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Subject: Re: Groups vs. Roles


> For all roles R, there exists a group G such that all members M of G have
> role R.

yes and no. I can see this to be the case in many situations but it is not

> It is not true that for all groups G, there exists a role R such that all
> members M of G have role R. For example, the group of workers in the US may
> have a certain set of privileges based on their group. However, they won't
> all have the same role.

that's right.

The difference between groups and roles is that groups are sets of users,
which you collectively refer to with a name (e.g., Employee, US-citizens,
Programmers, ....).

Roles are "sets of privileges". Intuitively, they are privileged hat that
users (authorized for that) can put on and take off as needed. In a
centralized organizational environment roles usually correspond to
organizaional responsibilities (e.g., manager, secretary, dean,
Dept-chair, ....) to which you associate privileges. In a role-based
context authorizations to access objects are granted to roles. Users are
then granted authorizations to activate roles. By activating a role, a
user can execute all accesses for which the role is authorized. For
instance, in a hospital you can have a "doctor" role that is authorized
to prescribe medicines. If Jane is authorized to activate the role, by
doing so, she will be allowed to prescribed medicine. This ability will go
away when she releases the role.

Simon, w.r.t. your point that for every role there is a group of users
authorized for that role, it can be so but it is not necessarily so.
For instance, you may have the same concept that you capture with a
semantic of a group and of role. As an example, you can have a group
"Doctors" and a role "doctor". Group "Doctors" will have all the
privileges that you intend to grant to all users who are doctors. Role
doctors will have all privileges that you intend to grant to users only
when they are operating in that specific role. If this is the case you can
indeed grant the authorization to activate role doctor to group Doctors
(instead than specifying one authorization for each individual).

do you agree?

In a distributed scenario i've seen the use of the term "role" also used
to intend a more generic form of "dynamic activation" of privileges, not
necessarily regulated by authorizations to activate roles. For instance,
in a distributed setting you may have a certificate-based access control
where access is granted depending on digital certificates requestor
presents.  For instance, you may say that users presenting a membership
certificate in ACM can access a given digital library. Although these
are not properly roles, the behavior is similar: by presenting the
certificate i can exercise given privileges that i would not otherwise (in
this sense it is like activating a role).



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