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Double Spend

2 min read

The problem

Double spending means using the same money twice. Physical cash is hard to spend twice because you hand over a token once. Digital information is easy to copy — before Bitcoin, the usual fix was a trusted ledger keeper (bank, card network) that checked balances and ordering.

The beginner guide: digital money “like a computer file” could be copied; banks prevented double spends by tracking accounts.

Bitcoin’s approach

Bitcoin makes ownership transfers public (pseudonymous addresses, not necessarily real names) and orders them in a single agreed history secured by Proof of Work and full-node validation.

Satoshi’s forum post (quoted in [[en/books/inventing-bitcoin|Inventing Bitcoin]]): the P2P network acts like a distributed timestamp server, marking the first spend of a coin; information is easy to spread and hard to suppress.

After confirmation: blocks bury spends under PoW; rewriting history requires redoing that work — prohibitively expensive deep in the chain.

51% context: The sources stress that miners cannot spend others’ coins without valid signatures; with majority hash power, an attacker’s realistic leverage is reordering or censoring their own spends — e.g. double-spending by building a private chain that later replaces the public one — not forging arbitrary balances.

Unconfirmed transactions

While a transaction is only in the mempool, acceptance is probabilistic — the recipient bears zero-confirmation risk. More confirmations reduce the probability of a chain reorganization undoing the payment.

Sources

Graph View

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