# 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. - **Patent:** US 4200770 - **Original title:** Cryptographic apparatus and method - **Owner:** Leland Stanford Junior University - **Granted:** 1980 - **Status:** Public domain (expired) - **Times cited:** 708 - **Field:** telecommunications, software, consumer_electronics, semiconductors ## What it does 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 does NOT 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. ## 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 ## 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). ## Frequently asked questions ### What does How to Create a Secret Code Key Without Meeting First cover? 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. ### Who owns patent US 4200770? Leland Stanford Junior University owns this patent, granted in 1980. ### When does this patent expire? This patent has expired and is now in the public domain — anyone can use the invention freely. ### What is patent US 4200770 cited by? This patent has been cited by 708 later patents that build on its ideas. ### What problem does this patent solve? 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). ### What does this patent NOT cover? Generating a secret key by having both parties meet in person beforehand. **Full plain-English explainer:** https://patentbrief.org/patent/us/4200770/diffie-hellman-public-key-exchange **Original patent:** https://patents.google.com/patent/US4200770 --- _Source: PatentBrief — https://patentbrief.org. Patent facts are from public records; the plain-English explanation is PatentBrief's._ ## Related patents Semantically similar inventions in the PatentBrief corpus: - [How RSA Public-Key Encryption Keeps Digital Messages Secret](https://patentbrief.org/patent/us/4405829/rsa-encryption) — This patent describes the foundational RSA algorithm, a method for securely sending messages where anyone can encrypt a message using a public key, but only the intended recipient can decrypt it using a secret private key. - [How Netscape Created the Secure Socket Layer (SSL) for Web Security](https://patentbrief.org/patent/us/5657390/secure-socket-layer-application-program-apparatus-and-method) — Netscape's 1995 patent defining the architecture for SSL, the foundational technology that allows browsers to securely transmit encrypted data over the internet. - [Using Transcendental Math to Generate Better Random Numbers](https://patentbrief.org/patent/us/12200123/starship-orbital-test-flight) — A method for creating high-quality random numbers by using complex transcendental math equations to process raw data. - [Hedy Lamarr and George Antheil's Frequency Hopping Secret Communication System](https://patentbrief.org/patent/us/2292387/hedy-lamarr-frequency-hopping) — A 1942 patent for a radio-controlled torpedo guidance system that used synchronized player piano rolls to hop between frequencies, preventing enemies from jamming the signal. - [How Computers Compress Data Using Dictionary Building](https://patentbrief.org/patent/us/4558302/lzw-compression) — This 1985 patent describes a method for making computer files smaller by building a dictionary of common data patterns and replacing them with shorter codes.