## Introduction
Replay attack, also known as double spending attack, is one of the key challenges that blockchain technologies have faced since their inception. Double spending involves an attacker attempting to spend the same currency more than once, which is impossible in the physical world. In this article we will look at the nature of this attack, its mechanisms and methods of protecting against it.
## Nature of the double spending attack
In the physical world, it is impossible to buy a resource from one seller and then spend the same coins from another seller. However, in the digital world, especially on blockchains, an attacker can try to execute a transaction, wait for it to be approved by the merchant, and then reverse it and spend the same currency in another transaction. This is achieved by representing the conflicting transaction, possibly on a different branch of the blockchain.
## Attack mechanism
1. Creating the first transaction: The attacker initiates the first transaction and sends it to the network.
2. Waiting for confirmation: The attacker waits for the merchant or recipient to approve the transaction.
3. Creating a conflicting transaction: The attacker creates a second transaction using the same funds and sends it to the network, possibly on a different branch of the blockchain.
4. Double Spending: If the second transaction is confirmed, the attacker will successfully spend the same currency twice.
## Protection methods
### Checking spent UTXO
UTXO (Unspent Transaction Output) is the output of a transaction that has not been spent. UTXO verification allows you to ensure that the funds used in the transaction have not been spent previously. This is one of the main methods to prevent double spending.
### Using nonce
Nonce is a unique number added to each transaction. It prevents the same transaction from being used again. Every time a new transaction is created, a new nonce is generated, making it impossible to perform the same transaction again.
### BFT systems with absolute finality
Systems that use Byzantine Fault Tolerance (BFT) algorithms with absolute finality are considered robust to the double-spending problem. In such systems, transactions are considered final and cannot be canceled or changed once confirmed.
## Conclusion
Transaction replay attacks pose a serious threat to blockchain systems. However, by using UTXO, nonce verification techniques, and BFT systems with absolute finality, one can effectively defend against this attack. Blockchain technologies continue to develop, and every day more and more advanced methods of protecting against double spending appear.
## Recommendations
1. Check if the UTXO has been spent: Make sure that the funds used in the transaction have not been spent previously.
2. Use nonce: Generate unique numbers for each transaction to prevent them from being used again.
