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fix: validate proof of work on block acceptance #115

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799550d
fix: validate proof of work on block acceptance
Jul 4, 2026
24636cf
fix: handle invalid proof of work explicitly
Jul 4, 2026
9423173
style: remove trailing whitespace
Jul 6, 2026
add5a77
fix: validate difficulty before proof-of-work checks
Jul 6, 2026
1fed17c
merge: incorporate upstream timestamp validation and CI workflows
Jul 16, 2026
fda0c04
test: anchor _chain_with_tx timestamps to previous block instead of w…
Jul 16, 2026
472fff1
Validate serialized block hashes during deserialization
Jul 18, 2026
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2 changes: 1 addition & 1 deletion minichain/block.py
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Original file line number Diff line number Diff line change
Expand Up @@ -152,7 +152,7 @@ def from_dict(cls, payload: dict):

# Verify the block hash
expected_hash = block.compute_hash()
if block.hash is not None and block.hash != expected_hash:
if block.hash != expected_hash:
raise ValueError("block hash does not match header")

# Recalculate and verify the Merkle root!
Expand Down
57 changes: 41 additions & 16 deletions minichain/chain.py
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Original file line number Diff line number Diff line change
Expand Up @@ -11,7 +11,15 @@
logger = logging.getLogger(__name__)


def validate_block_link_and_hash(previous_block, block):
class InvalidProofOfWorkError(ValueError):
pass


def validate_difficulty(difficulty, max_difficulty):

@Zahnentferner Zahnentferner Jul 6, 2026

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Do we really need this validation function? Don't we know that we will always call this function with arguments that satisfy the conditions in line 18?

Also, some conditions in line 18 seem redundant anyway. For example, if difficulty is greater than len(block.hash), line 39 will fail anyway.

@Zahnentferner Zahnentferner Jul 6, 2026

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Unless I'm overlooking something important, let's remove this validation function.

if type(difficulty) is not int or difficulty < 1 or difficulty > max_difficulty:
raise ValueError(f"invalid difficulty {difficulty}")

def validate_block(previous_block, block, expected_difficulty):
if block.previous_hash != previous_block.hash:
raise ValueError(
f"invalid previous hash {block.previous_hash} != {previous_block.hash}"
Expand All @@ -26,17 +34,26 @@ def validate_block_link_and_hash(previous_block, block):
if block.hash != expected_hash:
raise ValueError(f"invalid hash {block.hash}")

target = "0" * (block.difficulty or 1)
if not block.hash.startswith(target):
raise ValueError(f"invalid Proof of Work: hash {block.hash} does not satisfy difficulty {block.difficulty}")
if block.difficulty != expected_difficulty:
raise ValueError(
f"invalid difficulty {block.difficulty} != expected {expected_difficulty}"
)

@Zahnentferner Zahnentferner Jul 6, 2026

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In addition to the check in line 39, we must also check that block.difficulty is equal to the expected difficulty based on previous blocks.

@Zahnentferner Zahnentferner Jul 6, 2026

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Otherwise, the miner can circumvent the difficulty check, by simply writing a small difficulty in block.difficulty.

@SIDDHANTCOOKIE SIDDHANTCOOKIE Jul 6, 2026

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@Zahnentferner you're right. When I reviewed the attack was blocked by the separate difficulty checks in add_block and resolve_conflicts in the initial commit, but the architecture was flawed which I didn't notice since the checks already protected.
The difficulty check should have been moved inside the validator at the same time as the PoW check. That would have been the proper, complete fix. I focused too narrowly on, if the PoW check now exists or not.
The validation function and related issues came in later commits.

if not block.hash.startswith("0" * expected_difficulty):
raise InvalidProofOfWorkError(
f"invalid PoW: hash {block.hash} does not satisfy difficulty {expected_difficulty}"
)

if block.timestamp <= previous_block.timestamp:
raise ValueError(f"invalid timestamp: {block.timestamp} is not strictly greater than previous block timestamp {previous_block.timestamp}")
raise ValueError(
f"invalid timestamp: {block.timestamp} is not strictly greater than previous block timestamp {previous_block.timestamp}"
)

max_allowed_time = int(time.time() * 1000) + 15000
if block.timestamp > max_allowed_time:
raise ValueError(f"invalid timestamp: {block.timestamp} is too far in the future (max allowed: {max_allowed_time})")

