diff --git a/bin/template.ml b/bin/template.ml
index 2330ad2..72a33a1 100644
--- a/bin/template.ml
+++ b/bin/template.ml
@@ -77,17 +77,11 @@ let register_view origin user =
             let response = credential.response;
             let attestationObject = new Uint8Array(response.attestationObject);
             let clientDataJSON = new Uint8Array(response.clientDataJSON);
-            let rawId = new Uint8Array(credential.rawId);
 
             let body =
               JSON.stringify({
-                id: credential.id,
-                rawId: bufferEncode(rawId),
-                type: credential.type,
-                response: {
-                  attestationObject: bufferEncode(attestationObject),
-                  clientDataJSON: bufferEncode(clientDataJSON),
-                },
+                attestationObject: bufferEncode(attestationObject),
+                clientDataJSON: bufferEncode(clientDataJSON),
               });
 
             let headers = {'Content-type': "application/json; charset=utf-8"};
@@ -142,7 +136,6 @@ let authenticate_view challenge credentials user =
     navigator.credentials.get({ publicKey: request_options })
       .then(function (assertion) {
         let response = assertion.response;
-        let rawId = new Uint8Array(assertion.rawId);
         let authenticatorData = new Uint8Array(assertion.response.authenticatorData);
         let clientDataJSON = new Uint8Array(assertion.response.clientDataJSON);
         let signature = new Uint8Array(assertion.response.signature);
@@ -150,20 +143,15 @@ let authenticate_view challenge credentials user =
  
         let body =
           JSON.stringify({
-            id: assertion.id,
-            rawId: bufferEncode(rawId),
-            type: assertion.type,
-            response: {
-              authenticatorData: bufferEncode(authenticatorData),
-              clientDataJSON: bufferEncode(clientDataJSON),
-              signature: bufferEncode(signature),
-              userHandle: userHandle ? bufferEncode(userHandle) : null,
-            }
+            authenticatorData: bufferEncode(authenticatorData),
+            clientDataJSON: bufferEncode(clientDataJSON),
+            signature: bufferEncode(signature),
+            userHandle: userHandle ? bufferEncode(userHandle) : null,
            });
 
         let headers = {'Content-type': "application/json; charset=utf-8"};
         let username = window.location.pathname.substring("/authenticate/".length);
-        let request = new Request('/authenticate_finish/'+username, { method: 'POST', body: body, headers: headers } );
+        let request = new Request('/authenticate_finish/'+assertion.id+'/'+username, { method: 'POST', body: body, headers: headers } );
         fetch(request)
         .then(function (response) {
           if (!response.ok) {
diff --git a/bin/webauthn_demo.ml b/bin/webauthn_demo.ml
index 249d936..479c2e2 100644
--- a/bin/webauthn_demo.ml
+++ b/bin/webauthn_demo.ml
@@ -8,7 +8,7 @@ let find_username username =
     users None
 
 module KhPubHashtbl = Hashtbl.Make(struct
-    type t = Webauthn.key_handle * Mirage_crypto_ec.P256.Dsa.pub
+    type t = Webauthn.credential_id * Mirage_crypto_ec.P256.Dsa.pub
     let cs_of_pub = Mirage_crypto_ec.P256.Dsa.pub_to_cstruct
     let equal (kh, pub) (kh', pub') =
       String.equal kh kh' && Cstruct.equal (cs_of_pub pub) (cs_of_pub pub')
@@ -27,25 +27,25 @@ let check_counter kh_pub counter =
   then KhPubHashtbl.replace counters kh_pub counter;
   r
 
-let registration_challenges : (string, string * string list) Hashtbl.t = Hashtbl.create 7
+let registration_challenges : (string, string * Webauthn.challenge list) Hashtbl.t = Hashtbl.create 7
 
 let remove_registration_challenge userid challenge =
   match Hashtbl.find_opt registration_challenges userid with
   | None -> ()
   | Some (username, challenges) ->
-    let challenges = List.filter (fun c -> not (String.equal c challenge)) challenges in
+    let challenges = List.filter (fun c -> not (Webauthn.challenge_equal c challenge)) challenges in
     if challenges = [] then
       Hashtbl.remove registration_challenges userid
     else
       Hashtbl.replace registration_challenges userid (username, challenges)
 
