133 lines
7.5 KiB
Markdown
133 lines
7.5 KiB
Markdown
# PSBT Howto for Bitcoin Core
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Since Bitcoin Core 0.17, an RPC interface exists for Partially Signed Bitcoin
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Transactions (PSBTs, as specified in
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[BIP 174](https://github.com/bitcoin/bips/blob/master/bip-0174.mediawiki)).
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This document describes the overall workflow for producing signed transactions
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through the use of PSBT, and the specific RPC commands used in typical
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scenarios.
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## PSBT in general
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PSBT is an interchange format for Bitcoin transactions that are not fully signed
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yet, together with relevant metadata to help entities work towards signing it.
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It is intended to simplify workflows where multiple parties need to cooperate to
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produce a transaction. Examples include hardware wallets, multisig setups, and
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[CoinJoin](https://bitcointalk.org/?topic=279249) transactions.
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### Overall workflow
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Overall, the construction of a fully signed Bitcoin transaction goes through the
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following steps:
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- A **Creator** proposes a particular transaction to be created. They construct
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a PSBT that contains certain inputs and outputs, but no additional metadata.
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- For each input, an **Updater** adds information about the UTXOs being spent by
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the transaction to the PSBT. They also add information about the scripts and
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public keys involved in each of the inputs (and possibly outputs) of the PSBT.
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- **Signers** inspect the transaction and its metadata to decide whether they
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agree with the transaction. They can use amount information from the UTXOs
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to assess the values and fees involved. If they agree, they produce a
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partial signature for the inputs for which they have relevant key(s).
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- A **Finalizer** is run for each input to convert the partial signatures and
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possibly script information into a final `scriptSig` and/or `scriptWitness`.
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- An **Extractor** produces a valid Bitcoin transaction (in network format)
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from a PSBT for which all inputs are finalized.
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Generally, each of the above (excluding Creator and Extractor) will simply
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add more and more data to a particular PSBT, until all inputs are fully signed.
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In a naive workflow, they all have to operate sequentially, passing the PSBT
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from one to the next, until the Extractor can convert it to a real transaction.
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In order to permit parallel operation, **Combiners** can be employed which merge
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metadata from different PSBTs for the same unsigned transaction.
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The names above in bold are the names of the roles defined in BIP174. They're
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useful in understanding the underlying steps, but in practice, software and
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hardware implementations will typically implement multiple roles simultaneously.
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## PSBT in Bitcoin Core
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### RPCs
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- **`converttopsbt` (Creator)** is a utility RPC that converts an
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unsigned raw transaction to PSBT format. It ignores existing signatures.
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- **`createpsbt` (Creator)** is a utility RPC that takes a list of inputs and
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outputs and converts them to a PSBT with no additional information. It is
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equivalent to calling `createrawtransaction` followed by `converttopsbt`.
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- **`walletcreatefundedpsbt` (Creator, Updater)** is a wallet RPC that creates a
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PSBT with the specified inputs and outputs, adds additional inputs and change
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to it to balance it out, and adds relevant metadata. In particular, for inputs
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that the wallet knows about (counting towards its normal or watch-only
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balance), UTXO information will be added. For outputs and inputs with UTXO
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information present, key and script information will be added which the wallet
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knows about. It is equivalent to running `createrawtransaction`, followed by
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`fundrawtransaction`, and `converttopsbt`.
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- **`walletprocesspsbt` (Updater, Signer, Finalizer)** is a wallet RPC that takes as
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input a PSBT, adds UTXO, key, and script data to inputs and outputs that miss
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it, and optionally signs inputs. Where possible it also finalizes the partial
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signatures.
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- **`finalizepsbt` (Finalizer, Extractor)** is a utility RPC that finalizes any
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partial signatures, and if all inputs are finalized, converts the result to a
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fully signed transaction which can be broadcast with `sendrawtransaction`.
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- **`combinepsbt` (Combiner)** is a utility RPC that implements a Combiner. It
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can be used at any point in the workflow to merge information added to
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different versions of the same PSBT. In particular it is useful to combine the
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output of multiple Updaters or Signers.
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- **`decodepsbt`** is a diagnostic utility RPC which will show all information in
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a PSBT in human-readable form, as well as compute its eventual fee if known.
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### Workflows
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#### Multisig with multiple Bitcoin Core instances
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Alice, Bob, and Carol want to create a 2-of-3 multisig address. They're all using
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Bitcoin Core. We assume their wallets only contain the multisig funds. In case
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they also have a personal wallet, this can be accomplished through the
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multiwallet feature - possibly resulting in a need to add `-rpcwallet=name` to
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the command line in case `bitcoin-cli` is used.
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Setup:
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- All three call `getnewaddress` to create a new address; call these addresses
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*Aalice*, *Abob*, and *Acarol*.
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- All three call `getaddressinfo X`, with *X* their respective address, and
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remember the corresponding public keys. Call these public keys *Kalice*,
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*Kbob*, and *Kcarol*.
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- All three now run `addmultisigaddress 2 ["Kalice","Kbob","Kcarol"]` to teach
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their wallet about the multisig script. Call the address produced by this
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command *Amulti*. They may be required to explicitly specify the same
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addresstype option each, to avoid constructing different versions due to
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differences in configuration.
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- They also run `importaddress "Amulti" "" false` to make their wallets treat
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payments to *Amulti* as contributing to the watch-only balance.
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- Others can verify the produced address by running
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`createmultisig 2 ["Kalice","Kbob","Kcarol"]`, and expecting *Amulti* as
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output. Again, it may be necessary to explicitly specify the addresstype
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in order to get a result that matches. This command won't enable them to
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initiate transactions later, however.
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- They can now give out *D* as address others can pay to.
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Later, when *V* BTC has been received on *Amulti*, and Bob and Carol want to
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move the coins in their entirety to address *Asend*, with no change. Alice
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does not need to be involved.
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- One of them - let's assume Carol here - initiates the creation. She runs
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`walletcreatefundedpsbt [] {"Asend":V} 0 false {"subtractFeeFromOutputs":[0], "includeWatching":true}`.
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We call the resulting PSBT *P*. P does not contain any signatures.
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- Carol needs to sign the transaction herself. In order to do so, she runs
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`walletprocesspsbt P`, and gives the resulting PSBT *P2* to Bob.
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- Bob inspects the PSBT using `decodepsbt "P2"` to determine if the transaction
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has indeed just the expected input, and an output to *Asend*, and the fee is
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reasonable. If he agrees, he calls `walletprocesspsbt "P2"` to sign. The
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resulting PSBT *P3* contains both Carol's and Bob's signature.
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- Now anyone can call `finalizepsbt "P2"` to extract a fully signed transaction
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*T*.
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- Finally anyone can broadcast the transaction using `sendrawtransaction "T"`.
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In case there are more signers, it may be advantageous to let them all sign in
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parallel, rather passing the PSBT from one signer to the next one. In the
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above example this would translate to Carol handing a copy of *P* to each signer
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separately. They can then all invoke `walletprocesspsbt P`, and end up with
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their individually-signed PSBT structures. They then all send those back to
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Carol (or anyone) who can combine them using `combinepsbt`. The last two steps
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(`finalizepsbt` and `sendrawtransaction`) remain unchanged.
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