Re: [bitcoin-dev] 32-byte public keys in Schnorr and Taproot

[Karl-Johan Alm <karljohan-alm@garage.co.jp>]

Hello,

It makes no sense to me to not switch to 32-byte keys. There are
literally no (or very mild) disadvantages to this, from what it
appears like. I don't think refraining from updating a proposal just
because it's been out there for awhile is a valid reason, personally.

On Sat, Aug 10, 2019 at 3:17 AM Pieter Wuille via bitcoin-dev
<bitcoin-dev@lists.linuxfoundation.org> wrote:
>
> Hello all,
>
> It has been suggested [1] to drop the Y oddness bit in the witness
> program for Taproot outputs. This seems like a worthwhile change, as:
> * The bit doesn't actually contribute to security.
> * It avoids Taproot outputs from being more expensive to create than v0 P2WSH.
> * It doesn't preclude future changes that would still need the
> additional byte anyway.
>
> In exploring that option, Jonas Nick found that it seems cleanest [2]
> to actually introduce a type of 32-byte public keys (which implicitly
> have an even Y coordinate) in bip-schnorr, with associated signing and
> verification logic that are distinct from the 33-byte variant.
>
> This makes me wonder if we need 33-byte public keys at all.
>
> So I'd like to hear opinions about modifying bip-schnorr to only
> define 32-byte public keys. The implications of that would be:
> * bip-schnorr public keys wouldn't be exactly the same as ECDSA public
> keys, however all derivation logic would still apply (BIP32,
> mnemonics, xpubs, ... would still exist and be compatible - just the
> first pubkey byte would be dropped at the end).
> * The Q point in bip-taproot (the one going in the scriptPubKey) would
> just follow the 32-byte pubkey encoding, rather than needing a 33rd
> byte.
> * The P point in bip-taproot (the internal key revealed during script
> path) would become just a 32-byte public key (and we can drop the one
> bit in the control block to transmit the oddness of the Y coordinate
> of P).
> * In order to permit batch verification of the P to Q tweaking for
> script-path spending, another control block bit is now needed, namely
> one that indicates whether a negation was needed to make P+H(P||m)*G's
> Y coordinate even.
> * All public keys inside bip-tapscript would also become 32-bytes. If
> desired, the "upgradable public key" construction in bip-tapscript can
> be kept, by triggering based on the length of public keys (rather than
> based on their first byte).
>
> One question is whether it's worth such a change to bip-schnorr at
> this point. We've purposefully never progressed it past draft since
> publishing [3], but it has been a while. If necessary, it's possible
> to keep verification compatible by still hashing the implied "even"
> byte inside the scheme (which commits to the pubkey), but if we're
> going to change things, it's perhaps best to do it as cleanly as
> possible and also drop that byte.
>
> Opinions?
>
>   [1] https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2019-May/016943.html
>   [2] https://github.com/sipa/bips/pull/52
>   [3] https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2018-July/016203.html
>
> Cheers,
>
> --
> Pieter
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