Pr_half.eca
(* --------------------------------------------------------------------
* Copyright (c) - 2012--2016 - IMDEA Software Institute
* Copyright (c) - 2012--2021 - Inria
* Copyright (c) - 2012--2021 - Ecole Polytechnique
*
* Distributed under the terms of the CeCILL-B-V1 license
* -------------------------------------------------------------------- *)
require import AllCore Distr.
type input.
type output.
module type Adv = {
proc main(_:input) : output
}.
lemma equiv_not_pr_half (A<:Adv) (Pre: input -> glob A -> bool) (E: glob A -> output -> bool):
equiv [A.main ~ A.main : ={arg, glob A} /\ Pre arg{1} (glob A){1} ==> E (glob A){1} res{1} = !E (glob A){2} res{2}] =>
forall a &m,
Pre a (glob A){m} =>
Pr[A.main(a) @ &m : true] = 1%r =>
Pr[A.main(a) @ &m : E (glob A) res] = 1%r/2%r.
proof.
move => he a &m hpre hll.
have : Pr[A.main(a) @ &m : E (glob A) res] = Pr[A.main(a) @ &m : !E (glob A) res].
+ by byequiv he => // /#.
by rewrite Pr [mu_not] hll /#.
qed.