Methods for the authentication of a web site by a visitor to the web site. The visitor uses a device, such as a portable device like a cell phone to compute a dynamic identification string and a one-time password. The dynamic identification string is sent to a service provider, such as a certification service server associated with the web site. In response, the server computes a one-time password that is transmitted to the visitor's device. The device computed one-time password can then be compared to the server computed one-time password in order to authenticate the web site.