limitations
only proves timely delivery to encryptor
- sender? only makes sense if bundling network QoS or if ISP trusts sender - typically not so (e.g. VoIP)
- receiver's ISP? encrypt link for delay sensitive class
must have incentive to return receipt
- e.g. to claim refund after pre-payment
- (pragmatic solution to 'two generals' problem)
'tamper proof' & 'watermark' fallible
if don't need non-repud'n or watermarking
- limited key mgmt possible without smartcard