diff --git a/minichain/transaction.py b/minichain/transaction.py
index 27f41f3..298056e 100644
--- a/minichain/transaction.py
+++ b/minichain/transaction.py
@@ -6,82 +6,70 @@
 
 
 class Transaction:
+    _TX_FIELDS = frozenset({"sender", "receiver", "amount", "nonce", "data", "timestamp", "signature"})
+
+    def __setattr__(self, name, value) -> None:
+        if name in self._TX_FIELDS and getattr(self, "_sealed", False):
+            raise AttributeError(f"Transaction is sealed; cannot modify '{name}'")
+        super().__setattr__(name, value)
+        if name in self._TX_FIELDS and hasattr(self, "_cached_tx_id"):
+            super().__setattr__("_cached_tx_id", None)
+
+    @staticmethod
+    def _normalize_ts(ts) -> int:
+        ts = int(ts)
+        return ts * 1000 if ts < 1e12 else ts
+
     def __init__(self, sender, receiver, amount, nonce, data=None, signature=None, timestamp=None):
-        self.sender = sender        # Public key (Hex str)
-        self.receiver = receiver    # Public key (Hex str) or None for Deploy
+        self.sender = sender
+        self.receiver = receiver
         self.amount = amount
         self.nonce = nonce
-        self.data = data            # Preserve None (do NOT normalize to "")
-        if timestamp is None:
-            self.timestamp = round(time.time() * 1000)  # New tx: seconds → ms
-        elif timestamp > 1e12:
-            self.timestamp = int(timestamp)              # Already in ms (from network)
-        else:
-            self.timestamp = round(timestamp * 1000)     # Seconds → ms
-        self.signature = signature  # Hex str
+        self.data = data
+        self.timestamp = self._normalize_ts(timestamp) if timestamp is not None else round(time.time() * 1000)
+        self.signature = signature
+        self._cached_tx_id = None
+        self._sealed = False
 
     def to_dict(self):
-        return {
-            "sender": self.sender,
-            "receiver": self.receiver,
-            "amount": self.amount,
-            "nonce": self.nonce,
-            "data": self.data,
-            "timestamp": self.timestamp,
-            "signature": self.signature,
-        }
+        return {"sender": self.sender, "receiver": self.receiver, "amount": self.amount,
+                "nonce": self.nonce, "data": self.data, "timestamp": self.timestamp,
+                "signature": self.signature}
 
     def to_signing_dict(self):
-        return {
-            "sender": self.sender,
-            "receiver": self.receiver,
-            "amount": self.amount,
-            "nonce": self.nonce,
-            "data": self.data,
-            "timestamp": self.timestamp,
-        }
+        return {"sender": self.sender, "receiver": self.receiver, "amount": self.amount,
+                "nonce": self.nonce, "data": self.data, "timestamp": self.timestamp}
 
     @classmethod
     def from_dict(cls, payload: dict):
-        return cls(
-            sender=payload["sender"],
-            receiver=payload.get("receiver"),
-            amount=payload["amount"],
-            nonce=payload["nonce"],
-            data=payload.get("data"),
-            signature=payload.get("signature"),
-            timestamp=payload.get("timestamp"),
-        )
+        return cls(sender=payload["sender"], receiver=payload.get("receiver"),
+                   amount=payload["amount"], nonce=payload["nonce"],
+                   data=payload.get("data"), signature=payload.get("signature"),
+                   timestamp=payload.get("timestamp"))
 
     @property
     def hash_payload(self):
-        """Returns the bytes to be signed."""
         return canonical_json_bytes(self.to_signing_dict())
 
