Re: [bitcoin-dev] BIP Proposals for Output Script Descriptors

From: Andrew Chow <achow101-lists@achow101•com>
To: Jeremy <jlrubin@mit•edu>,
	Bitcoin Protocol Discussion
	<bitcoin-dev@lists•linuxfoundation.org>
Subject: Re: [bitcoin-dev] BIP Proposals for Output Script Descriptors
Date: Fri, 02 Jul 2021 20:05:45 +0000	[thread overview]
Message-ID: <08950fa0-36ba-5411-fee4-cf0d5c229ca6@achow101.com> (raw)
In-Reply-To: <CAD5xwhg9sno+aABgUtiwhBo8mLrmj5m9WowGjnUfv-f1pSbshw@mail.gmail.com>

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I've opened a PR against my own BIPs fork for review: https://github.com/achow101/bips/pull/3

Andrew

On 6/29/21 11:41 PM, Jeremy wrote:

> Kudos, this is fantastic!
>
> It might be easier, since there is a ton of content here, for you to open up some WIP PRs to collect feedback?
> --
> [@JeremyRubin](https://twitter.com/JeremyRubin)
>
> On Tue, Jun 29, 2021 at 2:15 PM Andrew Chow via bitcoin-dev <bitcoin-dev@lists•linuxfoundation.org> wrote:
>
>> Hi All,
>>
>> I've been working on formalizing the Output Script Descriptors that have
>> been available in Bitcoin Core for a while into BIPs. Since descriptors
>> are modular and have optional components, I've decided to split it into
>> 7 BIPs, rather than a single one. The first describes descriptors in
>> general and does not specify any particular descriptor. However it does
>> describe the general operation, key expressions (including derivation
>> paths and key origin info), and the descriptor checksum. The following 6
>> BIPs specify the actual descriptors themselves. These are non-segwit
>> descriptor (pk, pkh, sh), segwit descriptors (wpkh, wsh), multisig
>> descriptors (multi, sortedmulti), the taproot descriptor (tr), the combo
>> descriptor, and opaque descriptors (raw, addr). This separation is so
>> that implementors can choose to not implement some descriptors and still
>> say which descriptors they support without being too difficult to
>> understand.
>>
>> The text of all of the documents are below, and they can also be found
>> on github:https://github.com/achow101/bips/tree/descriptors/
>>
>> Thanks,
>> Andrew Chow
>>
>> ---
>>
>> <pre>
>> BIP: bip-descriptors-general
>> Layer: Applications
>> Title: Output Script Descriptors General Operation
>> Author: Pieter Wuille <pieter@wuille•net>
>> Andrew Chow <andrew@achow101•com>
>> Comments-Summary: No comments yet.
>> Comments-URI:
>> https://github.com/bitcoin/bips/wiki/Comments:BIP-descriptors-general
>> Status: Draft
>> Type: Informational
>> Created: 2021-06-27
>> License: BSD-2-Clause
>> </pre>
>>
>> ==Abstract==
>>
>> Output Script Descriptors are a simple language which can be used to
>> describe collections ofoutput scripts.
>> There can be many different descriptor fragments and functions.
>> This document describes the general syntax for descriptors, descriptor
>> checksums, and common expressions.
>>
>> ==Copyright==
>>
>> This BIP is licensed under the BSD 2-clause license.
>>
>> ==Motivation==
>>
>> Bitcoin wallets traditionally have stored a set of keys which are later
>> serialized and mutated to produce the output scripts that the wallet
>> watches and the addresses it provides to users.
>> Typically backups have consisted of solely the private keys, nowadays
>> primarily in the form of BIP 39 mnemonics.
>> However this backup solution is insuffient, especially since the
>> introduction of Segregated Witness which added new output types.
>> Given just the private keys, it is not possible for restored wallets to
>> know which kinds of output scripts and addresses to produce.
>> This has lead to incompatibilities between wallets when restoring a
>> backup or exporting data for a watch only wallet.
>>
>> Further complicating matters are BIP 32 derivation paths.
>> Although BIPs 44, 49, and 84 have specified standard BIP 32 derivation
>> paths for different output scripts and addresses, not all wallets
>> support them nor use those derivation paths.
>> The lack of derivation path information in these backups and exports
>> leads to further incompatibilities between wallets.
>>
>> Current solutions to these issues have not been generic and can be
>> viewed as being layer violations.
>> Solutions such as introducing different version bytes for extended key
>> serialization both are a layer violation (key derivation should be
>> separate from script type meaning) and specific only to a particular
>> derivation path and script type.
