A mobile device is configured to maintain data synchronization with a host server over a wireless network for user data of a host application program associated with a user account. The user account is associated with user account data which includes an encryption/decryption key, and the user data is communicated over the wireless network in an encrypted format in accordance with the encryption/decryption key. For the data synchronization, the mobile device regularly causes the user data associated with the user account to be stored in memory in a decrypted format for use with the client application program of the mobile device. The mobile device further causes a copy of the user data to be stored in the removable memory card in the encrypted format, for use in securely transferring the user data to an alternate mobile device for use with a client application program of the alternate mobile device.