Computer Security- Lecture 9- Access Control

Computer Security- Lecture 9- Access Control

What is access control?

Defining, managing and enforcing rights and permissions

Authorisation- who is allowed to do what

Authentication- who is what

Reference monitors

Concept in operating systems, refers to machine mediating access to resources

Example: a firewall

Example-

File 1- can be read and written by Bob

But only read by Phil

(o,℘((s,℘(p))))

Means set object ((set subject (permissions))))

Capabilities

Is a pair

List is the list of pairs for any subject

e.g. BOB having access to file 3 and file 1- can read file 2 but write and read file 3

(s,℘((o,℘(p))))

Access control Matrix

The ACL’s and capability lists are specific models of these matrixes

So can see Alice has access to read /etc/password

Discretionary Access control (DAC)

Model where the access control on objects is up to owner

Chmod command in unix- can choose is rights for users, groups and others

HRU model

Model- needs own reading

Unfortunately, these models are vulnerable to trojan horses!

Mandatory Access Control

Military style

Users cannot change policies

Multi-Level Security (MLS)

Works on partial ordering, L1 <L2 would mean L1 is less sensitive

Can use this to create a lattice

Policies within this

Bell and La Papudula

Biba

BLP

Enforces secrecy of information based on MLS

Example- subject can have read access to object if the object if the classification of the object is less or equal tothan the classification of the subject

Or if the classification of the subject is less than that if the object, it cannot be written to

Biba

Integrity rather than confidentiality

“A subject, S, can have a write access to an object, O, only if C(O)≤ C(S)”

Integrity- no read down example

“If a subject, S, can read an object, O, then S can have the write access only to objects, P, where C(P) ≤ C(O)”