def digest_type
      DIGESTS.each do |digest|
        return digest if source.key? digest
      end
      raise ChecksumMissing.new(self)
    end
def digest_type
      DIGESTS.each do |digest|
        return digest if source.key? digest
      end
      raise ChecksumMissing.new(self)
    end
The digest type defined in the software definition

 @raise [ChecksumMissing]
   if the checksum does not exist

 @return [Symbol]