Abstract: In order to achieve very secure transmission of sensitive information over a public communications medium such as the Internet to carry out transactions therebetween, an account initialization data set is developed in a client system and duplicated, by secure means, in a host system. At each of the client and host systems, an initial client control file data set including a plurality of control fields (at least one of which includes data developed from information which is unique to the client system hardware) is independently developed and stored using functionally identical algorithms and incorporating information included in at least a part of the account initialization data set.