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we saw, the consensus mechanism depends on having a majority of the miners acting honestly out of self-interest. However, if a miner or group of miners can achieve a significant share of the mining power, they can attack the consensus mechanism so as to disrupt the security and availability of the bitcoin network. It is important to note that consensus attacks can only affect future consensus, or at best the most recent past (tens of blocks). Bitcoin’s ledger becomes more and more immut‐ able as time passes. Beyond a certain “depth”, blocks are absolutely immutable, even under a sustained consensus attack that causes a fork. Consensus attacks also do not affect the security of the private keys and signing algorithm (ECDSA). A consensus attack cannot steal bitcoins, spend bitcoins without signatures, redirect bitcoins or otherwise change past transactions or ownership records. Consensus attacks can only affect the most recent blocks and cause denial-of-service disruptions on the creation of future blocks. One attack scenario against the consensus mechanism is called the “51% attack”. In this scenario a group of miners, controlling a majority (51%) of the total network’s hashing power, collude to attack bitcoin. With the ability to mine the majority of the blocks, the attacking miners can cause deliberate “forks” in the blockchain and double-spend transactions or execute denial-of-service attacks against specific transactions or ad‐ dresses. A fork/double-spend attack is one where the attacker causes previously con‐ firmed blocks to be invalidated by forking below them and re-converging on an alternate chain. With sufficient power, an attacker can invalidate six or more blocks in a row, causing transactions that were considered immutable (6 confirmations) to be invalida‐ ted. Note that a double-spend can only be done on the attacker’s own transactions, for which the attacker can produce a valid signature. Double-spending one’s own transac‐ tions is profitable if by invalidating a transaction the attacker can get a non-reversible exchange payment or product without paying for it. Let’s examine a practical example of a 51% attack. In the first chapter we looked at a transaction between Alice and Bob for a cup of coffee. Bob, the cafe owner, is willing to accept payment for cups of coffee without waiting for confirmation (mining in a block), because the risk of a double-spend on a cup of coffee is low in comparison to the con‐ venience of rapid customer service. This is similar to the practice of coffee shops that accept credit card payments without a signature for amounts below $25, as the risk of a credit-card chargeback is low while the cost of delaying the transaction to obtain a signature is comparatively larger. In contrast, selling a more expensive item for bitcoin runs the risk of a double-spend attack, where the buyer broadcasts a competing trans‐ action that spends the same inputs (UTXO) and cancels the payment to the merchant. A double-spend attack can happen in two ways: either before a transaction is confirmed, or if the attacker takes advantage of a blockchain fork to undo several blocks. A 51% attack allows an attacker to double-spend their own transactions in the new chain, thus undoing the corresponding transaction in the old chain. Consensus Attacks | 215PDF Image | Mastering Bitcoin
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