A system and method for protecting master transport encryption keys stored on a computing device. Master transport encryption keys are used to secure data communications between computing devices. In one example embodiment, there is provided a method in which a copy of a master transport encryption key is generated and stored in a volatile store of a first computing device (e.g. a mobile device). This copy of the master transport encryption key can be used to facilitate the decryption of data received at the first computing device from a second computing device (e.g. a data server), even while the first computing device is locked. The method also comprises encrypting the master transport encryption key, with a content protection key for example, and storing the encrypted master transport encryption key in a non-volatile store of the first computing device.