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ASCII notes - LSV_ Cachan


									First Pronobis Meeting
LSV, 22 mai 2006.


JGL: présentation générale + SECSI + LSV.


Catuscia: presentation of Comète.
Central theme: async pi-calc + a probabilistic input-choice construct.
Used to specify security protocols (fair exchange [Kostas, Catuscia]
      based on oblivious transfer,
      various anonymity protocols [Kostas, Catuscia].)
Model-checker based on Prism developed with Marta's group [Parker, Wu].

- study of a notion of strong probabilistic anonymity [Catuscia, Mohit
  combines non-determinism (anonymous agents) and probability (protocol
  Theory of evidence.
- probable innocence:
  - satisfied by "real protocols" like Crowds.
  - various definitions:
    - limit on the probability of detecting the culprit (Rubin);
    - limit on the probability of the agent to be the culprit
            (Halpern, O'Neil);
    -> developing a notion combining both [Kostas, Catuscia]
- future work:
  - other forms of non-determinism [with Purnina];
  - group anonymity [with Purnina];
  - extension to other paradigms of partial information hiding [with
  - relation with information theory [project Printemps, with Prakash];
  - logic based on conditional probability.

Concurrent Constraint Programming (CCP).
- applications to security:
  - constraints = partial knowledge accumulated by adversary;
  - monotonic evalution of the store = monotonic adversary;
    SPL (Winskel, Crazzolara), applied pi-calculus.
- advantages: elegant and simple denotational semantics
      based on closure operators (Panangaden, Saraswat 1991).
- CCP as a subset of the pi-calculus (Valencia, Saraswat, Victor,
- project followed by Valencia. Collaboration with Columbian

Roberto Segala.
Mostly work in progress, rather than finished work.
- comparative semantics
  - alternating and non-alternating models;
      all models are instances of the non-alternating case;
  - simulation and bisimulation relations;
- logical characterization: extensions of HM logic;
- non-discrete measures: stochastic transition measures;
      real-time systems (continuous);
- verification of cryptographic protocols:
  - task-based PIOAs; oblivious transfer;
  - approximate simulations: authentication, matching conversations.

Probabilistic Automata NA:
NA = (Q, q0, E, H, D),
      transition relation D \subseteq Q x (E U H) x Disc(Q)
      where Disc(Q) is space of discrete probability
      distributions over Q.
      H: internal (hidden) actions.
      E: external actions, E & H = empty.
Logical characterization:
      phi ::= true | not phi | phi and phi | <a> phi | [phi]_p
      where [phi]_p holds whenever prob (phi) >= phi
      is a complete logic for bisimulation.

Stochastic Transition Systems
      (Cattani, Segala, Kwiatkowska, Normal).
Same, with sigma-fields for actions and states, too.
      But need schedulers to be measurable, i.e.,
so as to define Markov kernels.
(Measurable) Markov kernels are preserved by projection
      -> modularity.

UC security (Canetti).   See talk by Pereira (notes reproduced below).

Approximate simulations [Segala, Turrini].
      to describe the notion of matching conversations
      (Bellare, Rogaway) in authentication.

     ---[snip]Talk by Olivier Pereira, LSV, 16 mai 2006[snip---

     Olivier Pereira (with Ran Canetti, Ling Cheung, Dilsun Kaynar,
           Moses Liskov, Nancy Lynch, Roberto Segala)
           Using Task-Structured PIOAs to Analyze Security Protocols

           (PIOA = Probabilistic Input-Output Automata, by Roberto

     PIOA : state variables, actions (input, output, internal),
           transitions: state x action -> Disc(states)_\bot
           [Disc = discrete distributions on states.]
     Add internal nondeterminism for output and internal actions,
           which is not algorithmically resolved, and
           kept unresolved in the analyzed systems.

     Tasks are used to resolve nondeterminism:
     - equivalence classes on actions (send message 1, select key,
           give turn to adv, etc.)
     - given a task, at most one possible (probabilistic) action.

      Task schedulers resolve nondeterminism and give probabilistic
      - are sequences of tasks;
      - constraint: task schedulers do not give extra power to adv;
      - execution: read first task, find and execute the action if
            then go to next task;
      - proofs quantify over all possible task schedulers.

