Technical reference
PSBT: moving an unsigned Bitcoin transaction between devices
An offline signer still needs to verify the outputs and fee of the transaction it signs.
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A transport format for signing
BIP-174 defines the Partially Signed Bitcoin Transaction format. It carries transaction information and the extra data participants need to prepare, sign and finalise a Bitcoin transaction. This allows separate devices or multiple signers to collaborate without putting all keys in one online application.
Offline is not the same as correct
A malicious online coordinator can prepare the wrong recipient or an excessive fee. Keeping a key offline prevents the coordinator from directly reading it, but does not make the proposed transaction trustworthy. The signer must inspect the transaction information it authorises.
The useful review boundary
Check each external output, its amount, the change treatment and the total fee. Verify that the device can establish the necessary input information rather than accepting unsupported claims from the coordinator. A successful signature proves that the signing conditions were met; it does not prove that the recipient corresponds to the invoice or person the user had in mind.