diff --git a/src/otaclient_common/shm_status.py b/src/otaclient_common/shm_status.py
index f20ca7cd4..f3b5c04aa 100644
--- a/src/otaclient_common/shm_status.py
+++ b/src/otaclient_common/shm_status.py
@@ -50,9 +50,24 @@ class RWBusy(Exception): ...
 class SHA512Verifier:
     """Base class for specifying hash alg related configurations."""
 
-    digest_alg = DEFAULT_HASH_ALG
-    digest_size = hashlib.new(digest_alg).digest_size
-    min_encap_msg_len = RWLOCK_LEN + digest_size + PAYLOAD_LEN_BYTES
+    DIGEST_ALG = DEFAULT_HASH_ALG
+    DIGEST_SIZE = hashlib.new(DIGEST_ALG).digest_size
+    MIN_ENCAP_MSG_LEN = RWLOCK_LEN + DIGEST_SIZE + PAYLOAD_LEN_BYTES
+
+    _key: bytes
+
+    def cal_hmac(self, _raw_msg: bytes) -> bytes:
+        return hmac.digest(key=self._key, msg=_raw_msg, digest=self.DIGEST_ALG)
+
+    def verify_msg(self, _raw_msg: bytes, _expected_hmac: bytes) -> bool:
+        return hmac.compare_digest(
+            hmac.digest(
+                key=self._key,
+                msg=_raw_msg,
+                digest=self.DIGEST_ALG,
+            ),
+            _expected_hmac,
+        )
 
 
 class MPSharedStatusReader(SHA512Verifier, Generic[T]):
@@ -78,7 +93,7 @@ def __init__(
             raise ValueError(f"failed to connect share memory with {name=}")
 
         self.mem_size = size = shm.size
-        self.msg_max_size = size - self.min_encap_msg_len
+        self.msg_max_size = size - self.MIN_ENCAP_MSG_LEN
         self._key = key
 
     def atexit(self) -> None:
@@ -103,8 +118,8 @@ def sync_msg(self) -> T:
         _cursor += RWLOCK_LEN
 
         # parsing the msg
-        input_hmac = bytes(buffer[_cursor : _cursor + self.digest_size])
-        _cursor += self.digest_size
+        input_hmac = bytes(buffer[_cursor : _cursor + self.DIGEST_SIZE])
+        _cursor += self.DIGEST_SIZE
 
         _payload_len_bytes = bytes(buffer[_cursor : _cursor + PAYLOAD_LEN_BYTES])
         payload_len = int.from_bytes(_payload_len_bytes, "big", signed=False)
@@ -114,9 +129,7 @@ def sync_msg(self) -> T:
             raise ValueError(f"invalid msg: {payload_len=} > {self.msg_max_size}")
 
         payload = bytes(buffer[_cursor : _cursor + payload_len])
-        payload_hmac = hmac.digest(key=self._key, msg=payload, digest=self.digest_alg)
-
-        if hmac.compare_digest(payload_hmac, input_hmac):
+        if self.verify_msg(payload, input_hmac):
             return pickle.loads(payload)
         raise ValueError("failed to validate input msg")
 
@@ -133,18 +146,18 @@ def __init__(
         key: bytes,
     ) -> None:
         if create:
-            _msg_max_size = size - self.min_encap_msg_len
+            _msg_max_size = size - self.MIN_ENCAP_MSG_LEN
             if _msg_max_size < 0:
-                raise ValueError(f"{size=} < {self.min_encap_msg_len=}")
+                raise ValueError(f"{size=} < {self.MIN_ENCAP_MSG_LEN=}")
             self._shm = shm = mp_shm.SharedMemory(name=name, size=size, create=True)
             self.mem_size = shm.size
         else:
             self._shm = shm = mp_shm.SharedMemory(name=name, create=False)
             self.mem_size = size = shm.size
-            _msg_max_size = size - self.min_encap_msg_len
+            _msg_max_size = size - self.MIN_ENCAP_MSG_LEN
             if _msg_max_size < 0:
                 shm.close()
-                raise ValueError(f"{size=} < {self.min_encap_msg_len=}")
+                raise ValueError(f"{size=} < {self.MIN_ENCAP_MSG_LEN=}")
 
         self.name = shm.name
         self._key = key
@@ -166,11 +179,10 @@ def write_msg(self, obj: T) -> None:
         if _pickled_len > self.msg_max_size:
             raise ValueError(f"exceed {self.msg_max_size=}: {_pickled_len=}")
 
-        _hmac = hmac.digest(key=self._key, msg=_pickled, digest=self.digest_alg)
         msg = b"".join(
             [
                 RWLOCK_LOCKED,
-                _hmac,
+                self.cal_hmac(_pickled),
                 _pickled_len.to_bytes(PAYLOAD_LEN_BYTES, "big", signed=False),
                 _pickled,
             ]