Subject: Re: [bitcoin-dev] Wallet vaults with pre-signed transactions but no ephemeral keys

From: darosior <[email-masked]>
Date: Thu, 26 Jan 2023 14:30:56 +0000

Hello Billy,

Yes it's basically a (simple) instantiation of Revault. You can find more at [https://github.com/revault](https://github.com/revault/) (you most likely want the `practical-revault` repo). There is a description of the concept, the specification of a communication protocol between the participants as well as the implementation of the whole.

Such a design presents some advantages, but it has two major issues:

- You need to make sure all your watchtowers received the Cancel signature before you sign the Unvault transaction. But how can you get this guarantee in the usual (and reasonable) model of an untrusted laptop?
- You can only have policies on the Unvault transaction (eg "You can only Unvault up to X BTC during working hours"), not on the Spend transaction (eg "You can only send coins to a Script with pubkey Y required in all spending paths"). Revault allows to use cosigning servers that act as anti-replay oracles to have policies on the spend, but this is obviously *very* suboptimal.

Both issues are solvable with covenants.

Antoine Poinsot
------- Original Message -------
Le lundi 23 janvier 2023 à 6:39 PM, Billy Tetrud via bitcoin-dev <[email-masked]> a écrit :

> In the discussion around James' OP_VAULT proposal, it was implied that precomputed vaults must use ephemeral keys that must be deleted as part of the vaulting protocol, like [Bryan Bishop's proposal](https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2019-August/017229.html). Looking around, I haven't been able to find any wallet vault proposal that doesn't require ephemeral keys, so at the risk of posting something that's obvious to everyone, I wanted to share a simple way to do a wallet vault without requiring any key deletion.
>
> The basic idea is to create an N-of-N multisig address, and pre-sign some transactions from it with N-1 keys to an address with several timelocked spend paths. This has the fallback that funds can always be spent immediately if you use all the keys, just like a normal N-of-N multisig address (since that's what it is at its core), but the usage of any of the pre-signed transactions leads to an address that guarantees a clawback within a time window. Here's a 3-of-3 example:
>
> Vault Initialization:
> 1. Create 3 of 3 Vault Address
> 2. Create an Interim Address that can send with:
> * 1 of 3 keys after a timelock of 1 month
> * 2 of 3 keys after a timelock of 1 week
> * 3 of 3 keys with no timelock
>
> Vaulting:
> 1. Create a transaction sending an output to the Vault Address
> 2. Create a transaction spending that Vault Address output to the Interim Address
> 3. Presign one copy of the step-2 transaction for each of the three combinations of two keys.
> 4. Store seeds separately, store the wallet config as well as the three presigned transactions with each seed.
>
> Unvaulting:
> 1. Sign one of the pre-signed transactions with the missing signature.
> 2. Broadcast
> 3. Wait the appropriate timelock for the number of keys you want to sign with.
> 4. Create a transaction sending from the Interim Address.
> 5. Broadcast
> Recovering (after unvaulting step 2 after the broadcasted transaction to the Interim Address has been mined):
> 1. Sign the utxo with all three keys to any destination. Alternatively sign with two keys, wait 1 week.
> 2. Broadcast it
>
> This has the usual downsides of pre-signed vaults that you need to backup these transactions for each vaulting, complications involving the flexibility (or lack thereof) of fees, and inflexibility in how much to unvault (must be the whole utxo, no change). This could of course be augmented with various techniques to make fee handling more flexible (anchor outputs, multiple versions of the presigned transactions with different fees, etc). More complicated presigning schemes could allow for some flexibility in unvaulting amount (eg by sending change back into the vault, and creating additional pre-signed transactions for that new output).
>
> It also has the same downside that OP_CTV vaults have, where a stolen key can steal funds from the interim address by racing the owner with their own transaction once the necessary delay has passed. Note that James' OP_VAULT opcode wouldn't have this problem.
>
> But not requiring any toxic waste keys seems like a pretty good benefit over Bryan Bishop's original proposal.
>
> Anyways sorry if this was already on people's radar and just too obvious to post about.