An apparatus for use in a digital messaging system includes a storage device and a processor coupled to the storage device. The storage device storing software instructions for controlling the processor that when executed by the processor configured the processor to: generate a master private and public key pair; associate the master private and public key pair with a first certificate; and derive at least one domain-specific key from the one of the master private and public key pair. The first certificate is registered to a group comprising a plurality of domains. The domain-specific key is associated with one of the plurality of domains.