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diff --git a/doc/descriptors.md b/doc/descriptors.md new file mode 100644 index 0000000000..de4d4e574f --- /dev/null +++ b/doc/descriptors.md @@ -0,0 +1,171 @@ +# Support for Output Descriptors in Bitcoin Core + +Since Bitcoin Core v0.17, there is support for Output Descriptors in the +`scantxoutset` RPC call. This is a simple language which can be used to +describe collections of output scripts. + +This document describes the language. For the specifics on usage for scanning +the UTXO set, see the `scantxoutset` RPC help. + +## Features + +Output descriptors currently support: +- Pay-to-pubkey scripts (P2PK), through the `pk` function. +- Pay-to-pubkey-hash scripts (P2PKH), through the `pkh` function. +- Pay-to-witness-pubkey-hash scripts (P2WPKH), through the `wpkh` function. +- Pay-to-script-hash scripts (P2SH), through the `sh` function. +- Pay-to-witness-script-hash scripts (P2WSH), through the `wsh` function. +- Multisig scripts, through the `multi` function. +- Any type of supported address through the `addr` function. +- Raw hex scripts through the `raw` function. +- Public keys (compressed and uncompressed) in hex notation, or BIP32 extended pubkeys with derivation paths. + +## Examples + +- `pk(0279be667ef9dcbbac55a06295ce870b07029bfcdb2dce28d959f2815b16f81798)` describes a P2PK output with the specified public key. +- `pkh(02c6047f9441ed7d6d3045406e95c07cd85c778e4b8cef3ca7abac09b95c709ee5)` describes a P2PKH output with the specified public key. +- `wpkh(02f9308a019258c31049344f85f89d5229b531c845836f99b08601f113bce036f9)` describes a P2WPKH output with the specified public key. +- `sh(wpkh(03fff97bd5755eeea420453a14355235d382f6472f8568a18b2f057a1460297556))` describes a P2SH-P2WPKH output with the specified public key. +- `combo(0279be667ef9dcbbac55a06295ce870b07029bfcdb2dce28d959f2815b16f81798)` describes any P2PK, P2PKH, P2WPKH, or P2SH-P2WPKH output with the specified public key. +- `sh(wsh(pkh(02e493dbf1c10d80f3581e4904930b1404cc6c13900ee0758474fa94abe8c4cd13)))` describes an (overly complicated) P2SH-P2WSH-P2PKH output with the specified public key. +- `multi(1,022f8bde4d1a07209355b4a7250a5c5128e88b84bddc619ab7cba8d569b240efe4,025cbdf0646e5db4eaa398f365f2ea7a0e3d419b7e0330e39ce92bddedcac4f9bc)` describes a bare *1-of-2* multisig output with keys in the specified order. +- `sh(multi(2,022f01e5e15cca351daff3843fb70f3c2f0a1bdd05e5af888a67784ef3e10a2a01,03acd484e2f0c7f65309ad178a9f559abde09796974c57e714c35f110dfc27ccbe))` describes a P2SH *2-of-2* multisig output with keys in the specified order. +- `wsh(multi(2,03a0434d9e47f3c86235477c7b1ae6ae5d3442d49b1943c2b752a68e2a47e247c7,03774ae7f858a9411e5ef4246b70c65aac5649980be5c17891bbec17895da008cb,03d01115d548e7561b15c38f004d734633687cf4419620095bc5b0f47070afe85a))` describes a P2WSH *2-of-3* multisig output with keys in the specified order. +- `sh(wsh(multi(1,03f28773c2d975288bc7d1d205c3748651b075fbc6610e58cddeeddf8f19405aa8,03499fdf9e895e719cfd64e67f07d38e3226aa7b63678949e6e49b241a60e823e4,02d7924d4f7d43ea965a465ae3095ff41131e5946f3c85f79e44adbcf8e27e080e)))` describes a P2SH-P2WSH *1-of-3* multisig output with keys in the specified order. +- `pk(xpub661MyMwAqRbcFtXgS5sYJABqqG9YLmC4Q1Rdap9gSE8NqtwybGhePY2gZ29ESFjqJoCu1Rupje8YtGqsefD265TMg7usUDFdp6W1EGMcet8)` describes a P2PK output with the public key of the specified xpub. +- `pkh(xpub68Gmy5EdvgibQVfPdqkBBCHxA5htiqg55crXYuXoQRKfDBFA1WEjWgP6LHhwBZeNK1VTsfTFUHCdrfp1bgwQ9xv5ski8PX9rL2dZXvgGDnw/1'/2)` describes a P2PKH output with child key *1'/2* of the specified xpub. +- `pkh([d34db33f/44'/0'/0']xpub6ERApfZwUNrhLCkDtcHTcxd75RbzS1ed54G1LkBUHQVHQKqhMkhgbmJbZRkrgZw4koxb5JaHWkY4ALHY2grBGRjaDMzQLcgJvLJuZZvRcEL/1/*)` describes a set of P2PKH outputs, but additionally specifies that the specified xpub is a child of a master with fingerprint `d34db33f`, and derived using path `44'/0'/0'`. +- `wsh(multi(1,xpub661MyMwAqRbcFW31YEwpkMuc5THy2PSt5bDMsktWQcFF8syAmRUapSCGu8ED9W6oDMSgv6Zz8idoc4a6mr8BDzTJY47LJhkJ8UB7WEGuduB/1/0/*,xpub69H7F5d8KSRgmmdJg2KhpAK8SR3DjMwAdkxj3ZuxV27CprR9LgpeyGmXUbC6wb7ERfvrnKZjXoUmmDznezpbZb7ap6r1D3tgFxHmwMkQTPH/0/0/*))` describes