From: Erik Aronesty <erik@q32•com>
To: Bitcoin Protocol Discussion <bitcoin-dev@lists•linuxfoundation.org>
Subject: Re: [bitcoin-dev] Multiparty signatures
Date: Mon, 9 Jul 2018 11:02:30 -0400 [thread overview]
Message-ID: <CAJowKgJ3K=wmCEtoZXJZhrnnA8XJcHYg788KP+7MCeP4Mxf-0w@mail.gmail.com> (raw)
In-Reply-To: <CAJowKg+=7nS4gNmtc8a4-2cu1uCOPqxjfchFwDVqUciKNMUYWQ@mail.gmail.com>
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Actually, it looks like in order to compute a multiparty signature you will
need to broadcast shares of r first, so it's not offline :(
It is still seems, to me, to be a simpler mechanism than musig - with
security assumptions that match the original Schnorr construction more
closely, and should therefore be easier to prove secure in a multiparty
context.
Shamir/Schnorr threshold multi-signature scheme:
Each party:
- Has a public key g*x', where x' is their private key, and where H(g*x)
can be considered their public index for the purposes of Shamir polynomial
interpolation
- Rolls a random k' and compute r' = g*k'
- Broadcast r' as a share
- Computes g*k, via lagrange interpolation across shares. At this point k
is not known to any party unless Shamir is vulnerable or DL is not hard
- Computes e' = H(M) * r'
- Computes s' = k'-x*e'
- Share of signature is (s', e')
Verification is the same as Scnhorr, but only after using interpolation to
get the needed (s, e, g*x) from shares of s', e' and g*x':
- Using lagrange interpolation, compute the public key g*x
- Again, using lagrange interpolation, compute (s, e)
- Verify the signature as per standard Schnorr
Security assumptions:
- Because this is not additive, and instead we are using Shamir
combination, the additional blinding and masking steps of musig are not
needed to create a secure scheme.
- The scheme is the same as Schnorr otherwise
- The only thing to prove is that H(M) * r does not reveal any information
about k ... which relies on the same DL assumptions as Bitcoin itself
- Overall, this seems, to me at least, to have a smaller attack surface
because there's fewer moving parts
On Mon, Jul 9, 2018 at 8:24 AM, Erik Aronesty <erik@q32•com> wrote:
> I was hoping that nobody in this group saw an obvious problem with it then
> I'd sit down and try to write up a paper.
>
> Not that hard to just reuse the work done on schnorr. And demonstrate
> that there are no additional assumptions.
>
> On Mon, Jul 9, 2018, 12:40 AM Pieter Wuille <pieter.wuille@gmail•com>
> wrote:
>
>> On Sun, Jul 8, 2018, 21:29 Erik Aronesty <erik@q32•com> wrote:
>>
>>> Because it's non-interactive, this construction can produce multisig
>>> signatures offline. Each device produces a signature using it's own
>>> k-share and x-share. It's only necessary to interpolate M of n shares.
>>>
>>> There are no round trips.
>>>
>>> The security is Shamir + discrete log.
>>>
>>> it's just something I've been tinkering with and I can't see an obvious
>>> problem.
>>>
>>> It's basically the same as schnorr, but you use a threshold hash to fix
>>> the need to be online.
>>>
>>> Just seems more useful to me.
>>>
>>
>> That sounds very useful if true, but I don't think we should include
>> novel cryptography in Bitcoin based on your not seeing an obvious problem
>> with it.
>>
>> I'm looking forward to seeing a more complete writeup though.
>>
>> Cheers,
>>
>> --
>> Pieter
>>
>>
>>
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next prev parent reply other threads:[~2018-07-09 15:02 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-07-08 14:19 Erik Aronesty
2018-07-08 15:16 ` Tim Ruffing
2018-07-08 18:23 ` Erik Aronesty
2018-07-08 21:01 ` Gregory Maxwell
2018-07-09 0:27 ` Erik Aronesty
2018-07-09 2:33 ` Pieter Wuille
2018-07-09 4:29 ` Erik Aronesty
2018-07-09 4:39 ` Pieter Wuille
[not found] ` <CAJowKg+=7nS4gNmtc8a4-2cu1uCOPqxjfchFwDVqUciKNMUYWQ@mail.gmail.com>
2018-07-09 15:02 ` Erik Aronesty [this message]
2018-07-09 15:57 ` Dan Robinson
2018-07-09 15:59 ` Gregory Maxwell
2018-07-09 16:33 ` Erik Aronesty
2018-07-09 16:58 ` Gregory Maxwell
2018-07-09 17:59 ` Erik Aronesty
2018-07-10 11:46 ` Erik Aronesty
2018-07-11 10:35 ` Adam Back
2018-07-11 14:45 ` Erik Aronesty
2018-07-19 12:16 ` Erik Aronesty
2018-07-19 12:24 ` Erik Aronesty
2018-07-19 13:11 ` Russell O'Connor
2018-07-20 16:25 ` Erik Aronesty
2018-07-20 17:34 ` Erik Aronesty
2018-07-20 20:18 ` Erik Aronesty
2018-07-26 2:05 ` Erik Aronesty
2018-07-09 16:21 ` Gregory Maxwell
2018-07-09 2:29 ` Pieter Wuille
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