Font Fingerprinting

Font fingerprinting detects which fonts are installed on the user's system. Since font sets vary between operating systems, languages, and user-installed software, they provide significant entropy.

How it works

Neoprint tests 48 font families by measuring text width differences against baseline fonts. For each test font, a string is rendered in three base fonts (monospace, sans-serif, serif) and then with the test font as the primary family. If the width changes, the font is installed.

const testString = 'mmmmmmmmmmlli1WwQqYy@#$%'
const fontSize = '72px'

// Measure baseline
const baseWidth = ctx.measureText(testString).width  // using "monospace"

// Test specific font
const testWidth = ctx.measureText(testString).width  // using "'Arial', monospace"

// If widths differ, Arial is installed

Tested font families (48)

The test list includes system fonts, cross-platform fonts, and OS-specific fonts:

• Cross-platform: Arial, Courier New, Georgia, Times New Roman, Verdana
• Windows: Calibri, Cambria, Consolas, Segoe UI, Tahoma
• macOS: Monaco, Menlo, SF Pro, SF Mono, Helvetica, Futura, Optima
• Linux: Roboto, Noto Sans, Ubuntu, DejaVu Sans, Liberation Sans, Fira Sans
• Popular web fonts: Open Sans, Lato, Source Sans Pro, PT Sans

Why it's effective

OS	Typical installed fonts	Unique fonts
macOS	~70-100	SF Pro, SF Mono, Helvetica Neue, Optima, Didot
Windows	~50-80	Calibri, Cambria, Segoe UI, Consolas
Linux (Ubuntu)	~30-50	Ubuntu, Noto Sans, Liberation Sans
Android	~15-25	Roboto, Noto Sans
iOS	~60-80	SF Pro, New York, Academy Engraved

Users who install design software (Adobe, Figma) or developer tools get additional fonts, further differentiating their profile.

Entropy and stability

Property	Value
Entropy	~12 bits
Stability	0.90
Typical duration	3-5ms

Fonts are highly stable (only change when user installs/removes fonts) and high-entropy (many possible combinations across 48 families).

Role in cross-browser ID

Fonts are installed at the OS level, not the browser level, making them a key signal for crossBrowserId. The same font set is visible from Chrome, Firefox, and Safari on the same machine.

Usage

const fp = await neoprint.get({ collectors: ['fonts'] })
const fonts = fp.components.fonts.value
// ["Arial", "Arial Black", "Comic Sans MS", "Courier", "Courier New", ...]

console.log(fonts.length)  // number of detected fonts from the test list