<pre>
  BIP: 43
  Layer: Applications
  Title: Purpose Field for Deterministic Wallets
  Authors: Marek Palatinus <slush@satoshilabs.com>
           Pavol Rusnak <stick@satoshilabs.com>
  Status: Deployed
  Type: Specification
  Assigned: 2014-04-24
</pre>

==Abstract==

This BIP introduces a "Purpose Field" for use in deterministic wallets
based on algorithm described in BIP-0032 (BIP32 from now on).

==Motivation==

Although Hierarchical Deterministic Wallet structure as described by BIP32
is an important step in user experience and security of the cryptocoin wallets,
the BIP32 specification offers implementers too many degrees of freedom.
Multiple implementations may claim they are BIP32 compatible, but in fact
they can produce wallets with different logical structures making them
non-interoperable. This situation unfortunately renders "BIP32 compatible"
statement rather useless.


==Purpose==

We propose the first level of BIP32 tree structure to be used as "purpose".
This purpose determines the further structure beneath this node.

<pre>
m / purpose' / *
</pre>

Apostrophe indicates that BIP32 hardened derivation is used.

We encourage different schemes to apply for assigning a separate BIP number
and use the same number for purpose field, so addresses won't be generated
from overlapping BIP32 spaces.

Purpose codes from 10001 to 19999 are reserved for [[https://github.com/satoshilabs/slips|SLIPs]].

Example: Scheme described in BIP44 should use 44' (or 0x8000002C) as purpose.

Note that m / 0' / * is already taken by BIP32 (default account), which
preceded this BIP.

Not all wallets may want to support the full range of features and possibilities
described in these BIPs. Instead of choosing arbitrary subset of defined features
and calling themselves BIPxx compatible, we suggest that software which needs
only a limited structure should describe such structure in another BIP and use
different "purpose" value.


==Node serialization==

Because this scheme can be used to generate nodes for more cryptocurrencies
at once, or even something totally unrelated to cryptocurrencies, there's no
point in using a special version magic described in section "Serialization
format" of BIP32. We suggest to use always 0x0488B21E for public and 0x0488ADE4
for private nodes (leading to prefixes "xpub" and "xprv" respectively).

==Reference==

* [[bip-0032.mediawiki|BIP32 - Hierarchical Deterministic Wallets]]
