An apparatus and method for authenticating a data transmission across a network. Variations in data rate of data received across the network are detected, and the received data are authenticated when the variations conform to a preselected variable data rate profile. Preferably, an average data rate is first determined, and the data rates of individual data rates of blocks of the received data are compared to the average data rate. Some of the data rates are preferably faster than a selected threshold. Different portions of the received data preferably take different, parallel paths through the network from a transmitter to a receiver. Preferably, the received data are transmitted via packets. The data rate is preferably varied by using different densities of data packets, or by selectively delaying selected packets. Data authentication preferably results in grant of user access to the received data.