https://github.com/EasyCrypt/easycrypt
Tip revision: 895e467d1beefdebcaa7bcf3306604947f525637 authored by Benjamin Gregoire on 25 January 2023, 14:28:17 UTC
cleanup
cleanup
Tip revision: 895e467
Birthday.ec
(* -------------------------------------------------------------------- *)
require import AllCore List Distr Ring.
require import StdRing StdOrder StdBigop FelTactic.
require (*--*) Mu_mem.
(*---*) import RField IntOrder RealOrder.
(** A non-negative integer q **)
op q : { int | 0 <= q } as ge0_q.
(** A type T equipped with its full uniform distribution **)
type T.
op uT: T distr.
op maxu : T.
axiom maxuP x: mu1 uT x <= mu1 uT maxu.
(** A module that samples in uT on queries to s **)
module Sample = {
var l:T list
proc init(): unit = {
l <- [];
}
proc s(): T = {
var r;
r <$ uT;
l <- r::l;
return r;
}
}.
module type Sampler = {
proc init(): unit
proc s(): T
}.
(** Adversaries that may query an s oracle **)
module type ASampler = {
proc s(): T
}.
module type Adv(S:ASampler) = {
proc a(): unit
}.
(** And an experiment that initializes the sampler and runs the adversary **)
module Exp(S:Sampler,A:Adv) = {
module A = A(S)
proc main(): unit = {
S.init();
A.a();
}
}.
(** Forall adversary A that makes at most q queries to its s oracle,
the probability that the same output is sampled twice is bounded
by q^2/|T| **)
section.
declare module A <: Adv {-Sample}.
declare axiom A_ll (S <: ASampler {-A}): islossless S.s => islossless A(S).a.
lemma pr_Sample_le &m:
Pr[Exp(Sample,A).main() @ &m : size Sample.l <= q /\ !uniq Sample.l]
<= (q*(q-1))%r/2%r * mu1 uT maxu.
proof.
fel 1 (size Sample.l) (fun x, x%r * mu1 uT maxu) q (!uniq Sample.l) []=> //.
+ by rewrite -Bigreal.BRA.mulr_suml Bigreal.sumidE 1:ge0_q.
+ by inline*; auto.
+ proc;wp; rnd (mem Sample.l); skip=> // /> &hr ???.
apply (Mu_mem.mu_mem_le_size (Sample.l{hr}) uT (mu1 uT maxu)).
by move=> x _;rewrite maxuP.
by move=> c; proc; auto=> /#.
qed.
lemma pr_Sample_le_q2 &m:
Pr[Exp(Sample,A).main() @ &m: size Sample.l <= q /\ !uniq Sample.l]
<= (q^2)%r * mu1 uT maxu.
proof.
apply (ler_trans _ _ _ (pr_Sample_le &m)).
apply ler_wpmul2r; 1: by apply ge0_mu.
have -> : q^2 = q*q by ring.
smt(ge0_q).
qed.
declare axiom A_bounded: hoare [A(Sample).a : size Sample.l = 0 ==> size Sample.l <= q].
local lemma aux &m :
Pr[Exp(Sample,A).main() @ &m: !uniq Sample.l] =
Pr[Exp(Sample,A).main() @ &m: size Sample.l <= q /\ !uniq Sample.l].
proof.
byequiv (_: ={glob A} ==> ={Sample.l} /\ size Sample.l{2} <= q)=> //=.
conseq (_: _ ==> ={Sample.l}) _ (_: _ ==> size Sample.l <= q)=> //=;2:by sim.
by proc;call A_bounded;inline *;auto.
qed.
lemma pr_collision &m:
Pr[Exp(Sample,A).main() @ &m: !uniq Sample.l]
<= (q*(q-1))%r/2%r* mu1 uT maxu.
proof. rewrite (aux &m); apply (pr_Sample_le &m). qed.
lemma pr_collision_q2 &m:
Pr[Exp(Sample,A).main() @ &m: !uniq Sample.l]
<= (q^2)%r * mu1 uT maxu.
proof. rewrite (aux &m); apply (pr_Sample_le_q2 &m). qed.
end section.
(*** The same result using a bounding module ***)
module Bounder(S:Sampler) = {
var c:int
proc init(): unit = {
S.init();
c <- 0;
}
proc s(): T = {
var r <- witness;
if (c < q) {
r <@ S.s();
c <- c + 1;
}
return r;
}
}.
module ABounder(S:ASampler) = {
proc s(): T = {
var r <- witness;
if (Bounder.c < q) {
r <@ S.s();
Bounder.c <- Bounder.c + 1;
}
return r;
}
}.
module Bounded(A:Adv,S:ASampler) = {
proc a(): unit = {
Bounder.c <- 0;
A(ABounder(S)).a();
}
}.
equiv PushBound (S <: Sampler {-Bounder}) (A <: Adv {-S,-Bounder}):
Exp(Bounder(S),A).main ~ Exp(S,Bounded(A)).main:
={glob A,glob S} ==>
={glob A,glob S}.
proof. by proc; inline*; sim. qed.
(** Forall adversary A with access to the bounded s oracle, the
probability that the same output is sampled twice is bounded by
q^2/|T| **)
section.
declare module A <: Adv {-Sample,-Bounder}.
declare axiom A_ll (S <: ASampler {-A}): islossless S.s => islossless A(S).a.
lemma pr_collision_bounded_oracles &m:
Pr[Exp(Bounder(Sample),A).main() @ &m: !uniq Sample.l]
<= (q^2)%r * mu1 uT maxu.
proof.
have ->: Pr[Exp(Bounder(Sample),A).main() @ &m: !uniq Sample.l] =
Pr[Exp(Sample,Bounded(A)).main() @ &m: !uniq Sample.l].
+ byequiv (PushBound Sample A) => //.
apply (pr_collision_q2 (Bounded(A)) _ _ &m).
+ move=> S HS;proc;call (A_ll (ABounder(S)) _);2:by auto.
by proc;sp;if;auto;call HS.
proc; call (_: size Sample.l <= Bounder.c <= q).
+ proc;sp;if=>//;inline *;auto=> /#.
auto;smt w=ge0_q.
qed.
end section.