>>
>> Output Script Descriptors introduces a generic solution to these issues.
>> Script types are specified explicitly through the use of Script Expressions.
>> Key derivation paths are specified explicitly in Key Expressions.
>> These allow for creating wallet backups and exports which specify the
>> exact scripts, subscripts (redeemScript, witnessScript, etc.), and keys
>> to produce.
>> With the general structure specified in this BIP, new Script Expressions
>> can be introduced as new script types are added.
>> Lastly, the use of common terminology and existing standards allow for
>> Output Script Descriptors to be engineer readable so that the results
>> can be understood at a glance.
>>
>> ==Specification==
>>
>> Descriptors consist of several types of expressions.
>> The top level expression is a <tt>SCRIPT</tt>.
>> This expression may be followed by <tt>#CHECKSUM</tt>, where
>> <tt>CHECKSUM</tt> is an 8 character alphanumeric descriptor checksum.
>>
>> ===Script Expressions===
>>
>> Script Expressions (denoted <tt>SCRIPT</tt>) are expressions which
>> correspond directly with a Bitcoin script.
>> These expressions are written as functions and take arguments.
>> Such expressions have a script template which is filled with the
>> arguments correspondingly.
>> Expressions are written with a human readable identifier string with the
>> arguments enclosed with parentheses.
>> The identifier string should be alphanumeric and may include underscores.
>>
>> The arguments to a script expression are defined by that expression itself.
>> They could be a script expression, a key expression, or some other
>> expression entirely.
>>
>> ===Key Expressions===
>>
>> A common expression used as an argument to script expressions are key
>> expressions (denoted <tt>KEY</tt>).
>> These represent a public or private key and, optionally, information
>> about the origin of that key.
>> Key expressions can only be used as arguments to script expressions.
>>
>> Key expressions consist of:
>> * Optionally, key origin information, consisting of:
>> ** An open bracket <tt>[</tt>
>> ** Exactly 8 hex characters for the fingerprint of the key where the
>> derivation starts (see BIP 32 for details)
>> ** Followed by zero or more <tt>/NUM</tt> or <tt>/NUM'</tt> path
>> elements to indicate the unhardened or hardened derivation steps between
>> the fingerprint and the key that follows.
>> ** A closing bracket <tt>]</tt>
>> * Followed by the actual key, which is either:
>> ** A hex encoded public key, which depending the script expression, may
>> be either:
>> *** 66 hex character string beginning with <tt>02</tt> or <tt>03</tt>
>> representing a compressed public key
>> *** 130 hex character string beginning with <tt>04</tt> representing an
>> uncompressed public key
>> *** 64 hex character string representing an x-only public key
>> ** A [[[https://en.bitcoin.it/wiki/Wallet_import_format|WIF](https://en.bitcoin.it/wiki/Wallet_import_format%7CWIF)]] encoded
>> private key
>> ** <tt>xpub</tt> encoded extended public key or <tt>xprv</tt> encoded
>> extended private key (as defined in BIP 32)
>> *** Followed by zero or more <tt>/NUM</tt> or <tt>/NUM'</tt> path
>> elements indicating BIP 32 derivation steps to be taken after the given
>> extended key.
>> *** Optionally followed by a single <tt>/*</tt> or <tt>/*'</tt> final
>> step to denote all direct unhardened or hardened children.
>>
>> If the <tt>KEY</tt> is a BIP 32 extended key, before output scripts can
>> be created, child keys must be derived using the derivation information
>> that follows the extended key.
>> When the final step is <tt>/*</tt> or <tt>/*'</tt>, an output script
>> will be produced for every child key index.
>> The derived key must be serialized as a compressed public key.
>>
>> In the above specification, the hardened indicator <tt>'</tt> may be
>> replaced with alternative hardnened indicators of <tt>h</tt> or <tt>H</tt>.
>>
>> ===Character Set===
>>
>> The expressions used in descriptors must only contain characters within
>> this character set so that the descriptor checksum will work.
>>
>> The allowed characters are:
>> <pre>
>> 0123456789()[],'/*abcdefgh@:$%{}
>> IJKLMNOPQRSTUVWXYZ&+-.;<=>?!^_|~
>> ijklmnopqrstuvwxyzABCDEFGH`#"\<space>
>> </pre>
>> Note that <tt><space></tt> on the last line is a space character.
>>
>> This character set is written as 3 groups of 32 characters in this
>> specific order so that the checksum below can identify more errors.
