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trezor-firmware/docs/misc/purpose48.md

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Purpose48 derivation scheme

Per BIP-43, the first level of the derivation path is used as a "purpose". The purpose number is usually selected to match the BIP number: e.g., BIP-49 uses purpose 49'.

There is no officially proposed BIP-48 standard. Despite that, a de-facto standard for the purpose 48' exists in the wild and is implemented by several HD wallets, most notably Electrum. This standard was never before formally specified, and this document aims to rectify the situation.

Motivation

Purpose48 is intended for multisig scenarios. It allows using multiple script types from a single logical account or root key, while keeping multisig keys separate from single-sig keys.

Specification

The following BIP-32 path levels are defined:

m / 48' / coin_type' / account' / script_type' / change / address_index

Meaning of all fields except script_type is defined in BIP-44

script_type can have the following values:

  • 0: raw BIP-11 (p2ms) multisig
  • 1: p2sh-wrapped segwit multisig (p2wsh-p2sh)
  • 2: native segwit multisig (p2wsh)

The path derivation is hardened up to and including the script_type field.

Trezor implementation

script_type value 0 corresponds to SPENDMULTISIG/PAYTOMULTISIG.

Value 1 corresponds to SPENDP2SHWITNESS/PAYTOP2SHWITNESS.

Value 2 corresponds to SPENDWITNESS/PAYTOWITNESS.

References

Electrum implementation: 9931df9f25/electrum/keystore.py (L862-L889)

Trezor implementation: TBD