The invention relates to a process of validation for transactions between a user terminal and a server of the type involving: "Request, Challenge, Response, Verification and Approval". In other aspects it also relates to a computer network, server or terminal for performing the method, as well as a physical key. It involves providing a code word made up of a first series of elements to a user. Providing a key to the user to use to scramble the code word. Holding the code word and key securely at the server; Receiving a request communication at the server from a user terminal. Responding to the request by issuing a second series of elements from the server to the user terminal. Displaying the second series of elements at the terminal. Inviting the user to enter a scrambled version of the code word by selecting the elements of the first series in order from the second series and for each element selected making an entry at the terminal in dependence on the key to create a series of entries. And using the series of entries to validate the transaction.