-let authentication_challenges : (string, string list) Hashtbl.t = Hashtbl.create 7
+let authentication_challenges : (string, Webauthn.challenge list) Hashtbl.t = Hashtbl.create 7
 
 let remove_authentication_challenge userid challenge =
   match Hashtbl.find_opt authentication_challenges userid with
   | None -> ()
   | Some challenges ->
-    let challenges = List.filter (fun c -> not (String.equal c challenge)) challenges in
+    let challenges = List.filter (fun c -> not (Webauthn.challenge_equal c challenge)) challenges in
     if challenges = [] then
       Hashtbl.remove authentication_challenges userid
     else
@@ -75,7 +75,7 @@ let add_routes t =
 
   let registration_challenge req =
     let user = Dream.param "user" req in
-    let challenge = Cstruct.to_string (Mirage_crypto_rng.generate 16) in
+    let challenge, challenge_b64 = Webauthn.generate_challenge () in
     let userid, credentials = match find_username user with
       | None -> gen_data ~alphabet:Base64.uri_safe_alphabet 8, []
       | Some (userid, (_, credentials)) -> userid, List.map (fun (_, cid, _) -> cid) credentials
@@ -85,7 +85,6 @@ let add_routes t =
       Option.value ~default:[]
     in
     Hashtbl.replace registration_challenges userid (user, challenge :: challenges);
-    let challenge_b64 = (Base64.encode_string challenge) in
     let json = `Assoc [
         "challenge", `String challenge_b64 ;
         "user", `Assoc [
@@ -109,49 +108,55 @@ let add_routes t =
       Logs.warn (fun m -> m "no challenge found");
       Dream.respond ~status:`Bad_Request "Bad request."
     | Some (username, challenges) ->
-      match Webauthn.register_response t body with
+      match Webauthn.register_response_of_string body with
       | Error e ->
         Logs.warn (fun m -> m "error %a" Webauthn.pp_error e);
         let err = to_string e in
         Flash_message.put_flash "" ("Registration failed " ^ err) req;
         Dream.json "false"
-      | Ok (challenge, _aaguid, credential_id, pubkey, _client_extensions, user_present,
-            user_verified, sig_count, _authenticator_extensions, attestation_cert) ->
-        if not (List.mem challenge challenges) then begin
-          Logs.warn (fun m -> m "challenge invalid");
-          Flash_message.put_flash "" "Registration failed: invalid challenge" req;
+      | Ok response ->
+        match Webauthn.register t response with
+        | Error e ->
+          Logs.warn (fun m -> m "error %a" Webauthn.pp_error e);
+          let err = to_string e in
+          Flash_message.put_flash "" ("Registration failed " ^ err) req;
           Dream.json "false"
-        end else begin
-          remove_registration_challenge userid challenge;
-          ignore (check_counter (credential_id, pubkey) sig_count);
-          Logs.info (fun m -> m "user present %B user verified %B" user_present user_verified);
-          Logs.app (fun m -> m "challenge for user %S" username);
-          match Dream.session "authenticated_as" req, Hashtbl.find_opt users userid with
-          | _, None ->
-            Logs.app (fun m -> m "registered %s: %S" username credential_id);
-            Hashtbl.replace users userid (username, [ (pubkey, credential_id, attestation_cert) ]);
-            Dream.invalidate_session req >>= fun () ->
