Secret sharing
Split a key into five parts where any three reconstruct it — and any two reveal nothing at all.
Adi Shamir's 1979 threshold scheme rests on the fact that a polynomial of degree k−1 is uniquely determined by k points. The secret is placed as the polynomial's constant term, each participant gets a point on the curve, and any k points suffice to interpolate it back. George Blakley published an equivalent solution using intersecting hyperplanes the same year.
The security is information-theoretic rather than computational: with k−1 shares every possible secret remains exactly as likely as before, no matter how much computing power the attacker brings. The method is used wherever no single person should be able to act alone — keys to HSMs, recovery of crypto wallets, and IANA's ceremonies for the DNSSEC root key, where the shares are held by appointed representatives from different countries.