diff --git a/lib/src/packet/key/ecdh_session_key_params.dart b/lib/src/packet/key/ecdh_session_key_params.dart
index 9723317b..1031827d 100644
--- a/lib/src/packet/key/ecdh_session_key_params.dart
+++ b/lib/src/packet/key/ecdh_session_key_params.dart
@@ -68,7 +68,10 @@ class ECDHSessionKeyParams extends SessionKeyParams {
         final privateKey = nacl.PrivateKey.fromSeed(
           Helper.secureRandom().nextBytes(TweetNaCl.seedSize),
         );
-        ephemeralKey = privateKey.publicKey.asTypedList.toBigIntWithSign(1);
+        ephemeralKey = Uint8List.fromList([
+          0x40,
+          ...privateKey.publicKey.asTypedList,
+        ]).toBigIntWithSign(1);
         sharedKey = TweetNaCl.crypto_scalarmult(
           Uint8List(TweetNaCl.sharedKeyLength),
           privateKey.asTypedList,