-            Flash_message.put_flash ""
-              (Printf.sprintf "Successfully registered as %s! <a href=\"/authenticate/%s\">[authenticate]</a>" username userid)
-              req;
-            Dream.json "true"
-          | Some session_user, Some (username', keys) ->
-            Logs.app (fun m -> m "user %S session_user %S" username session_user);
-            if String.equal username session_user && String.equal username username' then begin
+        | Ok (challenge, { user_present ; user_verified ; sign_count ; attested_credential_data ; certificate ; _ }) ->
+          let { Webauthn.credential_id ; public_key ; _ } = attested_credential_data in
+          if not (List.exists (Webauthn.challenge_equal challenge) challenges) then begin
+            Logs.warn (fun m -> m "challenge invalid");
+            Flash_message.put_flash "" "Registration failed: invalid challenge" req;
+            Dream.json "false"
+          end else begin
+            remove_registration_challenge userid challenge;
+            ignore (check_counter (credential_id, public_key) sign_count);
+            Logs.info (fun m -> m "register %S user present %B user verified %B"
+              username user_present user_verified);
+            match Dream.session "authenticated_as" req, Hashtbl.find_opt users userid with
+            | _, None ->
               Logs.app (fun m -> m "registered %s: %S" username credential_id);
-              Hashtbl.replace users userid (username, ((pubkey, credential_id, attestation_cert) :: keys)) ;
+              Hashtbl.replace users userid (username, [ (public_key, credential_id, certificate) ]);
               Dream.invalidate_session req >>= fun () ->
               Flash_message.put_flash ""
                 (Printf.sprintf "Successfully registered as %s! <a href=\"/authenticate/%s\">[authenticate]</a>" username userid)
                 req;
               Dream.json "true"
-            end else
-              (Logs.info (fun m -> m "session_user %s, user %s (user in users table %s)" session_user username username');
-               Dream.json ~status:`Forbidden "false")
-          | None, Some _keys ->
-            Logs.app (fun m -> m "no session user");
-            Dream.json ~status:`Forbidden "false"
-    end
+            | Some session_user, Some (username', keys) ->
+              if String.equal username session_user && String.equal username username' then begin
+                Logs.app (fun m -> m "registered %s: %S" username credential_id);
+                Hashtbl.replace users userid (username, ((public_key, credential_id, certificate) :: keys)) ;
+                Dream.invalidate_session req >>= fun () ->
+                Flash_message.put_flash ""
+                  (Printf.sprintf "Successfully registered as %s! <a href=\"/authenticate/%s\">[authenticate]</a>" username userid)
+                  req;
+                Dream.json "true"
+              end else
+                (Logs.info (fun m -> m "session_user %s, user %s (user in users table %s)" session_user username username');
+                 Dream.json ~status:`Forbidden "false")
+            | None, Some _keys ->
+              Logs.app (fun m -> m "no session user");
+              Dream.json ~status:`Forbidden "false"
+      end
   in
 
