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Date: Tue, 7 May 2024 14:34:02 +0000
From: Andrew Poelstra <apoelstra@wpsoftware.net>
To: "David A. Harding" <dave@dtrt.org>
Cc: Matthew Zipkin <pinheadmz@gmail.com>, Ethan Heilman <eth3rs@gmail.com>,
	Bitcoin Development Mailing List <bitcoindev@googlegroups.com>
Subject: Re: [bitcoindev] Signing a Bitcoin Transaction with Lamport
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On Mon, May 06, 2024 at 06:11:48PM -1000, David A. Harding wrote:
> On 2024-05-06 09:06, Andrew Poelstra wrote:
> > You can implement ECDSA. It will just take a *lot* of opcodes.
> 
> I'll accept that as a given, but how do you know that a given ECDSA
> signature actually commits to the transaction that contains it if
> OP_CHECKSIG only operates on fixed-size schnorr signatures?
> 

You need to connect your Lamport signature to an ECDSA CHECKSIG (in a
pre-Taproot output). So what I'm depending on here is that it's possible
to "copy the signature" from a pre-Taproot spend to a post-Taproot spend
by using Lamport signatures and some anti-equivocation scheme.

In pre-Taproot we confirm that the signature matches the pattern of
OP_SIZE outputs. In post-Taproot we reconstruct the signature and
constrain the transaction, checking that it spends *both* the
pre-Taproot and the post-Taproot output.

> Is this what you're describing: if the controlling signature is a lamport
> signature that commits to an ECDSA signature, it's safe to disclose the
> private key for the ECDSA signature; when you don't have to worry about
> private key disclosure, it's safe to construct a schnorr signature that uses
> the same private key, nonce, and message commitment as the ECDSA signature;
> if that schnorr signature makes OP_CHECKSIG return true, then you know the
> message is the current transaction?
> 

Nope, in this scheme we are avoiding Schnorr signatures entirely.

> That still leaves me confused.  If ECDSA can be implemented within
> tapscript, then I would expect that schnorr could also be implemented within
> tapscript; that gives you an OP_CSFS equivalent.  If being able to implement
> ECDSA in tapscript allows introspection, then I would expect implementing
> schnorr in tapscript would allow introspection; that gives you an OP_CAT
> equivalent.  If you have OP_CSFS and OP_CAT, you have covenants and there's
> no need for lamport signatures or ECDSA.
>

Implementing ECDSA in Tapscript *only* allows introspection in
conjunction with the ability to force a user to spend a Tapscript output
alongside a pre-Tapscript output containing the same ECDSA signature.
And I am waving my hands and saying that I think you can force this by
using covenant tricks.

> Apologies for my remaining confused in the face of something that's probably
> obvious,
> 

Lol. This whole thing is kinda insane.

-- 
Andrew Poelstra
Director, Blockstream Research
Email: apoelstra at wpsoftware.net
Web:   https://www.wpsoftware.net/andrew

The sun is always shining in space
    -Justin Lewis-Webster

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