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Multisig Derivation Standard.mediawiki
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Multisig Derivation Standard.mediawiki
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<pre>
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BIP: Number not assigned
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Layer: Applications
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Title: Modern Hierarchy for Deterministic Multisignature Wallets
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Author: Robert Spigler <RobertSpigler@ProtonMail.ch>
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Comments-Summary: No comments
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Comments-URI:
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Status: Proposed
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Type: Standards Track
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Created: 2020-03-11
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</pre>
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===Copyright===
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This BIP is licensed under the 2-clause BSD license.
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==Abstract==
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This BIP defines a sane hierarchy for deterministic multisig wallets based on an algorithm described in BIP-0032 (BIP32 from now on), purpose scheme described in
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BIP-0043 (BIP43 from now on), and multi-account hierarchy described in BIP-0044 (BIP44 from now on).
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This BIP is a particular application of BIP43.
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==Motivation==
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With the increase of more user friendly (offline) multisignature wallets, and adoption of new technologies such as
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[https://github.com/bitcoin/bitcoin/blob/master/doc/descriptors.md the descriptor language] and [https://github.com/bitcoin/bips/blob/master/bip-0174.mediawiki
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BIP-0174 (Partially Signed Bitcoin Transactions)] it is necessary to create a common derivation scheme that makes use of all new technologies.
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There are many issues with the current standards. As background, BIP 44/49/84 specifies:
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<pre>
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m / purpose' / coin_type' / account' / change / address_index
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</pre>
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where the BIP43 <code>purpose'</code> path is separate for each script (P2PKH, P2WPKH-in-P2SH, and P2WPKH respectively). However, these per-script derivations
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are made redundant with descriptors, which describe a collection of output scripts. Note also that these are single sig derivations (which souldn't be reused for
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multisig).
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Standardization is needed for multisig derivation paths. There are some in existence, but all have issues. For example, BIP45 specifies:
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<pre>
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m / purpose' / cosigner_index / change / address_index
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</pre>
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Like BIP44/49/84, BIP45 unecessarily demands a single script type (here, P2SH). In addition, BIP45 sets <code>cosigner_index</code> in order to sort the
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<code>purpose'</code> public keys of each cosigner. This too is redundant, as descriptors can set the order of the public keys with <code>multi</code> or have
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them sorted lexicographically (as described in [https://github.com/bitcoin/bips/blob/master/bip-0067.mediawiki BIP67) with <code>sortedmulti</code>. Sorting
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public keys between cosigners in order to create the full derivation path, prior to sending the key record to the coordinator to create the descriptor, merely adds
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additional unnecessary communication rounds.
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There is an additional multisig derivation path in use "BIP"48 (there is no real standard), which specifies:
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<pre>
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m / purpose' / coin_type' / account' / script_type' / change / address_index
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</pre>
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Rather than having a separate BIP per script type after P2SH (BIP45), vendors decided to insert <code>script_type'</code> into the derivation path (where
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P2SH-P2WSH=1, P2WSH=2, Future_Script=3, etc). As described previously, this is unnecessary, as the descriptor sets the script. While it attempts to reduce
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maintainence work by getting rid of new BIPs-per-script, it still requires maintaining an updated, redundant, script_type list.
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The hierarchy proposed in this paper is quite comprehensive. It allows the handling of multiple accounts, external and internal chains per account, and millions
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of addresses per chain.
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Any script that is supported by descriptors (and the specific wallet implementation) is compatible with this BIP.
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This paper was inspired from BIP44.
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==Key sorting==
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Any wallet that supports descriptors inherently supports deterministic key sorting as per BIP67 (through the <code>sortedmulti</code> function) so that all
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possible multisignature addresses/scripts are derived from deterministically sorted public keys.
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==Path levels==
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You should not be mixing keys and scripts. We define the following 5 levels in the BIP32 path:
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<pre>
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m / purpose' / coin_type' / account' / change / address_index
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</pre>
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<code>h</code> or <code>'</code> in the path indicates that BIP32 hardened derivation is used.
