diff --git a/lib/src/protobuf/omemo_message.dart b/lib/src/protobuf/omemo_message.dart
index 95dca67..6324cf5 100644
--- a/lib/src/protobuf/omemo_message.dart
+++ b/lib/src/protobuf/omemo_message.dart
@@ -9,7 +9,7 @@ class OmemoMessage {
     var i = 0;
     
     // required uint32 n          = 1;
-    if (data[0] != 8) {
+    if (data[0] != fieldId(1, fieldTypeUint32)) {
       throw Exception();
     }
     var decode = decodeVarint(data, 1);
@@ -17,7 +17,7 @@ class OmemoMessage {
     i += decode.length + 1;
 
     // required uint32 pn         = 2;
-    if (data[i] != 16) {
+    if (data[i] != fieldId(2, fieldTypeUint32)) {
       throw Exception();
     }
     decode = decodeVarint(data, i + 1);
@@ -25,7 +25,7 @@ class OmemoMessage {
     i += decode.length + 1;
 
     // required bytes  dh_pub     = 3;
-    if (data[i] != ((3 << 3) | 2)) {
+    if (data[i] != fieldId(3, fieldTypeByteArray)) {
       throw Exception();
     }
     final dhPub = data.sublist(i + 2, i + 2 + data[i + 1]);
@@ -34,7 +34,7 @@ class OmemoMessage {
     // optional bytes  ciphertext = 4;
     List<int>? ciphertext;
     if (i < data.length) {
-      if (data[i] != ((4 << 3) | 2)) {
+      if (data[i] != fieldId(4, fieldTypeByteArray)) {
         throw Exception();
       }
 
diff --git a/lib/src/protobuf/protobuf.dart b/lib/src/protobuf/protobuf.dart
index 3b871c0..74db070 100644
--- a/lib/src/protobuf/protobuf.dart
+++ b/lib/src/protobuf/protobuf.dart
@@ -4,6 +4,14 @@ const lsb7Mask = 0x7F;
 /// Constant for setting the MSB
 const msb = 1 << 7;
 
+/// Field types
+const fieldTypeUint32 = 0;
+const fieldTypeByteArray = 2;
+
+int fieldId(int number, int type) {
+  return (number << 3) | type;
+}
+
 class VarintDecode {
 
   const VarintDecode(this.n, this.length);