Oblivious transfer [1], especially as it pertains to secure multi-party computation (MPC). It's not as easily visualized as Diffie–Hellman or Shamir's Secret Sharing (both of which have already been mentioned), but a good chunk of what MPC allows for [2] seems like magic.

[1] https://en.wikipedia.org/wiki/Oblivious_transfer

[2] https://en.wikipedia.org/wiki/Socialist_millionaires