diff --git a/index.bs b/index.bs index 7cd55fe..6f75da9 100644 --- a/index.bs +++ b/index.bs @@ -411,49 +411,89 @@ Note: The high-entropy portions of the user agent information are retrieved thro Processing model {#processing} -------------- -

WindowOrWorkerGlobalScope

+

WindowOrWorkerGlobalScope

-Each [=user agent=] has an associated brands, which is a [=/list=] created by running [=create brands=]. +Each [=user agent=] has an associated brands, which is a [=/list=] created by running +[=create brands=]. -Every {{WindowOrWorkerGlobalScope}} object has an associated brands frozen array, which is a FrozenArray<NavigatorUABrandVersion>. It is initially the result of [=create a frozen array|creating a frozen array=] from the [=user agent=]'s [=brands=]. +Every {{WindowOrWorkerGlobalScope}} object has an associated brands frozen array, +which is a FrozenArray<NavigatorUABrandVersion>. It is initially the +result of [=create a frozen array|creating a frozen array=] from the [=user agent=]'s [=brands=]. -

Create brands

+

Create brands

When asked to run the create brands algorithm, the [=user agent=] MUST run the following steps: 1. Let |list| be a [=/list=]. -2. Collect pairs of [=user agent/brand=] and [=user agent/significant version=] which represent the [=user agent=], - its equivalence class and/or its rendering engine. +2. Collect pairs of [=user agent/brand=] and [=user agent/significant version=] which represent the [=user agent=] or + [=equivalence classes=]. 3. For each pair: 1. Let |dict| be a new {{NavigatorUABrandVersion}} dictionary, - with [=user agent/brand=] as {{NavigatorUABrandVersion/brand}} and [=user agent/significant version=] as {{NavigatorUABrandVersion/version}}. + with [=user agent/brand=] as {{NavigatorUABrandVersion/brand}} and [=user agent/significant version=] as + {{NavigatorUABrandVersion/version}}. 2. Append |dict| to |list|. 4. The [=user agent=] SHOULD execute the following steps: - 1. [=list/Append=] additional items to |list| containing {{NavigatorUABrandVersion}} objects, - initialized with arbitrary {{NavigatorUABrandVersion/brand}} and {{NavigatorUABrandVersion/version}} combinations. + 1. [=list/Append=] one additional [=list/item=] to |list| containing a {{NavigatorUABrandVersion}} dictionary, + initialized with arbitrary brand and + arbitrary version combinations. - 2. Randomize the order of the items in |list|. + 2. Randomize the order of the [=list/items=] in |list|. Note: One approach to minimize caching variance when generating these random components could be to determine them at build time, and keep them identical throughout the lifetime of the [=user agent=]'s significant - version. + version. Note: See [[#grease]] for more details on when and why these randomization steps might be appropriate. 5. Return |list|. -

Getters

+An equivalence class represents a group of browsers believed to be compatibile with +each other. A shared rendering engine may form an [=equivalence class=], for example. + +

Create arbitrary brand and version values

+ +To create an arbitrary brand, the [=user agent=] MUST run these steps: + + 1. Let |arbitraryBrand| be a [=/string=] composed of [=ASCII alpha=]. |arbitraryBrand| MUST contain one or more + 0x20 (SP) bytes and be no longer than twenty [=ASCII bytes=]. + 1. Let |arbitraryBrandList| be the result of splitting |arbitraryBrand| on ASCII + whitespace. + 1. Let |greaseyStack| be a [=stack=]. + 1. Let |greaseyChars| be the [=list=] of [=ASCII bytes=] « 0x20 (SP), 0x28 (left parenthesis), 0x29 + (right parenthesis), 0x2D (-), 0x2E (.), 0x2F (/), 0x3A (:), 0x3B (;), 0x3D (=), 0x3F (?), 0x5F (_) ». + 1. For each item of |arbitraryBrandList|, [=stack/push=] a randomly selected [=list/item=] from |greaseyChars| onto + |greaseyStack|. + 1. Let |greaseyBrandList| be a [=list=] and |index| be 0. + 1. While |greaseyStack| [=stack/is not empty=]: + 1. Let |item| be the result of popping from |greaseyStack|. + 1. Append |item| to |greaseyBrandList|. + 1. Append |arbitraryBrandList|[|index|] to |greaseyBrandList|. + 1. Increment |index| by 1. + 1. Return the result of stripping leading and trailing ASCII + whitespace from the [=concatenation=] of |greaseyBrandList| (with no separator). + + Note: [=Structured Headers=] allows for escaped 0x22 (\") and 0x5C (\\) inside a [=structured header/string=], but + these are known to not be web-compatible. + +To create an arbitrary version, return a [=/string=] that MUST match the format of the [=user agent=]'s +[=user agent/significant version=], but MUST NOT match the value. + +Note: User Agents may decide to send arbitrarily low versions to ensure proper version checking, and should vary them +over time. + +

