A Finney attack is a type of blockchain attack named after its creator, Hal Finney, one of the early contributors to Bitcoin. This attack is specific to blockchain networks that use proof-of-work consensus mechanisms, such as Bitcoin. A Finney attack exploits a time delay between the broadcast of a transaction and its inclusion in a block to profit from the attack.
Here’s how a Finney attack typically works:
The Attacker’s Steps
- Step 1: Preparation: The attacker first acquires a significant amount of cryptocurrency, typically Bitcoin, and sets up a miner or mining pool.
- Step 2: Send Funds: The attacker initiates a transaction to send their cryptocurrency to a recipient. This transaction is not immediately broadcast to the network.
- Step 3: Mine a Block: The attacker begins mining a new block privately, which contains the transaction from Step 2. This private mining process is hidden from the network and not broadcast.
- Step 4: Wait for a Victim: The attacker waits for a victim to perform a specific action. For example, the victim might make a large purchase that requires confirmation on the blockchain.
- Step 5: Release the Block: Once the victim’s transaction is broadcast to the network and included in the blockchain, the attacker releases their privately mined block, which contains the transaction from Step 2. This block is added to the blockchain before any other miners can compete because the attacker has already mined it privately.
- Step 6: Profit: The attacker’s transaction is confirmed on the blockchain after the victim’s transaction but before other miners can include competing blocks. This allows the attacker to double-spend their cryptocurrency, sending it to another address while still having the original funds.
Impact on the Victim
The victim, who received the cryptocurrency from the attacker’s initial transaction, believes the transaction is valid and irreversible. However, after the attacker releases their secretly mined block, the victim’s transaction is reversed, and they lose the funds they received.
Finney attacks rely on the fact that miners have control over the order in which transactions are included in blocks, and they can prioritize their own transactions. These attacks are generally considered less practical and profitable than other types of attacks, such as 51% attacks or double-spending attacks. Moreover, as blockchain networks and security mechanisms have evolved, the window of opportunity for Finney attacks has diminished.
Finney attacks are less common and less practical than other types of attacks, but they can still be a concern. Here are some remediation strategies to mitigate Finney attacks:
- Use More Confirmations: One of the most effective ways to mitigate Finney attacks is to wait for a higher number of block confirmations before considering a transaction as final. In Bitcoin, it is common to wait for at least six confirmations. The more confirmations you wait for, the more secure the transaction becomes, as the cost and difficulty of executing a Finney attack increase with each confirmation.
- Payment Protocols: Implement payment protocols or technologies like the Lightning Network (in the case of Bitcoin) for smaller, faster transactions. These off-chain solutions can provide faster confirmations and reduce the risk of Finney attacks for everyday transactions while relying on the underlying blockchain for security.
- Risk Assessment: Assess the risk associated with each transaction based on the value involved. For small-value transactions, waiting for one or two confirmations may be sufficient. For high-value transactions, a more extended confirmation period is advisable.
- Monitoring and Alert Systems: Implement real-time monitoring and alert systems to detect unusual or suspicious activity on the blockchain. This can help identify potential Finney attacks early and allow for immediate action.
- Multisignature Wallets: Use multisignature wallets, which require multiple private keys to authorize a transaction. This can add an extra layer of security by making it more difficult for an attacker to initiate a Finney attack.
- Transaction Replace-by-Fee (RBF): Enable RBF for your transactions if supported by your blockchain. RBF allows you to replace an unconfirmed transaction with a new one that pays a higher fee, making it less likely for the original transaction to be included in a block.