Trust Management


Trust Management


Trust Management is an approach to distributed access control and authorization based on the following concepts:





Policies determine when an entity or members of some group may perform an action on some object, such as a document, database, or purchase order. Deduction determines the consequences of a policy and produces “proofs” that can be enforced locally by access monitors. Digital signatures provide cryptographic security and eliminate the need for passwords and replicated password files.


Policy Language A policy language allows individuals to define interdependent information sharing and trust policies locally, referring to policies of other individuals or corporate hierarchies. Central concepts in role-based trust management are:


-          Authorization, granting a permission to all individuals occupying a certain role,

-          Delegation, allowing an individual to trust others to make specified decisions,

-          Roles, grouping individuals into categories for easy policy definition,

-          Revocation, terminating permission.


Access policies can be simple statements allowing all employees of a company to access all company documents, or complex policies that specify how different classifications of employees may read or update different parts of a complex database. Policies may further specify what kind of database updates are allowed by each kind of each employee, or give specific limits on how many times a right may be delegated.


Here is an example illustrating authorization and delegation. In this example, a hospital establishes the policy that the primary physician of any patient may access the patient’s records, where the association between patient and physician is determined by the clinic where the doctor normally sees the patient outside the hospital. When a doctor is not available, she may delegate her role as primary physician to another doctor:


    Hospital policy

            allow  access(medical_record(Y))  if  clinic  says  primary_physician(Y)

            define role clinic : Oak_Medical, Central_Clinic, …


    Central_Clinic policy

            define role primary_physician(Nancy): Susan allow delegation

            define role primary_physician(Albert): Susan allow delegation

define role   primary_physician(Tony): Susan allow delegation


    Susan policy when off-duty

            delegate Central_Clinic.primary_physician(Nancy)  to  Bob


In the condition, “clinic  says  primary_physician(Y)”, the word says  is interpreted as meaning that a clinic must sent a digitally signed certificate stating that the individual requesting access is the primary physician of the patient. Access requests come with digitally signed certificates that allow the source of the request to be identified.


Another example illustrates the use of policy logic in a peer-to-peer application, where

Richard shares snapshots with his friends and family over the network:


    Richard’s policy

            allow  access(photos)  if  photo_friends or family

            define role photo_friends: Edith, John, Fernando

            define role family: Vivi, Larry allow delegation


    Vivi’s policy

            delegate  to  Meredith


In this example, Richard identifies his brother and sister as members of his family and allows them to name other members of his family. However, he does not allow his photography friends to transfer their access to others.


Deduction. When an access request is received, the deduction engine uses deductive inference to determine whether the request should be granted or denied. If the engine determines that the request should be granted, then the engine can produce a sequence of signed policy statements that can be verified by a simple access guard. This allows a trust management engine to be located separately from an access guards. Since access guards do not need to perform deduction or search policy files, an access guard may be installed on an embedded or handheld computing device with limited computing power or limited network bandwidth.

The same deduction process that determines access can search distributed policy logic for conflicts. The deduction engine can also answer administrative queries, such as a request to display all employees who can sign purchase orders.


Distributed policies. In a distributed computing environment, policy statements governing a single resource may by stored at many locations. In addition, delegation allows one policy statement to refer to others. For example, Barbara, the owner of one folder, may allow Greg to delegate access to any file in this folder. Greg may then delegate access to anyone that Susan says is a member of her family. In this example, Greg trusts individuals that Susan trusts. Interrelated policy statements can be maintained using a peer-to-peer architecture, eliminating the need for a centralized server that contains all policy statements. By referring to each other, individuals may establish a “web of trust,” relying on each other to certify individuals and organizations.



Keys. Public-key cryptography allows users to generate key pairs, each consisting of a private signing key and a public verification key that may be distributed freely without compromising the secrecy of the signing key. The owner of a signing key may digitally sign a document by carrying out a cryptographic calculation. Once signed, anyone in possession of the verification key can check the signature.

Public-key infrastructure (PKI) refers to key-generation algorithms and certificate authorities that generate cryptographic keys, distribute them securely, and provide a record of who received specific keys.

Authentication is the process of identifying an individual or process. Password authentication asks a user to type in a user name and password. If the name-password combination matches an entry in the password file, the user is considered authenticated. Key-based authentication relies on key pairs and digital signatures instead of passwords; cryptography allows the same user key to be used for many applications. The X.509 certificate format is an industry standard format for presented certificates that tie a cryptographic key to a specific individual or device.