Getters

On getting, the {{NavigatorUAData/brands}} attribute MUST return [=this=]'s [=relevant global object=]'s [=WindowOrWorkerGlobalScope/brands frozen array=]. On getting, the {{NavigatorUAData/mobile}} attribute must return the [=user agent=]'s [=user agent/mobileness=]. -

getHighEntropyValues method

+

getHighEntropyValues method

The getHighEntropyValues(|hints|) method MUST run these steps: @@ -527,20 +567,20 @@ One approach which might be advisable could be for each [=user agent=] to lock t the version number, then shifting platform and model information to something reasonably generic in order to reduce the fingerprint the header provides. -GREASE-like UA Strings {#grease} +GREASE-like UA Brand Lists {#grease} ---------------------- History has shown us that there are real incentives for [=user agents=] to lie about their branding in order to thread the needle of sites' sniffing scripts, and prevent their users from being blocked by UA-based allow/block lists. -Resetting expectations may help to prevent abuse of the UA string's brand in the short term, but -probably won't help in the long run. The world of network protocols introduced the notion of GREASE [[I-D.ietf-tls-grease]]. We could borrow from that concept to tackle this problem. [=User agents=]' [=user agent/brands=] containing more than a single entry could encourage -standardized processing of the `UA` string. By randomly including additional, intentionally +standardized processing of the [=user agent/brands=] list. By randomly including additional, intentionally incorrect, comma-separated entries with arbitrary ordering, they would reduce the chance that we ossify on a few required strings. @@ -548,10 +588,10 @@ Let's examine a few examples: * In order to avoid sites from barring unknown browsers from their allow lists, Chrome could send a UA set that includes an non-existent browser, and which varies once in a while. - `"Chrome"; v="73", "NotBrowser"; v="12"` -* In order to enable equivalence classes based on Chromium versions, Chrome could add the rendering +* In order to enable [=equivalence classes=] based on Chromium versions, Chrome could add the rendering engine and its version to that. - `"Chrome"; v="73", "NotBrowser"; v="12", "Chromium"; v="73"` -* In order to encourage sites to rely on equivalence classes based on Chromium versions rather than +* In order to encourage sites to rely on [=equivalence classes=] based on Chromium versions rather than exact UA sniffing, Chrome might remove itself from the set entirely. - `"Chromium"; v="73", "NotBrowser"; v="12"` * Browsers based on Chromium may use a similar UA string, but use their own brand as part of the @@ -559,16 +599,11 @@ Let's examine a few examples: - `"Chrome"; v="73", "Awesome Browser"; v="60", "Chromium"; v="73"` -[=User agents=] MUST include more than a single value in [=user agent/brands=], where at least one -of these values is an arbitrary value. - -When adding arbitrary values to [=user agent/brands=], [=user agents=] MUST make sure that receivers -of the header adhere to [=Structured Header=] parsing, by adding escaped double-quotes, commas and -semi-colons to those values. The purpose of this is to make non-compliant server implementations -immediately aware that their parsing code is inadequate. +[=User agents=] MUST include more than a single value in [=user agent/brands=], where one of these values is an +arbitrary value. -The value order in [=user agent/brands=] MUST change over time, the prevent receivers of the header -from relying on certain values being in certain locations in the string. +The value order in [=user agent/brands=] MUST change over time to prevent receivers of the header +from relying on certain values being in certain locations in the list. When choosing GREASE strategies, [=user agents=] SHOULD keep caching variance in mind and minimize variance among identical [=user agent=] versions.