   let authenticate req =
@@ -162,51 +167,69 @@ let add_routes t =
       Dream.respond ~status:`Bad_Request "Bad request."
     | Some (username, keys) ->
       let credentials = List.map (fun (_, c, _) -> Base64.encode_string c) keys in
-      let challenge = Cstruct.to_string (Mirage_crypto_rng.generate 16) in
+      let challenge, challenge_b64 = Webauthn.generate_challenge () in
       let challenges = Option.value ~default:[] (Hashtbl.find_opt authentication_challenges userid) in
       Hashtbl.replace authentication_challenges userid (challenge :: challenges);
-      Dream.html (Template.authenticate_view (Base64.encode_string challenge) credentials username)
+      Dream.html (Template.authenticate_view challenge_b64 credentials username)
   in
 
   let authenticate_finish req =
-    let userid = Dream.param "userid" req in
-    Dream.body req >>= fun body ->
-    Logs.debug (fun m -> m "received body: %s" body);
-    match Hashtbl.find_opt authentication_challenges userid with
-    | None -> Dream.respond ~status:`Internal_Server_Error "Internal server error."
-    | Some challenges ->
-      match Hashtbl.find_opt users userid with
-      | None ->
+    let userid = Dream.param "userid" req
+    and b64_credential_id = Dream.param "credential_id" req
+    in
+    match Base64.decode ~alphabet:Base64.uri_safe_alphabet ~pad:false b64_credential_id with
+    | Error `Msg err ->
+      Logs.err (fun m -> m "credential id (%S) is not base64 uri safe: %s" b64_credential_id err);
+      Dream.json ~status:`Bad_Request "credential ID decoding error"
+    | Ok credential_id ->
+      Dream.body req >>= fun body ->
+      Logs.debug (fun m -> m "received body: %s" body);
+      match Hashtbl.find_opt authentication_challenges userid, Hashtbl.find_opt users userid with
+      | None, _ -> Dream.respond ~status:`Internal_Server_Error "Internal server error."
+      | _, None ->
         Logs.warn (fun m -> m "no user found with id %s" userid);
         Dream.respond ~status:`Bad_Request "Bad request."
-      | Some (username, keys) ->
-        let cid_keys = List.map (fun (key, credential_id, _) -> credential_id, key) keys in
-        match Webauthn.authentication_response t cid_keys body with
-        | Ok (challenge, credential, _client_extensions, _user_present, _user_verified, counter, _authenticator_extensions) ->
-          if not (List.mem challenge challenges) then begin
-            Logs.warn (fun m -> m "invalid challenge");
-            Flash_message.put_flash "" "Authentication failure: invalid challenge" req;
+      | Some challenges, Some (username, keys) ->
+        match List.find_opt (fun (_, cid, _) -> String.equal cid credential_id) keys with
+        | None ->
+          Logs.warn (fun m -> m "no key found with credential id %s" b64_credential_id);
+          Dream.respond ~status:`Bad_Request "Bad request."
+        | Some (pubkey, _, _) ->
+          match Webauthn.authenticate_response_of_string body with
+          | Error e ->
+            Logs.warn (fun m -> m "error %a" Webauthn.pp_error e);
+            let err = to_string e in
+            Flash_message.put_flash "" ("Authentication failure: " ^ err) req;
             Dream.json "false"
-          end else begin
-            remove_authentication_challenge userid challenge;
-            if check_counter credential counter
-            then begin
-              Flash_message.put_flash ""  "Successfully authenticated" req;
-              Dream.put_session "authenticated_as" username req >>= fun () ->
-              Dream.json "true"
-            end else begin
-              Logs.warn (fun m -> m "credential %S for user %S: counter not strictly increasing! \
-                Got %ld, expected >%ld. webauthn device compromised?"
-                (fst credential) username counter (KhPubHashtbl.find counters credential));
-              Flash_message.put_flash "" "Authentication failure: key compromised?" req;
+          | Ok authenticate_response ->
+            match Webauthn.authenticate t pubkey authenticate_response with
+            | Ok (challenge, { user_present ; user_verified ; sign_count ; _ }) ->
+              Logs.info (fun m -> m "authenticate %S user present %B user verified %B"
+                username user_present user_verified);
+              if not (List.exists (Webauthn.challenge_equal challenge) challenges) then begin
+                Logs.warn (fun m -> m "invalid challenge");
+                Flash_message.put_flash "" "Authentication failure: invalid challenge" req;
+                Dream.json "false"
+              end else begin
+                remove_authentication_challenge userid challenge;
+                if check_counter (credential_id, pubkey) sign_count
+                then begin
+                  Flash_message.put_flash ""  "Successfully authenticated" req;
+                  Dream.put_session "authenticated_as" username req >>= fun () ->
+                  Dream.json "true"
+                end else begin
+                  Logs.warn (fun m -> m "credential %S for user %S: counter not strictly increasing! \
+                    Got %ld, expected >%ld. webauthn device compromised?"
+                    b64_credential_id username sign_count (KhPubHashtbl.find counters (credential_id, pubkey)));
+                  Flash_message.put_flash "" "Authentication failure: key compromised?" req;
+                  Dream.json "false"
+                end
+              end
+            | Error e ->
+              Logs.warn (fun m -> m "error %a" Webauthn.pp_error e);
+              let err = to_string e in
+              Flash_message.put_flash "" ("Authentication failure: " ^ err) req;
               Dream.json "false"
-            end
-          end
-        | Error e ->
-          Logs.warn (fun m -> m "error %a" Webauthn.pp_error e);
-          let err = to_string e in
-          Flash_message.put_flash "" ("Authentication failure: " ^ err) req;
-          Dream.json "false"
   in
 