Access request. Web access requests and peer-to-peer access requests may be presented using different network protocols. In a web environment, the owner of a cryptographic key may establish a secure web connection using https, a secure version of the standard protocol http, augmented with steps that transmit the client’s key to the web server.


Comparison with other mechanisms

Authentication. Traditional access decisions have been based on a combination of authentication and access control. For example, in order to determine whether to answer a database query, a system may determine who sent the request and whether that person is allowed to ask that query. Traditionally, authentication is achieved using passwords. In a simple password-based system, a server may ask a client to transmit a password across the network. In Kerberos-based authentication, a challenge-response protocol is used that avoids sending passwords on the network, increasing security. The X.509 standard is a modern example of a mechanism for certifying identity. The X.509 standard specifies a format for digital certificates, but does not have associated policy logic or other decision-making aspect.

Access control. Traditional access control decisions are made using access control lists. For example, a secure web site may contain a list of authorized users and their passwords. If a client tries to access the site, the server asks for a user name and password. Access control lists are cumbersome to install and maintain, since a complete list of all authorized users must be installed at every site. If an employee leaves the company suddenly, then many access control lists must be updated.


Toward trust management standards

SDSI, A Simple Distributed Security Infrastructure], combines public-key infrastructure design with a means of defining groups and issuing group membership certificates. SDSI groups provide a basis for access-control lists and security policies, SDSI does not address the general problem of formulating, testing , following, or enforcing policies. However, a SDSI key can delegate to a group the authority to sign certificates on behalf of the key. SDSI certificates can time out, and they can be reconfirmed by an on-line agent acting for the issuer.

The Simple Public Key Infrastructure group has goals similar to the SDSI effort and now subsumes SDSI. The goals of the SPKI group are to develop Internet standards for an IETF sponsored public key certificate format, associated signature and other formats, and key acquisition protocols. An SPKI Certificate is intended to provide assurance if a public key is authorized to perform some action, e.g., access a system or  spend money from a given account, grant or revoke authorizations , delegate authority to temporary , associate a public key with a name , and to disseminate useful information, secure from tampering (e.g., one's own preferred e-mail address or name). SPKI certificates do not directly address the specification of complex policies or the distribution of keys and certificates.


Trust management vs access control lists: an example


Scenario:  Hi-Tech Sports  designs and markets golf clubs and tennis rackets. Hi-Tech Sports is a cut-throat business that guards its intellectual property and competitive advantages carefully.


Problem: CEO Ralph sees a new opportunity marketing a redesigned line of clubs to women. He asks VP of Marketing Susan to prepare a description of the new line of clubs, to be endorsed by a prominent woman golfer. Susan asks a manager, Bill, in the Golf Club Marketingdivision to coordinate the project and keep her and Ralph informed. Bill contacts Alice in the Golf Club Engineering division to determine technical specifications of the clubs and work out compromises between appearance, performance, and price. Bill and Alice delegate certain responsibilities to their staff and ask their staffs to collaborate closely.


Document collaboration: Several kinds of documents will provide the basis for collaboration on this project, including planning charts, cost spreadsheets, CAD drawings, and so on. Ralph believes in delegating responsibility, so he would like read permission for all documents related to the project, but not write permission. Susan is a micromanager so she wants read and write permission for all marketing documents. Bill would like to share some marketing documents with Alice and her team, and reserve control of others. Alice works under the engineering division policy that all team members share documents, but no one outside engineering is allowed to modify any engineering document without explicit permission from engineering personnel.


Traditional Access Control Lists: The access control lists for the Hi-Tech Sports project are shown in this table. There are two folders for marketing documents, one for the documents Bill chooses to allow Alice to modify and one for marketing-only documents.



Read Access

Write Access

Project management

Ralph, Susan



     Shared with engineering

     Unshared documents


Ralph, Susan, Bill, Alice

Ralph, Susan, Bill,Alice


Susan, Bill, Alice

Susan, Bill


Ralph, Susan, Bill, Alice



Characteristics of Access Control List (ACL) solution

·        IT will set up folders and establish permissions for each folder. If different divisions have separate file servers, this will involve folders shared between servers.

·        Since documents will be created without IT, all documents in a folder will have same permission. It is too cumbersome to maintain ACLs on a per-file basis.

