To: Jonas Schnelli <dev@jonasschnelli.ch>,
	Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>
References: <0d405f5d-c0a4-bad7-b6c3-08ba4424bf17@satoshilabs.com>
	<A806D2EB-744F-41A9-91C1-603F89E9005B@jonasschnelli.ch>
From: Pavol Rusnak <stick@satoshilabs.com>
Message-ID: <e4cb31ca-7552-760e-5029-d017f2616778@satoshilabs.com>
Date: Thu, 7 Sep 2017 20:38:37 +0200
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Subject: Re: [bitcoin-dev] Proposal: Extended serialization format for
 BIP-32 wallets
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From: Pavol Rusnak <stick@satoshilabs.com>
To: Jonas Schnelli <dev@jonasschnelli.ch>,
 Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>
Message-ID: <e4cb31ca-7552-760e-5029-d017f2616778@satoshilabs.com>
Subject: Re: [bitcoin-dev] Proposal: Extended serialization format for BIP-32
 wallets
References: <0d405f5d-c0a4-bad7-b6c3-08ba4424bf17@satoshilabs.com>
 <A806D2EB-744F-41A9-91C1-603F89E9005B@jonasschnelli.ch>
In-Reply-To: <A806D2EB-744F-41A9-91C1-603F89E9005B@jonasschnelli.ch>

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On 07/09/17 18:47, Jonas Schnelli wrote:
> But not sure if it=E2=80=99s 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 i=
t=E2=80=99s a BIP44 master key (and therefore avoid the bip32 path serial=
isation)? I know some =E2=80=9Ecentralised=E2=80=9C table need to be avai=
lable for that which may be not a good idea. But maybe the BIP could rese=
rve 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.

--=20
Best Regards / S pozdravom,

Pavol "stick" Rusnak
CTO, SatoshiLabs


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