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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 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))
        }