1. Chapter 2: Access Control Matrix
Overview
Access Control Matrix Model
Protection State Transitions
Commands
Conditional Commands
Introduction to Computer Security
© 2004 Matt Bishop

2. Overview
State of a system: the collection of the current values of all memory locations, all secondary storage, and all registers and other components of the system;
Protection state of system: describes current settings, values of system relevant to protection;

3. Access Control Matrix Describes protection state precisely;
Describes rights of each subject with respect to every other entity;
State transitions change elements of matrix;

4. Description objects (entities) Subjects S = { s1,…,sn }
o1 … om s1 … sn
Subjects S = { s1,…,sn }
Objects O = { o1,…,om } ∪ S
Rights R = { r1,…,rk }
Entries A[si, oj]  R
A[si, oj] = { rx, …, ry } means subject si has rights rx, …, ry over object oj

5. ACM – Process vs Files Processes p1, p2; Files f, g
Rights r, w, x, a, o f g p1 p2
rwo r
rwxo w a ro

6. ACM – Hosts in a LAN host names A B C own ftp ftp, nfs, mail, own
ftp, mail

7. ACM – Computer Program Functions: inc_ctr, dec_ctr, manager
Variable: counter
Rights +, –, call
counter
inc_ctr
dec_ctr - +
manager
call

8. State Transitions

9. State Transitions |– : represents transition
Xi |–  Xi+1: command  moves system from state Xi to Xi+1
Xi |– * Y: a sequence of commands moves system from state Xi to Y

10. Primitive Operations create subject s; create object o
destroy subject s;
destroy object o
enter r into A[s, o]
delete r from A[s, o]

11. Primitive Operations
create subject sn; sn

12. Primitive Operations
create object on on

13. Primitive Operations
destroy subject sd; sd

14. Primitive Operations
destroy object od od

15. Primitive Operations enter r into A[s, o]
Adds r rights for subject s over object o o s r

16. Primitive Operations delete r from A[s, o]
Removes r rights from subject s over object o o s r

17. Creating File Process p creates file f with r permission
command create•file(p, f)
create file f;
enter own into A[p, f];
enter r into A[p, f];
end

18. Creating Process command spawn•process(p, q) create process q;
enter own into a[p, q];
enter r into a[p, q];
enter r into a[q, p];
end

19. Mono-Operational Commands
Single primitive operation in this command
command make•owner(p, g)
enter own into a[p, g];
end

20. Conditional Commands command grant•read•file•1(p, f, q)
if own in A[p, f]
then
enter r into A[q, f];
end

21. Multiple Conditions command grant•read•file•2(p, f, q)
if own in A[p, f] and c in A[p, q]
then
enter r into A[q, f];
enter w into A[q, f];
end