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Comment by baby | Hacker News Reader

> seems to be contradictory

There are a lot of problems who seem contradictory until they are solved by cryptography.

It's not only true for crypto. In general the base of many inventions is finding a solution that transcends a core contradiction instead of just living with the tradeoff.

Can you give some examples? I'm honestly curious.

Public key cryptography is a good example, which solved the seemingly impossible to solve key-distribution problem:

https://www.coursera.org/learn/cryptography/lecture/be3ae/th...

public-key crypto as kristofferR pointed out, but it's so common nowadays that people don't realize how weird of a concept it might have been a few decades ago.

Last year I came across a "fair" version of the Socialist Millionaires problem. How can two persons interact with one another to compare their bank statement. If you think about it, you might consider that in any interaction, a party might abort the exchange as soon as he learns the result and would then have an advantage over the other party. But not too long ago cryptography found a way to make this fair :) (https://www.cryptologie.net/article/279/how-to-fairly-compar...)

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?

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.

Bitcoin solved the double-spending problem with its revolutionary blockchain tech.

Before Bitcoin came along a working decentralized P2P currency had been considered impossible, since nobody could figure out a way to make a P2P currency unforgeable (and therefore work as a currency) before that.

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