>> The first group are the most common "unprotected" characters (i.e.
>> things such as hex and keypaths that do not already have their own
>> checksums).
>> Case errors cause an offset that is a multiple of 32 while as many
>> alphabetic characters are in the same group while following the previous
>> restrictions.
>>
>> ===Checksum===
>>
>> Follwing the top level script expression is a single octothorpe
>> (<tt>#</tt>) followed by the 8 character checksum.
>> The checksum is an error correcting checksum similar to bech32.
>>
>> The checksum has the following properties:
>> * Mistakes in a descriptor string are measured in "symbol errors". The
>> higher the number of symbol errors, the harder it is to detect:
>> ** An error substituting a character from
>> <tt>0123456789()[],'/*abcdefgh@:$%{}</tt> for another in that set always
>> counts as 1 symbol error.
>> *** Note that hex encoded keys are covered by these characters. Extended
>> keys (<tt>xpub</tt> and <tt>xprv</tt>) use other characters too, but
>> also have their own checksum mechansim.
>> *** <tt>SCRIPT</tt> expression function names use other characters, but
>> mistakes in these would generally result in an unparsable descriptor.
>> ** A case error always counts as 1 symbol error.
>> ** Any other 1 character substitution error counts as 1 or 2 symbol errors.
>> * Any 1 symbol error is always detected.
>> * Any 2 or 3 symbol error in a descriptor of up to 49154 characters is
>> always detected.
>> * Any 4 symbol error in a descriptor of up to 507 characters is always
>> detected.
>> * Any 5 symbol error in a descriptor of up to 77 characters is always
>> detected.
>> * Is optimized to minimize the chance of a 5 symbol error in a
>> descriptor up to 387 characters is undetected
>> * Random errors have a chance of 1 in 2<super>40</super> of being
>> undetected.
>>
>> The checksum itself uses the same character set as bech32:
>> <tt>qpzry9x8gf2tvdw0s3jn54khce6mua7l</tt>
>>
>> Valid descriptor strings with a checksum must pass the criteria for
>> validity specified by the Python3 code snippet below.
>> The function <tt>descsum_check</tt> must return true when its argument
>> <tt>s</tt> is a descriptor consisting in the form <tt>SCRIPT#CHECKSUM</tt>.
>>
>> <pre>
>> INPUT_CHARSET =
>> "0123456789()[],'/*abcdefgh@:$%{}IJKLMNOPQRSTUVWXYZ&+-.;<=>?!^_|~ijklmnopqrstuvwxyzABCDEFGH`#\"\\
>> "
>> CHECKSUM_CHARSET = "qpzry9x8gf2tvdw0s3jn54khce6mua7l"
>> GENERATOR = [0xf5dee51989, 0xa9fdca3312, 0x1bab10e32d, 0x3706b1677a,
>> 0x644d626ffd]
>>
>> def descsum_polymod(symbols):
>> """Internal function that computes the descriptor checksum."""
>> chk = 1
>> for value in symbols:
>> top = chk >> 35
>> chk = (chk & 0x7ffffffff) << 5 ^ value
>> for i in range(5):
>> chk ^= GENERATOR[i] if ((top >> i) & 1) else 0
>> return chk
>>
>> def descsum_expand(s):
>> """Internal function that does the character to symbol expansion"""
>> groups = []
>> symbols = []
>> for c in s:
>> if not c in INPUT_CHARSET:
>> return None
>> v = INPUT_CHARSET.find(c)
>> symbols.append(v & 31)
>> groups.append(v >> 5)
>> if len(groups) == 3:
>> symbols.append(groups[0] * 9 + groups[1] * 3 + groups[2])
>> groups = []
>> if len(groups) == 1:
>> symbols.append(groups[0])
>> elif len(groups) == 2:
>> symbols.append(groups[0] * 3 + groups[1])
>> return symbols
>>
>> def descsum_check(s):
>> """Verify that the checksum is correct in a descriptor"""
>> if s[-9] != '#':
>> return False
>> if not all(x in CHECKSUM_CHARSET for x in s[-8:]):
>> return False
>> symbols = descsum_expand(s[:-9]) + [CHECKSUM_CHARSET.find(x) for x
>> in s[-8:]]
>> return descsum_polymod(symbols) == 1
>> </pre>
>>
>> This implements a BCH code that has the properties described above.
>> The entire descriptor string is first processed into an array of symbols.
>> The symbol for each character is its position within its group.
