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CoSi (Collective Signatures) are compact cryptographic proofs showing that several distinct secret key holders, called cosigners, have cooperated to sign a given message.

It is a protocol that enables a decentralized (potentially large) group of independent servers to efficiently issue aggregate Schnorr signatures. These collective signatures (co-signatures) convey the same information as a list of conventional signatures but are much more compact and efficient to verify against the aggregate public key of the server group. In practice, a co-signature is not much bigger than an individual Schnorr signature.

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