diff --git a/twofish/primitives.ml b/twofish/primitives.ml
index 2228dab..56a626c 100644
--- a/twofish/primitives.ml
+++ b/twofish/primitives.ml
@@ -351,10 +351,15 @@ let mds_rem a b =
   in
     mds_rem_ab ((Int64.of_int32 a) & 0xFFFFFFFFL) ((Int64.of_int32 b) & 0xFFFFFFFFL) 0
 
+let key_size = 32
+
 let init key =
   let keylength = String.length key in
-  if keylength != 32 then
-    failwith ("init: key length must be 32, got key length "^string_of_int (keylength))
+  if keylength != key_size then
+    failwith (Printf.sprintf
+                "init: key length must be %d, got key length %d"
+                key_size
+                (string_of_int keylength))
   else
   let le_longs = unpack_longs key 8 in
   
diff --git a/twofish/primitives.mli b/twofish/primitives.mli
index 8ad5ba4..bd4fc61 100644
--- a/twofish/primitives.mli
+++ b/twofish/primitives.mli
@@ -1,4 +1,11 @@
 type ctx
+(** Some context? *)
+(* XXX: Is this stateful?? *)
+val key_size : int
+(** [key_size] is the size of twofish keys. *)
 val init : string -> ctx
+(** [init key] is the context / state initialized from secret key [key]. *)
 val encrypt : ctx -> string -> string
+(** [encrypt ctx plain_text] is a single block encrypted with [ctx]. *)
 val decrypt : ctx -> string -> string
+(** [encrypt ctx cipher_text] is a single block decrypted with [ctx]. *)