Yes, it is theoretically possible to implement a double payment attack by manipulating the blockchain consensus mechanism, but the actual implementation needs to meet specific technical conditions, and the cost and risk are higher. The following is a step-by-step analysis of the key points： --- ### **1. The principle of double payment attack** The core of a double payment attack is to allow the same digital asset to be reused on the blockchain. The attacker destroys the immutability of the blockchain by creating **two conflicting transactions** (for example, paying the same amount of money to two different recipients) and trying to get both transactions confirmed by the network. --- ### **2. The relationship between attack method and consensus mechanism** The success of double payment depends on the consensus mechanism adopted by the blockchain： #### **a. Proof of work (PoW, such as Bitcoin)** -**51% attack**: If an attacker controls more than 50% of the computing power, he can temporarily monopolize the right to generate blocks and overwrite the original transactions through **Reorganization of the blockchain (Reorg)**. -**Implementation steps**： 1. The attacker sends transaction A (purchase of goods) to the merchant and waits for the transaction to be confirmed. 2. At the same time, a forked chain is secretly mined, containing another conflicting transaction B (transferring the same amount of money back to yourself). 3. When the length of the forked chain exceeds the main chain, the whole network accepts the forked chain as a valid chain, transaction A is rolled back, and transaction B takes effect. -**Difficulty**: It requires a huge cost of computing power, and the attack time window is limited (Bitcoin needs to be completed before 6 blocks are confirmed). #### **b. Proof of stake (PoS, such as Ethereum 2.0)** -**Long-Range Attack (Long-Range Attack)**: The attacker fakes a longer old chain by controlling the private key of the historical verifier. -**Implementation steps**： 1. The attacker buys or steals the private key of the old verification node. 2. A forked chain is reconstructed from the old block, including conflicting transactions. 3. Use the pledged equity to make the forked chain accepted. -**Difficulty**: It is necessary to control a large number of historical rights and interests, and modern PoS protocols (such as Ethereum's “final certainty”) restrict such attacks through a checkpoint mechanism. #### **c. Other consensus mechanisms (such as DPoS, PBFT)** -In consensus mechanisms with a small number of nodes (such as DPoS), an attacker may tamper with transactions by bribing or invading key verification nodes. -**Case **: EOS has been questioned about its security due to the centralization of 21 super nodes. --- ### **3. Feasibility of actual attack** -**Cost and benefit trade-off**： - A PoW attack requires millions of dollars in hardware and energy costs, and the price of tokens may plummet after the attack, and the gains outweigh the losses. -PoS attacks require a large number of tokens to be pledged (which may be confiscated), and they take a long time to lurk. -**Defense mechanism**： -**Transaction confirmation delay**: Such as Bitcoin's 6-block confirmation rule. -**Checkpoints**: Prevent old blocks from being tampered with. -**Network monitoring**: Detect abnormal forks or concentration of computing power. --- ### **4. Historical case** -**Bitcoin Gold (BTG) 51% attack** (2018): The attacker reorganized the blockchain by renting computing power and stole more than 18 million U.S. dollars. -**Multiple attacks on Ethereum Classic (ETC)**: From 2020 to 2021, it was repeatedly attacked by 51%, causing the exchange to suspend deposits and withdrawals. --- ### **5. Defense advice** -**Enterprise/user level**： -Wait for more block confirmations for large transactions. - Choose a blockchain with high computing power/high dispersion. -**Protocol design level**： -Adopt hybrid consensus (such as PoW+PoS). -Introduce punishment mechanisms (such as Slashing in PoS). --- ###**Conclusion** Double-payment attacks are theoretically and technically feasible, but are limited by resource costs, protocol defenses, and economic consequences after the attack. For mainstream blockchains (such as Bitcoin and Ethereum), the cost of attacks is extremely high and impractical; however, for small or less secure chains, the risks are significant. Therefore, choosing a blockchain with strong security and following best practices is the key to defending against such attacks.