Hi Jonas and list.

So I'm reading the paper and it's very interesting. I have other questions 
but this one seems more important so I'll just stick with this one:

Appendix A2 explains an attack on Musig2-IAS, in which you can forge a 
partial signature on a tweaked key of the honest signer. I don't understand 
why this same attack cannot be applied to MuSig2 itself?

the multisig-to-IAS "translation" makes sense, given the caveat of the 
weakness identified in the 2018 paper and explained here in detail, other 
than that it's basically about the message being a concat of the individual 
messages (and keys). But surely that doesn't change the structure of the 
attack? (i.e. multiply your R-vals by a2/a1, then take partial sig and 
multiply by a2/a1 and add the tweak). I note that 3 round musig is not 
vulnerable to it, nor would some PoK of R be.

Obviously I missed something.

Cheers,
AdamISZ/waxwing

On Thursday, April 17, 2025 at 10:38:46 AM UTC-6 Jonas Nick wrote:

> Hi list,
>
> Cross-Input Signature Aggregation (CISA) has been a recurring topic here, 
> aiming
> to reduce transaction sizes and verification cost [0]. Tim Ruffing, Yannick
> Seurin and I recently published DahLIAS, the first interactive aggregate
> signature scheme with constant-size signatures (64 bytes) compatible with
> secp256k1.
>
> https://eprint.iacr.org/2025/692.pdf
>
> Recall that in an aggregate signature scheme, each signer contributes 
> their own
> message, which distinguishes it from multi- and threshold signatures, 
> where all
> signers sign the same message. This makes aggregate signature schemes the
> natural cryptographic primitive for cross-input signature aggregation 
> because
> each transaction input typically requires signing a different message.
>
> Previous candidates for constant-size aggregate signatures either:
> - Required cryptographic assumptions quite different from the discrete 
> logarithm
> problem on secp256k1 currently used in Bitcoin signatures (e.g., groups 
> with
> efficient pairings).
> - Were "folklore" constructions, lacking detailed descriptions and security
> proofs.
>
> Besides presenting DahLIAS, the paper provides a proof that a class of 
> these
> folklore constructions are indeed secure if the signer does _not_ use key
> tweaking (e.g., no Taproot commitments or BIP 32 derivation). Moreover, we 
> show
> that there exists a concrete attack against a folklore aggregate signature
> scheme derived from MuSig2 when key tweaking is used.
>
> In contrast, DahLIAS is proven to be compatible with key tweaking. 
> Moreover, it
> requires two rounds of communication for signing, where the first round 
> can be
> run before the messages to be signed are known. Verification of DahLIAS
> signatures is asymptotically twice as fast as half-aggregate Schnorr 
> signatures
> and as batch verification of individual Schnorr signatures.
>
> We believe DahLIAS offers an attractive building block for a potential CISA
> proposal and welcome any feedback or discussion.
>
> Jonas Nick, Tim Ruffing, Yannick Seurin
>
>
> [0] See, e.g., https://cisaresearch.org/ for a summary of various CISA
> discussions.
>

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