Adds constant time equal functions for strings and bytes

src/cryptokit.ml

@@ -2024,3 +2024,21 @@ let mod_mult a b c =
   Bn.to_bytes ~numbits:(String.length c * 8)
     (Bn.mod_ (Bn.mult (Bn.of_bytes a) (Bn.of_bytes b))
              (Bn.of_bytes c))
+
+let const_time_eq eq len a b =
+  if len a = len b
+  then
+    let l = len a in
+    let lst = List.init l (fun x -> x) in
+    let fold acc i =
+      acc && (eq a b i) in
+    List.fold_left fold true lst
+  else false
+
+let const_time_eq_str = const_time_eq (fun a b i ->
+    String.get a i = String.get b i) String.length
+
+
+let const_time_eq_bytes = const_time_eq (fun a b i ->
+    Bytes.get a i = Bytes.get b i) Bytes.length
+

src/cryptokit.mli

@@ -1249,3 +1249,12 @@ val mod_mult: string -> string -> string -> string
     (** [mod_mult a b c] computes [a*b mod c], where the
         strings [a], [b], [c] and the result are viewed as
         arbitrary-precision integers in big-endian format. *)
+
+val const_time_eq_str : string -> string -> bool
+    (** [const_time_eq_str a b] compares the two strings [a] and  [b]
+        in constant time *)
+
+val const_time_eq_bytes : bytes -> bytes -> bool
+    (** [const_time_eq_bytes a b] compares the two byte strings [a] and [b]
+        in constant time *)
+

test/test.ml

@@ -945,6 +945,25 @@ let _ =
   end
 
 
+let _ =
+  testing_function "Constant time equal";
+  test 1 (const_time_eq_str "abc" "ab") false;
+
+  test 2 (const_time_eq_bytes (Bytes.of_string "abc") (Bytes.of_string "ab")) false;
+
+  test 3 (const_time_eq_str "coolcoolcool" "coolcoolnice") false;
+
+  test 4 (const_time_eq_bytes
+            (Bytes.of_string "coolcoolcool")
+            (Bytes.of_string "coolcoolnice")) false;
+
+  test 5 (const_time_eq_str "coolcoolcool" "coolcoolcool") true;
+
+  test 6 (const_time_eq_bytes
+            (Bytes.of_string "coolcoolcool")
+            (Bytes.of_string "coolcoolcool")) true
+;;
+
 (* End of tests *)
 
 let _ =
@@ -956,4 +975,3 @@ let _ =
     printf "All tests successful.\n";
     exit 0
   end
-