To: Gregory Maxwell <greg@xiph.org>, roconnor@blockstream.io,
	bitcoin-dev@lists.linuxfoundation.org
References: <51280a45-f86b-3191-d55e-f34e880c1da8@satoshilabs.com>
	<CAAS2fgRQk4EUp6FO2f+RkJpDTyZX0N4=uGp7ZF=0aUchZX8hSA@mail.gmail.com>
From: =?UTF-8?Q?Ond=c5=99ej_Vejpustek?= <ondrej.vejpustek@satoshilabs.com>
Message-ID: <4003eed1-584f-9773-8cf9-6300ebd1eac6@satoshilabs.com>
Date: Thu, 18 Jan 2018 14:50:41 +0100
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101
	Thunderbird/52.5.2
MIME-Version: 1.0
In-Reply-To: <CAAS2fgRQk4EUp6FO2f+RkJpDTyZX0N4=uGp7ZF=0aUchZX8hSA@mail.gmail.com>
Content-Type: text/plain; charset=utf-8
Content-Language: en-US
Content-Transfer-Encoding: 7bit
Subject: Re: [bitcoin-dev] Satoshilabs secret shared private key scheme
Precedence: list

Thank you for your comments, Gregory and Russell!

Gregory, thank you for you explanation of perfect secrecy, there is no
need for that, however. I'm professional mathematician and cryptographer.

> I read the above
> as "these are similar because they are based on math"...
They are based on algebra (group and commutative ring theory), which is
a great similarity. RSA and SHA, for example, are based on completely
distinct parts of mathematics.

> Complicated does not mean secure. And from an information theoretic
> perspective the hash does almost nothing (other then some small
> destruction of entropy due to its lack of perfect uniformity which is
> information theoretically equivalent to using a smaller perfect code).
> using error correcting codes and truncated hash functions create
identical amounts of information theoretic redundancy
I agree, see my last note in the previous mail. Adding redundancy by a
hash function is more secure than adding redundancy by a linear
relations. Just my opinion.

I see the difference between RSA and SSS you mentioned and I understand
your arguments about perfect secrecy. Just two comments:
  (1) Our proposal doesn't use SSS for the whole secret, but it divides
the secret into bytes and uses SSS for every byte separately. This
scheme is weaker because to reconstruct n-th byte it suffices to have
n-th bytes from k shares.
  (2) SSS is information-theoretic secure if you know k-1 or less
shares, where k is the threshold. But the proof doesn't hold if you know
for example a small part of every share.

> It is of no use to apply the precautionary principle against
impossible attacks, especially at the cost of losing the useful
properties of a real error correcting codes that would provide actual
guarantees against likely errors.
The discussion isn't about mathematics or about security proofs but
about cryptographic scheme design. In our use case you cannot assume
that all premises of security proof theorems (including SSS's perfect
secrecy) hold true (see the comment above).

In my opinion, to make a cryptographic scheme more robust it's better to
stick to general "intuitive" principles. Of course you have to consider
the advantages and disadvantages of this approach. That's why we
disclosed our draft and welcome all comments.

> The discussion of using a proper code was primarily related to the
> outer check value which protects the shares themselves and is sitting
> unprotected in plaintext
OK then. I was defending the hash in the inner check value.