A simplified version of the Bitcoin Consensus Algorithm is shown below: 1. New transactions are broadcast to all nodes 2. Each node collects new transactions into a block 3. In each round a random node gets to broadcast its block 4. Other nodes accept the block only if all transactions in it are valid (unspent, valid signatures) 5. Nodes express their acceptance of the block by including its hash in the next block they create Using this algorithm, can a user such as Alice, who really dislikes Bob, deny service to Bob by not including Bob’s transactions in any block that she proposes to get onto the blockchain?

Question

A simplified version of the Bitcoin Consensus Algorithm is shown below: 1. New transactions are broadcast to all nodes 2. Each node collects new transactions into a block 3. In each round a random ​node gets to broadcast its block 4. Other nodes accept the block only if all transactions in it are valid (unspent, valid signatures) 5. Nodes express their acceptance of the block by including its hash in the next block they create Using this algorithm, can a user such as Alice, who really dislikes Bob, deny service to Bob by not including Bob’s transactions in any block that she proposes to get onto the blockchain?

BlackTom AI · Accepted Answer

First, restating the scenario helps set the context: In this simplified Bitcoin-like protocol, transactions are broadcast to all nodes, blocks are proposed at random, and blocks are accepted only if all transactions are valid. Nodes indicate acceptance by including the block's hash in their next block. Now, let's evaluate each answer choice in turn.

Option 1: 'If Bob’s transaction doesn’t make it into the next block that Alice proposes, he will just wait until an honest node gets the chance to generate a block and then his transaction will get into that block.' This is plausible because, in a distributed consensus with random block airtime, any valid transaction will eventually be included by honest miners/nodes when they create a block. Provided Bob’s transaction remains valid (unspent, proper signatures), nothing about Alice’s proposal prevents it from appearing in a future block. So this statement aligns with the idea that denial by one participant does not permanently remove a valid transaction from the network.

Option 2: 'Alice cannot stop Bob from broadcasting to other nodes.' This is true in a decentralized network: Bob can always broadcast his transaction to the network, and other nodes will propagate it. Even if Alice tries to block or ignore Bob’s transaction in her own block proposals, Bob can still broadcast and have his transaction considered by other honest nodes when assembling their blocks. Hence, Alice’s actions do not prevent Bob from broadcasting to the wider network.

Option 3: 'Alice will be able to make all other nodes deny Bob's transaction.' This is incorrect. In a decentralized system, one participant’s block proposals do not have unilateral authority to compel all other nodes to deny a transaction. Valid transactions propagate through the network via peer-to-peer broadcasting, and other nodes independently decide which transactions to include in their blocks. No single actor can force universal denial simply by excluding a transaction from their own block proposals.

Option 4: 'Yes, Alice can deny Bob’s transaction.' This is misleading and inaccurate for a similar reason to Option 3. While Alice could attempt to omit Bob’s transaction from blocks she proposes, she cannot cause every other node to deny it or guarantee its exclusion across the network. The protocol relies on collective block creation by potentially many honest nodes, so denial would not be guaranteed or final and may be countered by other nodes including Bob’s transaction in their blocks.

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