NUT-12: Offline ecash signature validation

[KKA11010]

Additional context NUT-12 is an extension of Cashu's crypto system to allow a user Alice to verify the mint Bob's signature using only Bob's public keys. We explain how another user Carol who receives ecash from Alice can execute the DLEQ proof as well. This is achieved using a Discrete Log Equality (DLEQ) proof. Previously, Bob's signature could only be checked by himself using his own private keys (NUT-00).