Transaction Inputs

9 min readarticleIncludes quiz · 4 questions

Transaction inputs are where your Bitcoin comes from. Each input references a previous UTXO you own and includes a signature proving you have the right to spend it. Think of inputs as showing your "receipts" of where your money came from.

What is a Transaction Input?

  • •Definition: A reference to a previous unspent output (UTXO) that you're spending.
  • •Components: Previous transaction ID, output index, signature script (unlocking script).
  • •Proof of ownership: Contains your digital signature proving you own the private key.
  • •Multiple inputs: Transactions can have many inputs to combine smaller amounts.
  • •Must be unspent: You can only spend UTXOs that haven't been spent yet (no double-spending).
Transaction Input Structure
Transaction Input Structure

Key Definitions:

  • •Outpoint: The reference to a previous output (txid + index).
  • •Previous txid: The transaction ID where the UTXO was created.
  • •Output index (vout): Which output of that transaction (outputs are numbered starting from 0).
  • •scriptSig: The unlocking script containing your signature and public key.
  • •Witness: In SegWit transactions, signature data moved here (separate from base transaction).
  • •Sequence: A field for advanced features like Replace-By-Fee and timelocks.

Input Structure:

``` Input: Previous tx: 7f8a9b3c... (txid) Output index: 0 scriptSig: <signature> <public key> Sequence: 0xffffffff ```

This says: "I'm spending output #0 from transaction 7f8a9b3c..., and here's my signature to prove it."

Multiple Inputs Example:

Alice has:

She wants to send 0.6 BTC to Bob. Her wallet combines:

Inputs:

  • •Input 1: Reference to UTXO 1 (0.3 BTC) + signature
  • •Input 2: Reference to UTXO 2 (0.25 BTC) + signature

Total input value: 0.55 BTC (not enough!)

She needs to add:

  • •Input 3: Reference to UTXO 3 (0.15 BTC) + signature

Total input value: 0.70 BTC ✓ Output to Bob: 0.6 BTC Change back: 0.0999 BTC Fee: 0.0001 BTC

Multiple Inputs
Multiple Inputs

How Inputs Are Validated:

1. Check UTXO exists: Node looks up the previous transaction and output index. 2. Verify unspent: Ensure this UTXO hasn't been spent already. 3. Check signature: Verify the signature matches the public key and transaction data. 4. Match public key: Confirm the public key hashes to the address that locked the UTXO. 5. Sum values: Ensure total inputs ≥ total outputs + fee.

Coin Selection (How Wallets Choose Inputs):

Strategies:

  • •Largest first: Use biggest UTXOs to minimize number of inputs (smaller transaction).
  • •Smallest first: Use smallest UTXOs to consolidate dust.
  • •Privacy-focused: Avoid linking UTXOs from different sources.
  • •Fee-optimized: Balance between transaction size and future fee savings.

Trade-offs:

  • •More inputs = larger transaction = higher fees
  • •Fewer inputs = better, but might not have enough value
  • •Consolidating dust = good when fees are low

Input Locking Scripts:

UTXOs are locked with scripts that define spending conditions:

  • •P2PKH (Pay-to-Public-Key-Hash): Locked to a Bitcoin address. Requires signature + public key.
  • •P2WPKH (SegWit): Similar to P2PKH but witness data is separate.
  • •P2SH (Pay-to-Script-Hash): Locked to a script hash (e.g., multisig).
  • •P2TR (Taproot): Most flexible, privacy-preserving scripts.

Inputs must provide the correct "key" (scriptSig or witness) to unlock each type.

Test Your Knowledge

This lesson includes a 4-question quiz (passing score: 75%).

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