Check yourself: Sign without hot keys

Questions and answers, readable without JavaScript.

1. What is the point of a BIP 174 PSBT?

   • To keep keys on the same machine that broadcasts
   • So the online side can build and broadcast while the offline side signs, using a packet that carries what the signer needs
   • To hide the destination from the signer
   • To replace addresses with invoices
   Answer 1

   So the online side can build and broadcast while the offline side signs, using a packet that carries what the signer needs

   The online side watches, constructs, and later broadcasts. The offline side signs. The signer should need only what is inside the PSBT, not a live chain connection.

   Read the note → · PSBT and Air-Gapped Signing: Keep Keys Offline While the Network Watches

2. What must a Signer do with UTXO data, in BIP 174's role?

   • Fetch missing chain data from a side channel the host controls
   • Trust the host's displayed amount
   • Skip witness data for SegWit inputs
   • Use only the UTXOs provided in the PSBT, and check prevout hashes and script commitments before signing
   Answer 2

   Use only the UTXOs provided in the PSBT, and check prevout hashes and script commitments before signing

   The Signer accepts only a PSBT, uses only the UTXOs in it, and verifies commitments before signing. It must not invent missing chain data from a side channel the host controls.

   Read the note → · PSBT and Air-Gapped Signing: Keep Keys Offline While the Network Watches

3. If the offline device and the host show different destinations or amounts, what does the note say to do?

   • Stop
   • Prefer the host, which built the transaction
   • Average the two amounts
   • Re-seed the device from the online machine
   Answer 3

   Stop

   Clipboard malware rewrites what the host shows. It does not rewrite the offline device's display unless you ignore a disagreement. If they disagree, stop.

   Read the note → · PSBT and Air-Gapped Signing: Keep Keys Offline While the Network Watches