Fix generate_key (#44)

* Fix generate_key

* Add a functional test performing a whole key exchange

CHANGES.md

@@ -1,5 +1,10 @@
 ## unreleased
 
+### Fixed
+
+- Fix a bug in `generate_key` where it would never actually work when used with
+ a proper `rng` function (#44, @NathanReb)
+
 ## v0.1.0
 
 *2019-06-28*

fiat-p256.opam

@@ -27,6 +27,7 @@ depends: [
  "hex"
  "hex" {with-test}
  "ppx_deriving_yojson" {with-test}
+ "rresult" {with-test}
  "ppx_expect"
  "yojson" {with-test & >= "1.6.0"}
 ]

p256/fiat_p256.ml

@@ -56,7 +56,7 @@ type secret = Scalar.t
 let secret_of_cs = Scalar.of_cstruct
 
 let rec generate_private_key ~rng () =
- let candidate = rng 4 in
+ let candidate = rng 32 in
  match secret_of_cs candidate with
  | Ok secret ->
  secret

test/dune

@@ -0,0 +1,3 @@
+(test
+ (name test_fiat_p256)
+ (libraries alcotest fiat-p256 rresult))

test/test_fiat_p256.ml

@@ -0,0 +1,43 @@
+module Testable = struct
+ let cstruct =
+ let pp fmt t = Hex.pp fmt (Hex.of_cstruct t) in
+ Alcotest.testable pp Cstruct.equal
+end
+
+let list_init l f =
+ let rec go acc i =
+ if i = l then
+ List.rev acc
+ else
+ go ((f i)::acc) (i + 1)
+ in
+ go [] 0
+
+let int32_to_hex i = Printf.sprintf "%08Lx" i
+
+let prng len =
+ let i32_count = len / 4 + 1 in
+ let i32s = list_init i32_count (fun _ -> Random.int64 0x100000000L) in
+ let as_hex = String.concat "" (List.map int32_to_hex i32s) in
+ Cstruct.of_hex (String.sub as_hex 0 (len * 2))
+
+let whole_key_exchange =
+ let open Rresult.R.Infix in
+ let test_name = "whole_key_exchange" in
+ let test_fun () =
+ let res =
+ let secret1, public1 = Fiat_p256.gen_key ~rng:prng in
+ let secret2, public2 = Fiat_p256.gen_key ~rng:prng in
+ Fiat_p256.key_exchange secret1 public2 >>= fun shared1 ->
+ Fiat_p256.key_exchange secret2 public1 >>= fun shared2 ->
+ Ok (shared1, shared2)
+ in
+ match res with
+ | Ok (s1, s2) -> Alcotest.check Testable.cstruct test_name s1 s2
+ | Error e -> Alcotest.failf "Key exchange failed with error %a" Fiat_p256.pp_error e
+ in
+ [(test_name, `Quick, test_fun)]
+
+let () =
+ Random.self_init ();
+ Alcotest.run "Fiat_p256" [("Functional", whole_key_exchange)]