As electronic commerce will increase, players will increasingly wish to have signed documents and strong encryption. According to current knowledge, it must be expected that the costs of attacking, through Trojan horses, crypto impleme ntations on computers with mainstream operating systems, are low. This threatens legal certainty, either because such Trojan horses will emerge, or because their effect will be claimed. Also, they may attack encryption tools. It is discussed that it should be possible to build secure systems in a way that impersonation and eavesdropping is completely implausible. Some requirements for such solutions are discussed. It is concluded that on different market segments, different computers will be used, varying essentially in functionality, screen size, means of input, and degree of tamper resistance.