Method for decrypting an encrypted instruction and system thereof

Methods of preventing private information, which is hidden within data of a private domain reserved by an application program, from being easily accessed by a CPU and other devices, both where the data of the private domain is decrypted and the access to said data are restricted are disclosed, where the mentioned other devices do not include a decryption module utilized in the methods. Therefore, as long as agreements related to encryptions and decryptions are made in advance between the application program and the decryption module, private information can be well protected.