Abstract: The invention supplements the use of a conventional authentication token such as a MAC or DSG, to provide very strong evidence of the origin of an electronic message. A new type of authentication token, known as the "Non-Repudiation Vector" (NRV) is attached to each message sent across a data network. The NRV cryptographically links each message sent or received by a particular data processor on the network to previous and subsequent messages handled by that data processor, to create a chain of cryptographically linked messages. The burden of proof for the repudiation of a message is transferred to the sender of a message, who must demonstrate how the sending data processor could have moved through the time period during which the disputed message was sent, without generating the NRV corresponding to the disputed message.