Computer Security Access Control Mechanisms

1. Computer SecurityAccess Control Mechanisms

2. States of a Computer System The state of a system is the collection of current values of all components of the system: memory locations, secondary storage, registers etc. Protection states are those states that have to be protected. • .P = set of all protection states of the system • .Q = set of all authorized protection states • The system is not secure if the current state is in P -Q • A security policy characterizes the states in Q • A security mechanism prevents the system entering a state in P -Q

3. Access Control Matrix Model This is used to describe the protection states. It characterizes the rights of each subject of the system (entity/process) regarding the objects of the system (entities/processes) in terms of a matrix.

4. Butler-Lampson Model This describes the rights of users s (subjects) over files o (objects) by a matrix A whose rows are indexed by the subjects and whose columns are indexed by the objects. The rights belong to a set R. Each entry a[s,o] of matrix A is a subset of the set R, and is the set of rights of user s over file o.

5. Butler-Lampson Model The set of protection states P of a system is represented by a set of triples in (S,O,A), where S is the set of users, O the set of files and A the Access Control Matrix. The set of rights R (the entries in A) depends on the application.

6. Examples of ACMs file 1 file 2 process 1 process 2 process 1 read, write read read, write, write own execute, own process 2 append read, own read read, write execute, own Here R = { read, wright, own, append, execute } process 1 can read/write file 1, read file 2, communicate with process 2 by writing to it, etc.

7. Examples: rights on a LAN host names telegraph nob toadflex telegraph own ftp ftp nob ftp,nfs,mail,own ftp,nfs,mail toadflex ftp,mail ftp,nfs,mail,own Here R = { ftp, mail, nfs, own }, where ftp = the right to access the File Transfer Protocol mail = the right to send/receive using the Simple Mail Transfer Protocol (SMTP) nfs = the right to access file systems using the Network File System protocol

8. Examples: rights in a program to synchronize events host names counter inc_ctrdec_ctr manager inc_ctr+ dec_ctr- manager call callcall Here R = { +, -, call } (+,- represent the ability to add or subtract and call is the ability to invoke a procedure) inc_ctrincreases a counter and dec_ctrdecreases it manager calls the functions inc_ctr and dec_ctr

9. Other examples • Access Control by Boolean expression evaluation • Access Control by History See textbook

10. Protection State Transitions Initial state of the system: X0 = (S0,O0,A0 ) Transitions: t1, t2, … Corresponding states: X1, X2, … We use the notation: Xi ├─ ti+1 Xi+1 to indicate the state transition ti+1 moves the system from Xito Xi+1 X ├─* Y indicates that starting at X, after a series of transitions the system enters state Y.

11. Protection State Transitions Xi├─ci+1(pi+1,1 ,…, pi+1,m) Xi+1 indicates that the transition is caused by the command ci+1 with parameters pi+1,1 ,…, pi+1,m.

12. The Harrison-Ruzzo-Ullman Model This is based on a set of primitive commands. • create subject s [precondition: sS postcondition: S’ = S  { s }, O’ = O, no rights are assigned to s, all other rights are not affected ] • create object o [precondition: oO postcondition: S’= S, O’ = O  { o }, no rights are assigned to o all other rights are not affected ]

13. The Harrison-Ruzzo-Ullman Model • Enter right r into a[s,o] [precondition: sS, oO postcondition: S’ = S, O’ = O, a’ [s,o] = a [s.o] { r }, no other rights are affected ] • Delete right r from a[s,o] [precondition: sS, oO postcondition: S’ = S, O’ = O, a’ [s,o] = a [s.o]- { r }, no other rights are affected ]

14. The Harrison-Ruzzo-Ullman Model • destroy subject s [precondition: sS postcondition: S’ = S - { s }, O’= O, a’ [s,o]=  for all oO, no other rights are affected ] • destroy object o [precondition: oO postcondition: S’ = S, O’ = O - { o }, a’ [s,o] =  for all sS, no other rights are affected ]

15. The Harrison-Ruzzo-Ullman Model Example command create•file(p,f) create object f ; enter right owninto a [p,f]; enter right r into a [p,f]; enter right winto a [p,f]; end

16. The Harrison-Ruzzo-Ullman Model Example –conditional commands Suppose process p wants to give process q the right to read file f command grant•read•file1•(p,f,q) if ownin a [p,f] then enter rinto a [q,f]; end

17. The Harrison-Ruzzo-Ullman Model Example –conditional commands using and Suppose process p wants to give process q the right to read file f command grant•read•file2•(p,f,q) if rin a[p,f] and cin a[p,f] then enter rinto a[q,f]; end See textbook for other examples.

18. Copying and owning Rights • copy right(grant right) – augments existing rights • own right The copy right allows its possessor to grant rights (this right is often considered a flag attachment –hence flag right) The own right allows its possessor to add or delete privileges to themselves.

19. Copying Example Suppose process p has right r over object f , and let c be a copy right. The following command allows p to copy r over f to another process q only if p has copy right over f . command grant•r(p,f,q) if rin a[p,f] and cin a[p,f] then enter rinto a[q,f]; end

20. Attenuation of privilege The Principle of Attenuation of Privilege says that • a subject may not give rights it does not possess to another subject.