   let logout req =
@@ -225,7 +248,7 @@ let add_routes t =
     Dream.get "/registration-challenge/:user" registration_challenge;
     Dream.post "/register_finish/:userid" register_finish;
     Dream.get "/authenticate/:userid" authenticate;
-    Dream.post "/authenticate_finish/:userid" authenticate_finish;
+    Dream.post "/authenticate_finish/:credential_id/:userid" authenticate_finish;
     Dream.post "/logout" logout;
     Dream.get "/static/base64.js" base64;
   ]
diff --git a/src/webauthn.ml b/src/webauthn.ml
index 5e280a7..10adc10 100644
--- a/src/webauthn.ml
+++ b/src/webauthn.ml
@@ -1,12 +1,13 @@
-type key_handle = string
+type credential_id = string
 
+type json_decoding_error = [  `Json_decoding of string * string * string ]
 type error = [
-  | `Json_decoding of string * string * string
+  json_decoding_error
   | `Base64_decoding of string * string * string
   | `Client_data_type_mismatch of string
   | `Origin_mismatch of string * string
   | `Attestation_object of string
-  | `Rpid_hash_mismatch of Cstruct.t * Cstruct.t
+  | `Rpid_hash_mismatch of string * string
   | `Missing_credential_data
   | `Msg of string
 ]
@@ -23,7 +24,8 @@ let pp_error ppf = function
   | `Attestation_object msg ->
     Fmt.pf ppf "attestation object error %s" msg
   | `Rpid_hash_mismatch (should, is) ->
-    Fmt.pf ppf "rpid hash mismatch: expected %a received %a" Cstruct.hexdump_pp should Cstruct.hexdump_pp is
+    Fmt.pf ppf "rpid hash mismatch: expected %s received %s"
+      (Base64.encode_string should) (Base64.encode_string is)
   | `Missing_credential_data -> Fmt.string ppf "missing credential data"
   | `Msg msg -> Fmt.pf ppf "error %s" msg
 
@@ -33,12 +35,18 @@ type t = {
 
 type challenge = string
 
-let b64_enc = Base64.(encode_string ~pad:false ~alphabet:uri_safe_alphabet)
+let generate_challenge ?(size = 32) () =
+  if size < 16 then invalid_arg "size must be at least 16 bytes";
+  let ch = Mirage_crypto_rng.generate size |> Cstruct.to_string in
+  ch, Base64.encode_string ch
 
-let lift_err f = function Ok _ as a -> a | Error x -> Error (f x)
+let challenge_to_string c = c
+let challenge_of_string s = Some s
+
+let challenge_equal = String.equal
 
 let b64_dec thing s =
-  lift_err
+  Result.map_error
     (function `Msg m -> `Base64_decoding (thing, m, s))
     Base64.(decode ~pad:false ~alphabet:uri_safe_alphabet s)
 
@@ -52,16 +60,6 @@ let base64url_string_of_yojson = function
     Base64.(decode ~pad:false ~alphabet:uri_safe_alphabet b64)
     |> Result.map_error (function `Msg m -> m)
   | _ -> Error "base64url_string"
-let base64url_string_to_yojson s =
-  `String Base64.(encode_string ~pad:false ~alphabet:uri_safe_alphabet s)
-
-type typ = Public_key
-
-let typ_of_yojson = function
-  | `String "public-key" -> Ok Public_key
-  | _ -> Error "typ"
-
-let typ_to_yojson Public_key = `String "public-key"
 
 let extract_k_i map k : (_, string) result =
   Option.to_result ~none:"key not present"
@@ -120,10 +118,19 @@ let parse_attested_credential_data data =
   cose_pubkey pubkey >>= fun pubkey ->
   Ok ((aaguid, cid, pubkey), Cstruct.of_string rest)
 
