How Websites Use Distorted Text Riddles to Stop Bots
This patent describes the original method for creating CAPTCHAs, using distorted text or audio riddles to distinguish human users from automated bots.
Original patent title: “Method for selectively restricting access to computer systems”
This patent describes the original method for creating CAPTCHAs, using distorted text or audio riddles to distinguish human users from automated bots. Granted to Compaq Computer Corp in 2001 with 79 claims and 219 forward citations.
Key facts
Coverage
What does this patent actually cover?
The patent outlines a security process where a server challenges a user to solve a simple puzzle before granting access to a system. When a client requests access, the server generates random characters and modifies them—for example, by changing fonts, rotating letters, or adding background noise—to create a riddle that is easy for a human to read but difficult for a computer program to interpret. The server then checks if the user's answer matches the original string within a set time limit. If the answer is correct, the connection is allowed; if not, or if time runs out, the server assumes the requester is a bot and terminates the connection.
The gap
What does this patent NOT cover?
- Does not cover non-riddle based bot detection, such as behavioral mouse tracking or IP reputation analysis.
- Does not cover biometric authentication methods like fingerprint or facial recognition.
- Does not cover systems that rely on pre-existing user accounts or passwords for verification.
- Does not cover challenges that require solving logic puzzles or identifying objects in photos (like modern image-based reCAPTCHA).
These exclusions are unique to PatentBrief — derived from the actual claim language, not patent-office boilerplate.
What made this novel
The innovation lies in using the computer's own weakness—its difficulty with pattern recognition in noisy, distorted visual or audio data—as a security gate to verify human presence.
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
Early Yahoo! account registration forms
Classic text-based CAPTCHAs on web forums
Automated spam prevention on email sign-up pages
Why it matters
The bigger picture
This patent is the foundation of the CAPTCHA (Completely Automated Public Turing test to tell Computers and Humans Apart) technology that became standard across the early web. It was essential for preventing automated spam, bulk account registration, and brute-force attacks on web services during the late 90s and early 2000s.
Filed
April 13, 1998
Granted
February 27, 2001
Market context
Who's building on this
Companies in this space
The technology was famously refined by the reCAPTCHA team at Carnegie Mellon, which was later acquired by Google. Today, major cloud providers like Cloudflare and Akamai continue to build on this concept, evolving it into more sophisticated 'bot management' platforms.
Market impact
This patent effectively created the bot-detection market. It forced developers to implement standardized human-verification steps, which became a baseline requirement for any public-facing web service to prevent malicious automation.
Claim 1 — Plain English
What this patent covers
The patent outlines a security process where a server challenges a user to solve a simple puzzle before granting access to a system. When a client requests access, the server generates random characters and modifies them—for example, by changing fonts, rotating letters, or adding background noise—to create a riddle that is easy for a human to read but difficult for a computer program to interpret. The server then checks if the user's answer matches the original string within a set time limit. If the answer is correct, the connection is allowed; if not, or if time runs out, the server assumes the requester is a bot and terminates the connection.
The clever bit
The innovation lies in using the computer's own weakness—its difficulty with pattern recognition in noisy, distorted visual or audio data—as a security gate to verify human presence.
What it does not cover
- Does not cover non-riddle based bot detection, such as behavioral mouse tracking or IP reputation analysis.
- Does not cover biometric authentication methods like fingerprint or facial recognition.
- Does not cover systems that rely on pre-existing user accounts or passwords for verification.
- Does not cover challenges that require solving logic puzzles or identifying objects in photos (like modern image-based reCAPTCHA).
Patent timeline
Application submitted to the patent office
Application published, typically 18 months after filing
Patent officially issued
PatentBrief Score
Impact Score
Strong
Citation count
40/40
Highly cited
Claim breadth
20/20
Very broad protection
Recency
0/20
Older than 20 years
Assignee scale
0/20
Independent or smaller assigneeassigneeThe entity that owns the patent — usually the inventor's employer or a company.Read more →
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
$101K – $323K
Midpoint $202K · 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.
The original legal language
Original claims
79 claims as filed with the patent office.
Concepts involved
Citations
Patent lineage
Cite this patent
Broder, A. Z., Bharat, K., Lillibridge, M. D., & Abadi, M. (2001). How Websites Use Distorted Text Riddles to Stop Bots (U.S. Patent No. 6,195,698). U.S. Patent and Trademark Office. https://patentbrief.org/patent/us/6195698/google-search-engine
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 Websites Use Distorted Text Riddles to Stop Bots cover?
This patent describes the original method for creating CAPTCHAs, using distorted text or audio riddles to distinguish human users from automated bots.
Who owns patent US 6195698?
Compaq Computer Corp owns this patent, granted in 2001.
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 6195698 cited by?
This patent has been cited by 219 later patents that build on its ideas.
What problem does this patent solve?
This patent is the foundation of the CAPTCHA (Completely Automated Public Turing test to tell Computers and Humans Apart) technology that became standard across the early web. It was essential for preventing automated spam, bulk account registration, and brute-force attacks on web services during the late 90s and early 2000s.
What does this patent NOT cover?
Does not cover non-riddle based bot detection, such as behavioral mouse tracking or IP reputation analysis.
Patent monitoring