public bool IsValidBlock(Block NewBlock)
{
Block LastBlock = GetLastBlock();
// Before marking a block as valid we have to go through a serie of checks. Some can be quite
// hard to graps initially but they all make perfect sense once you get them. We do the cheap
// operations first since failing on those checks after a heavy operation is a waste of the
// cpu cycles.
// 1st) Check if the index is correct.
// 2nd) Check if the previous hash is my last block hash.
// 3rd) Check if the hash of the block is correct.
// 4th) Check if difficulty of new block is gte to the expected difficulty.
// 5th) Check if the block has at least one transaction (to prevent empty blocks from being
// mined).
// 6th) Check is block size does not exceed max block size.
// 7th) Check if there are only one reward and one fee transaction.
// More expensive operations:
// 8th) The transaction is not already in the blockchain.
// 9th) Check if all included transactions are valid.
// 10th) The transaction has only unspent inputs.
// 11th) The signature of the transaction is correct.
// 12th) Check if the sum of all input transactions equal the sum of all output transactions
// + block reward.
// 13th) Check if there are no double spent input transactions on the block.
// Some of these checks you can find under the transaction validation.
if (NewBlock.Index != LastBlock.Index + 1)
{
throw new BlockAssertion($"Not consecutive blocks. Expected new block index to be {LastBlock.Index + 1}, but got {NewBlock.Index}.");
}
if (NewBlock.PreviousHash != LastBlock.Hash)
{
throw new BlockAssertion($"New block points to a different block. Previous hash of new block is {NewBlock.PreviousHash}, while hash of last block is {LastBlock.Hash}.");
}
if (NewBlock.Hash != NewBlock.ToHash())
{
throw new BlockAssertion($"New blocks integrity check failed. Is {NewBlock.Hash}, should be: {NewBlock.ToHash()}");
}
if (NewBlock.GetDifficulty() >= GetDifficulty())
{
throw new BlockAssertion($"Expected the difficulty of the new block ({NewBlock.GetDifficulty()}) to be less than the current difficulty ({GetDifficulty()}).");
}
if (!NewBlock.HasTransactions())
{
throw new BlockAssertion($"New block does not have any transactions.");
}
Serializer Serializer = new Serializer();
if (Serializer.Size(NewBlock) > Config.MaximumBlockSizeInBytes)
{
throw new BlockAssertion($"New Block size (in bytes) is {Serializer.Size(NewBlock)} and the maximum is {Config.MaximumBlockSizeInBytes}.");
}
if (!NewBlock.HasRewardTransaction())
{
throw new BlockAssertion($"New block does not have a reward transaction.");
}
if (!NewBlock.GetRewardTransaction().IsRewardTransaction(Config.BlockReward) || !NewBlock.GetRewardTransaction().Verify())
{
throw new BlockAssertion($"New block does not have a valid reward transaction.");
}
// Somewhat more expensive operations
if (NewBlock.GetTransactions().Any(Tx => GetTransactionFromChain(Tx.Id) != null))
{
throw new BlockAssertion($"New block contains duplicate transactions.");
}
if (!NewBlock.GetTransactions().Filter(RTx => RTx.Type != Transaction.TransactionType.REWARD).All(Tx => Tx.Equates() && Tx.Verify()))
{
throw new BlockAssertion($"New block contains invalid transaction (inputs do not equate with outputs or signature invalid).");
}
// Double spending inputs check. Get one list of all transaction inputs of the new block and check
// if each one Input does not occur in transactions on the chain
if (GetTransactions()
.FlatMap(Tx => Tx.Inputs)
.Any(Input => NewBlock.GetTransactions().Any(Tx => Tx.ContainsInput(Input.Transaction, Input.Index))))
{
throw new BlockAssertion($"New block tries to spend already spent transaction inputs.");
}
// Todo: check if all inputs actually exist for each transaction
// Todo: check if block is not too old
return(true);
}