>> After every 3rd symbol, a 4th symbol is inserted which represents the
>> group numbers combined together.
>> This means that a change that only affects the position within a group,
>> or only a group number change, will only affect a single symbol.
>>
>> To construct a valid checksum given a script expression, the code below
>> can be used:
>>
>> <pre>
>> def descsum_create(s):
>> """Add a checksum to a descriptor without"""
>> symbols = descsum_expand(s) + [0, 0, 0, 0, 0, 0, 0, 0]
>> checksum = descsum_polymod(symbols) ^ 1
>> return s + '#' + ''.join(CHECKSUM_CHARSET[(checksum >> (5 * (7 -
>> i))) & 31] for i in range(8))
>>
>> </pre>
>>
>> ==Backwards Compatibility==
>>
>> Output script descriptors are an entirely new language which is not
>> compatible with any existing software.
>> However many components of the expressions reuse encodings and
>> serializations defined by previous BIPs.
>>
>> Output script descriptors are designed for future extension with further
>> fragment types and new script expressions.
>> These will be specified in additional BIPs.
>>
>> ==Reference Implemntation==
>>
>> Descriptors have been implemented in Bitcoin Core since version 0.17.
>>
>> ---
>>
>> <pre>
>> BIP: bip-descriptors-segwit
>> Layer: Applications
>> Title: segwit Output Script Descriptors
>> Author: Pieter Wuille <pieter@wuille•net>
>> Andrew Chow <andrew@achow101•com>
>> Comments-Summary: No comments yet.
>> Comments-URI:
>> https://github.com/bitcoin/bips/wiki/Comments:BIP-descriptors-segwit
>> Status: Draft
>> Type: Informational
>> Created: 2021-06-27
>> License: BSD-2-Clause
>> </pre>
>>
>> ==Abstract==
>>
>> This document specifies <tt>wpkh()</tt>, and <tt>wsh()</tt> output
>> script descriptors.
>> <tt>wpkh()</tt> descriptors take a key and produces a P2WPKH output script.
>> <tt>wsh()</tt> descriptors take a script and produces a P2WSH output script.
>>
>> ==Copyright==
>>
>> This BIP is licensed under the BSD 2-clause license.
>>
>> ==Motivation==
>>
>> Segregated Witness added 2 additional standard output script formats:
>> P2WPKH and P2WSH.
>> These expressions allow specifying those formats as a descriptor.
>>
>> ==Specification==
>>
>> Two new script expressions are defined: <tt>wpkh()</tt>, and <tt>wsh()</tt>.
>>
>> ===<tt>wpkh()</tt>===
>>
>> The <tt>wpkh(KEY)</tt> expression can be used as a top level expression,
>> or inside of a <tt>sh()</tt> descriptor.
>> It takes a single key expression as an argument and produces a P2WPKH
>> output script.
>> Only keys which are/has compressed public keys can be contained in a
>> <tt>wpkh()</tt> expression.
>>
>> The output script produced is:
>> <pre>
>> OP_0 <KEY_hash160>
>> </pre>
>>
>> ===<tt>wsh()</tt>===
>>
>> The <tt>wsh(SCRIPT)</tt> expression can be used as a top level
>> expression, or inside of a <tt>sh()</tt> descriptor.
>> It takes a single script expression as an argument and produces a P2WSH
>> output script.
>> <tt>wsh()</tt> expressions also create a witnessScript which is required
>> in order to spend outputs which use its output script.
>> This redeemScript is the output script produced by the <tt>SCRIPT</tt>
>> argument to <tt>wsh()</tt>.
>> Any key expression found in any script expression contained by a
>> <tt>wsh()</tt> expression must only produce compresed public keys.
>>
>> The output script produced is:
>> <pre>
>> OP_0 <SCRIPT_sha256>
>> </pre>
>>
>> ==Test Vectors==
>>
>> TBD
>>
>> ==Backwards Compatibility==
>>
>> <tt>wpkh()</tt>, and <tt>wsh()</tt> descriptors use the format and
>> general operation specified in
>> [[bip-descriptor-general.mediawiki|bip-descriptor-general]].
>> As these are a wholly new descriptors, they are not compatible with any
>> implementation.
>> However the scripts produced are standard scripts so existing software
>> are likely to be familiar with them.
>>
>> ==Reference Implemntation==
>>
>> <tt>wpkh()</tt>, and <tt>wsh()</tt> descriptors have been implemented in
>> Bitcoin Core since version 0.17.
