GAA API and specifies
      how the other arguments should be interpreted, in particular it
      describes the structure of the parameters for parameterized
      operation (see section 13).

   Output:

   YES   code (indicating authorization) is returned if all requested
         operations are authorized.

   NO    code (indicating denial of authorization) is returned if at
         least one operation is not authorized.

   MAYBE code (indicating a need for additional checks) is returned
         if there are some unevaluated conditions and additional
         application-specific checks are needed, or continuous
         evaluation is required.

   o  Mechanism-specific status code

   o  Detailed answer

Detailed answer contains:

   o  Authorization valid time period.
      The time period during which the authorization is granted is
      returned as condition to be checked by the application.
      Expiration time is calculated based on time-related restrictions
      expressed by the security attributes and restrictions in the
      authentication, authorization and delegated credentials.

   o  The requested operations are returned marked as granted or denied
      along with a list of corresponding conditions, if any.
      Each condition is marked as evaluated or not evaluated, if
      evaluated marked as met, not met or further evaluation or
      enforcement is required. This tells application which policies
      must be enforced.

   o  Information about additional security attributes required.
      Additional credentials might be required from clients to
      perform certain operations, e.g. group membership or delegated
      credentials.
      The application must understand the conditions that are returned
      unevaluated or it must reject the request.
      If understood, the application checks the conditions against
      information about the request, the target object, or environmental
      conditions to determine whether the conditions are met. Enforcement
      of the returned conditions is up to the application.

12. Creation of the GAA API security context

Prior to calling the gaa_check_authorization function, the application
must obtain the authenticated principal's identity and store it in the
security
context.  This context may be constructed from credentials obtained from
different mechanisms, e.g. GSS API, Kerberos or others.
This scenario places a heavy burden on the application programmer
to provide the integration of the security mechanism with the
application.  A second scenario is to obtain the authentication
credentials from a transport protocol that already has the
security context integrated with it. For example, the application can call
SSL or  authenticated RPC.  In this case, it is the implementation of the
transport mechanism (usually written by someone other than the application
programmer) which calls the security API requesting principal's identity,
and which constructs the security context.

The principal's authentication information is placed into the
security context and is passed to the GAA API. When additional security
attributes are required for the requested operation, the list of
required attributes is returned to be obtained by the application. The
application may provide GAA API with an upcall function for
requesting required additional credentials.

The credentials pulled by the GAA API are verified and added to
the security context by the upcall function. A reference to the
upcall function is passed to the GAA API as part of the security
context, and it added to the security context by the application or
transport.

13. Parameterized operation

Some operations for authorization may require parameters. For example,
request to transfer $20 will be expressed as operation: transfer
parameter: amount:$20.
Parameters are different from conditions and one can not merge lists
of each. Parameters must be typed to describe what they mean, and that
conditions that check parameters must define the types of the
parameters that they check.  Data in the GAA options structure
(see section 11.1) can be used to describe whether the single parameter
call in the GAA-API function signature is of a single type and what
type it is, or if it is a structure, and it a structure, what each of
the elements in the structure is.

When evaluating conditions that need to see parameters
they would then use this map from the GAA options structure to figure out
how to interpret the parameter and match each "variable" with the
particular parameter they are looking for.

14. Credential evaluation

Credentials are translated to the GAA API internal format and
placed into the GAA API security context.
When evaluating a policy, e.g., ACL, the necessary credentials
are looked for in the security context.

ACL entries containing principals that do not match the current
subject identity but grant the requested operation and the identities
that are associated with the subject, stored in the security
context, e.g. group memberships and delegated credentials, are taken
into account when evaluating an ACL.

In general, when an ACL grants requested operation and no additional
credentials are required, the GAA API will look for credentials that can
cause denial. A principal may chose to withhold credentials that it believes
may result in a denial.
There may be interactions when independent credentials are used, i.e.,
one set of credentials causes denial, but the other causes accept.
Administrator has to deal with these issues by carefully setting
policies in an ACL. It may be appropriate to specify more restricted
set of rights and require grant credentials to be presented.
A condition may specify whether grant or denial credentials take
precedence.

15. Extended example: simple Printer Manager application

To illustrate how the GAA API is used by application servers we
describe a simple Printer Manager application, where protected objects
are printers. The Printer Manager accepts requests from users to access
printers and invokes the GAA API routines to make authorization
decisions, under the assumption that the administrator of the resources
has specified the local policy regarding the use of the resources by
means of ACL files.
These files are stored in an Authorization Database, maintained by the
Printer Manager.

15.1 Conditions

Administrators will be more willing to grant access to the
printers if they can restrict the access to the resources to users or
organizations they trust. Further, the administrators should be
able to specify time availability, restrictions on resources consumed
by the clients and accounting for the consumed resources. To
specify these limits, the Printer Manager uses generic conditions, such as
time, location, payment and quota.

An example of Printer Manager-specific condition: printer load, expressed
as maximum number of jobs that allowed to be submitted to a printer.

15.2 Authorization walk-through

Here we present an authorization scenario to demonstrate the use of
the authorization framework for the case of printing a document.
Assume Kerberos V5 is used for principal authentication. Assume that
printer A has an ACL stored in the Printer Manager authorization database.

Let's consider a request from user Tom who is connecting from the
ORG.EDU domain to print a document on the printer A at 7:30 PM.

