Shamir's Secret Sharing: How to share a secret s to n parties so that a threshold >= t of them can reconstruct the secret.
Choose a random polynomial p of degree t-1 passing through (0,s). Share secrets p(x) to each party 1 <= x <= n.
Any threshold >= t parties can use Lagrange Interpolation to make a polynomial p1 and evaluate p1(0) to recover the secret s.
n.b. this works over finite fields used in cryptography #moonmath