>>
>> ---
>>
>> <pre>
>> BIP: bip-descriptors-non-segwit
>> Layer: Applications
>> Title: Non-segwit Output Script Descriptors
>> Author: Pieter Wuille <pieter@wuille•net>
>> Andrew Chow <andrew@achow101•com>
>> Comments-Summary: No comments yet.
>> Comments-URI:
>> https://github.com/bitcoin/bips/wiki/Comments:BIP-descriptors-non-segwit
>> Status: Draft
>> Type: Informational
>> Created: 2021-06-27
>> License: BSD-2-Clause
>> </pre>
>>
>> ==Abstract==
>>
>> This document specifies <tt>pk()</tt>, <tt>pkh()</tt>, and <tt>sh()</tt>
>> output script descriptors.
>> <tt>pk()</tt> descriptors take a key and produces a P2PK output script.
>> <tt>pkh()</tt> descriptors take a key and produces a P2PKH output script.
>> <tt>sh()</tt> descriptors take a script and produces a P2SH output script.
>>
>> ==Copyright==
>>
>> This BIP is licensed under the BSD 2-clause license.
>>
>> ==Motivation==
>>
>> Prior to the activation of Segregated Witness, there were 3 main
>> standard output script formats: P2PK, P2PKH, and P2SH.
>> These expressions allow specifying those formats as a descriptor.
>>
>> ==Specification==
>>
>> Three new script expressions are defined: <tt>pk()</tt>, <tt>pkh()</tt>,
>> and <tt>sh()</tt>.
>>
>> ===<tt>pk()</tt>===
>>
>> The <tt>pk(KEY)</tt> expression can be used in any context or level of a
>> descriptor.
>> It takes a single key expression as an argument and produces a P2PK
>> output script.
>> Depending on the higher level descriptors, there may be restrictions on
>> the type of public keys that can be included.
>> Such restrictions will be specified by those descriptors.
>>
>> The output script produced is:
>> <pre>
>> <KEY> OP_CHECKSIG
>> </pre>
>>
>> ===<tt>pkh()</tt>===
>>
>> The <tt>pkh(KEY)</tt> expression can be used as a top level expression,
>> or inside of either a <tt>sh()</tt> or <tt>wsh()</tt> descriptor.
>> It takes a single key expression as an argument and produces a P2PKH
>> output script.
>> Depending on the higher level descriptors, there may be restrictions on
>> the type of public keys that can be included.
>> Such restrictions will be specified by those descriptors.
>>
>> The output script produced is:
>> <pre>
>> OP_DUP OP_HASH160 <KEY_hash160> OP_EQUALVERIFY OP_CHECKSIG
>> </pre>
>>
>> ===<tt>sh()</tt>===
>>
>> The <tt>sh(SCRIPT)</tt> expression can only be used as a top level
>> expression.
>> It takes a single script expression as an argument and produces a P2SH
>> output script.
>> <tt>sh()</tt> expressions also create a redeemScript which is required
>> in order to spend outputs which use its output script.
>> This redeemScript is the output script produced by the <tt>SCRIPT</tt>
>> argument to <tt>sh()</tt>.
>>
>> The output script produced is:
>> <pre>
>> OP_HASH160 <SCRIPT_hash160> OP_EQUAL
>> </pre>
>>
>> ==Test Vectors==
>>
>> TBD
>>
>> ==Backwards Compatibility==
>>
>> <tt>pk()</tt>, <tt>pkh()</tt>, and <tt>sh()</tt> descriptors use the
>> format and general operation specified in
>> [[bip-descriptor-general.mediawiki|bip-descriptor-general]].
>> As these are a wholly new descriptors, they are not compatible with any
>> implementation.
>> However the scripts produced are standard scripts so existing software
>> are likely to be familiar with them.
>>
>> ==Reference Implemntation==
>>
>> <tt>pk()</tt>, <tt>pkh()</tt>, and <tt>sh()</tt> descriptors have been
>> implemented in Bitcoin Core since version 0.17.
>>
>> ---
>>
>> <pre>
>> BIP: bip-descriptors-tr
>> Layer: Applications
>> Title: tr() Output Script Descriptors
>> Author: Pieter Wuille <pieter@wuille•net>
>> Andrew Chow <andrew@achow101•com>
>> Comments-Summary: No comments yet.
>> Comments-URI:
>> https://github.com/bitcoin/bips/wiki/Comments:BIP-descriptors-tr
>> Status: Draft
>> Type: Informational
>> Created: 2021-06-27
>> License: BSD-2-Clause
>> </pre>
>>
>> ==Abstract==
>>
>> This document specifies <tt>tr()</tt> output script descriptors.
