The SET payment protocol uses digital signatures to authenticate messages and authorize transactions. It is assumed that these digital signatures make authorizations non-repudiable, i.e., provable to a third-party verifier. This paper evaluates what can be proved with the digital signatures in SET. The analysis shows that even a successful and completed SET protocol run does not give the parties enough evidence to prove certain important transaction features. A comparison with the similarly-structured iKP protocol shows a number of advantages of iKP as opposed to SET with respect to the use of its signatures as evidence tokens. It is shown that non-repudiation requires more than digitally signing authorization messages. Most importantly, protocols claiming non-repudiaton should explicitly specify the rules to be used for deriving authorization statements from digitally signed messages.
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