Entropy

A reading that splits all visitors into two equal halves carries one bit. Ten such readings, if none of them followed from another, would leave one visitor in a thousand.

• Rarity is the whole idea

  A value almost everyone shares narrows nothing, however technical it looks on a report.

• The figure belongs to a crowd

  Bits describe a population somebody measured, so the number depends on who was measured and when.

• Nobody sits at zero

  A browser answering ordinary values for everything still carries the readings a site cannot help seeing.

Two readings that follow each other carry far less together than apart. The claimed system and the set of installed fonts overlap heavily, so learning the second after the first adds little.

• Overlap costs you almost nothing

  Readings that agree by nature amount to close to one reading for the purpose of narrowing a crowd.

• Contradiction costs a great deal

  A pair that cannot occur on any real machine is worth more to a site than either reading on its own.

• Stability is a separate axis

  A rare value that changes every visit links nothing, and an ordinary one that never changes links everything.

These values are read in your browser on this page and nothing leaves it. The first three narrow a crowd sharply, and the last one barely at all.

Browsers split the fields they report about themselves into cheap ones sent with every request and costly ones answered only when a site asks. That split is written in terms of this idea, and it is where the word shows up in the plumbing rather than in an article.

• The cheap fields travel by default

  The browser name and its coarse version ride along with ordinary requests, so they are always aligned with the profile.

• The costly ones are requested

  The precise version, the architecture and the device model are answered on demand, and our engine rewrites them only when a request already carries them.

• Asking is itself a signal

  A site that requests the costly fields has declared its interest, and the request is visible to anything watching the traffic.

Every reading a profile answers with is derived from one seed belonging to that profile. Two profiles therefore describe two machines, and each profile stays the same machine for itself.

Agreement is the part that can be checked rather than argued about, and the self-test checks it reading by reading against that seed.

• One seed per profile

  The values are derived at launch from the profile, not stored as a file that can be copied away from it.

• Readings are compared against each other

  A stand asserts relationships instead of values, because an ordinary reading beside the wrong neighbour is the loudest thing on the page.

• The free check marks the sharp rows

  On the fingerprint check the rows that tell you apart carry a mark, and the mark is a name rather than a score.

Numbers in bits come from studies of the visitors of one site at one time. They describe that crowd, and your own browser is being compared against a population nobody has measured.

• Ask who was counted

  A figure from a privacy study describes the people who came to that study, who are not ordinary visitors.

• Ask when

  Browsers change what they report every few releases, so an old figure describes browsers that no longer exist.

• Ask what was added up

  Sums taken over readings that overlap are larger than the crowd they claim to narrow.