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Each level has a special meaning, described in the chapters below.
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===Purpose===
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Purpose is a constant set to XY' following the BIP43 recommendation.
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It indicates that the subtree of this node is used according to this specification.
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Hardened derivation is used at this level.
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===Coin type===
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One master node (seed) can be used for multiple Bitcoin networks.
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Sharing the same space for various networks has some disadvantages.
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Avoiding reusing addresses across networks and improving privacy issues.
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Coin type <code>0</code> for mainnet and <code>1</code> for testnet.
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Hardened derivation is used at this level.
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===Account===
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This level splits the key space into independent user identities, following the BIP44 pattern, so the wallet never mixes the coins across different accounts.
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Users can use these accounts to organize the funds in the same fashion as bank accounts; for donation purposes (where all addresses are considered public), for
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saving purposes, for common expenses, etc.
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Accounts are numbered from index 0 in sequentially increasing manner.
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This number is used as child index in BIP32 derivation.
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Hardened derivation is used at this level.
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===Change===
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Constant 0 is used for external chain and constant 1 for internal chain (also known as change addresses). External chain is used for addresses that are meant to be
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visible outside of the wallet (e.g. for receiving payments). Internal chain is used for addresses which are not meant to be visible outside of the wallet and is
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used for return transaction change.
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Public derivation is used at this level.
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===Index===
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Addresses are numbered from index 0 in sequentially increasing manner.
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This number is used as child index in BIP32 derivation.
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Public derivation is used at this level.
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==Examples==
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{|
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|network
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|account
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|chain
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|address
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|path
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|-
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|mainnet
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|first
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|external
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|first
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|m / XY' / 0' / 0' / 0 / 0
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|-
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|mainnet
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|first
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|external
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|second
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|m / XY' / 0' / 0' / 0 / 1
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|-
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|mainnet
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|first
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|change
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|first
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|m / XY' / 0' / 0' / 1 / 0
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|-
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|mainnet
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|first
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|change
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|second
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|m / XY' / 0' / 0' / 1 / 1
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|-
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|mainnet
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|second
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|external
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|first
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|m / XY' / 0' / 1' / 0 / 0
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|-
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|mainnet
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|second
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|external
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|second
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|m / XY' / 0' / 1' / 0 / 1
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|-
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|testnet
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|first
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|external
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|first
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|m / XY' / 1' / 0' / 0 / 0
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|-
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|testnet
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|first
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|external
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|second
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|m / XY' / 1' / 0' / 0 / 1
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|-
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|testnet
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|first
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|change
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|first
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|m / XY' / 1' / 0' / 1 / 0
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|-
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|testnet
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|first
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|change
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|second
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|m / XY' / 1' / 0' / 1 / 1
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|-
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|testnet
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|second
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|external
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|first
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|m / XY' / 1' / 1' / 0 / 0
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|-
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|testnet
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|second
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|external
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|second
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|m / XY' / 1' / 1' / 0 / 1
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|-
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|testnet
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|second
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|change
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|first
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|m / XY' / 1' / 1' / 1 / 0
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|-
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|testnet
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|second
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|change
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|second
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|m / XY' / 1' / 1' / 1 / 1
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|}
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==Acknowledgement==
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==References==
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Original mailing list thread: TBD
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* [[bip-0032.mediawiki|BIP32 - Hierarchical Deterministic Wallets]]
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* [[bip-0043.mediawiki|BIP43 - Purpose Field for Deterministic Wallets]]
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* [[bip-0044.mediawiki|BIP44 - Multi-Account Hierarchy for Deterministic Wallets]]
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* [[https://github.com/bitcoin/bitcoin/blob/master/doc/descriptors.md - Output Descriptors]]
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* [[bip-0174.mediawiki|BIP174 - Partially Signed Bitcoin Transaction Format]]
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* [[bip-0067.mediawiki|BIP67 - Deterministic Pay-to-script-hash multi-signature addresses through public key sorting]]
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