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A Signature corresponding to a Key. It is a sequence of bytes holding a public key signature from one of the three supported systems (ed25519, RSA-3072, ECDSA with p384). Or, it can be a list of signatures corresponding to a single threshold key. Or, it can be the ID of a smart contract instance, which is authorized to act as if it had a key. If an account has an ed25519 key associated with it, then the corresponding private key must sign any transaction to transfer cryptocurrency out of it. If it has a smart contract ID associated with it, then that smart contract is allowed to transfer cryptocurrency out of it. The smart contract doesn't actually have a key, and doesn't actually sign a transaction. But it's as if a virtual transaction were created, and the smart contract signed it with a private key. A key can also be a "threshold key", which means a list of M keys, any N of which must sign in order for the threshold signature to be considered valid. The keys within a threshold signature may themselves be threshold signatures, to allow complex signature requirements (this nesting is not supported in the currently, but will be supported in a future version of API). If a Signature message is missing the "signature" field, then this is considered to be a null signature. That is useful in cases such as threshold signatures, where some of the signatures can be null.