Re: [bitcoin-dev] Safer sighashes and more granular SIGHASH_NOINPUT

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

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:
> 
> 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.
> 
> 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.
> 
> 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.
> 
> 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.
> 
> P.S. Also very excited to combine the above idea with Taproot/Graftroot/g'Root.
> 
> --
> Cheers, Bob McElrath
> 
> "For every complex problem, there is a solution that is simple, neat, and wrong."
>    -- H. L. Mencken 
> 
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