[bitcoin-dev] Proposal: Extended serialization format for BIP-32 wallets

Pavol Rusnak stick at satoshilabs.com
Thu Sep 7 18:38:37 UTC 2017

On 07/09/17 18:47, Jonas Schnelli wrote:
> But not sure if it’s worth to save ~two bytes for that.

I think it's not worth complicating the field just to save two bytes.

But if we agree (for privacy reasons) that resolution of this field
should be reduced to week-level (as suggested by Jonas) or month-level
(as sugested by Peter), we could use just 16 bits for this.

TBH I think TREZOR will provide hardcoded constant for this field
(1.1.2014 for all its P2PKH xpubs and 1.8.2017 for all its
P2WPKH-in-P2SH xpubs). So no privacy is lost in this case, but if we
want to ENFORCE this on BIP level, we should decrease the resolution.

> 2.
> Would it make sense to have special depth bytes that directly implies it’s a BIP44 master key (and therefore avoid the bip32 path serialisation)? I know some „centralised“ table need to be available for that which may be not a good idea. But maybe the BIP could reserve a couple of depth-bytes (maybe 0xF0 to 0xFF) for predefined paths.

I think this is exactly what Thomas meant by "wallet developers are
going to use dirtier tricks" in his email, that's why I specifically
tried to avoid this. I see no good reason to do this, unless we want to
save some bytes and I don't think we are in need of doing this.

> 3.
> Would adding a version bit make sense to allow future extensions?

I think changing the human-readable part is the way to go. That way the
wallet can immediately say if it understands the format or not, without
parsing the binary data contents. Version bits were introduced in older
standards, because there was no such thing as human-readable prefix.

Best Regards / S pozdravom,

Pavol "stick" Rusnak
CTO, SatoshiLabs

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