Transaction verification

A client computer returns to a server, not only form data entered by the user representing an action to be taken by the server, but also a hash of the form data that is generated by a cryptographic hash function prior to returning the form data. As a result, the hash is generated before any man-in-the browser proxy has the opportunity to modify the form data. The server receives the hash of the form data generated before any man-in-the-browser proxy had access to the form data. If a hash of the form data does not match the received hash, the server detects modification of the form data, perhaps by a man-in the-browser proxy, and accordingly declines to perform the requested action.