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GetSigner returns a sign method that can be used to sign a payload using a key associated to the DID. This function also returns the verification method needed to verify the signature.
Providing the verification method allows the caller to provide the signature's recipient with a reference to the verification method needed to verify the payload. This is often done by including the verification method id either alongside the signature or as part of the header in the case of JSON Web Signatures.
The verifier can dereference the verification method id to obtain the public key needed to verify the signature.
This function takes a Verification Method selector that can be used to select a specific verification method from the DID Document if desired. If no selector is provided, the payload will be signed with the key associated to the first verification method in the DID Document.
The selector can either be a Verification Method ID or a Purpose. If a Purpose is provided, the first verification method in the DID Document that has the provided purpose will be used to sign the payload.
The returned signer is a function that takes a byte payload and returns a byte signature.