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Double Spend

2 min read

Double Spend

Tags: digital cash, consensus, mining

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.

Source: raw/Start/start.md

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 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.

Sources: raw/Start/start.md, raw/Books/izobretaem-bitkoin/glava-1.md, raw/Books/izobretaem-bitkoin/glava-5.md, raw/Books/izobretaem-bitkoin/glava-6.md, raw/Books/vojna-za-razmer-bloka/glava-7.md

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

Glossary | UTXO | mining | Proof of Work | mempool | blockchain

  • bitcoin — double spend as the headline problem solved
  • utxo — inputs consumed once per valid history
  • mining — ordering and confirmation
  • byzantine-generals-problem — agreement without trust
  • forks — reorganizations and chain choice

Graph View

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