When a client process running on behalf of the user "Tom" contacts the
Printer Manager with the request to  submit_print_job (which
is indicated by setting the appropriate bit in the bit vector) to
printer  A, the Printer Manager first calls the gaa_get_object_policy_info
function to obtain a sequence of security attributes representing the
security policy expressed in the ACL for the printer A.  The upcall
function for retrieving the sequence is passed to the GAA API and is
being called by the gaa_get_object_policy_info, which returns the
sequence.
Assume that the following sequence was returned:

----------------------------------------- first EACL entry

Token Type:         access_id_USER
Defining Authority: kerberos.V5
Value:              tom@ORG.EDU

Token Type:         pos_access_rights
Defining Authority: PrinterManager
Value:              PRINTER:submit_print_job

Token Type:         time_window
Defining Authority: pacific_time_zone
Value:              8:00AM-8:00PM

Token Type:         printer_load
Defining Authority: PrinterManager
Value:              20

----------------------------------------- second EACL entry

Token Type:         pos_access_id_ANYBODY
Defining Authority: none
Value:              none

Token Type:         pos_access rights
Defining Authority: PrinterManager
Value:              PRINTER:view_printer_capabilities

The Printer Manager must place the principal's authenticated identity in
the security context to pass it to the  gaa_check_authorization function.
This context may be constructed according to the first or second
scenario, described in section 12.
If Tom is authenticated successfully, then verified identity credentials
are placed into the security context, specifying Tom as the Kerberos
principal  tom@ORG.EDU.

Then the Printer Manager calls the gaa_check_authorization function.
In evaluating the security attribute sequence. The set, corresponding
to the first entry applies. It grants the requested operation, but there
two conditions that must be evaluated.

The first condition  time_window : 8AM-8PM is generic and is evaluated
directly by the GAA API. Since, the request was issued at 7:30 PM
this condition is satisfied.
The second condition printer_load : 20 is specific.
If the security context defined a condition evaluation function for
upcall, then this function is invoked and if this condition is met then
the final answer is YES (authorized) and the detailed answer contains:
Authorization expiration time : 8PM (assume that authentication
credential has expiration time 9PM).
Allowed operations:   submit_print_job
List of conditions:   time_window  : 8AM-8PM
                      printer_load : 20
Both conditions are marked as evaluated and met.

During the execution of the task the Printer Manager is enforcing
the limits imposed on the local resources and authorization time.

If the corresponding upcall function was not passed to the GAA
API, the answer is MAYBE and the second condition is marked as
not evaluated and must be checked by the Printer Manager.

When additional credential is needed, then if the security context
defines a credential retrieval function for upcall, then this function
is invoked. If the requested credential is obtained, then the final
answer is YES. If the upcall function was not passed to the GAA API,
the answer is NO.

16. Integration with existing authentication mechanisms

The framework supports various strengths of user authentication
mechanisms. An ACL may have entries associated with different
principals identifying the same subject using different authentication
methods.
A subject may be granted a different set of rights, depending on
the strength of the authentication method used for identification.
Specification of weaker authentication methods including network
address or username will allow the GAA API to be used with any
existing application that does not have support for strong
authentication.

17. Integration with existing authorization mechanisms

Existing tools and ACLs stored by the underlying system should be
usable in some sense - perhaps through a converter.

18. Integration with alternative authorization models

The proposed framework can be easily integrated with existing
access control models in a uniform and consistent manner.
[7] shows how this mechanism can implement role-based access control,
Clark-Wilson model and  lattice-based policies.

19.  References

[1] B.C. Neuman.
    Proxy-based authorization and accounting for distributed systems.
    Proceedings of the 13th International Conference on Distributed
    Computing Systems, Pittsburgh, May 1993.

[2] B.C. Neuman and Theodore Ts'o.
    Kerberos: An authentication service for computer networks.
    IEEE Communications Magazine, pages 33-38, September 1994

[3] M. Blaze, J. Feigenbaum and J. Lacy.
    Decentralized Trust Management.
    in Proc. IEEE Symp. on Security and Privacy, IEEE Computer Press,
    Los Angeles, pages 164-173, 1996.

[4] Large Scale Multicast Applications (lsma) working group.
    Taxonomy of Communication Requirements for Large-scale Multicast
    Applications.
    Internet draft.

[5] Department of Defense National Computer Security Center.
    Department of Defense Trusted Computer system Evaluation Criteria,
    December 1985. DoD 5200.28-STD

[6] Tom Parker and Denis Pinkas.
    Extended Generic Security Service APIs: XGSS-APIs Access control and
    delegation extensions
    Internet-Draft IETF Common Authentication Technology WG

[7] T.V. Ryutov and B.C. Neuman
    An Authorization Framework for Distributed Systems
    Paper was submitted for the NDSS'2000.
    http://gost.isi.edu/info/gaa_api.html

20. Acknowledgments

Karl Kesselman, Mike Swift, Denis Pinkas, Gene Tsudik, Brian Tung,
Bapa Rao, Ilia Ovsiannikov and the Xerox IS team have contributed
to discussion of ideas and material in this draft.

21. Authors' Addresses

Tatyana Ryutov
Clifford Neuman
USC/Information Sciences Institute
4676 Admiralty Way Suite 1001
Marina del Rey, CA 90292-6695
Phone: +1 310 822 1511
E-Mail: {tryutov, bcn}@isi.edu