bitcoin/doc/release-notes-16528.md

Wallet
------

### Experimental Descriptor Wallets

Please note that Descriptor Wallets are still experimental and not all expected functionality
is available. Additionally there may be some bugs and current functions may change in the future.
Bugs and missing functionality can be reported to the [issue tracker](https://github.com/bitcoin/bitcoin/issues).

0.21 introduces a new type of wallet - Descriptor Wallets. Descriptor Wallets store
scriptPubKey information using descriptors. This is in contrast to the Legacy Wallet
structure where keys are used to generate scriptPubKeys and addresses. Because of this
shift to being script based instead of key based, many of the confusing things that Legacy
Wallets do are not possible with Descriptor Wallets. Descriptor Wallets use a definition
of "mine" for scripts which is simpler and more intuitive than that used by Legacy Wallets.
Descriptor Wallets also uses different semantics for watch-only things and imports.

As Descriptor Wallets are a new type of wallet, their introduction does not affect existing wallets.
Users who already have a Bitcoin Core wallet can continue to use it as they did before without
any change in behavior. Newly created Legacy Wallets (which is the default type of wallet) will
behave as they did in previous versions of Bitcoin Core.

The differences between Descriptor Wallets and Legacy Wallets are largely limited to non user facing
things. They are intended to behave similarly except for the import/export and watchonly functionality
as described below.

#### Creating Descriptor Wallets

Descriptor Wallets are not created by default. They must be explicitly created using the
`createwallet` RPC or via the GUI. A `descriptors` option has been added to `createwallet`.
Setting `descriptors` to `true` will create a Descriptor Wallet instead of a Legacy Wallet.

In the GUI, a checkbox has been added to the Create Wallet Dialog to indicate that a
Descriptor Wallet should be created.

Without those options being set, a Legacy Wallet will be created instead. Additionally the
Default Wallet created upon first startup of Bitcoin Core will be a Legacy Wallet.

#### `IsMine` Semantics

`IsMine` refers to the function used to determine whether a script belongs to the wallet.
This is used to determine whether an output belongs to the wallet. `IsMine` in Legacy Wallets
returns true if the wallet would be able to sign an input that spends an output with that script.
Since keys can be involved in a variety of different scripts, this definition for `IsMine` can
lead to many unexpected scripts being considered part of the wallet.

With Descriptor Wallets, descriptors explicitly specify the set of scripts that are owned by
the wallet. Since descriptors are deterministic and easily enumerable, users will know exactly
what scripts the wallet will consider to belong to it. Additionally the implementation of `IsMine`
in Descriptor Wallets is far simpler than for Legacy Wallets. Notably, in Legacy Wallets, `IsMine`
allowed for users to take one type of address (e.g. P2PKH), mutate it into another address type
(e.g. P2WPKH), and the wallet would still detect outputs sending to the new address type
even without that address being requested from the wallet. Descriptor Wallets does not
allow for this and will only watch for the addresses that were explicitly requested from the wallet.

These changes to `IsMine` will make it easier to reason about what scripts the wallet will
actually be watching for in outputs. However for the vast majority of users, this change is
largely transparent and will not have noticeable effect.

#### Imports and Exports

In Legacy Wallets, raw scripts and keys could be imported to the wallet. Those imported scripts
and keys are treated separately from the keys generated by the wallet. This complicates the `IsMine`
logic as it has to distinguish between spendable and watchonly.

Descriptor Wallets handle importing scripts and keys differently. Only complete descriptors can be
imported. These descriptors are then added to the wallet as if it were a descriptor generated by
the wallet itself. This simplifies the `IsMine` logic so that it no longer has to distinguish
between spendable and watchonly. As such, the watchonly model for Descriptor Wallets is also
different and described in more detail in the next section.

To import into a Descriptor Wallet, a new `importdescriptors` RPC has been added that uses a syntax
similar to that of `importmulti`.

As Legacy Wallets and Descriptor Wallets use different mechanisms for storing and importing scripts and keys
the existing import RPCs have been disabled for descriptor wallets.
New export RPCs for Descriptor Wallets have not yet been added.

The following RPCs are disabled for Descriptor Wallets:

* importprivkey
* importpubkey
* importaddress
* importwallet
* dumpprivkey
* dumpwallet
* importmulti
* addmultisigaddress
* sethdseed

#### Watchonly Wallets

A Legacy Wallet contains both private keys and scripts that were being watched.
Those watched scripts would not contribute to your normal balance. In order to see the watchonly
balance and to use watchonly things in transactions, an `include_watchonly` option was added
to many RPCs that would allow users to do that. However it is easy to forget to include this option.

Descriptor Wallets move to a per-wallet watchonly model. Instead an entire wallet is considered to be
watchonly depending on whether it was created with private keys disabled. This eliminates the need
to distinguish between things that are watchonly and things that are not within a wallet itself.

This change does have a caveat. If a Descriptor Wallet with private keys *enabled* has
a multiple key descriptor without all of the private keys (e.g. `multi(...)` with only one private key),
then the wallet will fail to sign and broadcast transactions. Such wallets would need to use the PSBT
workflow but the typical GUI Send, `sendtoaddress`, etc. workflows would still be available, just
non-functional.

This issue is worsened if the wallet contains both single key (e.g. `wpkh(...)`) descriptors and such
multiple key descriptors as some transactions could be signed and broadast and others not. This is
due to some transactions containing only single key inputs, while others would contain both single
key and multiple key inputs, depending on which are available and how the coin selection algorithm
selects inputs. However this is not considered to be a supported use case; multisigs
should be in their own wallets which do not already have descriptors. Although users cannot export
descriptors with private keys for now as explained earlier.

#### BIP 44/49/84 Support

The change to using descriptors changes the default derivation paths used by Bitcoin Core
to adhere to BIP 44/49/84. Descriptors with different derivation paths can be imported without
issue.