# Descriptors
Descriptors are a compact and semi-standard way to easily encode, or "describe", how scripts (and subsequently, addresses) of a wallet should be generated. They can be especially helpful when working with multisigs or even more complex scripts, where the structure of the script itself is not trivial. They are a big step forward in making wallets more portable across different tools and apps, because for the first time they create a common language to describe a full bitcoin script that developers can use and integrate in their software.
The ecosystem around descriptors is still very much in its early stage, but they are starting to see some adoption in Bitcoin Core (opens new window) and other projects. BDK aims to produce the first "Native Descriptor" Bitcoin library that can be used by developers to build their own "Native Descriptor Wallets" (opens new window).
# Compatibility Matrix
Below are some tables to highlight the differences between Bitcoin Core's descriptor support, rust-miniscript's one and BDK's.
# Key Types
| Key Type | BDK | rust-miniscript | Bitcoin Core |
|---|---|---|---|
| Hex PublicKey | ✓ | ✓ | ✓ |
| WIF PrivateKey | ✓ | ✗ | ✓ |
| Extended Keys (xpub/xprv) | ✓ | ✗ | ✓ |
# Script Types (top level)
| Script Type | BDK | rust-miniscript | Bitcoin Core |
|---|---|---|---|
pk() | ✓ | ✓ | ✓ |
pkh() | ✓ | ✓ | ✓ |
wpkh() | ✓ | ✓ | ✓ |
sh(wpkh()) | ✓ | ✓ | ✓ |
sh() | ✓ | ✓ | ✓ |
wsh() | ✓ | ✓ | ✓ |
sh(wsh()) | ✓ | ✓ | ✓ |
combo() | ✗ | ✗ | ✓ |
addr() | ✗ | ✗ | ✓ |
raw() | ✗ | ✗ | ✓ |
| Bare scripts | ✓ | ✓ | ✗ |
# Operators
| Operator | BDK | rust-miniscript | Bitcoin Core |
|---|---|---|---|
pk() | ✓ | ✓ | ✓ |
pkh() | ✓ | ✓ | ✓ |
older() | ✓ | ✓ | ✓ |
after() | ✓ | ✓ | ✓ |
sha256() | ✓ | ✓ | ✓ |
hash256() | ✓ | ✓ | ✓ |
ripemd160() | ✓ | ✓ | ✓ |
hash160() | ✓ | ✓ | ✓ |
andor() | ✓ | ✓ | ✓ |
and_{v,b,n}() | ✓ | ✓ | ✓ |
or_{b,c,d,i}() | ✓ | ✓ | ✓ |
multi() | ✓ | ✓ | ✓ |
thresh() | ✓ | ✓ | ✓ |
sortedmulti() | ✓ | ✓ | ✓ |
# Modifiers
| Script Type | BDK | rust-miniscript | Bitcoin Core |
|---|---|---|---|
a: | ✓ | ✓ | ✓ |
s: | ✓ | ✓ | ✓ |
c: | ✓ | ✓ | ✓ |
t: | ✓ | ✓ | ✓ |
d: | ✓ | ✓ | ✓ |
v: | ✓ | ✓ | ✓ |
j: | ✓ | ✓ | ✓ |
n: | ✓ | ✓ | ✓ |
l: | ✓ | ✓ | ✓ |
u: | ✓ | ✓ | ✓ |
For a more thorough description of these operators and modifiers see Sipa's Miniscript Page (opens new window) and Bitcoin Core's (opens new window).
# Examples
Some examples of valid BDK descriptors are:
| Spending Policy | Descriptor | Address 0 | Address 1 |
|---|---|---|---|
| Static P2PKH | pkh(cSQPHDBwXGjVzWRqAHm6zfvQhaTuj1f2bFH58h55ghbjtFwvmeXR) | mrkwtj5xpYQjHeJe5wsweNjVeTKkvR5fCr | mrkwtj5xpYQjHeJe5wsweNjVeTKkvR5fCr |
| Static P2PKH, watch-only | pkh(02e96fe52ef0e22d2f131dd425ce1893073a3c6ad20e8cac36726393dfb4856a4c) | mrkwtj5xpYQjHeJe5wsweNjVeTKkvR5fCr | mrkwtj5xpYQjHeJe5wsweNjVeTKkvR5fCr |
| P2WSH 2-of-2 with one private key | wsh(multi(2,tprv8ZgxMBicQKsPePmENhT9N9yiSfTtDoC1f39P7nNmgEyCB6Nm4Qiv1muq4CykB9jtnQg2VitBrWh8PJU8LHzoGMHTrS2VKBSgAz7Ssjf9S3P/0/*,tpubDBYDcH8P2PedrEN3HxWYJJJMZEdgnrqMsjeKpPNzwe7jmGwk5M3HRdSf5vudAXwrJPfUsfvUPFooKWmz79Lh111U51RNotagXiGNeJe3i6t/1/*)) | tb1qqsat6c82fvdy73rfzye8f7nwxcz3xny7t56azl73g95mt3tmzvgs9a8vjs | tb1q7sgx6gscgtau57jduend6a8l445ahpk3dt3u5zu58rx5qm27lhkqgfdjdr |
| P2WSH-P2SH one key + 10 days timelock | sh(wsh(and_v(vc:pk_h(tprv8ZgxMBicQKsPePmENhT9N9yiSfTtDoC1f39P7nNmgEyCB6Nm4Qiv1muq4CykB9jtnQg2VitBrWh8PJU8LHzoGMHTrS2VKBSgAz7Ssjf9S3P/0/*),older(1440)))) | 2Mtk2nyS98MCi2P7TkoBGLaJviBy956XxB1 | 2MuEStKzYhqb5HCFgHz9153tZsL5sVqV5xC |
# Implementation Details
BDK extends the capabilities of rust-miniscript (opens new window) by introducing the concept of an ExtendedDescriptor: it represents a descriptor that contains one or more "derivable keys" like xpubs or xprvs
and can be "derived" from a normal Descriptor by deriving every single one of its keys. It is currently called "StringDescriptor" in the code, because it's implemented as a wrapped miniscript::Descriptor<String>.
ExtendedDescriptors are derived using a single index instead of a full derivation path: this is because normally most of the path is fixed and can be represented right after the xpub/xprv itself, and only the final index changes for each address. This is what's normally called a DescriptorExtendedKey in the codebase, it is represented with a similar syntax to Bitcoin Core's, such as:
[d34db33f/44'/0'/0']xpub6ERApfZwUNrhL.......rBGRjaDMzQLcgJvLJuZZvRcEL/0/*
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