diff --git a/src/crypto.ml b/src/crypto.ml
index 140d99c1..8b6ab805 100644
--- a/src/crypto.ml
+++ b/src/crypto.ml
@@ -64,6 +64,7 @@ module Binary_format_rsa = struct
         pub
     | _ -> assert false
 
+  (* custom private key binary format <> than gcrypt *)
   let priv_to_octets ({ e; d; n; p; q; dp; dq; q' } : Mirage_crypto_pk.Rsa.priv)
       =
     z_array_to_octets [| e; d; n; p; q; dp; dq; q' |]