A system and method for authorizing a client device to access a host device based on timestamps including preferably at least two time units. Both devices contain multiple sequence tables that relate an order of time units to the value of one of the time units. Both devices also contain multiple string tables that relate strings to values of the time units within the timestamps. When the client device wants to access the host, it generates a first timestamp and sends the host device the first timestamp and the character strings from host tables related to the value of time units of the first timestamp. The host tables are known to all authorized client devices within the network. The strings are ordered according to a sequence table in the client device and the host device. When received, the host device compares the received characters strings to the character strings within its host string table based on an order determined by its host sequence table. If the character strings and order match, the host sends the client a second timestamp and the process is repeated using the second timestamp and sequence and string tables associated with, and known only to, the client device and the host device. In addition, the client, which may be monitored or controlled by the host, is able to request ongoing re-verification of the hosts authorization to detect and prevent unauthorized access from a third party. The rapidly changing timestamps and network keys eliminate recursive efforts to access these protected devices from an unauthorized third party.