+let string_keys kv =
+  List.fold_right (fun (k, v) acc ->
+    match acc, k with
+    | Error _ as e, _ -> e
+    | Ok xs, `Text t -> Ok ((t, v) :: xs)
+    | _, _ -> Error "Map does contain non-text keys")
+    kv (Ok [])
+
 let parse_extension_data data =
    (try Ok (CBOR.Simple.decode_partial (Cstruct.to_string data))
    with CBOR.Error m -> Error m) >>= fun (data, rest) ->
    extract_map data >>= fun kv ->
+   string_keys kv >>= fun kv ->
    Ok (kv, Cstruct.of_string rest)
   
 type auth_data = {
@@ -132,7 +139,7 @@ type auth_data = {
   user_verified : bool ;
   sign_count : Int32.t ;
   attested_credential_data : (Cstruct.t * Cstruct.t * Mirage_crypto_ec.P256.Dsa.pub) option ;
-  extension_data : (CBOR.Simple.t * CBOR.Simple.t) list option ;
+  extension_data : (string * CBOR.Simple.t) list option ;
 }
 
 let flags byte =
@@ -185,20 +192,8 @@ let parse_attestation_object data =
   | _ -> Error "bad attestationObject CBOR"
   | exception CBOR.Error m -> Error m
 
-type response_raw = {
-  attestation_object : base64url_string [@key "attestationObject"];
-  client_data_json : base64url_string [@key "clientDataJSON"];
-} [@@deriving of_yojson]
-
-type public_key_credential_raw = {
-  id : string;
-  raw_id : base64url_string [@key "rawId"];
-  typ : typ [@key "type"];
-  response : response_raw;
-} [@@deriving of_yojson]
-
 let of_json_or_err thing p json =
-  lift_err
+  Result.map_error
     (fun msg -> `Json_decoding (thing, msg, Yojson.Safe.to_string json))
     (p json)
 
@@ -218,6 +213,10 @@ let json_string thing : Yojson.Safe.t -> (string, _) result = function
   | `String s -> Ok s
   | json -> Error (`Json_decoding (thing, "non-string", Yojson.Safe.to_string json))
 
+let json_assoc thing : Yojson.Safe.t -> ((string * Yojson.Safe.t) list, _) result = function
+  | `Assoc s -> Ok s
+  | json -> Error (`Json_decoding (thing, "non-string", Yojson.Safe.to_string json))
+
 (* XXX: verify [origin] is in fact an origin *)
 let create origin = { origin }
 
@@ -226,10 +225,32 @@ let rpid t =
   | [ _protocol ; "" ; host ] -> host
   | _ -> assert false
 
