BIP 328: add MuSig2 derivation BIP

Author: achow101

README.mediawiki

@@ -1016,6 +1016,13 @@ Those proposing changes should consider that ultimately consent may rest with th
 | Informational
 | Draft
 |-
+| [[bip-0328.mediawiki|328]]
+| Applications
+| Derivation Scheme for MuSig2 Aggregate Keys
+| Ava Chow
+| Informational
+| Draft
+|-
 | [[bip-0329.mediawiki|329]]
 | Applications
 | Wallet Labels Export Format

bip-0328.mediawiki

@@ -0,0 +1,80 @@
+<pre>
+  BIP: 328
+  Layer: Applications
+  Title: Derivation Scheme for MuSig2 Aggregate Keys
+  Author: Ava Chow <[email protected]>
+  Comments-Summary: No comments yet.
+  Comments-URI: https://github.com/bitcoin/bips/wiki/Comments:BIP-0328
+  Status: Draft
+  Type: Informational
+  Created: 2024-01-15
+  License: CC0-1.0
+</pre>
+
+==Abstract==
+
+This document specifies how BIP 32 extended public keys can be constructed from a BIP 327 MuSig2
+aggregate public key and how such keys should be used for key derivation.
+
+==Copyright==
+
+This BIP is licensed under the Creative Commons CC0 1.0 Universal license.
+
+==Motivation==
+
+Multiple signers can create a single aggregate public key with MuSig2 that is indistinguishable
+from a random public key. The cosigners need a method for generating additional aggregate pubkeys
+to follow the best practice of using a new address for every payment.
+
+The obvious method is for the cosigners to generate multiple public keys and produce a
+new aggregate pubkey every time one is needed. This is similar to how multisig using Bitcoin script
+works where all of the cosigners share their extended public keys and do derivation to produce
+the multisig script. The same could be done with MuSig2 and instead of producing a multisig script,
+the result would be a MuSig2 aggregate pubkey.
+
+However, it is much simpler to be able to derive from a single extended public key instead of having
+to derive from many extended public keys and aggregate them. As MuSig2 produces a normal looking
+public key, the aggregate public can be used in this way. This reduces the storage and computation
+requirements for generating new aggregate pubkeys.
+
+==Specification==
+
+A synthetic xpub can be created from a BIP 327 MuSig2 plain aggregate public key by setting
+the depth to 0, the child number to 0, and attaching a chaincode with the byte string
+<tt>868087ca02a6f974c4598924c36b57762d32cb45717167e300622c7167e38965</tt><ref>'''Where does this
+constant chaincode come from?''' It is the SHA256 of the text <tt>MuSig2MuSig2MuSig2</tt></ref>.
+This fixed chaincode should be used by all such synthetic xpubs following this specification.
+Unhardened child public keys can be derived from the synthetic xpub as with any other xpub. Since
+the aggregate public key is all that is necessary to produce the synthetic xpub, any aggregate
+public key that will be used in this way shares the same privacy concerns as typical xpubs.
+
+Furthermore, as there is no aggregate private key, only unhardened derivation from the aggregate
+public key is possible.
+
+When signing, all signers must compute the tweaks used in the BIP 32 derivation for the child key
+being signed for. The I<sub>L</sub> value computed in ''CKDpub'' is the tweak used at each
+derivation step. These are provided in the session context, each with a tweak mode of plain
+(''is_xonly_t = false''). When the ''Sign'' algorithm is used, the tweaks will be applied to the
+partial signatures.
+
+==Test Vectors==
+
+TBD
+
+==Backwards Compatibility==
+
+Once a synthetic xpub is created, it is fully backwards compatible with BIP 32 - only unhardened
+derivation can be done, and the signers will be able to produce a signature for any derived children.
+
+==Rationale==
+
+<references/>
+
+==Reference Implementation==
+
+TBD
+
+==Acknowledgements==
+
+Thanks to Pieter Wuille, Andrew Poelstra, Sanket Kanjalkar, Salvatore Ingala, and all others who
+participated in discussions on this topic.