raise ValueError(
f"invalid timestamp: {block.timestamp} is too far in the future (max allowed: {max_allowed_time})"
)

class Blockchain:
"""
Expand Down Expand Up @@ -130,7 +147,13 @@ def get_total_work(self, chain_list=None):
if chain_list is None:
with self._lock:
chain_list = self.chain
return sum(2 ** (block.difficulty or 1) for block in chain_list)

for block in chain_list:
if not isinstance(block.hash, str):
raise ValueError(f"invalid or missing hash on block {block.index}")
validate_difficulty(block.difficulty, len(block.hash))

return sum(2 ** block.difficulty for block in chain_list)
Comment on lines +151 to +156

@coderabbitai coderabbitai Bot Jul 16, 2026

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🩺 Stability & Availability | 🟠 Major | ⚡ Quick win

Reject malformed hashes before measuring total work.

Block.from_dict() permits a missing hash, so an untrusted candidate can reach len(None). That raises TypeError, which resolve_conflicts() does not catch. Using the supplied hash length as the difficulty bound also accepts non-SHA256 lengths.

Proposed fix 
     for block in chain_list:
-        validate_difficulty(block.difficulty, len(block.hash))
+        if not isinstance(block.hash, str) or len(block.hash) != 64:
+            raise ValueError("invalid block hash")
+        validate_difficulty(block.difficulty, 64)
📝 Committable suggestion

‼️ IMPORTANT
Carefully review the code before committing. Ensure that it accurately replaces the highlighted code, contains no missing lines, and has no issues with indentation. Thoroughly test & benchmark the code to ensure it meets the requirements.

Suggested change
for block in chain_list:
validate_difficulty(block.difficulty, len(block.hash))
return sum(2 ** block.difficulty for block in chain_list)
for block in chain_list:
if not isinstance(block.hash, str) or len(block.hash) != 64:
raise ValueError("invalid block hash")
validate_difficulty(block.difficulty, 64)
return sum(2 ** block.difficulty for block in chain_list)
🤖 Prompt for AI Agents 
Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

In `@minichain/chain.py` around lines 151 - 154, Update the chain validation loop
before the total-work calculation to validate each block’s hash as a required
SHA256-length value, rejecting missing or malformed hashes before calling
validate_difficulty. Avoid using len(block.hash) as the difficulty bound; use
the established SHA256 hash-length constraint, while preserving the existing
work summation for valid chains.


def _next_difficulty(self, difficulty, avg_block_time):
"""Advance the EMA difficulty control after a block, returning the new value."""
Expand All @@ -150,16 +173,15 @@ def _apply_block(self, prev_block, block, state, difficulty, avg_block_time):
from .validators import ValidationStatus

try:
validate_block_link_and_hash(prev_block, block)
validate_block(prev_block, block, difficulty)
except InvalidProofOfWorkError as exc:
logger.warning("Block %s rejected: %s", block.index, exc)
return ValidationStatus.INVALID, difficulty, avg_block_time
except ValueError as exc:
logger.warning("Block %s rejected: %s", block.index, exc)
status = ValidationStatus.INVALID if "hash" in str(exc) else ValidationStatus.FAILED
return status, difficulty, avg_block_time

if block.difficulty != difficulty:
logger.warning("Block %s rejected: Invalid difficulty. Expected %s, got %s", block.index, difficulty, block.difficulty)
return ValidationStatus.INVALID, difficulty, avg_block_time

receipts = []
for tx in block.transactions:
status, receipt = state.validate_and_apply_with_status(tx)
Expand Down Expand Up @@ -224,9 +246,12 @@ def resolve_conflicts(self, new_chain_list) -> tuple[bool, list]:
return False, []

with self._lock:
current_work = self.get_total_work()
new_work = self.get_total_work(new_chain_list)

try:
current_work = self.get_total_work()
new_work = self.get_total_work(new_chain_list)
except ValueError as exc:
logger.warning("Reorg rejected: %s", exc)
return False, []
if new_work <= current_work:
logger.debug("Incoming chain (work: %s) is not heavier than local chain (work: %s). Rejecting.", new_work, current_work)
return False, []
Expand Down
79 changes: 46 additions & 33 deletions minichain/persistence.py
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Original file line number Diff line number Diff line change
Expand Up @@ -24,7 +24,7 @@
from typing import Any

from .block import Block
from .chain import Blockchain, validate_block_link_and_hash
from .chain import Blockchain

logger = logging.getLogger(__name__)

Expand Down Expand Up @@ -64,7 +64,13 @@ def save(blockchain: Blockchain, path: str = ".") -> None:


def load(path: str = ".") -> Blockchain:
"""Restore a Blockchain from SQLite inside *path* (with legacy JSON fallback)."""
"""Restore a Blockchain from SQLite inside *path* (with legacy JSON fallback).