>> <tt>tr()</tt> descriptors take a key and optionally a tree of scripts
>> and produces a P2TR output script.
>>
>> ==Copyright==
>>
>> This BIP is licensed under the BSD 2-clause license.
>>
>> ==Motivation==
>>
>> Taproot added one additional standard output script format: P2TR.
>> These expressions allow specifying those formats as a descriptor.
>>
>> ==Specification==
>>
>> A new script expressions are defined: <tt>tr()</tt>.
>> A new expression is defined: Tree Expressions
>>
>> ===Tree Expression===
>>
>> A Tree Expression (denoted <tt>TREE</tt>) is an expression which
>> represents a tree of scripts.
>> The way the tree is represented in an output script is dependent on the
>> higher level expressions.
>>
>> A Tree Expression is:
>> * Any Script Expression that is allowed at the level this Tree
>> Expression is in.
>> * A pair of Tree Expressions consisting of:
>> ** An open brace <tt>{</tt>
>> ** A Tree Expression
>> ** A comma <tt>,</tt>
>> ** A Tree Expression
>> ** A closing brance <tt>}</tt>
>>
>> ===<tt>tr()</tt>===
>>
>> The <tt>tr(KEY)</tt> or <tt>tr(KEY, TREE)</tt> expression can only be
>> used as a top level expression.
>> All key expressions under any <tt>tr()</tt> expression must create
>> x-only public keys.
>>
>> <tt>tr(KEY</tt> takes a single key expression as an argument and
>> produces a P2TR output script which does not have a script path.
>> The keys produced by the key expression are used as the internal key as
>> specified by [[bip-0341.mediawiki#cite_ref-22-0|BIP 341]].
>> Specifically, "If the spending conditions do not require a script path,
>> the output key should commit to an unspendable script path instead of
>> having no script path.
>> This can be achieved by computing the output key point as ''Q = P +
>> int(hash<sub>TapTweak</sub>(bytes(P)))G''."
>>
>> <pre>
>> internal_key: lift_x(KEY)
>> 32_byte_output_key: internal_key + int(HashTapTweak(bytes(internal_key)))G
>> scriptPubKey: OP_1 <32_byte_output_key>
>> </pre>
>>
>> <tt>tr(KEY, TREE)</tt> takes a key expression as the first argument, and
>> a tree expression as the second argument and produces a P2TR output
>> script which has a script path.
>> The keys produced by the first key expression are used as the internal
>> key as specified by
>> [[bip-0341.mediawiki#Constructing_and_spending_Taproot_outputs|BIP 341]].
>> The Tree expression becomes the Taproot script tree as described in BIP 341.
>> A merkle root is computed from this tree and combined with the internal
>> key to create the Taproot output key.
>>
>> <pre>
>> internal_key: lift_x(KEY)
>> merkle_root: HashTapBranch(TREE)
>> 32_byte_output_key: internal_key + int(HashTapTweak(bytes(internal_key)
>> || merkle_root))G
>> scriptPubKey: OP_1 <32_byte_output_key>
>> </pre>
>>
>> ==Test Vectors==
>>
>> TBD
>>
>> ==Backwards Compatibility==
>>
>> <tt>tr()</tt> descriptors use the format and general operation specified
>> in [[bip-descriptor-general.mediawiki|bip-descriptor-general]].
>> As these are a wholly new descriptors, they are not compatible with any
>> implementation.
>> However the scripts produced are standard scripts so existing software
>> are likely to be familiar with them.
>>
>> Tree Expressions are largely incompatible with existing script
>> expressions due to the restrictions in those expressions.
>> As of 2021-06-27, the only allowed script expression that can be used in
>> a tree expression is <tt>pk()</tt>.
>> However there will be future BIPs that specify script expressions that
>> can be used in tree expressions.
>>
>> ==Reference Implemntation==
>>
>> <tt>tr()</tt> descriptors have been implemented in Bitcoin Core since
>> version 22.0.
>>
>> ---
>>
>> <pre>
>> BIP: bip-descriptors-multi
>> Layer: Applications
>> Title: Multisig Output Script Descriptors
>> Author: Pieter Wuille <pieter@wuille•net>
>> Andrew Chow <andrew@achow101•com>
>> Comments-Summary: No comments yet.
