# Mandatory Access Control, part 2
The policies we have examined so far have been primarily concerned with
maintaining secrecy of data. We are also interested in policies that
ensure integrity of data. For instance, a bank is probably much more
interested in ensuring that customers not be able to change account
balances than it is in ensuring that account balances be kept secret.
Although the latter is desirable, unauthorized data modification can
usually cause much more harm to a business than inadvertent disclosure
In 1987, Clark and Wilson examined how businesses manage the integrity
of their data in the "real world" and how such management might be
incorporated into a computer system. In the commercial world, we need
security policies that maintain the integrity of data. In this
environment, illegal data modification occurs due to fraud and error.
And there are two classical techniques to prevent fraud and error.
The first technique is the principle of *Well-formed Transactions*.
Users may not manipulate data arbitrarily, but only in constrained ways
that preserve or establish its integrity. An example of this is
double-entry bookkeeping. This involves making two sets of entries for
everything that happens. Another example is the use of logs. Rather than
erasing mistakes, the sequence of actions that reverses the mistake is
performed and recorded on the log. A record of everything that has
occurred is maintained. Using well-formed transactions makes it more
difficult for someone to maliciously or inadvertently change data.
Another technique to prevent fraud is the principle of *Separation of
Duty*. Here, transactions are separated into subparts that must be done
by independent parties. This works to maintain integrity as long as
there is no collusion between agents working on different subparts. A
typical transaction might look as follows:
- A purchasing agent creates an order. The agent sends the order to
both the supplier and the receiving agent.
- The supplier ships the goods to the receiving department. The
receiving clerk checks the goods that were received against the
original order and updates the inventory records.
- The supplier also sends an invoice to the accounting department. The
accountant checks the invoice against both the original order and
what the shipping clerk said was received. If they match, the
accountant pays the bill.
In this scenario, no one person can cause a problem that will go
unnoticed. The separation of duty rule being employed here is: A person
who can create or certify a well-formed transaction may not execute it.
The question now becomes: How can we employ these commercial principles
in computer systems? Clark and Wilson gave a set of rules for doing so.
They postulate *trusted procedures* (TPs), which are programs that
implement transactions. We summarize the Clark-Wilson rules as:
1. All subjects must be authenticated.
2. All TPs (and the operations they perform) must be logged—i.e., must
3. All TPs must be approved by a central authority.
4. No data may be changed except by a TP.
5. All subjects must be cleared to perform particular TPs by a central
Note how these rules exemplify the gold standard: they address
authentication, audit, and (the final three rules) authorization.
Two other features of the Clark-Wilson model are:
- Data managed by a system may be either *constrained*, meaning it is
governed by the Clark-Wilson rules, or *unconstrained*, meaning that
it is not governed by the rules.
- A system may implement *integrity verification procedures* (IVPs),
which test whether data is in a valid state. All TPs must guarantee
the truth of IVPs.
### Role-based Access Control
In the real world, security policies are dynamic. Access rights, whether
discretionary or mandatory, need to change as the responsibilities of
users change. This can make management of rights difficult. When a new
user is authorized for a system, the appropriate rights for that user
must be established. When a user changes job functions, some rights
should be deleted, some maintained, and some added.
Role-based access control (RBAC) addresses this problem by changing the
underlying subject–object model. A *role* is a job function or
title—i.e., a set of actions and responsibilities associated with a
particular working activity. Now, instead of an access control policy
being a relation on subjects, objects, and rights, a policy is a
relation on roles, objects, and rights; this is called a *right
assignment*. For example, the role "5430 TA" might be assigned the right
to grade 5430 homeworks. Further, subjects are now assigned to roles;
this is called a *role assignment*. Each subject may be assigned to many
roles, and each role may be assigned to many subjects. Finally, roles
are hierarchical. For example, the role "5430 Professor" should have all
the rights that a "5430 TA" does, and more.
Roles are similar to groups in Unix file system DAC. A group is a set of
users with which some rights are associated. The same is true of a role.
But in some implementations, a user is always a member of a group,
whereas a subject may activate or deactivate the rights associated with
any of the subject's roles. This enables finer-grained implementation of
the Principle of Least Privilege. Subjects may login with most of their
roles deactivated, and activate a role only when the rights associated
with the role are necessary. Also, roles may be hierarchical, inheriting
rights from other roles; that is not typically true of groups, even
though it would be possible to implement.