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Support for Output Descriptors in Bitcoin Cash Node

Since Bitcoin ABC v0.20.7, 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-script-hash scripts (P2SH), through the sh 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) represents a P2PK output.
  • pkh(02c6047f9441ed7d6d3045406e95c07cd85c778e4b8cef3ca7abac09b95c709ee5) represents a P2PKH output.
  • combo(0279be667ef9dcbbac55a06295ce870b07029bfcdb2dce28d959f2815b16f81798) represents a P2PK and P2PKH output.
  • multi(1,022f8bde4d1a07209355b4a7250a5c5128e88b84bddc619ab7cba8d569b240efe4,025cbdf0646e5db4eaa398f365f2ea7a0e3d419b7e0330e39ce92bddedcac4f9bc) represents a bare 1-of-2 multisig.
  • sh(multi(2,022f01e5e15cca351daff3843fb70f3c2f0a1bdd05e5af888a67784ef3e10a2a01,03acd484e2f0c7f65309ad178a9f559abde09796974c57e714c35f110dfc27ccbe)) represents a P2SH 2-of-2 multisig.
  • pk(xpub661MyMwAqRbcFtXgS5sYJABqqG9YLmC4Q1Rdap9gSE8NqtwybGhePY2gZ29ESFjqJoCu1Rupje8YtGqsefD265TMg7usUDFdp6W1EGMcet8) refers to a single P2PK output, using the public key part from the specified xpub.
  • pkh(xpub68Gmy5EdvgibQVfPdqkBBCHxA5htiqg55crXYuXoQRKfDBFA1WEjWgP6LHhwBZeNK1VTsfTFUHCdrfp1bgwQ9xv5ski8PX9rL2dZXvgGDnw/1'/2) refers to a single P2PKH output, using 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'.

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.
  • 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).
  • combo(KEY) (top level only): an alias for the collection of pk(KEY) and pkh(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:

  • Hex encoded public keys (66 characters starting with 02 or 03, or 130 characters starting with 04).
  • WIF encoded private keys may be specified instead of the corresponding public key, with the same meaning.

  • xpub encoded extended public key or xprv encoded private key (as defined in BIP 32).

    • 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.
    • Instead of a ', the suffix h can be used to denote hardened derivation.

  • 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 (66 characters starting with 02 or 03, or 130 characters starting with 04).
    • WIF encoded private keys may be specified instead of the corresponding public key, with the same meaning. -xpub encoded extended public key or xprv encoded private key (as defined in BIP 32).
    • 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...). Note that P2PKH addresses in descriptors cannot be used for P2PK outputs (use the pk function instead).
  • P2SH addresses (base58, of the form 3..., defined in BIP 13).

Explanation

Single-key scripts

Many single-key constructions are used in practice, generally including P2PK, and P2PKH. Many more combinations are imaginable, though they may not be optimal: P2SH-P2PK, P2SH-P2PKH.

To describe these, we model these as functions. The functions pk (P2PK) and pkh (P2PKH) take as input a public key in hexadecimal notation (which will be extended later), and return the corresponding scriptPubKey. The functions sh (P2SH) take as input a script, and return the script describing P2SH 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 public keys must sign.

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..2³¹-1) 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 instead.

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 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.

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 ABC wallets, a convenience function combo is provided, which takes as input a public key, and constructs the P2PK and P2PKH scripts for that key.