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Access control

Least privilege (principle)

The principle of least privilege means each actor in a system should be able to access no more than it needs.

For example, you can say

your web server should be denied to real all of the filesystem, except its own documents

your backup program should

be able to read most everything, but...

not be able to run anything (beyond its own components)

not be able to write anything except to the backup disk and its own logs

This is part of why functional accounts are often created for each such part: It's easier to handle this in broad terms even in simpler DAC setups, with just filesystem permissions.

When you want to crack down on this more thoroughly and more visibly, look at things like SELinux (because it adds MAC).

Note that the isolation in VMs and OS containers, are implicitly least-privilege as well: no connection to the outside unless permitted.

See also:

• https://en.wikipedia.org/wiki/Principle_of_least_privilege

Discretionary Access Control (model)

Discretionary access control (DAC) means access to an object is at the discretion of the object's owner.

...more precisely, the identity of subjects.

Usually that's owner-based for practical reasons, though e.g. capability systems often allow transfer to other parts.

DAC is mainly contrasted with MAC

Examples:

permissions in most filesystems

Mandatory Access Control (model)

Mandatory access control (MAC) means that the system that decides to allow access between objects, instead of the object owners (though some systems apply MAC on top of DAC)

Often means a model applied on top, labeling all objects with particular categories, and having rules based on these categories.

Such (often-broad) labeling is often done for practical reasons: it often makes the rules simpler, which makes it more clear they express what you intended.

MAC usually means design up front. And each large change should probably re-evaluate all implications.

While MAC is good at partitioning off parts of a system in broad terms (e.g. "web server may only read under /var/www regardless of permissions"), it is much less flexible at anything you can't describe fully at the design stage (e.g. people wanting to share files somewhat securely).

This is one reason MAC often assists DAC, using MAC to restrict the broad problems, and DAC for the more everyday things. (While you can sort of implement something much like DAC using MAC, this is often so messy that it may be harder to verify as being correct, and harder to administer in an everyday way)

Role-Based Access Control

✎ This article/section is a stub — some half-sorted notes, not necessarily checked, not necessarily correct. Feel free to ignore, or tell me about it.

Role-based access control (RBAC) is focused on roles and role hierarchy, subjects and sessions.

It's a moderately detailed abstraction, not yet a system in itself, but may be useful to architect into larger systems where you want to keep overview of what is allowed to happen and what is happening, particularly in larger organisations.

This makes it a potential piece of of DAC, MAC and others - probably replacing a simpler implementation of one of its parts (e.g. the role/account stuff).

https://en.wikipedia.org/wiki/Role-based_access_control

Related notes

If you squint, there is sometimes very little difference between a MAC category and a DAC group (whatever the preferred terms are).

That is, very similar to useand users - the remaining difference may lie largely in who may change the permissions - the admin, or the relevant user.

This is also roughly why there is value in combining MAC and DAC.

Unsorted

two time pad attack

If you know two different things have been encrypted with the same secret, then putting together those output may reveal things about the key, which can defeat the entire point of a one time pad.