From: Johnson Lau <jl2012@xbt.hk>
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Date: Wed, 28 Nov 2018 16:40:34 +0800
References: <20181128005416.GB22873@mcelrath.org>
To: Bob McElrath <bob@mcelrath.org>,
	bitcoin-dev <bitcoin-dev@lists.linuxfoundation.org>
In-Reply-To: <20181128005416.GB22873@mcelrath.org>
Message-Id: <8690D3D0-3815-4779-A571-C75AA75F707B@xbt.hk>
Subject: Re: [bitcoin-dev] Safer sighashes and more granular SIGHASH_NOINPUT
Precedence: list

This is incompatible with bip-schnorr, which intentionally disallow such =
use by always committing to the public key: =
https://github.com/sipa/bips/blob/bip-schnorr/bip-schnorr.mediawiki

With the recent fake Satoshi signature drama, and other potential ways =
to misuse and abuse, I think this is a better way to go, which =
unfortunately might disallow some legitimate applications.

Covenants could be made using OP_CHECKSIGFROMSTACK =
(https://fc17.ifca.ai/bitcoin/papers/bitcoin17-final28.pdf) or =
OP_PUSHTXDATA =
(https://github.com/jl2012/bips/blob/vault/bip-0ZZZ.mediawiki). I think =
this is the next step following the taproot soft fork

> On 28 Nov 2018, at 8:54 AM, Bob McElrath via bitcoin-dev =
<bitcoin-dev@lists.linuxfoundation.org> wrote:
>=20
> I have been working on an experimental wallet that implements Bitcoin
> Covenants/Vaults following a blog post I wrote about 2 years ago, =
using
> "Pay-to-Timelock Signed Transaction" (P2TST).  (Also mentioned =
recently by
> kanzure in a talk somewheres...)  The idea is that you deposit to an =
address for
> which you don't know the private key.  Instead you construct a second
> transaction sending that to a timelocked staging address for which you =
DO have
> the privkey (it also has an IF/ELSE condition with a second spending =
condition
> for use in case of theft attempt).  In order to do this you either =
have to
> delete the privkey of the deposit address (a difficult proposition to =
know it's
> actually been deleted), but instead one can construct a signature =
directly using
> a RNG, and use the SIGHASH to compute the corresponding pubkey via =
ECDSA
> recover, from which you compute the corresponding address.  In this =
way your
> wallet is a set of P2TST transactions and a corresponding privkey, =
with a (set
> of) emergency keys.
>=20
> This interacts with NOINPUT in the following way: if the input to the
> transaction commits to the pubkey in any way, you have a circular =
dependency on
> the pubkey that could only be satisfied by breaking a hash function.  =
This
> occurs with standard sighash's which commit to the txid, which in turn =
commit to
> the address, which commits to the pubkey, so this construction of
> covenants/vaults requires NOINPUT.
>=20
> AFAICT sipa's proposal is compatible with the above vaulted =
construction by
> using SIGHASH_NOINPUT | SIGHASH_SCRIPTMASK to remove the
> scriptPubKey/redeemScript from the sighash.  Putting the
> scriptPubKey/redeemScript in the sighash introduces the same circular
> dependency, but SIGHASH_SCRIPTMASK removes it.
>=20
> One would probably want to provide the fee from a separate wallet so =
as to be
> able to account for fluctuating fee pressures when the unvaulting =
occurs a long
> time after vaulting.  Thus you'd want to use SIGHASH_SINGLE so that a =
fee-wallet
> can add fees (or for composability of P2TSTs), and SIGHASH_NOFEE as =
well.
>=20
> P.S. Also very excited to combine the above idea with =
Taproot/Graftroot/g'Root.
>=20
> --
> Cheers, Bob McElrath
>=20
> "For every complex problem, there is a solution that is simple, neat, =
and wrong."
>    -- H. L. Mencken=20
>=20
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