1. A computer-implemented method for thwarting fraud perpetrated with a computer, the method comprising: receiving a request from a computer to perform a transaction, wherein said request is received together with transaction data and a cookie, and wherein said transaction data are separate from said cookie; by operation of one or more computer processors, determining in accordance with predefined validation criteria whether said cookie includes a valid representation of said transaction data; and performing said transaction only if said cookie includes a valid representation of said transaction data.