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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 pack(self):
 """ Return key and uid packed for sending in a command to the YubiHSM. """

        # # 22-bytes Yubikey secrets block
        # typedef struct {
        #   uint8_t key[KEY_SIZE];              // AES key
        #   uint8_t uid[UID_SIZE];              // Unique (secret) ID
        # } YUBIKEY_SECRETS;
        return self.key + self.uid.ljust(pyhsm.defines.UID_SIZE, chr(0))