Change Addresses and Privacy

• •Definition: A new address belonging to you where leftover funds are sent.
• •Purpose: Return excess value when spending UTXOs (since you must spend them entirely).
• •Privacy: Using a new change address prevents linking your transactions together.
• •Automatic: Good wallets automatically generate and use new change addresses.
• •Ownership: Change addresses are yours—derived from your seed phrase like any other address.

• •Change output: The output in a transaction that returns funds to the sender.
• •Address reuse: Using the same address multiple times (bad for privacy).
• •HD wallet: Hierarchical Deterministic wallet that generates new addresses from one seed.
• •Derivation path: The mathematical path used to generate each new address.
• •Gap limit: How many unused addresses a wallet checks when scanning the blockchain.
• •Coin control: Advanced feature letting you manually select which UTXOs to spend.

Transaction:

• •Input: Alice's 0.05 BTC UTXO + signature
• •Output 1: 0.01 BTC → Pizza shop (payment)
• •Output 2: 0.0399 BTC → Alice's new change address
• •Fee: 0.0001 BTC (implicit: 0.05 - 0.01 - 0.0399)

Result:

• •Alice's old UTXO (0.05 BTC) is spent
• •Pizza shop gets a new UTXO (0.01 BTC)
• •Alice gets a new UTXO (0.0399 BTC) at a fresh address
• •Miner collects 0.0001 BTC fee

Bad: Reusing same address for change

• •Alice pays Bob from address A → change back to address A
• •Alice pays Carol from address A → change back to address A
• •Now everyone can see all of Alice's transactions are linked
• •Balance is visible to anyone Alice transacts with

Good: New change address each time

• •Alice pays Bob from address A → change to new address B
• •Alice pays Carol from address B → change to new address C
• •Harder for observers to link Alice's transactions
• •Balance privacy improved

• •One seed phrase: Generates unlimited addresses
• •Deterministic: Same seed always generates same addresses in same order
• •Two chains: External (receiving) and internal (change) address chains
• •Derivation path: m/84'/0'/0'/0/5 (5th external address in first account)
• •Gap limit: Wallet scans ahead ~20 unused addresses to find all funds

Heuristics:

• •Round numbers: Payment (0.01 BTC) vs change (0.0399 BTC) → change is non-round
• •Address type: If sender uses SegWit but one output is legacy, that's probably payment
• •Output position: Some wallets always put change in position 1
• •Future spending: The output spent soon after is likely change (still in sender's control)

Mitigation:

• •Randomize output order
• •Use same address type for all outputs
• •CoinJoin (mix transactions with others)
• •Avoid round payment amounts when possible

• •Why: Preserve privacy by not linking certain UTXOs
• •Example: Don't mix KYC exchange coins with private CoinJoin coins
• •Labeling: Tag UTXOs by source (exchange, peer-to-peer, mining, etc.)
• •Consolidation: Choose when to combine small UTXOs (low fee times)