From: Matt Whitlock <bip@mattwhitlock.name>
To: bitcoin-development@lists.sourceforge.net
Date: Thu, 03 Apr 2014 17:42:32 -0400
Message-ID: <3837746.jqWvB0Uxrs@crushinator>
User-Agent: KMail/4.12.4 (Linux/3.12.13-gentoo; KDE/4.12.4; x86_64; ; )
In-Reply-To: <CAC7yFxSE8-TWPN-kuFiqdPKMDuprbiVJi7-z-ym+AUyA_f-xJw@mail.gmail.com>
References: <CAC7yFxSE8-TWPN-kuFiqdPKMDuprbiVJi7-z-ym+AUyA_f-xJw@mail.gmail.com>
MIME-Version: 1.0
Content-Transfer-Encoding: quoted-printable
Content-Type: text/plain; charset="utf-8"
Subject: Re: [Bitcoin-development] Presenting a BIP for Shamir's Secret
	Sharing of Bitcoin private keys
Precedence: list

On Thursday, 3 April 2014, at 4:41 pm, Nikita Schmidt wrote:
> I agree with the recently mentioned suggestion to make non-essential
> metadata, namely key fingerprint and degree (M), optional.  Their
> 4-byte and 1-byte fields can be added individually at an
> implementation's discretion.  During decoding, the total length will
> determine which fields are included.

The fingerprint field, Hash16(K), is presently specified as a 16-bit fi=
eld. Rationale: There is no need to consume 4 bytes just to allow share=
s to be grouped together. And if someone has more than 100 different se=
crets, they probably have a good system for managing their shares and w=
on't need the hash anyway.

> Encoding for the testnet is not specified.

Hmm, is that actually needed?

> Speaking of encoding, is it not wasteful to allocate three different
> application/version bytes just for the sake of always starting with
> 'SS'?  It would be OK if it were accepted as a BIP, but merely as a
> de-facto standard it should aim at minimising future chances of
> collision.

I agree on principle, however I think the more user-acceptable behavior=
 is for all base58-encoded Shamir shares to begin with a common prefix,=
 such as "SS". Users are accustomed to relying on the prefix of the bas=
e58 encoding to understand what the object is: "1" for mainnet pubkey h=
ash, "3" for mainnet script hash, "5" for uncompressed private key, "P"=
 for passphrase-protected private key, etc.

> I'd add a clause allowing the use of random coefficients instead of
> deterministic, as long as the implementation guarantees to never make=

> another set of shares for the same private key or master seed.

I'm not sure that's necessary, as this is an Informational BIP. Impleme=
ntations are free to ignore it. Shares with randomly selected coefficie=
nts would work just fine in a share joiner that conforms to the BIP, so=
 I would expect implementors to feel free to ignore the deterministic f=
ormula and use randomly selected coefficients.

> What about using the same P256 prime as for the elliptic curve?  Just=

> for consistency's sake.

The initial draft of this BIP used the cyclic order (n) of the generato=
r point on the secp256k1 elliptic curve as the modulus. The change to t=
he present scheme was actually done for consistency's sake, so all size=
s of secret can use a consistently defined modulus.

> Also, I'm somewhat inclined towards using the actual x instead of j i=
n
> the encoding.  I find it more direct and straightforward to encode th=
e
> pair (x, y).  And x=3D0 can denote a special case for future extensio=
ns.
>  There is no technical reason behind this, it's just for (subjective)=

> clarity and consistency.

There is a technical reason for encoding j rather than x[j]: it allows =
for the first 256 shares to be encoded, rather than only the first 255 =
shares.

If you want a sentinel value reserved for future extensions, then you m=
ight take notice that 0xFFFF is an invalid key fingerprint, along with =
several other values, and also that 0xFF is an unusable value of M=E2=88=
=922, as that would imply M=3D257, but the scheme can only encode up to=
 256 shares, so one would never have enough shares to meet the threshol=
d. I considered having the two optional fields be mandatory and allowin=
g 0xFFFF and 0xFF as "redacted" field values, but I like allowing the s=
hares to be shorter if the optional fields are omitted. (Imagine engrav=
ing Shamir secret shares onto metal bars by hand with an engraving tool=
. Fewer characters is better!)