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.mediawikispecifies the transaction input spending rules.
*
https://github.com/sipa/bips/blob/bip-schnorr/bip-tapscript.mediawikispecifies the changes to Script inside such spends.
*
https://github.com/sipa/bips/blob/bip-schnorr/bip-schnorr.mediawikiis 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