# 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.

- **Patent:** US 6195698
- **Original title:** Method for selectively restricting access to computer systems
- **Owner:** Compaq Computer Corp
- **Granted:** 2001
- **Status:** Public domain (expired)
- **Times cited:** 219
- **Field:** consumer_electronics, software, telecommunications, ai_ml

## What it does

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.

## 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).

## 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.

## Real-world examples

1. Early Yahoo! account registration forms
2. Classic text-based CAPTCHAs on web forums
3. Automated spam prevention on email sign-up pages

## Why it matters

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.

## 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.

**Full plain-English explainer:** https://patentbrief.org/patent/us/6195698/google-search-engine

**Original patent:** https://patents.google.com/patent/US6195698

---

_Source: PatentBrief — https://patentbrief.org. Patent facts are from public records; the plain-English explanation is PatentBrief's._