     Example: oblivious tranfer.
     Input actions: in(x)_Trans, x in {0,1} -> {0,1} (maps request to
                             requested bit)
                 in(i)_Rec, i in {0,1}
     Output actions: out(x)_Rec, x in {0,1}
     State: consists of two (imperative) variables:
           inval(Trans) in ({0,1} -> {0,1})_\bot, initially \bot
           inval(Rec) in {0,1,\bot}, initially \bot.
           in(x)_Trans: if inval(Trans)=\bot then inval(Trans):=x
           in(i)_Rec: if inval(Rec)=\bot then inval(Rec):=i
           out(x)_Rec: pre inval(Trans), inval(Rec) != \bot
                       and x=inval(Trans)(inval(Rec)).
     Tasks: {out(*)_Rec}

      UC-style security: protocol pi realizes functionality phi
            iff for every task-PIOA A,
            there is a task-PIOA S such that pi | A <= phi | S
      where <= is an implementation relation for task-PIOAs:
            A <= B means that
            for every environment E and task scheduler for A | E,
            there is a task scheduler for B | E st. E cannot distinguish
            A from B.
      <= is transitive and composable (ie, compatible with parallel
composition |).

      Two variants:
      - <=_0 for perfect indistinguishability;
            proved using a sound simulation relation;
            very systematic proofs;
      - <=_{neg, pt} for computational indistinguishability
            (poly time adv, negligible advantages).
            Computational assumptions are expressed in the form C1
<=_{neg, pt} C2.

     Example: hardcore predicates for trapdoor permutations.
           SH <=_{neg, pt} SHR
     where SH does:
              - given two random sources f (random permutation) and y,
                    sends f, and f (y), B(y) (hardcore bit)
        and SHR just draws f, z (claimed value of f(y)) and b (claimed
              at random.
        What if we use 2 hardcore bits (generated from same f)?
              By composition and transitivity, SH2 <=_{neg, pt} SHR2.

      Proving T1 <=_0 T2: by standard simulation techniques (also

        Case study on a simple oblivious transfer protocol [GMW87].

        ---[snip]end of talk by Olivier Pereira[snip]---

Angelo Troina.

Time and probability based information flow analysis.
(with Ruggero Lanotte [U. Insubria at Como],
      Andrea Maggiolo Schettini [U. Pisa]).

Multilevel security (non-interference).
      Low-level agents are not able to deduce
      anything about the activity of high level
      Timed case: (Focardi, Gorrieri, Martinelli 2000),
      (Evans, Schneider 2000), (Barbuti, Tesei 2003).
      Probabilistic case: (Gray, 1992), (Aldini,
      Bravetti, Gorrieri 2004), (Di Pierro, Hankin,
      Wiklicky 2004).
Model: probabilistic timed automata; bisimulation.
      (Sigma, X, Q, q0, delta, pi),
      where X is set of clocks,
      configs (q,v) where q in Q, and v is a valuation
      mapping clocks to their values.
A weak bisimulation is an equivalence relation R such
      that for every (s,s') in R and equivalence
      classes C of R, Pr [s, tau^* alpha, C] = Pr [s', tau^* alpha, C]
      for every alpha in Sigma U {tau} U \real^+ (last
      component is for idle waiting).
New definitions:
      A \ L restricts actions to set L, renormalizing probabilities;
      L / A (hiding) replaces every translation label a in L by tau;
      parallel composition A1 ||^p_L A2
      (A1 advances with probability p, A2 with 1-p)
      obtained by hiding wrt. L and renormalizing.
S satisfies non-interference (NI) iff S / Sigma_H is
      weakly bisimilar to S \ Sigma_H
      where Sigma_H is set of high-level actions.
      Decidable because weak bisim is decidable
      (finite state, guards as in timed automata).
Information flow analysis: probabilitistic and/or
      timed security properties.
      Good property: if we satisify (probabilistic, timed)
      NI then forgetting about probas/times gives
      us systems that satisfy the corresponding weaker
      notion of NI. The converse does not hold.
Non-deducibility on composition (NDC):
      S satisfied NDC iff for every high level agent Pi,
      for any p, for any L (high level),
      S / Sigma_H weakly bisim (S ||^p_L Pi) \ Sigma_H.
Decidable: split possible Pi's in finitely many equivalence classes.
Note: mNDC => mNI for every model m (probabilistic, timed, both).

