Re: [bitcoin-dev] Taproot proposal

From: Greg Sanders <gsanders87@gmail•com>
To: John Newbery <john@johnnewbery•com>,
	 Bitcoin Protocol Discussion
	<bitcoin-dev@lists•linuxfoundation.org>
Subject: Re: [bitcoin-dev] Taproot proposal
Date: Mon, 16 Sep 2019 12:18:35 -0400	[thread overview]
Message-ID: <CAB3F3DvdUhZXO+hWZxdS64hO3gQGwGUURur5CA5Fp4hgn7g5EQ@mail.gmail.com> (raw)
In-Reply-To: <CAFmfg2tV+_M2_HD-GO1jbnufSLAW+K36LCXRNL9R_-0FPpNQVA@mail.gmail.com>

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> I'd prefer to not support P2SH-nested TR. P2SH wrapping was useful for
segwit
v0 for compatibility reasons. Most wallets/exchanges/services now support
sending
to native segwit addresses (https://en.bitcoin.it/wiki/Bech32_adoption) and
that
will be even more true if Schnorr/Taproot activate in 12+ months time.

Apologies for necroing an ancient thread, but I'm echoing my agreement with
John here.
We still have plenty of time to have ecosystem upgrade by the time taproot
is likely to activate.

On Wed, May 22, 2019 at 10:30 AM John Newbery via bitcoin-dev <
bitcoin-dev@lists•linuxfoundation.org> wrote:

> Hi,
>
> > A Taproot output is a SegWit output [...]  with
> > version number 1, and a 33-byte witness program whose first byte is 0 or
> 1.
>
> Given a secret key k and public key P=(x,y), a signer with the knowledge
> of k
> can sign for -P=(x,p-y) since -k is the secret key for that point.
> Encoding the
> y value of the public key therefore adds no security. As an alternative to
> providing the y value of the taproot output key Q when constructing the
> taproot
> output, the signer can provide it when signing. We can also restrict the y
> value
> of the internal key P to be even (or high, or a quadratic residue). That
> gives
> us 4 options for how to set the y signs for P and Q.
>
> 1. Q sign is explictly set in the witness program, P sign is explicitly
> set in the control block
>     => witness program is 33 bytes, 32 possible leaf versions (one for
> each pair of 0xc0..0xff)
> 2. Q sign is explictly set in the witness program, P sign is implicitly
> even
>     => witness program is 33 bytes, 64 possible leaf versions (one for
> each 0xc0..0xff)
> 3. Q sign is explictly set in the control block, P sign is explicitly set
> in the control block
>     => witness program is 32 bytes, 16 possible leaf versions (one for
> each 4-tuple of 0xc0..0xff)
> 4. Q sign is explictly set in the control block, P sign is implicitly even
>     => witness program is 32 bytes, 32 possible leaf versions (one for
> pair of 0xc0..0xff)
>
> The current proposal uses (1). Using (3) or (4) would reduce the size of a
> taproot output by one byte to be the same size as a P2WSH output. That
> means
> that it's not more expensive for senders compared to sending to P2WSH.
>
> (Credit to James Chiang for suggesting omitting the y sign from the public
> key and
> to sipa for pointing out the 4 options above)
>
> > (native or P2SH-nested, see BIP141)
>
> I'd prefer to not support P2SH-nested TR. P2SH wrapping was useful for
> segwit
> v0 for compatibility reasons. Most wallets/exchanges/services now support
> sending
> to native segwit addresses (https://en.bitcoin.it/wiki/Bech32_adoption)
> and that
> will be even more true if Schnorr/Taproot activate in 12+ months time.
>
> On Mon, May 6, 2019 at 2:36 PM Pieter Wuille via bitcoin-dev <
> bitcoin-dev@lists•linuxfoundation.org> wrote:
>
>> Hello everyone,
>>
>> Here are two BIP drafts that specify a proposal for a Taproot
>> softfork. A number of ideas are included:
>>
>> * Taproot to make all outputs and cooperative spends indistinguishable
>> from eachother.
>> * Merkle branches to hide the unexecuted branches in scripts.
>> * Schnorr signatures enable wallet software to use key
>> aggregation/thresholds within one input.
>> * Improvements to the signature hashing algorithm (including signing
>> all input amounts).
>> * Replacing OP_CHECKMULTISIG(VERIFY) with OP_CHECKSIGADD, to support
>> batch validation.
>> * Tagged hashing for domain separation (avoiding issues like
>> CVE-2012-2459 in Merkle trees).
>> * Extensibility through leaf versions, OP_SUCCESS opcodes, and
>> upgradable pubkey types.
>>
>> The BIP drafts can be found here:
>> * https://github.com/sipa/bips/blob/bip-schnorr/bip-taproot.mediawiki
>> specifies the transaction input spending rules.
>> * https://github.com/sipa/bips/blob/bip-schnorr/bip-tapscript.mediawiki
>> specifies the changes to Script inside such spends.
>> * https://github.com/sipa/bips/blob/bip-schnorr/bip-schnorr.mediawiki
>> is the Schnorr signature proposal that was discussed earlier on this
>> list (See
>> https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2018-July/016203.html
>> )
>>
>> An initial reference implementation of the consensus changes, plus
>> preliminary construction/signing tests in the Python framework can be
>> found on https://github.com/sipa/bitcoin/commits/taproot. All
>> together, excluding the Schnorr signature module in libsecp256k1, the
>> consensus changes are around 520 LoC.
>>
>> While many other ideas exist, not everything is incorporated. This
>> includes several ideas that can be implemented separately without loss
>> of effectiveness. One such idea is a way to integrate SIGHASH_NOINPUT,
>> which we're working on as an independent proposal.
>>
>> The document explains basic wallet operations, such as constructing
>> outputs and signing. However, a wide variety of more complex
>> constructions exist. Standardizing these is useful, but out of scope
>> for now. It is likely also desirable to define extensions to PSBT
>> (BIP174) for interacting with Taproot. That too is not included here.
>>
>> Cheers,
>>
>> --
>> Pieter
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