Change randomness interface

Go back to make_randomness, since the module initialization time is
noticeable otherwise.

lib/crypto.ml

@@ -62,19 +62,20 @@ let check_election p =
 let prng = lazy (Cryptokit.Random.(pseudo_rng (string secure_rng 32)))
 
 module MakeSimpleMonad (G : GROUP) = struct
-  type 'a t = 'a
+  type 'a t = unit -> 'a
   let ballots = ref []
-  let return x = x
-  let bind x f = f x
+  let return x () = x
+  let bind x f = f (x ())
 
   let random q =
     let size = Z.size q * Sys.word_size / 8 in
-    let r = Cryptokit.Random.string (Lazy.force prng) size in
-    Z.(of_bits r mod q)
+    fun () ->
+      let r = Cryptokit.Random.string (Lazy.force prng) size in
+      Z.(of_bits r mod q)
 
   type ballot = G.t Serializable_t.ballot
-  let cast x = ballots := x :: !ballots
-  let fold f x = List.fold_left (fun accu b -> f b accu) x !ballots
+  let cast x () = ballots := x :: !ballots
+  let fold f x () = List.fold_left (fun accu b -> f b accu ()) x !ballots
 end
 
 (** Homomorphic elections *)
@@ -248,7 +249,7 @@ struct
     overall_proof >>= fun overall_proof ->
     return {choices; individual_proofs; overall_proof}
 
-  let randomness =
+  let make_randomness () =
     sswap (Array.map (fun q ->
       Array.init (Array.length q.q_answers) (fun _ -> random G.q)
     ) params.e_questions)

lib/crypto.mli

@@ -15,8 +15,10 @@ val check_election : (module ELECTION_PARAMS) -> bool
 
 module MakeSimpleMonad (G : GROUP) : ELECTION_MONAD
   with type ballot = G.t Serializable_t.ballot
-  and type 'a t = 'a
-(** Simple election monad that keeps all ballots in memory. *)
+  and type 'a t = unit -> 'a
+(** Simple election monad that keeps all ballots in memory. It uses a
+    secure random number generator lazily initialized by a seed shared
+    by all instances. *)
 
 module MakeElection
   (P : ELECTION_PARAMS)

lib/crypto_sigs.mli

@@ -125,7 +125,7 @@ module type ELECTION = sig
   type randomness = Z.t array array
   (** Randomness needed to create a ballot. *)
 
-  val randomness : randomness m
+  val make_randomness : unit -> randomness m
   (** Creates randomness for [create_ballot] below. The result can be
       kept for Benaloh-style auditing. *)
 

tests/sandbox.ml

@@ -186,9 +186,9 @@ let nballots = Array.map Serializable_compat.of_ballot ballots;;
 assert (Array.forall Election.check_ballot nballots);;
 assert (Array.forall2 (fun b b' -> b = Compat.to_ballot b') ballots nballots);;
 
-let create_ballot b = Election.(create_ballot randomness b)
+let create_ballot b = Election.(create_ballot (make_randomness () ()) b)
 
-let test_ballot = create_ballot [| [| 1; 0; 0; 0 |] |];;
+let test_ballot = create_ballot [| [| 1; 0; 0; 0 |] |] ();;
 assert (Election.check_ballot test_ballot);;
 
 let result =
@@ -206,7 +206,7 @@ let y = ys.(0);;
 let x = Z.of_string "45298523167338358817538343074024028933886309805828157085973885299032584889325";;
 assert P.G.(g **~ x =% y);;
 
-let test_factor = Election.compute_factor tally x;;
+let test_factor = Election.compute_factor tally x ();;
 assert (Election.check_factor tally y test_factor);;
 assert (Serializable_t.(test_factor.decryption_factors) = result.partial_decryptions.(0).decryption_factors);;