CS283 - Lecture 4 - Part 3 - CapabilityBasedAccessControl

CS283 - Lecture 4 - Part 3 - CapabilityBasedAccessControl -...

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GWU CS 172/283 Autumn 2009 Sources: Memon’s notes, Brooklyn Poly Bishop’s Text, Chapter 15 Bishop’s slides, Chapter 15 Text by Pfleeger and Pfleeger, Chapter 4 Lecture 4 – Part 3 - Capability-Based Access Control
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GWU CS 172/283 - Autumn 2009 Holmblad - Lecture 04 – Part 3- Rev 20090929 2 Access Control Mechanisms Access Control Matrix Access Control List Capability based access control Lock and Key based access control. Rings-based access control
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GWU CS 172/283 - Autumn 2009 Holmblad - Lecture 04 – Part 3- Rev 20090929 3 Capability based access control. Conceptually, capability is row of ACM i.e. list of rights for a subject. Definition: Let O be set of objects, and R the set of rights of a system. A capability list c is a set of pairs C = {(o, r): o O, r R} Let cap be function that determines capability list c associated with subject s. Then cap(s) = {(o i , r i ):1 i n} is that subject s may access o i using any right in r i .
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GWU CS 172/283 - Autumn 2009 Holmblad - Lecture 04 – Part 3- Rev 20090929 4 Capabilities Capability-based control: turn ACL on its head: indexed by subject and not object A capability is a “license” of sorts, stored as a token Stored by OS, secure, cryptographic protection, transferable Eg: digital rights associated with a media asset
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GWU CS 172/283 - Autumn 2009 Holmblad - Lecture 04 – Part 3- Rev 20090929 5 Capability Lists Rows of access control matrix file1 file2 file3 Andy rx r rwo Betty rwxo r Charlie rx rwo w C-Lists: Andy: { (file1, rx) (file2, r) (file3, rwo) } Betty: { (file1, rwxo) (file2, r) } Charlie: { (file1, rx) (file2, rwo) (file3, w) }
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GWU CS 172/283 - Autumn 2009 Holmblad - Lecture 04 – Part 3- Rev 20090929 6 Semantics Like a bus ticket Mere possession indicates rights that subject has over object Object identified by capability (as part of the token) Name may be a reference, location, or something else Subject without capability cannot name object satisfactorily Must prevent process from altering capabilities Otherwise subject could change rights encoded in capability or object to which they refer Major difference from ACL, which is controlled by OS
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GWU CS 172/283 - Autumn 2009 Holmblad - Lecture 04 – Part 3- Rev 20090929 7 Implementation Tagged architecture Bits protect individual hardware words from being modified E.g. B5700 (Burroughs processor): tag was 3 bits and indicated how word was to be treated (pointer, type, descriptor, etc .) Paging/segmentation protections Like tags, but put capabilities in a read-only segment or page Programs must refer to capabilities by pointers • Otherwise, program could use a copy of the capability—which it could modify
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GWU CS 172/283 - Autumn 2009 Holmblad - Lecture 04 – Part 3- Rev 20090929 8 Implementation ( con’t ) Cryptography Associate with each capability a cryptographic checksum (digital signature) enciphered using a key known to OS
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CS283 - Lecture 4 - Part 3 - CapabilityBasedAccessControl -...

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