The motivation for this BIP proposal states that encoding the script type in the derivation is redundant because output descriptors are becoming commonplace. However, I think it's important to note that this shifts more burden of wallet backup onto the user, who now has an additional item (the output descriptor) to backup correctly. This is particularly important when we consider that performing this backup is often laborious, including stamping onto metal etc.

There are two ways to go here:
1. Use derivation paths that do not encode the script type, and conduct a global education effort to encourage users to backup output descriptors in addition to their seed words, without which their funds may be lost.
2. Use a standard set of derivation paths that encode the script type, ensuring that even if users do not back this up, they can check against a handful of common derivations and recover the funds.

Option 1 is obviously cleaner and more precise from a technical POV, but I feel it is less practical. The approach of using common words to encode the seed was chosen because these words are short and familiar - an output descriptor on the other hand is technical and longer form, and thus more difficult to record correctly, particularly on damage-resistant media. 

Craig

On Tue, Mar 16, 2021 at 2:19 PM Robert Spigler via bitcoin-dev <bitcoin-dev@lists.linuxfoundation.org> wrote:
No, wallets don't and shouldn't have to check all script types on recovery.  Descriptor Wallets solve all of this.

To back up a multisignature wallet, each cosigner stores their xprv (how you do this; BIP39, WIF, etc, is out of scope). and the wallet descriptor.  This is done, for example, in Yeti. To recover, simply combine `M` private keys, and check the script designated in 1 of the descriptor copies.

For single signature wallets, it is the same, except only one signature is needed.  Store xprv and descriptor.

It is not fair nor accurate to say that currently, in order to recover, wallets need "just the seed words".  They also need all public keys, and derivation paths.

Descriptors (and this BIP), is a much cleaner way to handle wallet creation and backup, by separating the two layers (keys and scripts) and getting rid of redundant information.


Personal Fingerprint:  BF0D 3C08 A439 5AC6 11C1 5395 B70B 4A77 F850 548F



‐‐‐‐‐‐‐ Original Message ‐‐‐‐‐‐‐
On Sunday, March 14, 2021 11:13 AM, SomberNight <somber.night@protonmail.com> wrote:

> See some replies inline. (quoted text from BIP draft)
>
> > Date: Sun, 14 Mar 2021 01:51:15 +0000
> > From: Robert Spigler RobertSpigler@protonmail.ch
> > Subject: [bitcoin-dev] Signature and Script Independent Hierarchy for Deterministic Wallets.
>
> > There are many issues with the current standards. As background, BIP 44/49/84 specifies:
> > `m / purpose' / coin_type' / account' / change / address_index`
> > where the BIP43 `purpose'` path is separate for each script (P2PKH, P2WPKH-in-P2SH, and P2WPKH respectively). However, these per-script derivations are made redundant with descriptors
>
> > We should not be mixing keys and scripts in the same layer. The wallet should create extended private/public keys independent of the script or signature type
>
> You say that keys and scripts should not be mixed in the same layer, and imply that this was solely done due to these standards predating output script descriptors. Even if this was the case, it is not the only reason for doing it. BIP44/49/84 mixing scripts and keys in the same layer makes recovery from seed/mnemonic much easier.
> Note the significant overlap between the authors of BIP39 and BIP44. I am fairly certain BIP44 was designed with recovering from a BIP39 seed (and no additional information backed up) in mind. Note the "Account discovery" section of BIP44.
> (Electrum seeds go even further, as such seeds contain a version number that encodes both the script type and the key derivation path to use.)
>
> > We define the following 5 levels in the BIP32 path:
> > `m / purpose' / coin_type' / account' / change / address_index`
>
> > [Account]
> > It is crucial that this level is increased for each new wallet joined or private/public keys created; for both privacy and cryptographic purposes.
> > For example, in multisignature wallets, before sending a new key record to a coordinator, the wallet must increment the `account'` level. Before creating it's own single signature wallet, the `account'` level must again be incremented.
>
> Imagine a user who has a BIP39 (or similar) seed. Even today, recovering most non-singlesig scripts from that is obviously infeasible. However, all singlesig scripts at least can be discovered if the keys are using the suggested derivation paths.
> By trying to create a standard that mixes discoverable and non-discoverable scripts in the same derivation scheme and incrementing a single index, you are turning all scripts into being non-discoverable.
> Note that even if a user only used singlesig scripts and followed this proposal, during recovery from seed the wallet would have to check all script types for all account indices (which is only ever going to get more expensive as new script types come).
> The workaround and I imagine your suggested solution is clearly to backup both seed words and output script descriptors; and to keep appending new output script descriptors to existing backups when the account index is incremented. While much less user-friendly than backing up just a seed, it is more generic and extendable.
>
> My point is simply that your proposal is making a tradeoff here. The tradeoff itself seems easy to miss on first read of the text, so I just wanted to explicitly point it out for the record.
>
> ghost43 / SomberNight


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