-let register_response t data =
-  of_json "response" public_key_credential_raw_of_yojson data >>= fun credential ->
+type credential_data = {
+  aaguid : string ;
+  credential_id : credential_id ;
+  public_key : Mirage_crypto_ec.P256.Dsa.pub ;
+}
+
+type registration = {
+  user_present : bool ;
+  user_verified : bool ;
+  sign_count : Int32.t ;
+  attested_credential_data : credential_data ;
+  authenticator_extensions : (string * CBOR.Simple.t) list option ;
+  client_extensions : (string * Yojson.Safe.t) list ;
+  certificate : X509.Certificate.t option ;
+}
+
+type register_response = {
+  attestation_object : base64url_string [@key "attestationObject"];
+  client_data_json : base64url_string [@key "clientDataJSON"];
+} [@@deriving of_yojson]
+
+let register_response_of_string =
+  of_json "register response" register_response_of_yojson
+
+let register t response =
   (* XXX: credential.getClientExtensionResults() *)
-  let response = credential.response in
   let client_data_hash = Mirage_crypto.Hash.SHA256.digest
     (Cstruct.of_string response.client_data_json) in
   begin try Ok (Yojson.Safe.from_string response.client_data_json)
@@ -245,19 +266,19 @@ let register_response t data =
   json_get "origin" client_data >>= json_string "origin" >>= fun origin ->
   guard (String.equal t.origin origin)
     (`Origin_mismatch (t.origin, origin)) >>= fun () ->
-  json_get "clientExtensions" client_data >>= fun client_extensions ->
+  json_get "clientExtensions" client_data >>= json_assoc "clientExtensions" >>= fun client_extensions ->
   Result.map_error (fun m -> `Attestation_object m)
     (parse_attestation_object response.attestation_object) >>= fun (auth_data, attestation_statement) ->
   let rpid_hash = Mirage_crypto.Hash.SHA256.digest (Cstruct.of_string (rpid t)) in
   guard (Cstruct.equal auth_data.rpid_hash rpid_hash)
-    (`Rpid_hash_mismatch (rpid_hash, auth_data.rpid_hash)) >>= fun () ->
+    (`Rpid_hash_mismatch (Cstruct.to_string rpid_hash, Cstruct.to_string auth_data.rpid_hash)) >>= fun () ->
   (* verify user present, user verified flags in auth_data.flags *)
   Option.to_result ~none:`Missing_credential_data
-    auth_data.attested_credential_data >>= fun (aaguid, credential_id, pubkey) ->
+    auth_data.attested_credential_data >>= fun (aaguid, credential_id, public_key) ->
   begin match attestation_statement with
    | None -> Ok None
    | Some (cert, signature) ->
-     let pub_cs = Mirage_crypto_ec.P256.Dsa.pub_to_cstruct pubkey in
+     let pub_cs = Mirage_crypto_ec.P256.Dsa.pub_to_cstruct public_key in
      let sigdata = Cstruct.concat [
        Cstruct.create 1 ; rpid_hash ; client_data_hash ; credential_id ; pub_cs
      ] in
@@ -266,28 +287,46 @@ let register_response t data =
      in
      X509.Public_key.verify `SHA256 ~signature pk (`Message sigdata) >>= fun () ->
      Ok (Some cert)
-  end >>= fun cert ->
+  end >>= fun certificate ->
   (* check attestation cert, maybe *)
   (* check auth_data.attested_credential_data.credential_id is not registered ? *)
-  Ok (challenge, aaguid, Cstruct.to_string credential_id, pubkey, client_extensions, auth_data.user_present, auth_data.user_verified, auth_data.sign_count, auth_data.extension_data, cert)
+  let registration =
+    let attested_credential_data = {
+      aaguid = Cstruct.to_string aaguid ;
+      credential_id = Cstruct.to_string credential_id ;
+      public_key
+    } in
+    {
+      user_present = auth_data.user_present ;
+      user_verified = auth_data.user_verified ;
+      sign_count = auth_data.sign_count ;
+      attested_credential_data ;
+      authenticator_extensions = auth_data.extension_data ;
+      client_extensions ;
+      certificate ;
+    }
+  in
+  Ok (challenge, registration)
 
-type auth_response_raw = {
+type authentication = {
+  user_present : bool ;
+  user_verified : bool ;
+  sign_count : Int32.t ;
+  authenticator_extensions : (string * CBOR.Simple.t) list option ;
+  client_extensions : (string * Yojson.Safe.t) list ;
+}
+
+type authenticate_response = {
   authenticator_data : base64url_string [@key "authenticatorData"];
   client_data_json : base64url_string [@key "clientDataJSON"];
   signature : base64url_string ;
   userHandle : base64url_string option ;
 } [@@deriving of_yojson]
 
-type auth_assertion_raw = {
-  id : string;
-  raw_id : base64url_string [@key "rawId"];
-  typ : typ [@key "type"];
-  response : auth_response_raw;
-} [@@deriving of_yojson]
+let authenticate_response_of_string =
+  of_json "authenticate response" authenticate_response_of_yojson
 
