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Internet Security CSCE 813 Communicating Sequential Processes

Internet Security CSCE 813 Communicating Sequential Processes. Project. Related Work Need to know by now: What is the problem domain? What is the specific problem you’re addressing? What solutions are out there (if there is any)? What are the limitations of these solutions?

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Internet Security CSCE 813 Communicating Sequential Processes

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  1. Internet Security CSCE 813Communicating Sequential Processes

  2. Project • Related Work • Need to know by now: • What is the problem domain? • What is the specific problem you’re addressing? • What solutions are out there (if there is any)? • What are the limitations of these solutions? • How your proposed approach overcome some of these limitations? CSCE 813 - Farkas

  3. Related Work • Format: • Problem Overview • Related work 2.1 Research on problem domain 2.2 Research on specific problem 2.3 Limitation of existing research References CSCE 813 - Farkas

  4. Related Work • Organize existing work into categories, e.g., on what specific problem they solve, what is the nature of the proposed solution, etc. • Don’t just list the different papers in a sequential order! • Briefly explain what problems they address and what the main contributions are. • Be critical! CSCE 813 - Farkas

  5. References • Be precise! • Use full references, with authors, title, where it was published, when, and the page numbers • If you supply URLs, list when the URL was downloaded • Organize references in alphabetical order • Use one of the accepted bibliography format • See http://www.asij.ac.jp/middle/lib/BibliographyFormat/Bibliography%20Format.htm for more formatting on references CSCE 813 - Farkas

  6. Back to CSP

  7. Reading • Today: • Modelling and analysis of security protocols: Chapter 1 • Next Class: • Modelling and analysis of security protocols: Chapter 1 and 2 CSCE 813 - Farkas

  8. CSP Objectives • Model dynamics • Model and analyze concurrency • E.g., calculation intensive systems, distributed applications • Support parallelism CSCE 813 - Farkas

  9. Prefix • Offering a single action • Offering of choice: any set of visible actions • If A  , ?x : A → P(x) represent all the actions in A • x is the parameter of P -- parameters can beused in events or manipulated • When a  A is chosen, it behaves like P(a) CSCE 813 - Farkas

  10. Choice Operator • Choice operator:  • Gives the option between the actions of two processes then • Behaves like the one chosen • Revisit: if A = B  C then ?x : A → P(x) = (?x : B → P(x))  (?x : C → Q(x) ) • If B and C are disjoint: together they give all the choices in A • What happens if B and C overlap? CSCE 813 - Farkas

  11. Non-Deterministic Choice • P Q • behaves like P or like Q • User has no control over which • Can be implemented using two internal actions • Implementer is not required to implement this way (can choose either P or Q or (P or Q)) • Useful for model degree of unpredictability, like communication medium that transmits data correctly or loose it. CSCE 813 - Farkas

  12. Time-Based Choice • P t Q • Chose choices offered by P for t time units and • If nothing is chosen, it behaves like Q • Similar traces than other choice if no time is recorded • Can be P  Q where t is non-deterministic CSCE 813 - Farkas

  13. Conditional Choice • If-then-else • Choice is based on condition • if b then P else Q • Example: FW(s) = in?x → (if valid(x,s) then out!x → FW(newstate(s,x)) else FW(newstate(s,x)) ) • Revisit non-deterministic machine: NDM = in?x → (NDM  out!x → NDM) CSCE 813 - Farkas

  14. Parallel Operators • Put sequential processes parallel • System state: state of each component • Number of possible states increases exponentially with the size of the network • How to put processes together for parallel network? • How to check whether such a network satisfies a specification? CSCE 813 - Farkas

  15. Parallel Combination • Just an other process to which any of the previous operators can be applied. • Each parallel process is equivalent to a sequential one (with infeasibly large number of states) • CSP processes influence each other by affecting what communications they can perform. CSCE 813 - Farkas

  16. Parallel Combination • Synchronize all visible actions • P || Q can perform a   only when P and Q can • (?x : A → P(x)) || (?x : B → Q(x)) = ?x : A B→ (P(x) || Q(x)) CSCE 813 - Farkas

  17. Parallel Combinations • Interfaces parallel operator: P ||X Q • Synchronize all events in X • Example: • P = ?x : A → P’(x) • Q = ?x : B → Q’(x) • P ||X Q = ?x : X  A  B → (P’(x) || Q’(x))  ?x : A \ X → (P’(x) ||X Q)  ?x : B \ X → (P||X Q’(x)) CSCE 813 - Farkas

  18. General Interleaving • P ||| Q when P ||Ø Q • P and Q use disjoint sets of events CSCE 813 - Farkas

  19. Alphabet Controlled • P X ||Y Q • Each process is given control of a particular set of events • No process is ever permitted to communicate outside of its own alphabet • Interface between two processes: intersection of their alphabet CSCE 813 - Farkas

  20. Use of Parallel Operators • Achieve a particular overall behavior • For example, build constraints on traces • P ||X Q, where P is any process, and all Q’s processes belong to X => P is only allowed to do things in X that Q permits. • E.g., example on page 54 CSCE 813 - Farkas

  21. Hiding and Renaming • Hiding: • Internal details are not visible to outsiders • If X in  and P is a process than P \ X behaves like P but all events in X are hidden (turned into invisible actions) • Renaming: • Alphabet replacement (relation) • P[[R]] behaves like P but all visible events a from P are renamed by whatever R associates a with • Use to make copies • e.g., P[[a,a/b,c]] – both b and c are mapped to a • e.g., P[[b,c/a,a]] – both a is mapped to b and c (offers the choice of b and c to the environment but the state after either of these choices is the same CSCE 813 - Farkas

  22. Additional operators • Sequential composition P ; Q • Does whatever P does until terminates and then does what Q does • Process Skip : successful termination • Special event:  -- always the final event • e.g., a → b → Skip, terminates successfully after events a and b • e.g., (a → Skip) ; P same external behavior as a → P CSCE 813 - Farkas

  23. CSP Operators • Stop process does nothing • a → P event prefix • ?x:A → P event prefix choice • P  Q choice between two processes • P  Q nondeterministic choice • P || Q lockstep parallel • P ||X Q interface parallel • P X ||Y Q synchronizing parallel CSCE 813 - Farkas

  24. CSP Operators • P \ X event hiding • P[[R]]process relationrenaming • Skipsuccessful termination • P ; Q sequential composition CSCE 813 - Farkas

  25. Process Behavior • Concurrent processes may lead to: • Deadlock: each process is willing to do something but the entire system cannot agree on any action • Livelock: infinite sequence of internal (hidden) communication occur between the components. Similar external appearance to deadlock • Non-determinism: both processes P1 and p2 are willing to talk to a third one Q which has to make a choice. CSCE 813 - Farkas

  26. Traces • Sequences of visible events until an arbitrary finite time • E.g., • traces(Stop) = { < > } • traces(a → P  b → Skip) = { <a > n, <a > n^ <b >, <a > n^ <b, > n in N } • Traces model • Nonempty • Prefix closed (if s^t is in trace, so is s) • We can calculate traces(P) for any CSP P CSCE 813 - Farkas

  27. Next Class: Modeling security protocols in CSP CSCE 813 - Farkas

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