def secret_key(format: :z85)
      case format
      when :z85
        key = ffi_delegate.secret_txt.force_encoding(Encoding::ASCII)
        return nil if key.count("0") == 40
      when :binary
        key = ffi_delegate.secret_key.read_string(32)
        return nil if key.count("\0") == 32
      else
        raise ArgumentError, "invalid format: %p" % format
      end
      key
    end
def secret_key(format: :z85)
      case format
      when :z85
        key = ffi_delegate.secret_txt.force_encoding(Encoding::ASCII)
        return nil if key.count("0") == 40
      when :binary
        key = ffi_delegate.secret_key.read_string(32)
        return nil if key.count("\0") == 32
      else
        raise ArgumentError, "invalid format: %p" % format
      end
      key
    end
Returns the secret key either as Z85-encoded ASCII string (default) or
 binary string.
 @param format [Symbol] +:z85+ for Z85, +:binary+ for binary
 @return [String] secret key
 @return [nil] if secret key is undefined (like after loading from a file
   created using {#save_public})