Re: [bitcoindev] Allowing Duplicate Keys in BIP 390 musig() Expressions

[Nagaev Boris <bnagaev@gmail.com>]

Hi Ava,

Is it safe to allow multiple participants to have the same public key?
If deterministic nonce generation is used (deriving each participant's
nonce from the message, the set of public keys, and the participant's
private key), duplicate public keys would lead to identical nonces.

While this may not be catastrophic (since they are signing the same
message and the private key likely can't be extracted) it still seems
risky. Identical nonces can have unexpected consequences, and I'm not
sure if all security assumptions would still hold.

Curious what you think.

Best,
Boris

On Tue, Jun 3, 2025 at 6:08 PM 'Ava Chow' via Bitcoin Development
Mailing List <bitcoindev@googlegroups.com> wrote:
>
> Hi All,
>
> In implementing musig() descriptor expressions, I realized that the
> restriction "Repeated participant public keys are not allowed" is a bit
> complicated to implement. While I don't see why anyone would want to
> duplicate keys, MuSig2 does allow duplicate participant keys and
> allowing them would make the implementation of musig() expressions much
> easier. Thus I'd like to propose changing the BIP to remove this
> restriction.
>
> Has anyone implemented musig() expressions yet with this restriction,
> and would removing it be a significant breaking change to anyone? If
> not, I'll make the change to the BIP in a few days.
>
> Thanks,
>
> Ava
>
>
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-- 
Best regards,
Boris Nagaev

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