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Support for Output Descriptors in Dash Core

Since Dash Core v0.17, there is support for Output Descriptors. This is a simple language which can be used to describe collections of output scripts. Supporting RPCs are:

  • scantxoutset takes as input descriptors to scan for, and also reports specialized descriptors for the matching UTXOs.
  • getdescriptorinfo analyzes a descriptor, and reports a canonicalized version with checksum added.
  • deriveaddresses takes as input a descriptor and computes the corresponding addresses.
  • listunspent outputs a specialized descriptor for the reported unspent outputs.
  • getaddressinfo outputs a descriptor for solvable addresses (since v0.18).
  • importmulti takes as input descriptors to import into a legacy wallet (since v0.18).
  • generatetodescriptor takes as input a descriptor and generates coins to it (regtest only, since v0.19).
  • utxoupdatepsbt takes as input descriptors to add information to the psbt (since v0.19).
  • createmultisig and addmultisigaddress return descriptors as well (since v0.20).
  • importdescriptors takes as input descriptors to import into a descriptor wallet (since v0.21).
  • listdescriptors outputs descriptors imported into a descriptor wallet (since v22).

This document describes the language. For the specifics on usage, see the RPC documentation for the functions mentioned above.

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.
  • Multisig scripts where the public keys are sorted lexicographically, through the sortedmulti 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.
  • combo(0279be667ef9dcbbac55a06295ce870b07029bfcdb2dce28d959f2815b16f81798) describes any P2PK, P2PKH with the specified public key.
  • multi(1,022f8bde4d1a07209355b4a7250a5c5128e88b84bddc619ab7cba8d569b240efe4,025cbdf0646e5db4eaa398f365f2ea7a0e3d419b7e0330e39ce92bddedcac4f9bc) describes a bare 1-of-2 multisig with the specified public key.
  • sortedmulti(1,022f8bde4d1a07209355b4a7250a5c5128e88b84bddc619ab7cba8d569b240efe4,025cbdf0646e5db4eaa398f365f2ea7a0e3d419b7e0330e39ce92bddedcac4f9bc) describes a bare 1-of-2 multisig with keys sorted lexicographically in the resulting redeemScript.
  • 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'.

Reference

Descriptors consist of several types of expressions. The top level expression is either a SCRIPT, or SCRIPT#CHECKSUM where CHECKSUM is an 8-character alphanumeric descriptor checksum.

SCRIPT expressions:

  • 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).
  • sh(SCRIPT) (top level only): P2SH embed the argument.
  • 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.
  • sortedmulti(k,KEY_1,KEY_2,...,KEY_n) (anywhere): k-of-n multisig script with keys sorted lexicographically in the resulting 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 (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 X...). Note that P2PKH addresses in descriptors cannot be used for P2PK outputs (use the pk function instead).
  • P2SH addresses (base58, of the form 7..., defined in BIP 13).

Explanation

Single-key scripts

Many single-key constructions are used in practice, generally including P2PK and P2PKH. More combinations are imaginable, though they may not be optimal: P2SH-P2PK and 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 sh (P2SH) function takes as input a script, and returns the script describing P2SH outputs with the input as embedded script. The name of the function does 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) and sortedmulti(k,key_1,key_2,...,key_n) functions. They represents a k-of-n multisig policy, where any k out of the n provided public keys must sign.

Key order is significant for multi(). 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 ith child key from each wildcard path in lockstep, rather than scripts with any combination of child keys from each wildcard path.

Key order does not matter for sortedmulti(). sortedmulti() behaves in the same way as multi() does but the keys are reordered in the resulting script such that they are lexicographically ordered as described in BIP67.

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 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 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, for signing transactions, or for dumping wallet descriptors including private key material.

For example, after importing the following 2-of-3 multisig descriptor into a wallet, one could use signrawtransactionwithwallet to sign a transaction with the first key:

sh(multi(2,xprv.../84'/0'/0'/0/0,xpub1...,xpub2...))

Note how the first key is an xprv private key with a specific derivation path, while the other two are public keys.

Compatibility with old wallets

In order to easily represent the sets of scripts currently supported by existing Dash Core wallets, a convenience function combo is provided, which takes as input a public key, and describes a set of P2PK and P2PKH scripts for that key.

Checksums

Descriptors can optionally be suffixed with a checksum to protect against typos or copy-paste errors.

These checksums consist of 8 alphanumeric characters. As long as errors are restricted to substituting characters in 0123456789()[],'/*abcdefgh@:$%{} for others in that set and changes in letter case, up to 4 errors will always be detected in descriptors up to 501 characters, and up to 3 errors in longer ones. For larger numbers of errors, or other types of errors, there is a roughly 1 in a trillion chance of not detecting the errors.

All RPCs in Dash Core will include the checksum in their output. Only certain RPCs require checksums on input, including deriveaddress and importmulti. The checksum for a descriptor without one can be computed using the getdescriptorinfo RPC.