>> Comments-URI:
>> https://github.com/bitcoin/bips/wiki/Comments:BIP-descriptors-multi
>> Status: Draft
>> Type: Informational
>> Created: 2021-06-27
>> License: BSD-2-Clause
>> </pre>
>>
>> ==Abstract==
>>
>> This document specifies <tt>multi()</tt>, and <tt>sortedmulti()</tt>
>> output script descriptors.
>> Both functions take a threshold and one or more public keys and produce
>> a multisig output script.
>> <tt>multi()</tt> specifies the public keys in the output script in the
>> order given in the descriptor while <tt>sortedmulti()</tt> sorts the
>> public keys lexicographically when the output script is produced.
>>
>> ==Copyright==
>>
>> This BIP is licensed under the BSD 2-clause license.
>>
>> ==Motivation==
>>
>> The most common complex script used in Bitcoin is a threshold multisig.
>> These expressions allow specifying multisig scripts as a descriptor.
>>
>> ==Specification==
>>
>> Two new script expressions are defined: <tt>multi()</tt>, and
>> <tt>sortedmulti()</tt>.
>> Both expressions produce the scripts of the same template and take the
>> same arguments.
>> They are written as <tt>multi(k,KEY_1,KEY_2,...,KEY_n)</tt>.
>> <tt>k</tt> is the threshold - the number of keys that must sign the
>> input for the script to be valid.
>> <tt>KEY_1,KEY_2,...,KEY_n</tt> are the key expressions for the multisig.
>> <tt>k</tt> must be less than or equal to <tt>n<tt>.
>>
>> <tt>multi()</tt> and <tt>sortedmulti()</tt> expressions can be used as a
>> top level expression, or inside of either a <tt>sh()</tt> or
>> <tt>wsh()</tt> descriptor.
>> Depending on the higher level descriptors, there may be restrictions on
>> the type of public keys that can be included.
>>
>> Depending on the higher level descriptors, there are also restrictions
>> on the number of keys that can be present, i.e. the maximum value of
>> <tt>n</tt>.
>> When used at the top level, there can only be at most 3 keys.
>> When used inside of a <tt>sh()</tt> expression, there can only be most
>> 15 compressed public keys (this is limited by the P2SH script limit).
>> Otherwise the maximum number of keys is 20.
>>
>> The output script produced also depends on the value of <tt>k</tt>. If
>> <tt>k</tt> is less than or equal to 16:
>> <pre>
>> OP_k KEY_1 KEY_2 ... KEY_n OP_CHECKMULTISIG
>> </pre>
>>
>> if <tt>k</tt> is greater than 16:
>> <pre>
>> k KEY_1 KEY_2 ... KEY_n OP_CHECKMULTISIG
>> </pre>
>>
>> ===<tt>sortedmulti()</tt>===
>>
>> The only change for <tt>sortedmulti()</tt> is that the keys are sorted
>> lexicographically prior to the creation of the output script.
>> This sorting is on the keys that are to be put into the output script,
>> i.e. after all extended keys are derived.
>>
>> ===Multiple Extended Keys</tt>===
>>
>> When one or more the key expressions in a <tt>multi()</tt> or
>> <tt>sortedmulti()</tt> expression are extended keys, the derived keys
>> use the same child index.
>> This changes the keys in lockstep and allows for output scripts to be
>> indexed in the same way that the derived keys are indexed.
>>
>> ==Test Vectors==
>>
>> TBD
>>
>> ==Backwards Compatibility==
>>
>> <tt>multi()</tt>, and <tt>sortedmulti()</tt> descriptors use the format
>> and general operation specified in
>> [[bip-descriptor-general.mediawiki|bip-descriptor-general]].
>> As these are a wholly new descriptors, they are not compatible with any
>> implementation.
>> However the scripts produced are standard scripts so existing software
>> are likely to be familiar with them.
>>
>> ==Reference Implemntation==
>>
>> <tt>multi()</tt>, and <tt>multi()</tt> descriptors have been implemented
>> in Bitcoin Core since version 0.17.
>>
>> ---
>>
>> <pre>
>> BIP: bip-descriptors-combo
>> Layer: Applications
>> Title: combo() Output Script Descriptors
>> Author: Pieter Wuille <pieter@wuille•net>
>> Andrew Chow <andrew@achow101•com>
>> Comments-Summary: No comments yet.
>> Comments-URI:
>> https://github.com/bitcoin/bips/wiki/Comments:BIP-descriptors-combo
>> Status: Draft
>> Type: Informational
>> Created: 2021-06-27
>> License: BSD-2-Clause
>> </pre>
>>
>> ==Abstract==
>>
>> This document specifies <tt>combo()</tt> output script descriptors.
