Correct number of limbs in gen tables

ec/gen_tables/gen_tables.ml

@@ -20,8 +20,11 @@ let pp_array elem_fmt fmt arr =
   done;
   fout "@]@,}"
 
-let pp_string_words ~wordsize fmt str =
-  let limbs = String.length str * 8 / wordsize in
+let div_round_up a b =
+  a / b + (if a mod b = 0 then 0 else 1)
+
+let pp_string_words ~wordsize ~byte_length fmt str =
+  let limbs = div_round_up (byte_length * 8) wordsize in
   assert (String.length str * 8 mod wordsize = 0);
   let bytes = Bytes.unsafe_of_string str in
   fprintf fmt "@[<2>{@\n";
@@ -51,22 +54,22 @@ let check_shape tables =
         x)
     tables
 
-let print_tables tables ~wordsize =
+let print_tables tables ~wordsize ~byte_length =
   let fe_len = String.length tables.(0).(0).(0) in
   printf "@[<2>static WORD generator_table[%d][15][3][LIMBS] = @," (fe_len * 2);
   pp_array
-    (pp_array (pp_array (pp_string_words ~wordsize)))
+    (pp_array (pp_array (pp_string_words ~wordsize ~byte_length)))
     std_formatter tables;
   printf "@];@,"
 
 let print_toplevel name (module P : Mirage_crypto_ec.Dh_dsa) =
-  let tables = P.Dsa.Precompute.generator_tables () in
+  let tables, byte_length = P.Dsa.Precompute.generator_tables () in
   check_shape tables;
   print_header name;
   printf "@[<v>#ifdef ARCH_64BIT@,";
-  print_tables ~wordsize:64 tables;
+  print_tables ~wordsize:64 ~byte_length tables;
   printf "#else // 32-bit@,";
-  print_tables ~wordsize:32 tables;
+  print_tables ~wordsize:32 ~byte_length tables;
   printf "@]#endif@."
 
 let curves =

ec/mirage_crypto_ec.ml

@@ -63,7 +63,7 @@ module type Dsa = sig
     val generate : key:priv -> Cstruct.t -> Cstruct.t
   end
   module Precompute : sig
-    val generator_tables : unit -> string array array array
+    val generator_tables : unit -> (string array array array * int)
   end
 end
 
@@ -428,7 +428,7 @@ module type Scalar = sig
   val to_octets : scalar -> string
   val scalar_mult : scalar -> point -> point
   val scalar_mult_base : scalar -> point
-  val generator_tables : unit -> field_element array array array
+  val generator_tables : unit -> (field_element array array array * int)
 end
 
 module Make_scalar (Param : Parameters) (P : Point) : Scalar = struct
@@ -465,7 +465,8 @@ module Make_scalar (Param : Parameters) (P : Point) : Scalar = struct
   (* Specialization of [scalar_mult d p] when [p] is the generator *)
   let scalar_mult_base = P.scalar_mult_base
 
-  (* Pre-compute multiples of the generator point *)
+  (* Pre-compute multiples of the generator point
+     returns the tables along with the number of significant bytes *)
   let generator_tables () =
     let len = Param.fe_length * 2 in
     let one_table _ = Array.init 15 (fun _ -> P.at_infinity ()) in
@@ -482,7 +483,7 @@ module Make_scalar (Param : Parameters) (P : Point) : Scalar = struct
       base := P.double !base
     done;
     let convert {f_x; f_y; f_z} = [|f_x; f_y; f_z|] in
-    Array.map (Array.map convert) table
+    (Array.map (Array.map convert) table, Param.byte_length)
 end
 
 module Make_dh (Param : Parameters) (P : Point) (S : Scalar) : Dh = struct

ec/mirage_crypto_ec.mli

@@ -135,10 +135,11 @@ module type Dsa = sig
   (** Operations to precompute useful data meant to be hardcoded in
       [mirage-crypto-ec] before compilation *)
   module Precompute : sig
-    val generator_tables : unit -> string array array array
+    val generator_tables : unit -> (string array array array * int)
     (** Return an array of shape (Fe_length * 2, 15, 3) containing multiples of
         the generator point for the curve. Useful only to bootstrap tables
-        necessary for scalar multiplication. *)
+        necessary for scalar multiplication. Returns the tables and the number
+        of significant bytes for each element.*)
   end
 end