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UTXOs (Unspent Transaction Outputs)

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UTXOs (Unspent Transaction Outputs)

Tags: protocol, accounting, privacy

What a UTXO Is

Bitcoin has no accounts and no balances in the traditional sense. Instead, the ledger tracks Unspent Transaction Outputs (UTXOs) — discrete chunks of bitcoin that have been created by a transaction output and not yet spent.

Your “balance” is simply the sum of all UTXOs that your private keys can spend.

Source: raw/Theory/protocol/utxo/

How Transactions Work

Inputs consume existing UTXOs; outputs create new UTXOs.

Example: Alice has a 0.5 BTC UTXO and wants to send 0.3 BTC to Bob.

  1. Alice creates a transaction with her 0.5 BTC UTXO as input
  2. Output 1: 0.3 BTC → Bob’s address
  3. Output 2: 0.19999 BTC → Alice’s change address (she gets back the remainder, minus fee)
  4. The 0.5 BTC UTXO is consumed; two new UTXOs (0.3 and 0.19999) are created

The UTXO set = all currently unspent outputs. Every full node maintains this set to validate new transactions.

Why UTXOs Matter for Privacy

The UTXO model has important privacy implications:

Address reuse is bad: If Alice always uses the same address, analysts can track all her UTXOs together.

Change addresses: The change output going back to Alice reveals which of the two outputs is “hers” to analysts — defeating some privacy.

CIOH (Common Input Ownership Heuristic): Multiple inputs in one transaction likely come from the same wallet — allowing address clustering. See blockchain analysis for how this is exploited.

UTXO management: Thoughtfully combining (or not combining) UTXOs affects privacy. Poor UTXO management can link previously separate coin histories.

See privacy] for countermeasures.

Coin Control

Coin control is the ability to manually select which UTXOs to use as inputs in a transaction. Important for:

  • Avoiding mixing KYC and no-KYC coins (which reveals the no-KYC address to the KYC cluster)
  • Preserving privacy by not combining outputs from unrelated origins
  • Managing UTXO sizes (avoid dust)

Wallets with coin control: Electrum, Sparrow Wallet, Samourai.

UTXO Set and Full Node Validation

Every Bitcoin full node maintains the complete UTXO set (~5-6 GB as of 2024). When validating a new transaction, the node checks:

  • The input UTXOs exist in the UTXO set
  • The signatures are valid for those UTXOs
  • The total output ≤ total input (no new coins created)

This is Bitcoin’s trustless validation — no need to trust anyone, the math checks out.

Lightning Channels as UTXOs

A Lightning channel is a single 2-of-2 multisig UTXO on-chain. The entire channel balance lives in one UTXO; off-chain commitment transactions determine the split between parties. Closing a channel creates one or two UTXOs (one per party).

This is why Lightning is efficient: thousands of off-chain payments settle to a single on-chain UTXO pair.

Sources

Glossary | Bitcoin | privacy | self-custody | Lightning Network | SegWit | address types | blockchain analysis

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