##### https://github.com/EasyCrypt/easycrypt
Revision 9f6a2f9c698e6c47fa841a0b040bc45d82a601c5 authored by Benjamin Gregoire on 07 July 2016, 06:32 UTC, committed by Benjamin Gregoire on 07 July 2016, 06:32 UTC
1 parent 776af38
Tip revision: 9f6a2f9
FundamentalLemma.ec
``````require import Real Distr.

op max (x y:real) = if x <= y then y else x.

type t.

(* We want the bad event to be defined on both sides, *
* so we assume that all the variables that are used  *
* to define victory conditions and bad events are    *
* stored in a separate module. (Note: the empty      *
* signature could be instantiated with anything,     *
* including the concrete experiment themselves       *
* if their glob types match.)                        *)
module type Mem = { }.

module type Exp = {
proc main(): t
}.

lemma Pr_split (G <: Exp) (Mem <: Mem) (A: (glob Mem) -> t -> bool) (F: (glob Mem) -> t -> bool) &m:
Pr[G.main() @ &m: A (glob Mem) res /\ F (glob Mem) res]
+ Pr[G.main() @ &m: A (glob Mem) res /\ !F (glob Mem) res]
= Pr[G.main() @ &m: A (glob Mem) res].
proof.
cut: Pr[G.main() @ &m: A (glob Mem) res /\ F (glob Mem) res]
+ Pr[G.main() @ &m: A (glob Mem) res /\ !F (glob Mem) res]
= Pr[G.main() @ &m: (A (glob Mem) res /\ F (glob Mem) res) \/ (A (glob Mem) res /\ !F (glob Mem) res)]
by (rewrite Pr [mu_disjoint]; smt).
cut ->: Pr[G.main() @ &m: (A (glob Mem) res /\ F (glob Mem) res) \/ (A (glob Mem) res /\ ! F (glob Mem) res)]
= Pr[G.main() @ &m: A (glob Mem) res]
by (rewrite Pr [mu_eq]; smt).
by move => <-.
qed.

lemma FundamentalLemma (G1 <: Exp) (G2 <: Exp) (Mem <: Mem)
(A: (glob Mem) -> t -> bool) (B: (glob Mem) -> t -> bool)
(F: (glob Mem) -> t -> bool) &m:
Pr[G1.main() @ &m: A (glob Mem) res /\ !F (glob Mem) res] =
Pr[G2.main() @ &m: B (glob Mem) res /\ !F (glob Mem) res] =>
`|Pr[G1.main() @ &m: A (glob Mem) res] - Pr[G2.main() @ &m: B (glob Mem) res]|
<= max Pr[G1.main() @ &m: F (glob Mem) res] Pr[G2.main() @ &m: F (glob Mem) res].
proof.
rewrite -(Pr_split G1 Mem A F &m) -(Pr_split G2 Mem B F &m)=> ->.
cut ->: forall (x y z:real), x + y - (z + y) = x - z by smt.
apply (Trans _ (max Pr[G1.main() @ &m: A (glob Mem) res /\ F (glob Mem) res]
Pr[G2.main() @ &m: B (glob Mem) res /\ F (glob Mem) res]));
first smt full.
cut H: forall (x y x' y':real), x <= x' => y <= y' => max x y <= max x' y' by smt.
apply H.
rewrite -(Pr_split G1 Mem F A &m) andC; smt.
rewrite -(Pr_split G2 Mem F B &m) andC; smt.
qed.
`````` Computing file changes ...