[drog]

Thanks to other people pointing out, I now see that there is still a fundamental problem — crush can see that you appointed them to be your crush without liking you back, and the solution is that: 1) everobody precommits to your crush set in advance 2) users use mpc protocol that will ensure that your crush reveals if both of you precommitted to each other. (I guess it's similar to "Yao millionaire problem" where two parties calculate "x < y" without revealing x and y. but you calculate "x == y" where x and y is values that you committed to previously and you dont reveal x and y)

Previous variant does not need any blockchain because you can just embed encrypted message to the web page similarly to the original hash variant (really it's the same as sending private message to your crush) but MPC variant probably needs blockhain because that's a perfect way to publicly precommit to something.

Note: there is still the possibility that you can precommit to "x is my crush" without x being your real crash to lure out if you are crush of x.

P.S. I think that's a good illustration of a service that can't be done without crypto and have similar properties.