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UTXOs (Unspent Transaction Outputs)

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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: UTXO — Part 1

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.

Source: UTXO — Part 2

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 countermeasures for how to address these risks.

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.

Source: UTXO — Part 3

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.

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