Computer Security Access Control Mechanisms 7/23/2019

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 7/23/2019

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. 7/23/2019

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. 7/23/2019

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. 7/23/2019

Examples of ACMs Here R = { read, wright, own, append, execute } 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. 7/23/2019

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 7/23/2019

Examples: rights in a program to synchronize events host names counter inc_ctr dec_ctr manager inc_ctr + dec_ctr - manager call call call Here R = { +, -, call } (+,- represent the ability to add or subtract and call is the ability to invoke a procedure) inc_ctr increases a counter and dec_ctr decreases it manager calls the functions inc_ctr and dec_ctr 7/23/2019

Other examples Access Control by Boolean expression evaluation Access Control by History See textbook 7/23/2019

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 Xi to Xi+1 X ├─* Y indicates that starting at X, after a series of transitions the system enters state Y. 7/23/2019

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 . 7/23/2019

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 ] 7/23/2019

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] postcondition: S’ = S, O’ = O, a’ [s,o] = a [s.o] - { r }, no other rights are affected ] 7/23/2019

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 ] 7/23/2019

The Harrison-Ruzzo-Ullman Model Example command create • file (p,f) create object f ; enter right own into a [p,f] ; enter right r into a [p,f] ; enter right w into a [p,f] ; end 7/23/2019

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 own in a [p,f] then enter r into a [q,f] ; end 7/23/2019

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 r in a [p,f] and c in a [p,f] then enter r into a [q,f] ; end See textbook for other examples. 7/23/2019

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. 7/23/2019

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 r in a [p,f] and c in a [p,f] then enter r into a [q,f] ; end 7/23/2019

Attenuation of privilege The Principle of Attenuation of Privilege says that a subject may not give rights it does not possess to another subject. 7/23/2019