-let authentication_response t cid_keys data =
-  of_json "response" auth_assertion_raw_of_yojson data >>= fun assertion ->
-  let response = assertion.response in
+let authenticate t public_key response =
   let client_data_hash = Mirage_crypto.Hash.SHA256.digest
     (Cstruct.of_string response.client_data_json) in
   begin try Ok (Yojson.Safe.from_string response.client_data_json)
@@ -303,16 +342,21 @@ let authentication_response t cid_keys data =
   json_get "origin" client_data >>= json_string "origin" >>= fun origin ->
   guard (String.equal t.origin origin)
     (`Origin_mismatch (t.origin, origin)) >>= fun () ->
-  json_get "clientExtensions" client_data >>= fun client_extensions ->
+  json_get "clientExtensions" client_data >>= json_assoc "clientExtensions" >>= fun client_extensions ->
   Result.map_error (fun m -> `Msg m)
     (parse_auth_data response.authenticator_data) >>= fun auth_data ->
   let rpid_hash = Mirage_crypto.Hash.SHA256.digest (Cstruct.of_string (rpid t)) in
   guard (Cstruct.equal auth_data.rpid_hash rpid_hash)
-    (`Rpid_hash_mismatch (rpid_hash, auth_data.rpid_hash)) >>= fun () ->
-  Option.to_result ~none:(`Msg "no key found")
-    (List.assoc_opt assertion.raw_id cid_keys) >>= fun pubkey ->
+    (`Rpid_hash_mismatch (Cstruct.to_string rpid_hash, Cstruct.to_string auth_data.rpid_hash)) >>= fun () ->
   let sigdata = Cstruct.concat [ Cstruct.of_string response.authenticator_data ; client_data_hash ]
   and signature = Cstruct.of_string response.signature
   in
-  X509.Public_key.verify `SHA256 ~signature (`P256 pubkey) (`Message sigdata) >>= fun () ->
-  Ok (challenge, (assertion.raw_id, pubkey), client_extensions, auth_data.user_present, auth_data.user_verified, auth_data.sign_count, auth_data.extension_data)
+  X509.Public_key.verify `SHA256 ~signature (`P256 public_key) (`Message sigdata) >>= fun () ->
+  let authentication = {
+    user_present = auth_data.user_present ;
+    user_verified = auth_data.user_verified ;
+    sign_count = auth_data.sign_count ;
+    authenticator_extensions = auth_data.extension_data ;
+    client_extensions ;
+  } in
+  Ok (challenge, authentication)
diff --git a/src/webauthn.mli b/src/webauthn.mli
new file mode 100644
index 0000000..071038d
--- /dev/null
+++ b/src/webauthn.mli
@@ -0,0 +1,65 @@
+type t
+
+val create : string -> t
+
+val rpid : t -> string
+
+type json_decoding_error = [ `Json_decoding of string * string * string ]
+
+type error = [ 
+    json_decoding_error
+  | `Base64_decoding of string * string * string
+  | `Client_data_type_mismatch of string
+  | `Origin_mismatch of string * string
+  | `Attestation_object of string
+  | `Rpid_hash_mismatch of string * string
+  | `Missing_credential_data
+  | `Msg of string
+]
+
+val pp_error : Format.formatter -> [< error ] -> unit
+
+type challenge
+
+val generate_challenge : ?size:int -> unit -> challenge * string
+
+val challenge_to_string : challenge -> string
+val challenge_of_string : string -> challenge option
+val challenge_equal : challenge -> challenge -> bool
+
+type credential_id = string
+
+type credential_data = {
+  aaguid : string ;
+  credential_id : credential_id ;
+  public_key : Mirage_crypto_ec.P256.Dsa.pub ;
+}
+
+type registration = {
+  user_present : bool ;
+  user_verified : bool ;
+  sign_count : Int32.t ;
+  attested_credential_data : credential_data ;
+  authenticator_extensions : (string * CBOR.Simple.t) list option ;
+  client_extensions : (string * Yojson.Safe.t) list ;
+  certificate : X509.Certificate.t option ;
+}
+
+type register_response
+val register_response_of_string : string -> (register_response, json_decoding_error) result
+
+val register : t -> register_response -> (challenge * registration, error) result
+
+type authentication = {
+  user_present : bool ;
+  user_verified : bool ;
+  sign_count : Int32.t ;
+  authenticator_extensions : (string * CBOR.Simple.t) list option ;
+  client_extensions : (string * Yojson.Safe.t) list ;
+}
+
+type authenticate_response
+val authenticate_response_of_string : string -> (authenticate_response, json_decoding_error) result
+
+val authenticate : t -> Mirage_crypto_ec.P256.Dsa.pub -> authenticate_response ->
+  (challenge * authentication, error) result