Font fingerprint

A page asks for a string in one named family and names a fallback after it. A family that is not installed never draws, the fallback draws instead, and the width gives the answer away.

Nothing appears on screen and no permission is asked. Hundreds of families are measured in the time the rest of the page takes to load.

1. 1

   Draws a short string

   One line in the named family, set large so a small difference in shape becomes a measurable difference in width.

2. 2

   Compares it to the fallback

   Equal width means the family is not there. A different width means it is.

3. 3

   Collects the set

   The families that answered are folded into one value, and on an untouched machine that value is the same on every visit.

A font list is the history of a machine. Software brings families with it, and so do the language packs the system was set up with.

• The software you installed

  A design suite, an office suite and a developer toolchain each leave their own families behind.

• The languages you work in

  Scripts the system was never set up for are simply absent, and absence reads as clearly as presence.

• The system underneath

  Every operating system ships families of its own, so the list also states which system it came from.

• The rare family

  One unusual typeface narrows a machine faster than a dozen common ones, because almost nobody else answers for it.

Every profile declares a set of its own: the families that system always ships, plus about half of the optional ones drawn from the profile seed. Two profiles made on one computer answer with two different sets.

A named family outside the set reports as unavailable, so text falls back exactly as it would on a machine where that family had never been installed.

• The set varies and the order does not

  Families keep the order of the full list, because a shuffled order would be a signal of its own.

• Other names for the same files

  On macOS and Linux the set also admits the internal names of families it already carries, because those systems are asked that way.

• Fonts a page brings itself

  A typeface the page loads from its own address is resolved before any of this and renders as the designer intended.

The lines below come from the acceptance run of the build shipping today. It ran on a Windows profile whose set holds thirty four families.

• The declared family renders

  The first family of the set is measured against the fallback and comes back wider than it.

  visible

• Families outside the set stay away

  Five well known families are probed one by one, and those the set does not hold must stay silent.

  0 leaked

• Generic families follow the set

  Cursive, fantasy and math are assigned from what the profile declares instead of from the host system.

  from the profile

• Canvas fingerprint

  Text drawn on a canvas depends on the fonts, but the value a site keeps is the picture. Separate surface and separate defence.

• Web fonts

  A typeface the page downloads is the same for every visitor, so it says nothing about you. Only local families do.

• The list in a document editor

  An editor shows you the whole system list. What a page measures without permission is narrower than that.

The free check on this site counts the families it finds in the browser you are reading this in and names the first of them.

Inside the app the self-test runs in a profile against the engine that profile launched. It measures the declared family, then probes five well known families, and prints what came back.