fix: validate proof of work on block acceptance

minichain/chain.py

@@ -10,6 +10,10 @@
 logger = logging.getLogger(__name__)
 
 
+class InvalidProofOfWorkError(ValueError):
+    pass
+
+
 def validate_block_link_and_hash(previous_block, block):
     if block.previous_hash != previous_block.hash:
         raise ValueError(
@@ -25,6 +29,10 @@ def validate_block_link_and_hash(previous_block, block):
     if block.hash != expected_hash:
         raise ValueError(f"invalid hash {block.hash}")
 
+    if not block.hash.startswith("0" * block.difficulty):
+        raise InvalidProofOfWorkError(
+            f"invalid PoW: hash {block.hash} does not satisfy difficulty {block.difficulty}"
+        )
 
 class Blockchain:
     """
@@ -130,6 +138,9 @@ def add_block(self, block):
         with self._lock:
             try:
                 validate_block_link_and_hash(self.last_block, block)
+            except InvalidProofOfWorkError as exc:
+                logger.warning("Block %s rejected: %s", block.index, exc)
+                return ValidationStatus.INVALID
             except ValueError as exc:
                 logger.warning("Block %s rejected: %s", block.index, exc)
                 return ValidationStatus.INVALID if "hash" in str(exc) else ValidationStatus.FAILED

tests/test_pow_validation.py

@@ -0,0 +1,37 @@
+from minichain import Blockchain, Block
+from minichain.validators import ValidationStatus
+
+
+def _make_block_with_invalid_pow(chain):
+    block = Block(
+        index=chain.last_block.index + 1,
+        previous_hash=chain.last_block.hash,
+        transactions=[],
+        difficulty=chain.current_difficulty,
+        state_root=chain.state.state_root(),
+    )
+
+    while True:
+        block.hash = block.compute_hash()
+        if not block.hash.startswith("0" * block.difficulty):
+            return block
+        block.nonce += 1
+
+
+def test_add_block_rejects_invalid_pow():
+    chain = Blockchain()
+    block = _make_block_with_invalid_pow(chain)
+
+    assert chain.add_block(block) == ValidationStatus.INVALID
+    assert len(chain.chain) == 1
+
+
+def test_resolve_conflicts_rejects_invalid_pow():
+    chain = Blockchain()
+    block = _make_block_with_invalid_pow(chain)
+
+    success, _ = chain.resolve_conflicts([chain.chain[0], block])
+
+    assert success is False
+    assert len(chain.chain) == 1
+