Re: [Bitcoin-development] Presenting a BIP for Shamir's Secret Sharing of Bitcoin private keys

[Alan Reiner <etotheipi@gmail.com>]

Armory has had "Fragmented Backups" for over a year, now.  Advanced
users love it.  Though, I would say it's kind of difficult to
standardize the way I did it since I was able to implement all the
finite field math with recursion, list comprehensions and python
arbitrary-big-integers in about 100 lines.  I'm not sure how "portable"
it is to other languages.  There's obviously better ways to do it, but I
didn't need a better way, because I don't need to support fragmentation
above M=8 and this was 100% sufficient for it.  And I was the only one
doing it, so there was no one to be compatible with.

I won't lie, there's a lot of work that goes into making an interface
that makes this feature "usable."  The user needs clear ways to identify
which fragments are associated with which wallet, and which fragments
are compatible with each other.  They need a way to save some fragments
to file, print them, or simply write them down.  They need a way to
re-enter fragment, reject duplicates, identify errors, etc.  Without it,
the math fails silently, and you end up restoring a different wallet.   
And they need a way to test that it all works.   Armory did all this,
but it was no trivial task.  Including an interface that will test up to
50 subsets of make sure the math produces the same values every time
(which still is not sufficient for some users, who won't be satisified
til they see they're wallet actually restored from fragments.

Also I put the secret in the highest-order coefficient of the
polynomial, and made sure that the other coefficients were
deterministic.  This meant that if print out an M-of-N wallet, I can
later print out an M-of-(N+1) wallet and the first N fragments will be
the same.  I'm not sure how many users would trust this, but we felt it
was important in case a user needs to export some fragments, even if
they don't increase N.

You might consider loading Armory in offline mode, create a wallet, and
then do a fragmented backup to see how we did it.  I am extremely
satisfied with the interface, but it's most definitely an "advanced"
tool.  But so is Armory ... which made it a good fit.  But it might not
be for everyone.

-Alan



On 03/29/2014 11:44 AM, Matt Whitlock wrote:
> On Saturday, 29 March 2014, at 11:08 am, Watson Ladd wrote:
>> https://freedom-to-tinker.com/blog/stevenag/new-research-better-wallet-security-for-bitcoin/
> Thanks. This is great, although it makes some critical references to an ACM paper for which no URL is provided, and thus I cannot implement it.
>
> A distributed ECDSA notwithstanding, we still need a way to decompose a BIP32 master seed into shares. I am envisioning a scenario in which I might meet my sudden and untimely demise, and I wish to allow my beneficiaries to reconstruct my wallet's master seed after my death. I would like to distribute seed shares to each of my beneficiaries and some close friends, such that some subset of the shares must be joined together to reconstitute my master seed. Shamir's Secret Sharing Scheme is perfect for this use case. I am presently working on extending my draft BIP so that it also applies to BIP32 master seeds of various sizes.
>
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