diff --git a/twofish/cbc.ml b/twofish/cbc.ml
index f592c56..7e1265a 100644
--- a/twofish/cbc.ml
+++ b/twofish/cbc.ml
@@ -36,11 +36,13 @@ let xor_strings b c =
     
     
 let encrypt cbc enc p =
+  assert (String.length p = Primitive.key_ize);
   let p' = xor_strings p cbc.prev_v in
   let c = enc p' in
     (cbc.prev_v <- c; c)
       
 let decrypt cbc dec c =
+  assert (String.length p = Primitive.key_ize);
   let p = dec c in
   let p' = xor_strings p cbc.prev_v in
     (cbc.prev_v <- c; p')
diff --git a/twofish/cbc.mli b/twofish/cbc.mli
index 947fdea..12b5353 100644
--- a/twofish/cbc.mli
+++ b/twofish/cbc.mli
@@ -4,5 +4,8 @@ type state =
     }
 
 val init : string -> state
+(** [init iv] is the initial state using [iv] a the initialization vector. *)
 val encrypt : state -> (string -> string) -> string -> string
+(** Encrypt a single block in CBC mode. *)
 val decrypt : state -> (string -> string) -> string -> string
+(** Decrypt a single block in CBC mode. *)