From: Matt Whitlock <bip@mattwhitlock.name>
To: jan.moller@gmail.com
Date: Tue, 22 Apr 2014 05:13:01 -0400
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Cc: bitcoin-development <bitcoin-development@lists.sourceforge.net>
Subject: Re: [Bitcoin-development] Presenting a BIP for Shamir's Secret
	Sharing of Bitcoin private keys
Precedence: list

On Tuesday, 22 April 2014, at 10:39 am, Jan M=F8ller wrote:
> On Tue, Apr 22, 2014 at 10:29 AM, Matt Whitlock <bip@mattwhitlock.nam=
e>wrote:
> > On Tuesday, 22 April 2014, at 10:27 am, Jan M=F8ller wrote:
> > > > >  - Please allow M=3D1. From a usability point of view it make=
s sense to allow
> > > > > the user to select 1 share if that is what he wants.
> > > >
> > > > How does that make sense? Decomposing a key/seed into 1 share i=
s
> > > > functionally equivalent to dispensing with the secret sharing s=
cheme
> > > > entirely.
> > > >
> > > I agree that it may look silly to have just one-of-one share from=
 a
> > > technical point of view, but from an end-user point of view there=
 could be
> > > reasons for just having one piece of paper to manage. If M can be=
 1 then
> > > the software/hardware doesn't have to support multiple formats,
> > > import/export paths + UI  (one for SIPA keys in one share, one fo=
r HD seeds
> > > in one share, one for SIPA keys + HD seeds in multiple shares).
> > >
> > > Less complexity & more freedom of choice.
> >
> > Alright. It's a fair argument. Do you agree with encoding M using a=
 bias
> > of -1 so that M up to and including 256 can be encoded in one byte?=

>=20
> Necessary Shares =3D M+1, not a problem
>=20
> I would probably encode N-of-M in 1 byte as I don't see good use case=
s with
> more than 17 shares. Anyway, I am fine with it as it is.

Encoding bias of M changed to -1, and test vectors updated:
https://github.com/whitslack/btctool/blob/bip/bip-xxxx.mediawiki