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However, this sounds like wishing to violate the known laws of physics (specifically, their time invariance). Unless some of the information available at the time T is unavailable at some time T'>T, how could you delete something already shared with another party?

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Yeah, HDCP (high-definition copy protection) makes an attempt to make content viewable, but not copyable, but it only works by certifying every single piece of the chain and it's still vulnerable to someone using a simple video camera to capture images of the displayed message.

Barring a human-computer interface that conveys messages directly into someone's brain, I don't see this as possible. If you can see it or if you can hear it, then it is copy-able.

I mostly meant the fact that if you can decode a message that you have with a key and algorithm that you have, you can do it later again, with the same result. Of course, HDCP is an also example of this, since you still have the message and you still have the key, even after some time (as long as you still have both).

> However, this sounds like wishing to violate the known laws of physics

Some crypto work is focused on proving that certain things are impossible. But so far I haven't seen any paper proving that this was impossible.

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