Public key infrastructure (Computer security) Security measures 6 Access control 4 Cell phones 3 Mobile communication systems 3 Cellular telephone systems 1 Computer networks 1 Computer security 1 Computers 1 Data protection 1 Electronic filing systems 1 Electronic information resources 1 Electronic mail systems 1 Equipment and supplies 1 Instant messaging 1 Security systems 1 Wireless communication systems 1 XML (Document markup language) 1