Access to a data storage arrangement is provided in dependence upon a public/private key pair, the public key being held by the data storage arrangement and the private key by a removable memory device. The public key is securely held in a user directory of authorized users on the data storage arrangement, and is associated with a local version number. Another copy of the user directory is securely held by the removable memory device and is associated with an external version number. When the removable memory device is connected to the data storage arrangement to enable authentication, these version numbers are compared and the older version of the user directory is replaced with the later version. In this fashion updates to the list of authorized users can be propagated among suitably adapted devices without the use of a network.