Internet Security CSCE 813 Communicating Sequential Processes

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Internet Security CSCE 813Communicating
Sequential Processes
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Project

• Related Work
• Need to know by now
• What is the problem domain?
• What is the specific problem youre 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?

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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

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Related Work

• Organize existing work into categories, e.g., on
  what specific problem they solve, what is the
  nature of the proposed solution, etc.
• Dont just list the different papers in a
  sequential order!
• Briefly explain what problems they address and
  what the main contributions are.
• Be critical!

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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/BibliographyF
  ormat/Bibliography20Format.htm for more
  formatting on references

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Back to CSP
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Reading

• Today
• Modelling and analysis of security protocols
  Chapter 1
• Next Class
• Modelling and analysis of security protocols
  Chapter 1 and 2

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CSP Objectives

• Model dynamics
• Model and analyze concurrency
• E.g., calculation intensive systems, distributed
  applications
• Support parallelism

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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 be
  used in events or manipulated
• When a ? A is chosen, it behaves like P(a)

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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?

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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.

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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

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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)

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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?

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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.

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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))

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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 ? (PX Q(x))

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General Interleaving

• P Q when P Ø Q
• P and Q use disjoint sets of events

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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

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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 Qs
  processes belong to X gt P is only allowed to do
  things in X that Q permits.
• E.g., example on page 54

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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)
• PR behaves like P but all visible events a
  from P are renamed by whatever R associates a
  with
• Use to make copies
• e.g., Pa,a/b,c both b and c are mapped to a
• e.g., Pb,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

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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

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CSP Operators

• Stop process does nothing
• a ? P event prefix
• ?xA ? 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

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CSP Operators

• P \ X event hiding
• PR process relation renaming
• Skip successful termination
• P Q sequential composition

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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.

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Traces

• Sequences of visible events until an arbitrary
  finite time
• E.g.,
• traces(Stop) lt gt
• traces(a ? P ? b ? Skip)
• lta gt n, lta gt n ltb gt, lta gt n ltb,? gt n in N
• Traces model
• Nonempty
• Prefix closed (if st is in trace, so is s)
• We can calculate traces(P) for any CSP P

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Next Class Modeling security protocols in CSP