A method and device performing encryption/decryption of data on a mass storage media. A plurality of different encryption/decryption algorithms with associated keys can be utilised on different storage areas of the media, such as blocks/sectors on a hard disk drive, thereby increasing the security of the data significantly. In addition, combining the keys with random numbers for a further increase of the security of the data. The block/sector number is used to select an algorithm with associated key and random number. The device and method encrypts/decrypts emails, networking traffic etc. and other types of electronic data. In addition, the device provides an authentication session when booting a computer from a hard disk drive, and even be used to select a certain operating system and/or environment associated with a certain key carrier by changing the Master Boot Record of the disk system.