The Sign method provides a way to sign a given Aleph message using a substrate account. The full message is not used as the payload, only fields of the BaseMessage type are.
The sign method of the package 'polkadot' is used as the signature method.
The Aleph message to sign, using some of its fields.
DOTAccount implements the Account class for the substrate protocol. It is used to represent a substrate account when publishing a message on the Aleph network.