Transactions
A transaction is a signed statement: "move these inputs to these outputs". No account numbers, no balances — just cryptographic proof of authorisation.
In this lesson
- Break a transaction into inputs, outputs and signatures
- Explain UTXO and account models
- Follow the broadcast-to-confirmation journey
A signed statement
Every node validates a transaction the same way: 1) every input carries a valid signature from its owner, 2) the inputs exist and are unspent, 3) inputs ≥ outputs (the difference is fee/change), 4) no double-spend already in the chain. No identity check, no reversal button.
- Transactions spend unspent outputs like cash notes
- Parallel-friendly, simple double-spend check
- Privacy is better by default
- Transactions debit/credit balances like a bank ledger
- Smart-contract friendly, stateful
- Easier balances, harder history hygiene
Example · Paying a café
You scan a QR code; your wallet builds a transaction spending a 0.015 BTC output you received earlier, sending 0.01 to the café and 0.0049 back to yourself as change, paying 0.0001 in fee. Signed with your key, gossiped, and picked up by a miner within seconds.
{\n "inputs": [{ "txid": "9f2b...", "vout": 0, "scriptSig": "3045...0221..." }],\n "outputs": [\n { "value": 0.01000000, "address": "bc1q..." },\n { "value": 0.00490000, "address": "bc1qm... (change)" }\n ]\n}Fees are the market
Fees compete for block space: when demand spikes, users attach higher fees and miners prioritise them. This is the mechanism that keeps spam expensive.
Transactions are final by design
There is no chargeback. Sending to the wrong address, signing a malicious contract approval, or falling for address-poisoning malware all result in permanent loss. Verify the first and last characters of addresses — or use hardware-wallet verification.
Sign and broadcast
Build a transaction in the lab, sign it, then corrupt the signature and watch every node reject it.
How fast is confirmation?
First inclusion is typically seconds to a minute; "safe" depth depends on the network and the value at stake.
Can transactions be censored?
Individual transactions can be delayed by fee pressure or miner policy; protocol-level censorship resistance is a decentralisation metric, not a guarantee.
What reveals my identity?
The chain never sees names — but exchanges, reuse of addresses and off-chain metadata can deanonymise you.