diff --git a/src/util.ml b/src/util.ml
index 35fad636..8d5d1f8b 100644
--- a/src/util.ml
+++ b/src/util.ml
@@ -57,7 +57,7 @@ module Bin_rsa = struct
     let z_array_of_octets ~nb s =
       (*Fmt.epr "z_array_of_octets, nb=%d@." nb;*)
       let s_len = String.length s in
-      let* () = check (s_len <= 2 * nb) in
+      let* () = check (s_len > 2 * nb) in
       let pos = ref 0 in
       let len_arr =
         Array.init nb (fun _i ->
@@ -66,8 +66,8 @@ module Bin_rsa = struct
             len)
       in
       let* () =
-        let total_len = (2 * nb) + Array.fold_left ( + ) 0 len_arr in
-        check (s_len = total_len)
+        let len = (2 * nb) + Array.fold_left ( + ) 0 len_arr in
+        check (s_len = len)
       in
       let z_arr =
         Array.init nb (fun i ->
@@ -81,36 +81,6 @@ module Bin_rsa = struct
 
   open Internal
 
-  module Implementation_1 = struct
-    let pub_to_octets ({ n; e } : Mirage_crypto_pk.Rsa.pub) =
-      let n = z_to_bits_be n in
-      let e = z_to_bits_be e in
-      let n_len = String.length n in
-      let e_len = String.length e in
-      let len = 4 + n_len + e_len in
-      let b = Bytes.make len '\x00' in
-      Bytes.set_uint16_be b 0 n_len;
-      Bytes.set_uint16_be b 2 e_len;
-      Bytes.blit_string n 0 b 4 n_len;
-      Bytes.blit_string e 0 b (4 + n_len) e_len;
-      Bytes.unsafe_to_string b
-
-    let pub_of_octets s =
-      let pub_of_octets s =
-        let len = String.length s in
-        let* () = check (len >= 4) in
-        let n_len = String.get_uint16_be s 0 in
-        let e_len = String.get_uint16_be s 2 in
-        let* () = check (len = n_len + e_len + 4) in
-        let n = z_of_bits_be s 4 n_len in
-        let e = z_of_bits_be s (4 + n_len) e_len in
-        Mirage_crypto_pk.Rsa.pub ~n ~e
-      in
-      match pub_of_octets s with
-      | Error (`Msg e) -> Fmt.failwith "rsa pub_of_octets failure: %s@." e
-      | Ok v -> v
-  end
-
   let pub_to_octets ({ n; e } : Mirage_crypto_pk.Rsa.pub) =
     z_array_to_octets [| n; e |]
 
@@ -121,7 +91,7 @@ module Bin_rsa = struct
       | [| n; e |] ->
           let* pub = Mirage_crypto_pk.Rsa.pub ~n ~e in
           Ok pub
-      | _ -> Fmt.error_msg "rsa pub_of_octets invalid data"
+      | _ -> assert false
     in
     match pub_of_octets s with
     | Error (`Msg e) -> Fmt.failwith "rsa pub_of_octets failure: %s@." e
@@ -138,7 +108,7 @@ module Bin_rsa = struct
       | [| e; d; n; p; q; dp; dq; q' |] ->
           let* priv = Mirage_crypto_pk.Rsa.priv ~e ~d ~n ~p ~q ~dp ~dq ~q' in
           Ok priv
-      | _ -> Fmt.error_msg "rsa priv_of_octets invalid data"
+      | _ -> assert false
     in
     match priv_of_octets s with
     | Error (`Msg e) -> Fmt.failwith "rsa priv_of_octets failure: %s@." e
diff --git a/test/test.ml b/test/test.ml
index 7d6b9785..4e9efd4d 100644
--- a/test/test.ml
+++ b/test/test.ml
@@ -1,3 +1,20 @@
+let () = Mirage_crypto_rng_unix.use_default ()
+
+let () =
+  let priv = Mirage_crypto_pk.Rsa.generate ~bits:2048 () in
+  let pub = Mirage_crypto_pk.Rsa.pub_of_priv priv in
+  let () =
+    let open Crypto.RsaPrivateKey in
+    let priv' = priv |> to_octets |> of_octets in
+    assert (to_octets priv = to_octets priv')
+  in
+  let () =
+    let open Crypto.RsaPublicKey in
+    let pub' = pub |> to_octets |> of_octets in
+    assert (to_octets pub = to_octets pub')
+  in
+  ()
+
 let () =
   let open Api in
   let check jsont s =