Access Control Cunsheng Ding HKUST, Hong Kong, CHINA C. Ding - - - PowerPoint PPT Presentation
Access Control Cunsheng Ding HKUST, Hong Kong, CHINA C. Ding - COMP4631 - L17 1 Agenda of this Lecture The basic concepts of access control, ACLs, capabilities, etc. Two approaches to access control Further reading C. Ding -
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home directory, have total control over all files in all directories and subdirectories.
all files in “Public_html”, but should not do other
in this subdirectory.
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cding Public_html private Question: How do I do the access controls?
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à subject: active entity --- user or process à object: passive entity --- file or resource à access operation: read, write, ...
à What is the subject allowed to do? (1st approach) à What may be done with an object? (2nd approach)
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Subject Access request Security reference monitor Object The security reference monitor will check the access control policy and will grant or reject the request. Real World Examples ?
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model:
à execute à read à append, also called blind write à write
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write read append execute Observe X X Alter X X
can be opened for read access or for write access so that the O/S can avoid potential conflicts.
user editing a file should not be asked to open it
Alter mode.
an object without observing its content is rarely useful (exception: audit log).
Example: use of a cryptographic key. This motivates the execute right, which includes neither Observe nor Alter mode.
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policies are expressed in terms of three
à read: read from a file à write: write to a file à execute: execute a file
the access operations take this meaning: à read: list contents à write: create or rename files in the directory à execute: search the directory
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These operations differ from the Bell-LaPadula model. E.G., Unix write access does not imply read access. Creation and deletion of files are governed by access control to the directory.
à read à write à execute à delete à change permission à change ownership
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Access operations in the NTFS (New Technology File System) file system of Windows NT:
handle deletion of files or change of access rights.
ingredient you may want to use when setting security policies.
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policies
set of policies?
– The access control structure should allow you to express the access control policy you want to enforce. – You should be able to check that your policy has been captured correctly.
combination of subject and object.
structures are cumbersome to manage. Intermediate levels of control are preferable.
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– S … set of subjects – O … set of objects – A … set of access operations
may perform on object o.
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Alice Bob
bill.doc {exec} {exec} edit.exe {exec,read} {exec,read,write} fun.com
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– access rights are stored with the subject – capabilities º rows of the access control matrix
Subjects may grant the right to grant rights.
– How to check who may access a specific object? – How to revoke a capability?
interest in capabilities.
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Alice edit.exe: {exec} fun.com: {exec,read}
Room 001 Computer 111 Printer 1234 …………… TV 999 Yes No Yes ………. No
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Capability for Cunsheng Ding in HKUST
– access rights are stored with the object – ACLs º columns of the access control matrix
– Unix: owner, group, others
subject?
systems.
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fun.com Alice: {exec} Bill: {exec,read,write}
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ACL for Color Printer 111111
Cunsheng Ding Yes
John Wong No Paul Wu Yes ………. ……. Alice Fu No
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users groups
users groups
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relation on L (i.e., subset of L x L) that is – reflexive: for all aÎL, a£a – transitive: for all a,b,cÎL, if a£b and b£c, then a£c – antisymmetric: for all a,bÎL, if a£b and b£a, then a=b
relation Í on a power set P(C).
access control policy.
××× .in where i1,…,in are integers.
relation – Ability a2 is a prefix of ability a1 if there exists another ability a3 so that a1 = a2a3. In this case, we write a2 £ a1.
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Photo
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Read: all member
UST Libarary
Borrow: academic and stud. Badminton: staffs, stud. Basketball: acad. staffs, and their family memb.
UST Sport Facility CSD color printer Academic staffs Postgradautes Any other object in UST
Access Control List
Who can access? What are you allowed to do?
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