Subject: Re: [bitcoindev] Signing a Bitcoin Transaction with Lamport Signatures (no changes needed)

From: Ethan Heilman <[email-masked]>
Date: Tue, 30 Apr 2024 16:43:51 -0400

One could redesign this scheme to minimize the number of opcodes.

Back of the napkin scheme follows:

Alice, rather than Lamport signing the length of each ECDSA signature,
instead Lamport (or WOTS) signs a vector of the positions of the low-length
ECDSA signatures (low length here means length=58 rather than length=59).
Then the redeem script would only check the length of those particular
signatures and could drop the other other public keys. This saves
significantly on the number of opcodes because we only need to check the
Lamport signature on the vector, no one each signature length and we can
drop unused checked signatures without evaluating them.

Alice's advantage over the attacker is that she gets to fix the positions
of the low length ECDSA signatures after she generates them. This means if
the total number of signatures is N and the number of low length signatures
is M, her advantage over the attacker is (N choose M). For instance if
N=M=10, to generate 10 59-length signatures, Alice needs to perform
2^(8*10) work. This is because we assume the probability of generating a
58-byte ECDSA signature is 1/256 (1/2^8). However if N=40, M=10 she only
needs to perform 2^(8*10)/(40 choose 10) work.

If we assume that we want 80 bits of security, this means we need M=10 low
length ECDCSA signatures (2^(8*10)). The next parameter is how cheap we
want this to be for Alice to compute, we can boost Alice's signing time by
increasing N and remove log2(N choose 10) from the work she needs to
compute the signature. If we want to ensure Alice has to do no more than
2^32 work to sign, then we need N=46 or 46 ecdsa signatures.

On Tue, Apr 30, 2024 at 10:21 AM Andrew Poelstra <[email-masked]>
wrote:

> On Tue, Apr 30, 2024 at 08:32:42AM -0400, Matthew Zipkin wrote:
> > > if an attacker managed to grind a 23-byte r-value at a cost of 2^72
> > computations, it would provide the attacker some advantage.
> >
> > If we are assuming discrete log is still hard, why do we need Lamport
> > signatures at all? In a post-quantum world, finding k such that r is 21
> > bytes or less is efficient for the attacker.
> >
>
> Aside from Ethan's point that a variant of this technique is still
> secure in the case that discrete log is totally broken (or even
> partially broken...all we need is that _somebody_ is able to find the
> discrete log of the x=1 point and for them to publish this).
>
> Another reason this is useful is that if you have a Lamport signature on
> the stack which is composed of SIZE values, all of which are small
> enough to be manipulated with the numeric script opcodes, then you can
> do covenants in Script.
>
> (Sadly(?), I think none of this works in the context of the 201-opcode
> limit...and absent BitVM challenge-response tricks it's unlikely you can
> do much in the context of the 4MWu block size limit..), but IMO it's a
> pretty big deal that size limits are now the only reason that Bitcoin
> doesn't have covenants.)
>
> --
> 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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