docs: def serialize(self, backend):
          """
          Use this method to prepare the data to be uploaded directly to some PEP node. This
          method applies an additional encoding step to the public keys in the bundle. The
          result is a dictionary with the keys ik, spk, spk_signature and otpks. The values
          are structured the same way as the inputs of the constructor.
          """
  
          return {
              "ik": backend.encodePublicKey(self.ik, "25519"),
              "spk": {
                  "id"  : self.spk["id"],
                  "key" : backend.encodePublicKey(self.spk["key"], "25519"),
              },
              "spk_signature": self.spk_signature,
              "otpks": list(map(lambda otpk: {
                  "id"  : otpk["id"],
                  "key" : backend.encodePublicKey(otpk["key"], "25519")
              }, self.otpks))
          }