Why would you use symmetric encryption instead of asymmetric one?

Cryptography: What are the best strategies to store a encryption key on the computer?

• I am developing a side project for supporting data encryption on Linux platform. My code encrypts the files using AES256CBC. (I am using OpenSSL Crypto API). The user can encrypt the files and at some later time can decrypt the files. An AES key is generated which I use to encrypt the files. The issue is I need to save the key somewhere so as to enable the user to decrypt the files at some later point in time. Currently I am saving the key in a text file which is hidden the home directory of the user and I am encrypting the file by another AES key which is generated using the password as one of the inputs. What are the best strategies to store a key securely on the local machine? Are there any standards for this? I have seen that some commercial encryption products from McAfee and Symantec use a sort of key store on the local machine to store encryption keys. The key store are also password protected. Any suggestions will be highly appreciated. P.S : I am looking for suggestion more from design point of view.

• Answer:

  While this might not answer your question, I just thought I'd share something I came across. In ARM devices there is something known as TrustZone technology which is the following: The Security Extensions, marketed as TrustZone Technology, is found in  ARMv6KZ and later application profile architectures. It provides a low  cost alternative to adding an additional dedicated security core to an SoC,  by providing two virtual processors backed by hardware based access  control. This enables the application core to switch between two states,  referred to as worlds (to reduce confusion with other names for  capability domains), in order to prevent information from leaking from  the more trusted world to the less trusted world. This world switch is  generally orthogonal to all other capabilities of the processor, thus  each world can operate independently of the other while using the same  core. Memory and peripherals are then made aware of the operating world  of the core and may use this to provide access control to secrets and  code on the device. (From Wikipedia) So if you manage your cryptographic keys only from the more trusted "secure world", which has a region of the memory mapped for itself with hardware support, no rootkit or any software of that sort could actually get access to your cryptographic keys. Again, I'm not an expert in this area. Just in my learning stage.