Privacy/Features/Pref to limit number of fonts loaded per tab

Status

Team

Open issues/risks

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Stage 1: Definition

1. Feature overview

EFF's Panoptclock project showed that the fonts installed (and their order) provide 14 bits of entropy to identify users.

2. Users & use cases

-Privacy enhancing tools/addons such as the Tor browsing bundle that could enable this setting without needing to have a firefox fork. -Web Users wanting to reduce their web fingerprint (maybe triggered by private mode) -Web developers wanting to check the fallbacks of their sites.

3. Dependencies

None

4. Requirements

- Must not break (as in make unreadable) international sites. - Must not leak any extra information per page load (solutions might leak information to the same site between loads are not acceptable). - It must stop CSS and Javascript leaks. - Must not prevent chrome from using local fonts.

Non-goals

-Will not try to prevent bad web renderings (bad sizes due the assumptions on the fonts rendered) -Will not prevent OS identification. Each OS has different default fonts and redenering engines, which have layout differences. -Will not prevent font enumeratioon via plugins such as flash.

Stage 2: Design

5. Functional specification

When the setting is enabled, the system will only render w3c generic fonts (serif, sans-serif, monospace, cursive, or fantasy) or downloadable fonts. This will be more expressive than the 'use_document_fonts' preference.

6. User experience design

There will be no UX. The change will be a preference that would be disabled by default.

Stage 3: Planning

7. Implementation plan

We will make changes on "layout/style/nsRuleNode.cpp" so that the CSS fonts would be limited to our font selection. This will be done in two stages:

1. Only allow generic fonts, this would reuse most of the code on related to use_document_fonts. 2. Also allow webfonts, This will be tricky as we need to create a list of valid dowloaded fonts per document. This list would be then checked at the same point to see of we need font replacement or not.

8. Reviews

Security review

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Privacy review

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Localization review

This is my largest concern. Currently I manually test against several non-western pages, but I think need a more automated and more comprehensive test/

Accessibility

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Quality Assurance review

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Operations review

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Stage 4: Development

9. Implementation

This is tagged with bug 732096 https://bugzilla.mozilla.org/show_bug.cgi?id=732096

Stage 5: Release

10. Landing criteria

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Feature details

Team status notes