Initially, the Internet was designed to be used by government and academic users, but now it is rapidly becoming commercialized. It has on-line "shops", even electronic "shopping malls". Customers, browsing at their computers, can view products, read descriptions, and sometimes even try samples. What they lack is the means to buy from their keyboard, on impulse. They could pay by credit card, transmitting the necessary data by modem; but intercepting messages on the Internet is trivially easy for a smart hacker, so sending a credit-card nu ....