DATA NON-REPUDIATION

Method of and apparatus for two-party data non-repudiation. For applications which have a (near) real-time operation, the slowness engendered by the straightforward cryptographic signing of messages may well be intolerable. Accordingly the invention provides a method of operating a terminal to provi de non-repudiation in respect of data exchanged between the terminal and at lea st one other terminal, the data exchanged via a link in which at least a portio n of the exchanged data may be lost, wherein a common block of received data i s agreed between the participants. Rather than signing every packet, only the agreed data possessed in common after reception is signed.