A customer computer 12, vendor computer 16 and verification computer 14 are interconnected by means of a network 18, such as the internet The customer 12 can initiate a transaction, such as the purchase of information from the vendor 16. However, the vendor 16 will not proceed until verification of the transaction has been received from the site 14. This is not provided until the customer 12 has sent a unique fingerprint of data to the site 14, identifying the customer machine by reference to hardware device types or serial numbers, software types or licences, e-mail addressed or the like. This fingerprint is stored for future reference in showing that the transaction was validly implemented by the customer machine 12.