Re: [bitcoin-dev] Schnorr signatures BIP

From: Erik Aronesty <erik@q32•com>
To: Gregory Maxwell <greg@xiph•org>,
	 Bitcoin Protocol Discussion
	<bitcoin-dev@lists•linuxfoundation.org>
Subject: Re: [bitcoin-dev] Schnorr signatures BIP
Date: Tue, 11 Sep 2018 12:34:11 -0400	[thread overview]
Message-ID: <CAJowKg+-45h6vraL1PpnqfhHSbG+G40L+FD7xN+C-Dn1E6Y_Vg@mail.gmail.com> (raw)
In-Reply-To: <CAAS2fgT0uBGbLBOW4TxA-qCzOLwoQ1qSV-R0dMKRzPLAm_UOqQ@mail.gmail.com>

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 To answer points:

- I switched to the medium article so that I could correct, edit and
improve things to make them more clear.
- I responded to feedback by modifying the protocol to make it work - not
by ignoring it.
- I coded it up in python so I could be sure it worked, because I was
concerned that it was broken
- Yes, coding it up showed me that it's definitely interactive, and no
different than a "standard shnorr sig" in any meaningful way regarding the
security
- No special protocol support is needed over Schnorr signing itself.  The
e, s version can be made at least as secure as schnorr + DLP.  I haven't
researched the R,s version.
- An M-1 rogue-key attack would require the attacker would to either

  - attack the hash function to produce a predictable R based on a known
mesage
  - attack the DLP to influence x or k

Neither attack gives any particular advantage to someone who has M-1 keys.

I haven't tested whether the R,s version is susceptible though.

On Thu, Sep 6, 2018 at 9:15 AM Gregory Maxwell via bitcoin-dev <
bitcoin-dev@lists•linuxfoundation.org> wrote:

> On Wed, Sep 5, 2018 at 1:49 PM Erik Aronesty via bitcoin-dev
> <bitcoin-dev@lists•linuxfoundation.org> wrote:
> > Detailed explanation with code snippets:
> >
> > https://medium.com/@simulx/an-m-of-n-bitcoin-multisig-scheme-[snip]
>
> This appears to be a repost of the broken scheme you posted about on
> Bitcointalk, but then failed to respond to the response.
>
> https://bitcointalk.org/index.php?topic=4973123.0
>
> > The more I look into it and speak to professors about i, the more it
> seems "so trivial nobody really talks about it".
>
> I think you might be falling into the trap of ignoring feedback you
> don't like and and accepting that which sounds like "yea yea,
> something like that".
>
> Something "like that" does work: and is expressly and explicitly
> anticipated by the BIP but to be both secure and functional requires
> proper delineation (E.g. musig) _and_ interaction. What you're
> proposing is continually vague.  My best efforts at making sense of
> what you've written indicate that either it's non-interactive and
> not-actually functional at all,  OR it's interactive and just a less
> secure subset (no proper delinearization to prevent rogue key attacks)
> of what we already propose.
>
> When Poelstra suggests a CAS implementation he means something like
> this Sage notebook: http://bitcoin.ninja/secp256k1.ecdsa.sage  This
> provides for a method of communicating in both directions which is
> completely precise.
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