>> These take a key and produce P2PK, P2PKH, P2WPKH, and P2SH-P2WPKH output
>> scripts if applicable to the key.
>>
>> ==Copyright==
>>
>> This BIP is licensed under the BSD 2-clause license.
>>
>> ==Motivation==
>>
>> In order to make the transition from traditional key based wallets to
>> descriptor based wallets easier, it is useful to be able to take a key
>> and produce the scripts which have traditionally been produced by wallet
>> software.
>>
>> ==Specification==
>>
>> A new top level script expression is defined: <tt>combo(KEY)</tt>.
>> This expression can only be used as a top level expression.
>> It takes a single key expression as an argument and produces either 2 or
>> 4 output scripts, depending on the key.
>> A <tt>combo()</tt> expression always produces a P2PK and P2PKH script,
>> the same as putting the key in both a <tt>pk()</tt> and a <tt>pkh()</tt>
>> expression.
>> If the key is/has a compressed public key, then P2WPKH and P2SH-P2WPKH
>> scripts are also produced, the same as putting the key in both a
>> <tt>wpkh()</tt> and <tt>sh(wpkh())</tt> expression.
>>
>> ==Test Vectors==
>>
>> TBD
>>
>> ==Backwards Compatibility==
>>
>> <tt>combo()</tt> descriptors use the format and general operation
>> specified in [[bip-descriptor-general.mediawiki|bip-descriptor-general]].
>> As this is a wholly new descriptor, it is not compatible with any
>> implementation.
>> However the scripts produced are standard scripts so existing software
>> are likely to be familiar with them.
>>
>> ==Reference Implemntation==
>>
>> <tt>combo</tt> descriptors have been implemented in Bitcoin Core since
>> version 0.17.
>>
>> ---
>>
>> <pre>
>> BIP: bip-descriptors-encap
>> Layer: Applications
>> Title: raw() and addr() Output Script Descriptors
>> Author: Andrew Chow <andrew@achow101•com>
>> Pieter Wuille <pieter@wuille•net>
>> Comments-Summary: No comments yet.
>> Comments-URI:
>> https://github.com/bitcoin/bips/wiki/Comments:BIP-descriptors-raw
>> Status: Draft
>> Type: Informational
>> Created: 2021-06-27
>> License: BSD-2-Clause
>> </pre>
>>
>> ==Abstract==
>>
>> This document specifies <tt>raw()</tt> and <tt>addr()</tt> output script
>> descriptors.
>> <tt>raw()</tt> encapsulates a raw script as a descriptor.
>> <tt>addr()</tt> encapsulates an address as a descriptor.
>>
>> ==Copyright==
>>
>> This BIP is licensed under the BSD 2-clause license.
>>
>> ==Motivation==
>>
>> In order to make descriptors maximally compatible with scripts in use
>> today, it is useful to be able to wrap any arbitrary output script or an
>> address into a descriptor.
>>
>> ==Specification==
>>
>> Two new script expressions are defined: <tt>raw()</tt> and <tt>addr()</tt>.
>>
>> ===<tt>raw()</tt>===
>>
>> The <tt>raw(HEX)</tt> expression can only be used as a top level descriptor.
>> As the argument, it takes a hex string representing a Bitcoin script.
>> The output script produced by this descriptor is the script represented
>> by <tt>HEX</tt>.
>>
>> ===<tt>addr()</tt>===
>>
>> The <tt>addr(ADDR)</tt> expression can only be used as a top level
>> descriptor.
>> It takes an address as its single argument.
>> The output script produced by this descriptor is the output script
>> produced by the address <tt>ADDR</tt>.
>>
>> ==Test Vectors==
>>
>> TBD
>>
>> ==Backwards Compatibility==
>>
>> <tt>raw()</tt> and <tt>addr()</tt> descriptors use the format and
>> general operation specified in
>> [[bip-descriptor-general.mediawiki|bip-descriptor-general]].
>> As this is a wholly new descriptor, it is not compatible with any
>> implementation.
>> The reuse of existing Bitcoin addresses allows for this to be more
>> easily implemented.
>>
>> ==Reference Implemntation==
>>
>> <tt>raw()</tt> and <tt>addr</tt> descriptors have been implemented in
>> Bitcoin Core since version 0.17.
>>
>> _______________________________________________
>> bitcoin-dev mailing list
>> bitcoin-dev@lists•linuxfoundation.org
>> https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev

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