Attacks may start from the simplest operation, if the key is stored in the external memory, the attacker simply access the address and data bus to steal the key. Even if the key is not stored continuously in the memory, the attacker can still break the external code to determine which memory contains the password. Boot loading, debugging and measurement are vulnerable to a simple attack. These operating states typically allow for microprocessor memory and register read / write operations. Even if the documents are not open, it is easy to get the information through the microprocessor firm's employees who have a wide range of social experience.