diff --git a/src/github.com/matrix-org/dendrite/clientapi/writers/register.go b/src/github.com/matrix-org/dendrite/clientapi/writers/register.go
index c7547ca34..40901bbf5 100644
--- a/src/github.com/matrix-org/dendrite/clientapi/writers/register.go
+++ b/src/github.com/matrix-org/dendrite/clientapi/writers/register.go
@@ -4,7 +4,6 @@ import (
 	"context"
 	"crypto/hmac"
 	"crypto/sha1"
-	"encoding/hex"
 	"errors"
 	"fmt"
 	"net/http"
@@ -46,9 +45,9 @@ type registerRequest struct {
 }
 
 type authDict struct {
-	Type    authtypes.LoginType `json:"type"`
-	Session string              `json:"session"`
-	Mac     string              `json:"mac"`
+	Type    authtypes.LoginType         `json:"type"`
+	Session string                      `json:"session"`
+	Mac     gomatrixserverlib.HexString `json:"mac"`
 	// TODO: Lots of custom keys depending on the type
 }
 
@@ -72,7 +71,7 @@ type legacyRegisterRequest struct {
 	Username string              `json:"user"`
 	Admin    bool                `json:"admin"`
 	Type     authtypes.LoginType `json:"type"`
-	Mac      string              `json:"mac"`
+	Mac      gomatrixserverlib.HexString           `json:"mac"`
 }
 
 func newUserInteractiveResponse(sessionID string, fs []authFlow) userInteractiveResponse {
@@ -301,7 +300,8 @@ func completeRegistration(
 func isValidMacLogin(
 	username, password string,
 	isAdmin bool,
-	givenMacStr, sharedSecret string,
+	givenMac []byte,
+	sharedSecret string,
 ) (bool, error) {
 	// Double check that username/passowrd don't contain the HMAC delimiters. We should have
 	// already checked this.
@@ -328,10 +328,5 @@ func isValidMacLogin(
 	}
 	expectedMAC := mac.Sum(nil)
 
-	givenMac, err := hex.DecodeString(givenMacStr)
-	if err != nil {
-		return false, err
-	}
-
 	return hmac.Equal(givenMac, expectedMAC), nil
 }
diff --git a/vendor/manifest b/vendor/manifest
index e30b2f2e7..9200690e8 100644
--- a/vendor/manifest
+++ b/vendor/manifest
@@ -116,7 +116,7 @@
 		{
 			"importpath": "github.com/matrix-org/gomatrixserverlib",
 			"repository": "https://github.com/matrix-org/gomatrixserverlib",
-			"revision": "40b35e1c997fc7e35342aeb39187ff6bf3e10b2e",
+			"revision": "ce6f4766251e31487906dfaaebd7d7cfea147252",
 			"branch": "master"
 		},
 		{
diff --git a/vendor/src/github.com/matrix-org/gomatrixserverlib/hex_string.go b/vendor/src/github.com/matrix-org/gomatrixserverlib/hex_string.go
new file mode 100644
index 000000000..883307d53
--- /dev/null
+++ b/vendor/src/github.com/matrix-org/gomatrixserverlib/hex_string.go
@@ -0,0 +1,44 @@
+/* Copyright 2017 New Vector Ltd
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package gomatrixserverlib
+
+import (
+	"encoding/hex"
+	"encoding/json"
+)
+
+// A HexString is a string of bytes that are hex encoded when used in JSON.
+// The bytes encoded using hex when marshalled as JSON.
+// When the bytes are unmarshalled from JSON they are decoded from hex.
+type HexString []byte
+
+// MarshalJSON encodes the bytes as hex and then encodes the hex as a JSON string.
+// This takes a value receiver so that maps and slices of HexString encode correctly.
+func (h HexString) MarshalJSON() ([]byte, error) {
+	return json.Marshal(hex.EncodeToString(h))
+}
+
+// UnmarshalJSON decodes a JSON string and then decodes the resulting hex.
+// This takes a pointer receiver because it needs to write the result of decoding.
+func (h *HexString) UnmarshalJSON(raw []byte) (err error) {
+	var str string
+	if err = json.Unmarshal(raw, &str); err != nil {
+		return
+	}
+
+	*h, err = hex.DecodeString(str)
+	return
+}
diff --git a/vendor/src/github.com/matrix-org/gomatrixserverlib/hex_string_test.go b/vendor/src/github.com/matrix-org/gomatrixserverlib/hex_string_test.go
new file mode 100644
index 000000000..6cb048622
--- /dev/null
+++ b/vendor/src/github.com/matrix-org/gomatrixserverlib/hex_string_test.go
@@ -0,0 +1,82 @@
+/* Copyright 2016-2017 Vector Creations Ltd
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package gomatrixserverlib
+
+import (
+	"encoding/json"
+	"testing"
+)
+
+func TestMarshalHex(t *testing.T) {
+	input := HexString("this\xffis\xffa\xfftest")
+	want := `"74686973ff6973ff61ff74657374"`
+	got, err := json.Marshal(input)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if string(got) != want {
+		t.Fatalf("json.Marshal(HexString(%q)): wanted %q got %q", string(input), want, string(got))
+	}
+}
+
+func TestUnmarshalHex(t *testing.T) {
+	input := []byte(`"74686973ff6973ff61ff74657374"`)
+	want := "this\xffis\xffa\xfftest"
+	var got HexString
+	err := json.Unmarshal(input, &got)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if string(got) != want {
+		t.Fatalf("json.Unmarshal(%q): wanted %q got %q", string(input), want, string(got))
+	}
+}
+
+func TestMarshalHexStruct(t *testing.T) {
+	input := struct{ Value HexString }{HexString("this\xffis\xffa\xfftest")}
+	want := `{"Value":"74686973ff6973ff61ff74657374"}`
+	got, err := json.Marshal(input)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if string(got) != want {
+		t.Fatalf("json.Marshal(%v): wanted %q got %q", input, want, string(got))
+	}
+}
+
+func TestMarshalHexMap(t *testing.T) {
+	input := map[string]HexString{"Value": HexString("this\xffis\xffa\xfftest")}
+	want := `{"Value":"74686973ff6973ff61ff74657374"}`
+	got, err := json.Marshal(input)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if string(got) != want {
+		t.Fatalf("json.Marshal(%v): wanted %q got %q", input, want, string(got))
+	}
+}
+
+func TestMarshalHexSlice(t *testing.T) {
+	input := []HexString{HexString("this\xffis\xffa\xfftest")}
+	want := `["74686973ff6973ff61ff74657374"]`
+	got, err := json.Marshal(input)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if string(got) != want {
+		t.Fatalf("json.Marshal(%v): wanted %q got %q", input, want, string(got))
+	}
+}