Replay Attack Alarm Grows as BIP-110 Backers Brand Bitcoin 'Spamcoin'

The issue matters because Luke Dashjr and other BIP-110 supporters are discussing a hard fork that would replace the minority chain’s SHA-256d mining algorithm with BLAKE2b. Such a move would permanently create a whole new design, but the two networks would still share their transaction history from before the split.

What Exactly Is a Replay Attack?

Imagine someone holds 1 bitcoin before a hard fork. When the blockchain divides, the same historical unspent transaction output, or UTXO, exists on both networks. In practical terms, the owner controls corresponding coins on each chain with the same private key.

Trouble begins if both networks also recognize the same transaction and signature rules. Suppose the owner sends the coin to an exchange on Chain A. If that signed transaction is also valid on Chain B, another party can copy it and broadcast it there. Chain B may accept the transaction because, cryptographically, nothing distinguishes the authorization from one intended for its network.

That is a replay. Nobody steals the private key or breaks Bitcoin’s cryptography. The problem is simpler: the user created one valid authorization, but two blockchains recognize it. And while the BIP-110 chain sits at block height 961636, people spending BTC are moving BIP-110 coins, or whatever they will be called going forward, at the same time, without even knowing.