Computer Security Access Control Matrix

Computer SecurityAccess Control Matrix

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

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.

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 belongs to the set R, and is the right of user s over file o.

Butler-Lampson Model In this model set of protection states P is 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 M) depends on the application.

Examples of ACMs file 1 file 2 process 1 process 2 process 1 R, W, O R R, W, E, O W process 2 A R, O R R, W, E, O 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.

Examples: rights on a LAN host names telegraph nob toadflex telegraph own ftp ftp nob ftp, nfs, amil own ftp, nfs, mail toadflex ftp, mail ftp, nfs, amil 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

Examples: rights in a program host names counter inc_ctr dec_ctr manager inc_ctr + dec_ctr - manager call call call Here inc_ctrincreases a counter and dec_ctr decreases it. R = { +, -, call }

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

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 from Xito Xi+1 X├─* Y indicates that starting at X, after a series of transitions the system enters state Y.

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

The Harrison-Ruzzo-Ullman Model This is based on a set of primitive commands. • create subject s • create object o • Enter right r into a[s,o] • Delete right r from a[s,o] • destroy subject s • destroy object o

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

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

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.

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.

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

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

Computer Security Access Control Matrix