[bitcoin-dev] Defending against empty or near empty blocks from malicious miner takeover?

Nick ODell nickodell at gmail.com
Fri Mar 24 17:29:54 UTC 2017

Two concerns:

1) This makes block validity depend on things that aren't in the
blockchain. If you and I have different minrelaytxfee's, we will have
different mempool sizes. Your node will discard a block, but my node will
think it is valid, and our nodes will not come to consensus.

2) This is trivially bypassed. A miner can take some coins they own
already, and create a zero-value transaction that has a scriptPubKey of
OP_1. (i.e. anyone-can-spend.) Then, they create another transaction
spending the first transaction, with an empty scriptSig, and the same
scriptPubKey. They do this over and over until they fill up the block.

The last OP_1 output can be left for the next miner. Since the above
algorithm is deterministic, a merkle tree containing every transaction
except the coinbase can be precomputed. The 'malicious' miners do not need
to store this fake block.

On Fri, Mar 24, 2017 at 10:03 AM, CANNON via bitcoin-dev <
bitcoin-dev at lists.linuxfoundation.org> wrote:

> Hash: SHA512
> When the original white paper was written the idea was that nodes
> would be miners at same time. That the distribution of mining power
> being mostly on par with the distribution of nodes if I understand
> correctly. The problem we face now I fear, is the mining power
> becoming centralized. Even if every bitcoin node invested a $1000
> into mining power and mined at a loss, it still would not even
> make a dent in hash distribution. Currently there are around 6000
> known nodes. If each node invested $1000 for say 10 ths of hashing
> power. At current hashrate of around 3,674,473,142 GH/s this would
> only make up %16 of hash power. This is out of balance as while
> nodes are distributed mining power is becoming very centralized
> due to the creation of monopolization of ASICs. The problem we
> are facing is a small group of a couple people whom control a
> large amount and growing of hash power. At time of this writing
> it has quickly risen to 39% and at current rate will soon become
> 50% of hashing power that is controlled by a small group of a few
> people. Their intentions are too hijack the bitcoin network to a
> cryptocurrency that suits their dangerous agenda. Dangerous because
> their plan would centralize power of consensus as I understand it,
> to themselves the miners. Dangerous also because the code base of
> the attempting subverters is buggy, insecure, and reckless from a
> technological standpoint. Even though they only have very minute
> amount of nodes compared to legitimate bitcion nodes, the danger
> is that they are very quickly taking over in mining power. While
> it is true that nodes will not accept invalid blocks that would be
> attempted to be pushed by the conspirators, they are threatening to
> attack the valid (or in their words, "minority chain") by dedicating
> some mining power soley to attacking the valid chain by mining empty
> blocks and orphaning the valid chain. So even though the majority
> of nodes would be enforcing what blocks are valid and as a result
> block the non-compliant longer chain, the conspiring miner can simply
> (as they are currently threatening to) make the valid chain unuseable
> by mining empty blocks.
> If a malicious miner with half or majority control passes invalid
> blocks, the worst case scenario is a hardfork coin split in which
> the non-compliant chain becomes an alt. However the problem is that
> this malicious miner is very recently threatening to not just simply
> fork, but to kill the valid chain to force economic activity to the
> adversary controlled chain. If we can simply defend against attacks
> to the valid chain, we can prevent the valid chain from dying.
> While empty or near empty blocks would generally be protected by
> the incentive of miners to make money. The threat is there if the
> malicious miner with majority control is willing to lose out on
> these transaction fees and block reward if their intention is to
> suppress it to force the majority onto their chain.
> Proposal for potential solution Update nodes to ignore empty blocks,
> so this way mined empty blocks cannot be used to DOS attack the
> blockchain. But what about defense from say, blocks that are
> not empty but intentionally only have a couple transactions
> in it? Possible to have nodes not only ignore empty blocks,
> but also blocks that are abnormally small compared to number of
> valid transactions in mempool?
> For example would be something like this:
> If block = (empty OR  <%75 of mempool) THEN discard
> This threshold just an example.
> What would be any potentials risks
> and attacks resulting from both having such new rulesets and not
> doing anything?
> Lets assume that the first problem of blocking empty or near empty
> blocks has been mitigated with the above proposed solution. How
> likely and possible would it be for a malicious miner with lots of
> mining power to orphan the chain after so many blocks even with
> non empty blocks? Is there a need to mitigate this?
> If so is it possible?
> Time is running short I fear. There needs to be discussion on various
> attacks and how they can be guarded against along with various
> other contingency plans.
> - --
> Cannon
> PGP Fingerprint: 2BB5 15CD 66E7 4E28 45DC 6494 A5A2 2879 3F06 E832
> Email: cannon at cannon-ciota.info
> If this matters to you, use PGP.
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