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...)