Support for SLIP132's custom BIP32 prefixes

[afk11]

At least trezor and electrum support SLIP132 base58 prefixes for extended keys, which indicate the script type to be derived.

I think it could be worth adding a flags parameter to bip32_key_from_base58[,_alloc], otherwise could expose the behavior through a separate function and avoid the BC break?

[greenaddress]

@afk11 thanks for raising this - agreed, libwally should allow serialization and deserialization for non-bip32
versions. Maybe adding a flag (eg BIP32_ALLOW_ARBITRARY_VERSION) could do the
job, however bip32_key_unserialize (and all the APIs based on that, such as
bip32_key_from_base58) currently do not accept flags in input so that would
require an API change. Before proceeding we should discuss if it's the best way
of doing it.

If needed, at the moment it could be implemented with a small workaround, eg in
py3:

import wallycore as wally

def bip32_from_base58_arbitrary_version(b58key, version, is_private):
    decoded = wally.base58check_to_bytes(b58key)
    assert decoded[:4] == version
    bip32_version = wally.BIP32_VER_MAIN_PRIVATE if is_private else wally.BIP32_VER_MAIN_PUBLIC
    return wally.bip32_key_unserialize((bip32_version).to_bytes(4, 'big') + decoded[4:])

def bip32_to_base58_arbitrary_version(xkey, flags, version):
    encoded = wally.bip32_key_serialize(xkey, flags)
    return wally.base58check_from_bytes(version + encoded[4:])

[afk11]

Thanks for the response!

I experimented with allowing developer provided definitions, and adding a pointer to a definition on ext_key. If no definition is set, the previous code working with BIP32_VER_MAIN_PUBLIC/BIP32_VER_MAIN_PRIVATE/etc is used: master...afk11:bip32-versions

I had the same thoughts regarding bip32_key_unserialize, but since ^ adds extra arguments I went with separate functions for now. I'm pretty sure bip32_key_unserialize could populate the definition for whatever network it determines, and hence we could rule out special cases for keys with and without definitions.

Let me know what you think anyway!