[bitcoin-dev] BIP proposal for Lightning-oriented multiaccount multisig HD wallets

[Simone Bronzini <simone.bronzini@chainside.net>]

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Hi all,
last month we started looking for feedback (here and on other channels)
about a proposal for a new structure to facilitate the management of
different multisig accounts under the same master key, avoiding key
reuse but still allowing cosigners to independently generate new
addresses. While previously multiaccount multisig wallets were little
used, now that LN is becoming a reality it is extremely important to
have a better multiaccount management method to handle multiple payment
channels.
Please have a look at the draft of the BIP at the link below:

https://github.com/chainside/BIP-proposal/blob/master/BIP.mediawiki

Any feedback is highly appreciated, but in particular we would like to
collect opinions about the following issues:

1. coin_type level:
this level is intended to allow users to manage multiple
cryptocurrencies or forks of Bitcoin using the same masterkey (similarly
to BIP44). We have already received some legit objections that, since we
are talking about a Bitcoin Improvement Proposal,  it shouldn't care
about alt-coins. While we can agree with such objections, we also
believe that having a coin_type level improves interoperability with
muti-currency wallets (which is good), without any major drawback.
Moreover, even a Bitcoin maximalist may hold multiple coins for whatever
reason (short term speculation, testing, etc).

2. SegWit addresses:
since mixing SegWit and non-SegWit addresses on the same BIP44 structure
could lead to UTXOs not being completely recognised by old wallets,
BIP49 was proposed to separate the key space. Since this is a new
proposal, we can assume that wallets implementing it would be
SegWit-compatible and so there should be no need to differetiate between
SegWit and non-SegWit pubkeys. Anyway, if someone believes this problem
still holds, we thought about two possible solutions:
    a. Create separate purposes for SegWit and non SegWit addresses
(this would keep the same standard as BIP44 and BIP49)
    b. Create a new level on this proposed structure to divide SegWit
and non SegWit addresses: we would suggest to add this new level between
cosigner_index and change

We believe solution b. would be better as it would give the option of
having a multisig wallet with non SegWit-aware cosigners without having
to use two different subtrees.

This proposal is a work in progess so we would like to receive some
feedback before moving on with proposing it as a BIP draft.

Simone Bronzini

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