·        Most policies can be implemented

o       CEO Ralph can read all files

o       VP Marketing Susan can read and write project management and marketing files; she cannot write files in the engineering folder.

o       Martketing project manager Bill can read and write marketing files and read engineering files.

o       Bill can move files between the shared and unshared marketing folders to control which files Alice can modify.

o       Engineering project manager Alice can read and write engineering files and can read marketing files.

·        Some policies cannot or will not be implemented

o       Engineering policy allows Bill to make changes to engineering files if Alice approves them, but there is no way for Alice to grant Bill permission

o       If Bill and Alice allocated some staff to the project initially, IT can add these employees to the access control lists. If Bill and Alice reassign staff, the access control lists must be changed.

Access control policies can be implemented at the file level instead of the folder level, at a cost of producing and maintaining a larger number of access control lists.


Trust Management Solution

The TM solution uses local policy logic, combined by a deduction engine. Each employee has a cryptographic key, installed in their computer, that identifies them. File access can be described by policy logic statements. Sample Hi-Tech Sports policy statements that apply to the golf project are shown in this table:



Policy Logic

CEO Ralph

allow read(anyfile) if CEO

allow set_policy(mktg_file) if VP_mktg

allow set_policy(engr_file) if VP_engr

VP Mktg Susan

allow readwrite(mktg_file) if VP_mktg

allow create(mktg_file) if mktg_division

Golf Mktg Mgr Bill

allow readwrite(golf_proj_file) if golf_prof or shared(golf_proj_file)

VP Engr

allow readwrite(eng_file) if VP_mktg or authorized(eng_file)

Golf Engr  Mgr Alice



Characteristics of Deductive Policy Logic (DPL) solution

·        The policy logic describes company policy, not individual files and folders. The same corporate policies will be applied to all folders when they are created; no basic policy changes are necessary for this project, or any other project!

·        The VP of Engineering can set a policy for all engineering files. This policy can allow engineers to delegate authority over files. No IT action is needed.

·        Golf Engineering manager Alice does not have to specify any policy; the deduction engine knows that Alice is in Golf Engineering and applies the policy established by the VP of Engineering.


Policy checking and conflict resolution

Policy statements, written in policy logic, can be tested using the deduction engine. Some examples are:

·        List all of the individuals that are authorized to perform a given action,

·        Make sure no person is authorized to review his own purchase orders,

·        Find any conflicts in the policy.


The first example is the simplest. When someone tries to perform an action, the deduction engine decides whether this action is allowed. Since the policy determines the permitted set of person-action pairs, the deduction engine can also determine the set of people who are allowed to do a certain action. More generally, the deduction engine can answer a set of SQL-like queries about the policy logic within an organization or distributed among members of an interest group.


The second example illustrates an unintended consequence of a policy. More specifically, a company may have a policy about how purchase orders are reviewed. After this policy is written in policy logic, the logic designer may wish to test the policy for unintended properties. If the set of policy rules allow someone to approve his own purchase order, the deduction engine can determine this.


Policy conflicts can occur if policy logic with negation is used. For example, suppose Inez’s policy says that Janice cannot access any file she creates. However, Inez’s supervisor might have a policy that allows Janice’s supervisor to delegate access to anyone in her group. Then it may be possible for Janice to read Inez’s files, in violation of Inez’s policy. Policy conflicts can be avoided by assigning a priority to each statement. If a supervisor’s policy takes priority, then every policy is logically consistent. Even though priority is used to avoid logical inconsistency, it is valuable to use the deduction engine to find conflicting policy statements.


Example: Differential Pricing




This example shows a conference registration site, on the right, with a pricing policy that distinguishes academic participants from regular industrial participants. In particular, a student may register for the conference for $100. This pricing policy reflects the registration costs for the RSA Data Security Conference, for example.


The left column of the figure shows a root certificate authority (CA), which could be Verisign or similar site. The CA issues a certificate to Stanford University and identifies Stanford as an accredited university.


Stanford University issues a certificate to Prof. Mitchell, identifying him as a faculty member. Stanford also has a policy that faculty can identify their students.


Mitchell issues a certificate to Ajay Chander identifying him as a PhD student in the Computer Science department.


When student Ajay Chander registers for the conference, he registers as a student and sends payment for the $100 fee. The registration site can verify that he is a student by combining the information from the four certificates. The process of retrieving and combining certificates to answer a query (such as, “Is Chander a registered student at an accredited university?”) is a function of the certificate and policy infrastructure.


Comparison with access control: In the traditional approach based on access control lists, the registration site would either have to have a complete list of all students at all universities, or decide not to verify that student registrants are truly students.