Replay attack, also known as replay attack, is a network attack in which a malicious entity intercepts valid data and transmits it repeatedly in the network. Due to the validity of the original data (usually from an authorized user), the network's security protocols usually treat this attack as a normal data transmission. At the same time, since hackers performing replay attacks will intercept the original information and retransmit it unchanged, hackers usually do not need to crack the data.

What kind of damage can a hacker do using a replay attack?

Replay attacks can gain access to other information stored within a protected network by transmitting credentials that appear to be valid. They can also trick financial institutions into making repeat transactions, allowing attackers to withdraw funds directly from victim accounts. In some cases, hackers will combine different parts of different encrypted messages and upload the resulting ciphertext to the network. This is a so-called cut-and-paste attack. Hackers often use this type of attack to obtain more valuable information on the network and use this information to further damage the system.

Replay attacks do pose some obvious risks, but hackers who only use them have limited gains. An attacker cannot tamper with data that is being transmitted and has not been rejected by the network, so the effectiveness of this attack will be limited to retransmitting previous data. And these attacks are usually easy to defend against. Basic defenses such as adding timestamps to data transfers can prevent simple replay attacks. Servers can also cache duplicate messages and cut off connections to such messages after a certain number of times, thus limiting the number of consecutive replays an attacker can make.

Why are replay attacks critical in the cryptocurrency space?

While such attacks are not unique to the cryptocurrency world, they are particularly relevant to the security environment of cryptocurrency transactions and blockchain ledgers. Because blockchain ledgers often undergo protocol changes or upgrades called "hard forks." When a hard fork occurs, the existing ledger is split into two parts, with one part running the legacy version of the software and the other running the updated version. Some hard forks are simply for ledger upgrades, while others are for more efficient formation of new cryptocurrencies. The most obvious example of the latter effect is the hard fork that occurred on August 1, 2017, an update that allowed Bitcoin Cash to branch off the Bitcoin ledger.

When these hard forks occur, an attacker could theoretically conduct a replay attack on the blockchain ledger. Transactions processed on one ledger may be valid on another chain after the hard fork. So, someone who receives a certain amount of cryptocurrency on one ledger can switch to another ledger and copy the transaction, and fraudulently receive the same amount of cryptocurrency a second time. And because the wallet is not part of the shared history of the ledger, users who access the blockchain after the hard fork will not be vulnerable to such attacks.

How does blockchain protect against such attacks?

Yes, the vulnerability of a forked blockchain ledger to possible replay attacks deserves our attention, but most hard forks include security protocols specifically designed to prevent such attacks. Effective measures against blockchain replay attacks are divided into two categories, one is mandatory replay protection, and the other is selective replay protection. In mandatory replay protection, special marks are added to the new ledger generated by the hard fork to ensure that transactions performed on the new ledger are invalid on the old ledger, and vice versa. This method was implemented to protect Bitcoin Cash when it forked from Bitcoin.

This mandatory replay protection is automatically enforced as soon as a hard fork occurs. Selective replay protection requires users to manually change their transactions to ensure that these transactions cannot be replayed. Selective replay protection is effective when a hard fork is an update to the cryptocurrency's main ledger rather than a new ledger.

In addition to these master ledger solutions, individual users can take steps on their own to protect themselves from replay attacks. One method is to lock the tokens being transferred until the ledger has a certain number of blocks, thereby preventing network validation for replay attacks by those with the same number of tokens. However, it should be noted that not all wallets or ledgers provide this function.

Summarize

When a replay attack is successfully implemented, it does pose a threat to network security. Unlike other types of attacks, replay attacks do not rely on the decryption of data, which makes them an effective "solution" for malicious actors to combat increasingly sophisticated security protocols. But there are also some powerful solutions that can effectively prevent blockchain systems from being affected by them. In particular, the use of mandatory replay protection can effectively prevent attackers from copying transactions after a hard fork occurs.