Allowing Duplicate Keys in BIP 390 musig() Expressions

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

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
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Best regards,
Boris Nagaev

Hi Boris,

BIP 327 explicitly allows for duplicate participant pubkeys, so as long
as all signing procedures follow the BIP, everything will be fine. Also,
BIP 327 explicitly warns against deterministic nonces for reasons
unrelated to duplicate pubkeys.

Although, allowing duplicates does bring up an additional issue with the
MuSig2 PSBT fields as these inherently do not allow duplicate pubkeys.

Ava