You can freely build on How to Create a Secret Code Key Without Meeting First

What this patent 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 is now free to use

All 0 claims of US 4200770 are in the public domain. Specifically:

The 0 dependent claims add narrowing limitations and are also free.

What is NOT covered

Patent expiry frees this specific invention. Separately-patented improvements made after expiry may still be protected.

• 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.

Who is building on this today

The principles of Diffie-Hellman key exchange are implemented by virtually all major technology companies involved in networking and security, including Google, Apple, Microsoft, Amazon, and countless cybersecurity firms. It's a fundamental building block for secure communication protocols used across the internet.

Products built on expired version of this technology

How to cite this patent in your documentation

Leland Stanford Junior University. US Patent 4200770. Cryptographic apparatus and method. Granted 1980, expired 1997. Now in the public domain.

Note: This is a convenience citation. Consult a patent attorney for formal freedom-to-operate analysis.

PatentBrief is an educational resource and does not provide legal advice. Patent expiration information is derived from USPTO records and may not reflect continuation patents, divisional filings, or separately-patented improvements. For commercial use or production decisions, obtain a formal freedom-to-operate (FTO) opinion from a registered patent attorney.