[bitcoindev] Re: BIP draft: Full-Aggregation of BIP 340 Signatures

[waxwing/ AdamISZ <ekaggata@gmail.com>]

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A couple of other minor comments:

On list ordering, I was tempted to write "why not include a default 
ordering algorithm", but ... I can see why that's not worth bothering with, 
since the design includes the untrusted coordinator role, so in that sense 
it really doesn't matter.

*Edit; leaving this paragraph in, but, it's not worth much; you already 
have a description that covers most everything I mention here:

Security argument: the paper's security claim is a reduction to the 
algebraic variant of the one-more-discrete-log assumption under the random 
oracle model for the hash_sig (AOMDL under ROM). Obviously there's only so 
much to be written there, but following on from BIP340 and BIP327 I think 
it's reasonable to briefly describe the security claim somewhere, and what 
its kind of "ancestry" is. As there, pointing out that AOMDL is a weaker 
(better) assumption is probably worth mentioning. You could also add a 
note, though it's unlikely any BIP reader would be confused about this 
point, that the scheme is *not* intended to be post-quantum secure.

(Is it worth mentioning the co-EUF-CMA definition here? Perhaps in a 
footnote? While it's both in the weeds, and also has no direct implication 
for implementation, it nicely shows why the technical problem to solve here 
is different from what the paper calls "IMS" vs "IAS".)

About nonce gen: this is obviously a tricky but hugely important point, 
just as it was for BIP327. The first comment I want to make is, why do you 
link to 
https://medium.com/blockstream/musig-dn-schnorr-multisignatures-with-verifiably-deterministic-nonces-27424b5df9d6#e3b6 
in the section where you're saying "don't use deterministic nonce 
generation"? The main point of that blog post is to show a way that that 
*can* done in MuSig2, even though, by default, it's insecure. But aren't 
you mainly trying to point out that, as in BIP327, in this BIP, we don't 
have security with deterministic nonces? (rather than making a Musig-DN 
recommendation)?

(Hmm, I guess you used that link because it nicely describes the attack? If 
so maybe another link's better as it could be misleading perhaps).

Separately you do point out the statelessness requirement can be dropped 
for one signer, which is a nice detail. ... I'm just wondering, why does 
this not apply to BIP327 also? (I guess in some general sense it does, but 
maybe it was not interesting there for some reason? Is it just because the 
'special last signer deterministic' subcase subsumes it?)

Cheers,
AdamISZ/waxwing


On Tuesday, July 7, 2026 at 4:49:39 PM UTC-3 waxwing/ AdamISZ wrote:

> Hi Fabian,
>
> Great! Thanks for the progress on this.
>
> Can I suggest one addition: in the Sign algo, you have "*Index lookup and 
> uniqueness check*: Find the unique index *j* in *0..u-1* such that *pubnoncej[33:66] 
> = cbytes(R2)*. Fail if no such index exists or if more than one such 
> index exists." (and then the pubkey and message check after). Perhaps add a 
> footnote warning, as per the paper, that these 2 checks are *not* 
> equivalent to checking that the tuple (pubkey, message, R_2) is unique. I 
> mean it's not technically needed but seems like a very good idea, since as 
> per the paper there is an attack there, and it would not be an 
> unanticipatable implementation footgun.
>
> You have the checks in the reference.py implementation, obviously; but 
> putting that "tripwire" in the test vectors seems very advisable, though (I 
> admit my check of your generated test vectors was not thorough but I think 
> it's not there yet?)
>
> Second point goes from the ultra-specific to the ultra-general: I'm 
> probably being dumb here, but it seems a little strange to have a BIP that 
> specifies the algo but not the consensus change at all? Hmm, as I write 
> this ... I guess this is mirroring BIP340 vs BIP341? That the former just 
> formalizes the exact algo of the signature scheme and a later BIP specifies 
> the latter? It is a bit different though; taproot was a whole new scripting 
> scheme, this will just (I guess?) have basically a new OP_CODE (or rather, 
> redefinition) or similar?
>
> About motivation:
>
> > CISA with full-aggregation can even create an economic incentive for 
> privacy since using CoinJoin and PayJoin transactions can become cheaper 
> than sending individual transactions.
>
> It's a can of worms, since there are so many nuances, but I think the 
> tendency to assume that this incentivization is real is a *little* 
> misleading: CISA does *not* incentivize batching payments in a way that 
> creates privacy; it only incentivizes consolidation and batching (amongst 
> many spenders as well as for one spender). What is undeniable is that *if* 
> you're doing a coinjoin of some flavor, you are incentivized to switch to 
> CISA. Anyway, don't take this as a big criticism particularly, because 
> *whatever* you write here will be arguable. My personal intuition is that 
> CISA *does* aid privacy but more via second and third order effects than 
> via first order. But, meh, not simple :)
>
> I'd also like to repeat a request on the earlier discussion on this 
> mailing list [1] on this topic:
>
> (Quoting Jonas' answer to me here: )
> >> The side note also raises this point: would it be a good idea to 
> explicitly 
> >> write down ways in which the usage of the scheme/structure can, and 
> cannot, 
> >> be optimised for the single-party case? 
> >
> >This is a very interesting point, probably out of scope for the paper. A 
> >single-party signer, given secret keys xi, ..., xn for public keys X1, 
> ..., Xn 
> >can draw r at random, compute R := r*G and then set s := r + c1*x1 + ... 
> + 
> >cn*xn. So this would only require a single group multiplication. 
>
> .. but, I relegate this to a "comment" here, because I don't know if there 
> is a written down "optimization for single signer" apart from what Jonas 
> said there. If it doesn't exist, you can't put it in the BIP :) What I 
> think would really be worth avoiding is : there's a folklore optimization 
> for coding the use of DahLIAS in a single wallet that is not properly 
> analyzed by the people here who know what they're talking about -- because, 
> "minefield" etc etc.
>
> Cheers,
> waxwing/AdamISZ
>
> [1] https://groups.google.com/g/bitcoindev/c/eothFkxAvK0/m/gfGZ8NxXEQAJ 
> and surrounding conversation
> On Monday, July 6, 2026 at 2:10:57 PM UTC-3 Fabian wrote:
>
>> Hi list,
>>
>> I would like to share a BIP draft for full-aggregation of BIP 340
>> signatures, a standard for the DahLIAS interactive aggregate signature
>> scheme by Jonas Nick, Tim Ruffing, and Yannick Seurin:
>> https://eprint.iacr.org/2025/692
>>
>> Full-aggregation allows a group of signers to produce a single 64-byte
>> signature for a list of public key and message pairs in a two-round
>> signing protocol very similar to MuSig2/BIP327. The primary application
>> in Bitcoin would be CISA (cross-input signature aggregation).
>>
>> The BIP draft text can be found here:
>>
>> https://github.com/fjahr/bips/blob/4b34e86d0040edad6cba3f7518b33472a11ebeaf/bip-XXXX.mediawiki
>>
>> For inline comments, I have opened a mock pull request on my BIPs repo 
>> fork:
>> https://github.com/fjahr/bips/pull/4
>>
>> Feedback of any kind is much appreciated.
>>
>> Best,
>> Fabian
>>
>

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