Subject: Re: [bitcoin-dev] On adaptor security (in protocols)

From: Lloyd Fournier <[email-masked]>
Date: Mon, 1 May 2023 12:23:30 +0800

Hi waxwing,

I think your view of the uselessness of single signer adaptors is too
pessimistic. The claim you make is that they "don't provide a way to
create  enforcement that the publication of signature on a pre-defined
message will reveal a secret'' and so are useless. I think this is wrong.
If I hold a secret key for X and create a signature adaptor with some
encryption key Y with message m and do not create any further signatures
(adaptor or otherwise) on m, then any signature on m that is published
necessarily reveals the secret on Y to me. This is very useful and has
already been used for years by DLCs in production.

I haven't read the proofs in detail but I am optimistic about your
approach. One thing I was considering while reading is that you could make
a general proof against all secure Schnorr signing scheme in the ROM by
simply extending the ROM forwarding approach from Aumayer et al to all
"tweak" operations on the elements that go into the Schnorr challenge hash
i.e. the public key and the nonce. After all whether it's MuSig2, MuSig,
FROST they all must call some RO. I think we can prove that if we apply any
bijective map to the (X,R) tuple before they go into the challenge hash
function then any Schnorr-like scheme that was secure before will be secure
when bip32/TR tweaking (i.e. tweaking X) and adaptor tweaking (tweaking R)
is applied to it. This would be cool because then we could prove all these
variants secure for all schemes past and present in one go. I haven't got a
concrete approach but the proofs I've looked at all seem to share this
structure.

Cheers,

LL

On Sun, 30 Apr 2023 at 00:20, AdamISZ via bitcoin-dev <
[email-masked]> wrote:

> Hi list,
> I was motivated to look more carefully at the question of the security of
> using signature adaptors after recently getting quite enthused about the
> idea of using adaptors across N signing sessions to do a kind of multiparty
> swap. But of course security analysis is also much more important for the
> base case of 2 party swapping, which is of .. some considerable practical
> importance :)
>
> There is work (referenced in Section 3 here) that's pretty substantial on
> "how secure are adaptors" (think in terms of security reductions) already
> from I guess the 2019-2021 period. But I wanted to get into scenarios of
> multiple adaptors at once or multiple signing sessions at once with the
> *same* adaptor (as mentioned above, probably this is the most important
> scenario).
>
> To be clear this is the work of an amateur and is currently unreviewed -
> hence (a) me posting it here and (b) putting the paper on github so people
> can easily add specific corrections or comments if they like:
>
> https://github.com/AdamISZ/AdaptorSecurityDoc/blob/main/adaptorsecurity.pdf
>
> I'll note that I did the analysis only around MuSig, not MuSig2.
>
> The penultimate ("third case"), that as mentioned, of "multiple signing
> sessions, same adaptor" proved to be the most interesting: in trying to
> reduce this to ECDLP I found an issue around sequencing. It may just be
> irrelevant but I'd be curious to hear what others think about that.
>
> If nothing else, I'd be very interested to hear what experts in the field
> have to say about security reductions for this primitive in the case of
> multiple concurrent signing sessions (which of course has been analyzed
> very carefully already for base MuSig(2)).
>
> Cheers,
> AdamISZ/waxwing
>
>
>
>
> Sent with Proton Mail secure email.
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