Date: Thu, 11 Feb 2021 20:31:13 +0000
To: Dr Maxim Orlovsky <orlovsky@protonmail.com>
From: Pieter Wuille <bitcoin-dev@wuille.net>
Reply-To: Pieter Wuille <bitcoin-dev@wuille.net>
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Cc: Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>
Subject: Re: [bitcoin-dev] BIP32/43-based standard for Schnorr signatures &
	decentralized identity
Precedence: list

=E2=80=90=E2=80=90=E2=80=90=E2=80=90=E2=80=90=E2=80=90=E2=80=90 Original Me=
ssage =E2=80=90=E2=80=90=E2=80=90=E2=80=90=E2=80=90=E2=80=90=E2=80=90
On Thursday, February 11, 2021 6:38 AM, Dr Maxim Orlovsky <orlovsky@protonm=
ail.com> wrote:

> Thank you very much for all the clarifications; it=E2=80=99s good to have=
 them sorted out and clearly structured. From what you wrote it follows tha=
t we still need to reserve a dedicated purpose (with new BIP) for BIP340 si=
gnatures to avoid key reuse, am I right?

Maybe, but it would be for a particular way of using keys (presumably: sing=
le-key pay-to-taproot), not just the signature scheme itself. If you go dow=
n this path you'll also want dedicated branches for multisig participation,=
 and presumably several interesting new policies that become possible with =
Taproot.

The only thing ECDSA/Schnorr specific about this is that - if you want to m=
aintain provable security - the keys used for ECDSA and BIP340 should be se=
parated by a hardened step. It seems however that all approaches people act=
ually use to prevent reuse do that already.

And as I said, dedicated branches only help for the simple case. For exampl=
e, it doesn't address the more general problem of preventing reuse of keys =
in multiple distinct groups of multisig sets you participate in. If you wan=
t to solve that you need to keep track of  index is for participating in wh=
at - and once you have something like that you don't need dedicated purpose=
 based derivation at all anymore.

So I'm not sure I'd state it as us *needing* a dedicated purpose/branch for=
 single-key P2TR (and probably many other useful ways of using taproot base=
d spending policies...). But perhaps it's useful to have.

Greg Maxwell pointed out to me that there may be another reason to want non=
-reuse across ECDSA and BIP340 keys: if someone were to do all of these wro=
ng:
* not follow BIP340 and re-use RFC6979 for BIP340 nonce generation
* reuse the same keys for both
* sign the same message with both
... you would actually leak your private key. This isn't a concern for Bitc=
oin transaction signing however, as the sighash (message) indirectly commit=
s to BIP341 or not, and thus it'd be impossible to construct colliding mess=
ages. Still, it's a consideration to factor in.

Cheers,

--
Pieter