diff --git a/packages/core/src/transport/cryptography.ts b/packages/core/src/transport/cryptography.ts
index 2faeecc..56a40bb 100644
--- a/packages/core/src/transport/cryptography.ts
+++ b/packages/core/src/transport/cryptography.ts
@@ -19,5 +19,6 @@ export const newKey: () => Promise<Bytes> =
 const HMAC_BLAKE2s = makeHMAC(BLAKE2s);
 
 export function mac(secretKey: Bytes, data: Bytes): Bytes {
-    return Bytes.from(HMAC_BLAKE2s(underlying(secretKey), underlying(data)));
+    return Bytes.from(HMAC_BLAKE2s(underlying(secretKey), underlying(data))
+        .subarray(0, KEY_LENGTH));
 }