Transparent Encryption for External Storage Media with Key Management Adapted to Mobile Use

External storage devices, such as USB-sticks, SD-cards, or other flash memory devices, are widely used today. These devices may pose a threat if they contain sensitive data and are then lost or forgotten somewhere. Usability of existing encryption solutions for such external storage media is limited because of the need to reenter keys or authentication credentials every time the media is plugged to a different device. We present a solution to the usability problem by caching the key with time delayed deletion. We implemented two variations of this concept. One solution does not require any explicit user interaction, thus limiting the use of the external storage device to short term data transfer, only. The second solution allows multiple encryption keys for different files of the file system to be managed on a trusted host, using the timed key caching to avoid handing out keys or passwords.