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Computer SecurityAccess Control Mechanisms

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States of a Computer SystemThe 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

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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.

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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.

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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.

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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.

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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 Protocolmail = 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

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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 itmanager calls the functions inc_ctr and dec_ctr

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Other examples

• Access Control by Boolean expression evaluation• Access Control by History

See textbook

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Protection State Transitions

Initial state of the system: X0 = (S0,O0,A0 )Transitions: 1, 2, …Corresponding states: X1, X2, …

We use the notation: Xi ├─ i+1 Xi+1

to indicate the state transition i+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.

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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 .

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The Harrison-Ruzzo-Ullman Model

This is based on a set of primitive commands.• create subject s [precondition: sS postcondition: S’ = S { s }, O’ = O, no rights are assigned to

s, all other rights are not affected ]• create object o [precondition: oO postcondition: S’= S, O’ = O { o }, no rights are assigned to o

all other rights are not affected ]

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The Harrison-Ruzzo-Ullman Model• Enter right r into a[s,o] [precondition: sS, oO 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: sS, oO postcondition: S’ = S, O’ = O, a’ [s,o] = a [s.o] - { r },

no other rights are affected ]

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The Harrison-Ruzzo-Ullman Model• destroy subject s [precondition: sS postcondition: S’ = S - { s }, O’= O, a’ [s,o]= for all oO,

no other rights are affected ]• destroy object o [precondition: oO postcondition: S’ = S, O’ = O - { o }, a’ [s,o] = for all sS,

no other rights are affected ]

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The Harrison-Ruzzo-Ullman Model

Examplecommand 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

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The Harrison-Ruzzo-Ullman Model

Example –conditional commands

Suppose process p wants to give process q the right to read file fcommand grant•read•file1•(p,f,q) if own in a [p,f] then

enter r into a [q,f] ; end

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The Harrison-Ruzzo-Ullman ModelExample –conditional commands using and

Suppose process p wants to give process q the right to read file fcommand 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.

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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.

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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

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Attenuation of privilege

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

another subject.