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Lesson 05 / 21 · Advanced

Signatures and what they authorize

A valid signature is only as meaningful as the message it covers.

14 MIN WITH PRACTICEREAD → TRY → REFLECTNO WALLET NEEDED

By the end, you’ll be able to…

  • Distinguish authentication from informed authorization.
  • Explain why transaction details matter to a signer.
Your learning map

Three questions to carry into this lesson.

01Distinguish authentication from informed authorization.
02Explain why transaction details matter to a signer.
Use these goals to guide your reading. Try the paper exercise, then explain the result in your own words.

A signature binds a key to a message

Bitcoin uses digital signatures to authorize spending under applicable script rules. BIP 340 specifies Schnorr signatures used in Taproot contexts; earlier output types use other established rules. A valid signature establishes a mathematical relation among a message, signature and public key. It does not establish that a human understood the transaction shown by an interface.

The covered data matters

Transaction signature rules determine which parts are committed to. Different signature-hash modes have different meanings. A signer must therefore inspect the actual transaction and intended policy rather than assuming that every signature authorizes exactly one intuitive payment. A request described as verification can still deserve scrutiny if its content is unclear.

Make the display useful

For a fictional hardware-signing exercise, show the recipient outputs, change recognition, network and fee before approval. If a device cannot meaningfully display what matters, that is a limitation to address, not an invitation to click through. A signature workflow should support refusal and an independent review route. This course never asks for a real signature; use a written transaction summary to practice explaining what would be authorized.

Your turn / A paper experiment

Practice on paper

A signer sees a 20,000-sat recipient payment but the proposed transaction also has an unfamiliar second external output. What is the right review question?

I’ve tried it — show the worked answer

Ask what every output represents and whether change is correctly identified. A correct first output does not establish that the whole transaction matches the intended action.

Want to explore with buttons and instant feedback? Try the practice lab ↗

Think it through

Make a choice. Discover why.

Choose an answer and check the explanation. You can retry as often as you like. These are practice questions, not a test of mastery; answers are not saved or sent.

1. Does cryptographic validity prove informed human consent?
  • Yes
  • No
Read the explanation

No. The human may have been shown misleading information.

2. Should unfamiliar signature modes be ignored because the wallet recognizes them?
  • Yes
  • No
Read the explanation

No. Their commitments need to match the intended authorization.

One idea to take with you

Review the message and spending effect, not merely the signature prompt.

Your learning, at your pace

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