Blocks are replayed through the canonical ``Blockchain._apply_block()``
pipeline so that difficulty adjustment, PoW, receipt validation, state-root
validation and transaction validation are all performed by the same code
path used at runtime. No parallel validation logic is maintained here.
"""
db_path = os.path.join(path, _DB_FILE)
legacy_path = os.path.join(path, _LEGACY_DATA_FILE)

Expand All @@ -90,6 +96,9 @@ def load(path: str = ".") -> Blockchain:
raise ValueError(f"Invalid or empty chain data in '{path}'")
if not isinstance(raw_accounts, dict):
raise ValueError(f"Invalid accounts data in '{path}'")
for address, account in raw_accounts.items():
if not isinstance(address, str) or not isinstance(account, dict):
raise ValueError(f"Invalid accounts data in '{path}'")

blocks = []
for raw_block in raw_blocks:
Expand All @@ -100,17 +109,44 @@ def load(path: str = ".") -> Blockchain:
except (KeyError, TypeError, ValueError) as exc:
raise ValueError(f"Invalid chain data in '{path}'") from exc

normalized_accounts = {}
for address, account in raw_accounts.items():
if not isinstance(address, str) or not isinstance(account, dict):
raise ValueError(f"Invalid accounts data in '{path}'")
normalized_accounts[address] = account
blockchain = Blockchain()

from .pow import calculate_hash
genesis = blocks[0]
if genesis.index != 0:
raise ValueError("Invalid genesis block")
if genesis.hash != calculate_hash(genesis.to_header_dict()):
raise ValueError("Invalid genesis block hash")
if genesis.hash != blockchain.chain[0].hash:
raise ValueError(
f"Persisted genesis hash {genesis.hash!r} does not match "
f"local genesis {blockchain.chain[0].hash!r}"
)

_verify_chain_integrity(blocks)
from .validators import ValidationStatus
from .state import State

temp_state = State()
temp_state.chain_id = blockchain.chain_id
temp_state.restore(blockchain._genesis_state_snapshot)

temp_difficulty = blocks[0].difficulty
temp_avg_block_time = blockchain.target_block_time

for i in range(1, len(blocks)):
status, temp_difficulty, temp_avg_block_time = blockchain._apply_block(
blocks[i - 1], blocks[i], temp_state, temp_difficulty, temp_avg_block_time
)
if status != ValidationStatus.VALID:
raise ValueError(
f"Block #{blocks[i].index} failed validation during load "
f"(status={status.name})"
)

blockchain = Blockchain()
blockchain.chain = blocks
blockchain.state.accounts = normalized_accounts
blockchain.state = temp_state
blockchain.current_difficulty = temp_difficulty
blockchain.avg_block_time = temp_avg_block_time

logger.info(
"Loaded %d blocks and %d accounts from '%s'",
Expand All @@ -121,29 +157,6 @@ def load(path: str = ".") -> Blockchain:
return blockchain


# ---------------------------------------------------------------------------
# Integrity verification
# ---------------------------------------------------------------------------


def _verify_chain_integrity(blocks: list[Block]) -> None:
"""Verify genesis, hash linkage, and block hashes."""
genesis = blocks[0]
if genesis.index != 0:
raise ValueError("Invalid genesis block")
from .pow import calculate_hash
if genesis.hash != calculate_hash(genesis.to_header_dict()):
raise ValueError("Invalid genesis block hash")

for i in range(1, len(blocks)):
block = blocks[i]
prev = blocks[i - 1]
try:
validate_block_link_and_hash(prev, block)
except ValueError as exc:
raise ValueError(f"Block #{block.index}: {exc}") from exc


# ---------------------------------------------------------------------------
# SQLite helpers
# ---------------------------------------------------------------------------
Expand Down
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