which a user communicates with the program the term user represents generally any individual mechanism or other programming desiring to communicate with the program for example in the client server model defined above the server usually generates and delivers to a client an interface for communicating with a program operating on or controlled by the server device where the server is a web server the interface is a web page the web page when displayed by the client device presents a user with controls for selecting options issuing commands and entering text the controls displayed can take many forms they may include push buttons radio buttons text boxes scroll bars or pull down menus accessible using a keyboard and or a pointing device such as a mouse connected to a client device in a non graphical environment the controls may include command lines allowing the user to enter textual commands where the user is another program an interface may be a programmatic interface i ntroduction in distributed computing environments a user employs a client to request access to a network resource the request includes the user 39 s credentials which are required to be verified before access to the resource is granted the various embodiments of the present invention described below help prevent a third party from intercepting and later resubmitting the request in a replay attack although the various embodiments of the invention will be described with reference to the computer network 10 shown schematically in fig1 the invention is not limited to use with network 10 the invention may be implemented in or used with any computer system in which it is necessary or desirable to access electronic data the following description and the drawings illustrate.