[bitcoin-dev] Blinded 2-party Musig2

[Tom Trevethan <tom@commerceblock.com>]

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@moonsettler

Your scheme for blinding the challenge (e in your notation) works as far as
I can tell. It is better than the way I suggested as it doesn't require
modifying the aggregated pubkey (and the blinding nonce can be different
for each signature).

@AdamISZ and @Jonas

It is not necessarily the server that would need to verify that the
challenge is 'well formed', but the receiver of a statecoin. The concept of
having a blinded statechain server is that each signature generated for a
shared public key must be verified by the receiver of the corresponding
coin. So a receiver would retrieve the number of co-signings performed by
the server (K) and then verify each of the K signatures, and K transactions
that they have received from the sender. They can additionally verify that
each of the K R values has been correctly formed with a proof of secret
value for creating R2 (along with the R1 from the server).

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