What is Canvas fingerprinting?

In today's digital age, websites use various methods to identify and track users' online activities, and one common technique is Canvas fingerprinting.

Canvas is a tag element in HTML5 used for drawing graphics and images on a webpage. Through JavaScript code, developers can dynamically generate, display, and modify these images. Canvas fingerprinting leverages the subtle differences in how different browsers render Canvas elements to generate a unique device identifier.

Simply put, Canvas is like an electronic canvas. When different devices draw the same pattern on this canvas, the final result will be slightly different due to hardware and software variations. These 'differences, imperceptible to the naked eye,' are precisely what constitute your device fingerprint.

How does Canvas fingerprinting work?

A website obtains a user's Canvas fingerprint through the following five steps:

Step 1: Create a hidden Canvas element

The website creates an invisible Canvas element on the page to draw graphics, text, and images. This is like placing a canvas on the webpage that the user cannot see.

Step 2: Draw graphics

Using the Canvas API, the website draws various graphics or shapes on this hidden canvas, such as lines, rectangles, circles, or text, and also applies different colors and transformation effects.

Step 3: Extract Canvas data

After the drawing is complete, the image data is recorded as pixel values, with each pixel containing a specific numerical value. The website extracts this numerical image data via the Canvas API and JavaScript—essentially converting the graphics on the canvas into numbers and recording the color of each pixel.

Step 4: Generate a unique fingerprint

The website inputs these color values into an algorithm, which, after complex calculations, generates a unique identifier (i.e., a hash value). Due to differences in users' operating systems, browsers, graphics cards, drivers, and rendering processes, subtle variations occur during graphic rendering. These minor differences ultimately lead to different hash values. Each user can thus have a unique Canvas fingerprint.

Step 5: Store and compare the fingerprint

The website stores this unique fingerprint on its server. When the user visits again, the website recaptures the Canvas data, recalculates the fingerprint, and then compares it with the previously stored fingerprint to identify the user.

Use Cases for Canvas Fingerprinting

Canvas fingerprinting is widely used for several main reasons:

Differences in device hardware and browser environments: Each device's hardware and browser environment is unique, including factors like the graphics card, operating system, and browser version. These factors cause subtle differences in Canvas drawing operations—for example, different devices may have minor variations in rendering colors, text, and gradient effects.
Randomness in drawing operations: When executing Canvas drawing operations, there is a degree of randomness due to device and browser differences. The same drawing operation can produce slightly different results on different devices, or even on different browser versions on the same device.
Difficult to simulate and tamper with: A Canvas fingerprint is generated based on differences in the device and browser environment, making it generally difficult to simulate or tamper with.

Based on these characteristics, Canvas fingerprinting is mainly applied in the following areas:

Account Security: Websites can monitor user account behavior and promptly restrict account access in the event of a malicious attack.
Personalized Advertising: Advertisers can push targeted ad content based on user fingerprint information to increase click-through rates.
Game Fairness: Gaming platforms can identify devices with similar or identical fingerprints to exclude interference from cheating devices, ensuring game fairness.

How to Avoid Canvas Fingerprinting?

We all know that disabling cookies can prevent cookie tracking. But disabling Canvas tracking is not that simple.

What happens if you completely disable the Canvas element? The webpages you visit might not load correctly because you are preventing the normal display of Canvas elements. Moreover, from the website's perspective, all other users are displaying Canvas elements normally, but you are not—this could actually arouse the website's suspicion.

In most cases, a 'replacement' solution is better than a 'disabling' one. If a website needs to obtain user data, the user can provide a randomly generated Canvas fingerprint to replace the real one. This satisfies the website's need for data while also protecting personal privacy and security, achieving the best of both worlds.

There are some plugins on the market specifically for generating Canvas fingerprints, but their effectiveness and security are still under observation. In most cases, using a fingerprint browser is the most convenient option—it can almost completely mask the real device's Canvas fingerprint.

One of the detection results from realfp is the Canvas fingerprint hash value. If you are using a fingerprint browser, you can visit realfp to run a test and compare the Canvas hash values obtained from browsers like Chrome, Firefox, and Safari.

Summary

Canvas fingerprinting remains a widely used fingerprinting technology with both advantages and disadvantages. We need to stay vigilant, understand its potential impact, and choose appropriate privacy protection measures based on personal needs and a trade-off of pros and cons.

Advantages:

Accurate Identification: Canvas fingerprinting technology can generate unique digital fingerprints to accurately identify and track devices, providing a basis for personalized services and security verification.
Enhanced Security: It can be used to prevent fraud and malicious activities, enhancing the security of systems and applications.
Personalized Content: It can be used to push personalized content, meeting users' demands for a personalized experience.

Disadvantages:

Privacy Invasion: It may collect and analyze user device information, raising concerns about personal privacy.
Risk of Misidentification: Due to various factors in devices and browsers that can cause subtle changes in drawing operations, there is a risk of misidentification, which could lead to legitimate users being incorrectly identified or false positives.