* Referer: :class:`~stem.descriptor.server_descriptor.ServerDescriptor` **extra_info_sha256_digest** attribute
    * Format: **SHA256/BASE64**

.. versionchanged:: 1.8.0
   Added the hash_type and encoding arguments.

:param stem.descriptor.DigestHash hash_type: digest hashing algorithm
:param stem.descriptor.DigestEncoding encoding: digest encoding

:returns: **hashlib.HASH** or **str** based on our encoding argument
zUUnsupported Operation: this should be implemented by the ExtraInfoDescriptor subclass)