Module 6: What a Bitcoin wallet actually does

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6.1 Introduction

• A bank account
• A file storage system
• A record of balances

• Manage private keys securely
• Derive public keys and addresses
• Detect UTXOs belonging to those keys
• Build and sign transactions
• Monitor the blockchain for payments and confirmations

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6.2 Core responsibilities of a Bitcoin wallet

1. Private Key Management
   • Securely generate private keys (often via a mnemonic seed phrase).
   • Encrypt private keys at rest.
   • Make private keys available only when signing transactions.
   • Never leak private keys to the network, servers, or logs.
   Critical: If the wallet fails at key management, Bitcoin is lost.
2. Address Derivation and Management
   • Use private keys to derive public keys.
   • Create Bitcoin addresses based on public keys (Legacy, SegWit, Taproot).
   • Generate new receiving addresses automatically.
   • Rotate addresses after use to enhance privacy.
   Wallets must expose addresses, but keep private keys hidden.
3. UTXO Tracking
   • Monitor the Bitcoin blockchain (directly or via APIs like Bitnob).
   • Identify which UTXOs belong to the wallet’s addresses.
   • Update wallet balance by summing up all unspent UTXOs.
   • Mark UTXOs as spent when building new transactions.
   Key point: There is no balance stored on the blockchain, the wallet calculates it by tracking UTXOs.
4. Transaction Construction and Signing
   • Select UTXOs as inputs when sending bitcoin.
   • Create transaction outputs (payment + change).
   • Calculate and apply network fees.
   • Sign the transaction locally using the private key.
   • Ensure the transaction is properly structured and valid.
   Building and signing transactions is the heart of the wallet’s sending capability.
5. Broadcasting and Monitoring Transactions
   • Broadcast signed transactions to the Bitcoin network.
   • Track transaction status:
     • Unconfirmed (in mempool)
     • Confirmed (in blocks)
     • Replaced (if RBF used)
   • Update UI to reflect confirmations and finality.
   • Handle dropped or reorganized transactions gracefully.
   Wallets must connect users to the live Bitcoin network reliably but securely.

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6.3 Important constraints

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6.4 Flowchart: What a Bitcoin wallet really does

User Requests Action (e.g., View Balance, Send)

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Wallet Derives Necessary Keys and Addresses (from Seed)

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Wallet Monitors Blockchain (via Bitnob RPC or node)

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Wallet Tracks UTXOs Owned by User

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User Initiates Send Bitcoin

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Wallet Selects UTXOs, Builds Transaction, Signs Locally (with Private Key)

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Wallet Broadcasts Signed Transaction to Network

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Wallet Monitors Transaction for Confirmations & Updates UI

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6.5 Product implications for PMs

• Onboarding must center around seed backup, not just “account creation.”
• Balances must update in real time based on UTXOs detected.
• Transaction history must reflect mempool status, confirmation depth, and reorg risks.
• Backup, security, privacy must be treated as first-class product features.

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6.6 Real world example

1. His device generates a 12-word seed phrase.
2. The app derives his master private key from the seed.
3. The app derives his first few Bitcoin addresses.
4. The app registers these addresses for monitoring.

1. The app detects UTXOs at those addresses.
2. Adds the UTXOs to his wallet.
3. Calculates his “balance” by summing UTXO values.

1. The app selects his UTXOs.
2. Builds a transaction.
3. Signs it using his private key.
4. Broadcasts it to the Bitcoin network.
5. Monitors confirmations.

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6.7 Common misconceptions to avoid

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6.8 Summary of module 6

• Securely manage private keys.
• Derive and rotate public addresses.
• Track UTXOs on the Bitcoin network.
• Build, sign, and broadcast transactions.
• Monitor blockchain confirmations.

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Module 6 complete