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