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.
Original patent title: “Cryptographic apparatus and method”
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. Granted to Leland Stanford Junior University in 1980 with 15 claims and 708 forward citations, and it is now in the public domain.
Coverage
What does this patent actually cover?
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.
The gap
What does this patent 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.
These exclusions are unique to PatentBrief — derived from the actual claim language, not patent-office boilerplate.
Key facts
What made this novel
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.
The Patent Drawing
Schematic visualization of the patent's claim structure. Hand-drawn diagrams in progress for each landmark patent.
Where you've seen this
Real-world examples
TLS/SSL (used in HTTPS for secure websites)
SSH (secure remote login)
VPNs (Virtual Private Networks)
Encrypted messaging apps
Why it matters
The bigger picture
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).
Filed
September 6, 1977
Granted
April 29, 1980
Market context
Who's building on this
Companies in this space
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.
Market impact
This patent created the possibility of truly secure communication over untrusted networks, enabling the growth of e-commerce and the modern internet. It directly led to the development of public-key cryptography systems and became essential for securing online transactions and data transmission, fundamentally changing how digital information is protected.
Claim 1 — Plain English
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.
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.
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.
Patent timeline
Application submitted to the patent office
Application published, typically 18 months after filing
Patent officially issued
Patent enters public domain
This patent is in the public domain
See the Freedom to Build guide — what is free to use, what is not, and how to cite this patent.
PatentBrief Score
Impact Score
Strong
Citation count
40/40
Highly cited
Claim breadth
10/20
Broad claimsclaimsThe numbered statements at the end of a patent that legally define what the inventor owns.Read more →
Recency
0/20
Older than 20 years
Assignee scale
20/20
Major company or institution
PatentBrief Impact Score — based on citation count, claim breadth, recency, and assignee scale. Not a legal assessment.
Heuristic Value Estimate
What this patent might be worth
$53K – $168K
Midpoint $105K · expired or expiring · industry ×1.4
Heuristic only — blends forward/backward citation counts, claim scope, time remaining, litigation history, and CPC-derived industry baseline. Real valuations need a professional appraisal.
Claim text not yet imported for this patent
The original legal language
Original claims
15 claims as filed with the patent office.
Concepts involved
Citations
Patent lineage
Cite this patent
Hellman, M. E., Merkle, R. C., & Diffie, B. W. (1980). How to Create a Secret Code Key Without Meeting First (U.S. Patent No. 4,200,770). U.S. Patent and Trademark Office. https://patentbrief.org/patent/us/4200770/diffie-hellman-public-key-exchange
Auto-generated from the patent record. Double-check author order and the issue date against the official USPTO document before submitting.
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Common Questions
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.
Same assignee
More from Leland Stanford Junior University
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