How to Create a Secret Code Key Without Meeting First

This 1980 patent describes a way for two people to create a secret code key over a public channel, like the internet, without ever meeting or sharing the key directly.

What it covers

This patent explains a method and apparatus for generating a secret cryptographic key between two parties, let's call them Alice and Bob, who can only communicate over an insecure channel. Alice starts with a secret number (signal A) and Bob starts with his own secret number (signal B). They each transform their secret number using a special mathematical process that's easy to do but incredibly hard to undo (infeasible to invert). Alice sends her transformed number (transformed A) to Bob, and Bob sends his transformed number (transformed B) to Alice. Then, Alice takes Bob's transformed number (transformed B) and combines it with her original secret number (signal A) to create a shared secret key. Bob does the same, taking Alice's transformed number (transformed A) and combining it with his original secret number (signal B). The magic is that both Alice and Bob end up with the exact same secret key, but an eavesdropper who only sees the transformed numbers (transformed A and transformed B) cannot figure out the key or the original secret numbers.

What it doesn't cover

• —Generating a secret key by having both parties meet in person beforehand.
• —Using a pre-agreed secret key that is shared via a secure channel before communication begins.
• —Methods where the transformation process is easily reversible or invertible by an eavesdropper.
• —Systems where the final shared secret key can be deduced solely from the publicly exchanged transformed signals.
• —Creating a secure key that is not identical for both parties involved in the communication.

The clever bit

The core innovation is that two parties can compute a shared secret key without ever transmitting the key itself, and without having any prior shared secrets. They use public, easily transformable but hard-to-reverse operations on their private secrets, allowing them to derive the same secret key from publicly exchanged information.

Why it matters

This patent, often referred to as the Diffie-Hellman key exchange, is a foundational piece of modern cryptography. It solved the critical problem of how to establish a secure communication channel over an insecure network, a problem that was a major hurdle for the early internet. Its principles underpin secure online transactions, encrypted messaging, and secure web browsing (HTTPS).

Real-world examples

• 1.TLS/SSL (used in HTTPS for secure websites)
• 2.SSH (secure remote login)
• 3.VPNs (Virtual Private Networks)
• 4.Encrypted messaging apps