     @property
     def tx_id(self):
-        """Deterministic identifier for the signed transaction."""
-        return canonical_json_hash(self.to_dict())
+        if self._cached_tx_id is None:
+            self._cached_tx_id = canonical_json_hash(self.to_dict())
+        return self._cached_tx_id
 
     def sign(self, signing_key: SigningKey):
-        # Validate that the signing key matches the sender
         if signing_key.verify_key.encode(encoder=HexEncoder).decode() != self.sender:
             raise ValueError("Signing key does not match sender")
-        signed = signing_key.sign(self.hash_payload)
-        self.signature = signed.signature.hex()
+        self.signature = signing_key.sign(self.hash_payload).signature.hex()
+        self._sealed = True
 
     def verify(self):
         if not self.signature:
             return False
-
         try:
-            verify_key = VerifyKey(self.sender, encoder=HexEncoder)
-            verify_key.verify(self.hash_payload, bytes.fromhex(self.signature))
-            return True
-
+            VerifyKey(self.sender, encoder=HexEncoder).verify(
+                self.hash_payload, bytes.fromhex(self.signature))
         except (BadSignatureError, CryptoError, ValueError, TypeError):
-            # Covers:
-            # - Invalid signature
-            # - Malformed public key hex
-            # - Invalid hex in signature
             return False
+        else:
+            return True
\ No newline at end of file
diff --git a/tests/test_cache.py b/tests/test_cache.py
new file mode 100644
index 0000000..0e7265e
--- /dev/null
+++ b/tests/test_cache.py
@@ -0,0 +1,78 @@
+import pytest
+from unittest.mock import patch
+from minichain.transaction import Transaction
+from nacl.signing import SigningKey
+from nacl.encoding import HexEncoder
+
+def test_tx_caching_efficiency():
+    """
+    Verifies that the expensive hashing math is only performed once 
+    and skipped on subsequent accesses (Memoization proof).
+    """
+    sk = SigningKey.generate()
+    sender_hex = sk.verify_key.encode(encoder=HexEncoder).decode()
+    tx = Transaction(sender=sender_hex, receiver="addr", amount=100, nonce=1)
+
+    # We 'patch' the hashing function to count how many times it's called
+    with patch('minichain.transaction.canonical_json_hash') as mock_hash:
+        mock_hash.return_value = "mocked_hash_value"
+
+        # 1. First Access: Should trigger the hash calculation
+        res1 = tx.tx_id
+        assert res1 == "mocked_hash_value"
+        assert mock_hash.call_count == 1
+
+        # 2. Second Access: Should return the cached value (count remains 1)
+        res2 = tx.tx_id
+        assert res2 == "mocked_hash_value"
+        assert mock_hash.call_count == 1  # <--- THIS proves the cache worked!
+
+        # 3. Comprehensive Invalidation: Changing ANY field must clear the cache
+        mutations = {
+            "sender": "new_sender_hex",
+            "receiver": "new_receiver",
+            "amount": 200,
+            "nonce": 2,
+            "data": "new_data",
+            "timestamp": 1234567890,
+            "signature": "fake_signature_hex"
+        }
+
+        expected_calls = 1
+        for field, new_value in mutations.items():
+            # Mutate the field dynamically
+            setattr(tx, field, new_value)
+            
+            # Prove the cache was instantly killed
+            assert tx._cached_tx_id is None, f"Cache failed to clear when mutating {field}"
+            
+            # Access ID again, which forces a re-calculation
+            _ = tx.tx_id
+            
+            # Prove the hashing math ran exactly one more time
+            expected_calls += 1
+            assert mock_hash.call_count == expected_calls, f"Hash did not recalculate for {field}"
+
+def test_signed_tx_is_sealed():
+    """Verifies that a signed transaction clears cache, changes ID, and cannot be modified."""
+    sk = SigningKey.generate()
+    sender_hex = sk.verify_key.encode(encoder=HexEncoder).decode()
+    tx = Transaction(sender=sender_hex, receiver="bob", amount=100, nonce=1)
+    
+    # 1. Grab the ID before signing
+    unsigned_id = tx.tx_id
+    assert tx._cached_tx_id == unsigned_id
+    
+    # 2. Sign it
+    tx.sign(sk)
+    
+    # 3. Prove signing killed the old cache
+    assert tx._cached_tx_id is None
+    
+    # 4. Prove the new ID is totally different
+    signed_id = tx.tx_id
+    assert signed_id != unsigned_id
+    
+    # 5. Prove it's locked down (Sealed)
+    with pytest.raises(AttributeError, match="Transaction is sealed"):
+        tx.amount = 500
\ No newline at end of file