Merkle's Puzzles

In cryptography, Merkle's Puzzles is an early construction for a public-key cryptosystem, devised by Ralph Merkle in 1974 and published in 1978. The protocol allows two parties to agree on a shared secret by exchanging messages, even if they share no secret beforehand. The scheme provides a quadratic security gap between legitimate parties and an eavesdropper, and is recognized as a precursor to public-key cryptography in the modern sense.