a set of *1-of-2* P2WSH multisig outputs where the first multisig key is the *1/0/`i`* child of the first specified xpub and the second multisig key is the *0/0/`i`* child of the second specified xpub, and `i` is any number in a configurable range (`0-1000` by default). + +## Reference + +Descriptors consist of several types of expressions. The top level expression is always a `SCRIPT`. + +`SCRIPT` expressions: +- `sh(SCRIPT)` (top level only): P2SH embed the argument. +- `wsh(SCRIPT)` (not inside another 'wsh'): P2WSH embed the argument. +- `pk(KEY)` (anywhere): P2PK output for the given public key. +- `pkh(KEY)` (anywhere): P2PKH output for the given public key (use `addr` if you only know the pubkey hash). +- `wpkh(KEY)` (not inside `wsh`): P2WPKH output for the given compressed pubkey. +- `combo(KEY)` (top level only): an alias for the collection of `pk(KEY)` and `pkh(KEY)`. If the key is compressed, it also includes `wpkh(KEY)` and `sh(wpkh(KEY))`. +- `multi(k,KEY_1,KEY_2,...,KEY_n)` (anywhere): k-of-n multisig script. +- `addr(ADDR)` (top level only): the script which ADDR expands to. +- `raw(HEX)` (top level only): the script whose hex encoding is HEX. + +`KEY` expressions: +- Optionally, key origin information, consisting of: + - An open bracket `[` + - Exactly 8 hex characters for the fingerprint of the key where the derivation starts (see BIP32 for details) + - Followed by zero or more `/NUM` or `/NUM'` path elements to indicate unhardened or hardened derivation steps between the fingerprint and the key or xpub/xprv root that follows + - A closing bracket `]` +- Followed by the actual key, which is either: + - Hex encoded public keys (either 66 characters starting with `02` or `03` for a compressed pubkey, or 130 characters starting with `04` for an uncompressed pubkey). + - Inside `wpkh` and `wsh`, only compressed public keys are permitted. + - [WIF](https://en.bitcoin.it/wiki/Wallet_import_format) encoded private keys may be specified instead of the corresponding public key, with the same meaning. + - `xpub` encoded extended public key or `xprv` encoded extended private key (as defined in [BIP 32](https://github.com/bitcoin/bips/blob/master/bip-0032.mediawiki)). + - Followed by zero or more `/NUM` unhardened and `/NUM'` hardened BIP32 derivation steps. + - Optionally followed by a single `/*` or `/*'` final step to denote all (direct) unhardened or hardened children. + - The usage of hardened derivation steps requires providing the private key. + +(Anywhere a `'` suffix is permitted to denote hardened derivation, the suffix `h` can be used instead.) + +`ADDR` expressions are any type of supported address: +- P2PKH addresses (base58, of the form `1...` for mainnet or `[nm]...` for testnet). Note that P2PKH addresses in descriptors cannot be used for P2PK outputs (use the `pk` function instead). +- P2SH addresses (base58, of the form `3...` for mainnet or `2...` for testnet, defined in [BIP 13](https://github.com/bitcoin/bips/blob/master/bip-0013.mediawiki)). +- Segwit addresses (bech32, of the form `bc1...` for mainnet or `tb1...` for testnet, defined in [BIP 173](https://github.com/bitcoin/bips/blob/master/bip-0173.mediawiki)). + +## Explanation + +### Single-key scripts + +Many single-key constructions are used in practice, generally including +P2PK, P2PKH, P2WPKH, and P2SH-P2WPKH. Many more combinations are +imaginable, though they may not be optimal: P2SH-P2PK, P2SH-P2PKH, +P2WSH-P2PK, P2WSH-P2PKH, P2SH-P2WSH-P2PK, P2SH-P2WSH-P2PKH. + +To describe these, we model these as functions. The functions `pk` +(P2PK), `pkh` (P2PKH) and `wpkh` (P2WPKH) take as input a `KEY` expression, and return the +corresponding *scriptPubKey*. The functions `sh` (P2SH) and `wsh` (P2WSH) +take as input a `SCRIPT` expression, and return the script describing P2SH and P2WSH +outputs with the input as embedded script. The names of the functions do +not contain "p2" for brevity. + +### Multisig + +Several pieces of software