Observations and future work:
- introduce approximate version of weak bisim for PTA;
- we can formulate other well-known information flow
      security properties within our framwork;
- extend the model with cryptographic primitives in
      order to analyze security protocols;
- develop an automatic technique to adjust insecure systems.


On capacities, games, and previsions.


Catuscia Palamidessi (with Mohit Bhargava)
Probabilistic and Nondeterministic Aspects of Anonymity.

The concurrency approach to anonymity (Schneider-Sidiropoulos)
Let A = {a(i) | i in Anonymous agents} = anonymous actions
B = actions visible to observers
C = Actions - (A U B) actions we want to hide
Defn: P is anon iff its set of traces is invariant wrt any permutation
      p of the actions in A, i.e. p(Traces(P)) = Trace(P)
      (after projection onto B?)
Mix proba and nondet, using proba async pi-calc (Herescu, Palamidessi
      semantics based on probabilistic automata of Segala and Lynch,
S-S encode proba as non-det.

Encoding "beyond suspicion" level of anonymity (strongly probabilistic
      (in the sense of Reiter and Rubin.)

What Chaum proves:
for every i, o if o => a then p(a(i)|o) = p(a(i)|a).
           (o => a meaning that we can observe that some
            agent paid)
      provided the coins are fair.
      Ie, the observation of o does not add anything to the
      knowledge of p(a(i)), except that a has been performed
      ~ conditional anon by Halpern and O'Neill.
Pbs: may depend on the probs of the agents;
      not applicable to non-det users.

Formalize strongly prob anon. in terms of notion of evidence.
Given a set of exhaustive and mutually exclusive hypothesis, and an event o, what is the evidence, given o, that h1 holds?

- prob: evidence (hi,o) = p(o|hi)
- nondet: evidence (hi,o) = p_{hi}(o) / sum_j p_{hj} (o)
      = prob case with uniform distribution.


Dave Parker

- Research activities at Bham.
      Focus: probabilistic verification.
      MDPs + discrete/continuous time MCs.
      Handles PCTL, and CSL, plus extensions.
      Efficient MTBDD-based (multi-terminal) implementation.
Discrete event simulation engine for MCs (not MDPs).
Cost/reward-based property analysis.
Improved tool links: eg, CADP (for bisimulation; Verimag, i.e.,
Counterexample generation.

Research areas: efficiency improvements (symbolic
      impl., parallelisation, grid computing),
      model-checking algorithms (symmetry reduction,
      abstraction techniques for MDPs, partial order
      reduction [Baier et al.], compositionality),
      additional models and formalisms
      (real-time probabilistic model-checking (PTAs),
      probabilistic calculi for mobility (pi-calculus,
Also applications of probabilistic model-checking:
      - ubiquitous computing (manet's);
      - security protocols: prob. contract signing
      (with Shmatikov), anonymity;
      - systems biology (CTMCs, signalling pathways:
            cyclin, FGF, e-coli(sigma_32)).

- Probabilistic pi-calculus model-checking [with Catuscia, Peng].
      - control-finite (no recursion within parallel)
      - input-closed (no inputs from environment)
      Combine MMC (Stony Brook; pi-calculus model-checker
     for mu-calculus) with PRISM.

- Game-based abstraction for MDPs.
      ie., CEGAR.
      Use Simple Stochastic Games [Condon 92],
      where we have 3 players: 1, 2, and probabilistic.
      Encode abstraction by having player 1 control
      non-determinism from abstraction, player 2
      control original non-determinism.
Analysis of SSGs: reachability of goal vertex set F.
      p_{a1, a2} (F) = prob. to reach F under player
            strategies a1, a2.
      Optimal probs sup_{a1} inf_{a2} p_{a1, a2} (F)
      and sup_{a2} inf_{a1} p_{a1, a2} (F)
      computable by iterative methods, as in MDPs.
Then compute bounds for pmin(F) and pmax(F) in MDP:
      - inf_{a1,a2} p_{a1, a2} (F) <= pmin(F) <= sup_{a1} inf_{a2} p_{a1,
a2} (F)
      - sup_{a2} inf_{a1} p_{a1, a2} (F) <= pmax(F) <= sup_{a1,a2} p_{a1,
a2} (F)
Case study: the zeroconf protocol (decentralized IP address config.

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