use multi-signature (multisig) scripts based +on Bitcoin's OP_CHECKMULTISIG opcode. To support these, we introduce the +`multi(k,key_1,key_2,...,key_n)` function. It represents a *k-of-n* +multisig policy, where any *k* out of the *n* provided `KEY` expressions must +sign. + +Key order is significant. A `multi()` expression describes a multisig script +with keys in the specified order, and in a search for TXOs, it will not match +outputs with multisig scriptPubKeys that have the same keys in a different +order. Also, to prevent a combinatorial explosion of the search space, if more +than one of the `multi()` key arguments is a BIP32 wildcard path ending in `/*` +or `*'`, the `multi()` expression only matches multisig scripts with the `i`th +child key from each wildcard path in lockstep, rather than scripts with any +combination of child keys from each wildcard path. + +### BIP32 derived keys and chains + +Most modern wallet software and hardware uses keys that are derived using +BIP32 ("HD keys"). We support these directly by permitting strings +consisting of an extended public key (commonly referred to as an *xpub*) +plus derivation path anywhere a public key is expected. The derivation +path consists of a sequence of 0 or more integers (in the range +*0..2<sup>31</sup>-1*) each optionally followed by `'` or `h`, and +separated by `/` characters. The string may optionally end with the +literal `/*` or `/*'` (or `/*h`) to refer to all unhardened or hardened +child keys in a configurable range (by default `0-1000`, inclusive). + +Whenever a public key is described using a hardened derivation step, the +script cannot be computed without access to the corresponding private +key. + +### Key origin identification + +In order to describe scripts whose signing keys reside on another device, +it may be necessary to identify the master key and derivation path an +xpub was derived with. + +For example, when following BIP44, it would be useful to describe a +change chain directly as `xpub.../44'/0'/0'/1/*` where `xpub...` +corresponds with the master key `m`. Unfortunately, since there are +hardened derivation steps that follow the xpub, this descriptor does not +let you compute scripts without access to the corresponding private keys. +Instead, it should be written as `xpub.../1/*`, where xpub corresponds to +`m/44'/0'/0'`. + +When interacting with a hardware device, it may be necessary to include +the entire path from the master down. [BIP174](https://github.com/bitcoin/bips/blob/master/bip-0174.mediawiki) standardizes this by +providing the master key *fingerprint* (first 32 bit of the Hash160 of +the master pubkey), plus all derivation steps. To support constructing +these, we permit providing this key origin information inside the +descriptor language, even though it does not affect the actual +scriptPubKeys it refers to. + +Every public key can be prefixed by an 8-character hexadecimal +fingerprint plus optional derivation steps (hardened and unhardened) +surrounded by brackets, identifying the master and derivation path the key or xpub +that follows was derived with. + +Note that the fingerprint of the parent only serves as a fast way to detect +parent and child nodes in software, and software must be willing to deal with +collisions. + +### Including private keys + +Often it is useful to communicate a description of scripts along with the +necessary private keys. For this reason, anywhere a public key or xpub is +supported, a private key in WIF format or xprv may be provided instead. +This is useful when private keys are necessary for hardened derivation +steps, or for dumping wallet descriptors including private key material. + +### Compatibility with old wallets + +In order to easily represent the sets of scripts currently supported by +existing Bitcoin Core wallets, a convenience function `combo` is +provided, which takes as input a public key, and describes a set of P2PK, +P2PKH, P2WPKH, and P2SH-P2WPH scripts for that key. In case the key is +uncompressed, the set only includes P2PK and P2PKH scripts. |