How Hash-Primarily based Protected Shopping Works in Google Chrome


By Rohit Bhatia, Mollie Bates, Google Chrome Safety

There are numerous threats a person faces when shopping the online. Customers could also be tricked into sharing delicate data like their passwords with a deceptive or faux web site, additionally referred to as phishing. They might even be led into putting in malicious software program on their machines, referred to as malware, which might acquire private knowledge and in addition maintain it for ransom. Google Chrome, henceforth referred to as Chrome, allows its customers to guard themselves from such threats on the web. When Chrome customers browse the online with Protected Shopping protections, Chrome makes use of the Protected Shopping service from Google to establish and keep at bay varied threats.

Protected Shopping works in numerous methods relying on the person’s preferences. In the most typical case, Chrome makes use of the privacy-conscious Replace API (Software Programming Interface) from the Protected Shopping service. This API was developed with person privateness in thoughts and ensures Google will get as little details about the person’s shopping historical past as potential. If the person has opted-in to “Enhanced Safety” (coated in an earlier put up) or “Make Searches and Shopping Higher“, Chrome shares restricted further knowledge with Protected Shopping solely to additional enhance person safety.

This put up describes how Chrome implements the Replace API, with applicable tips to the technical implementation and particulars in regards to the privacy-conscious facets of the Replace API. This must be helpful for customers to know how Protected Shopping protects them, and for builders to flick thru and perceive the implementation. We’ll cowl the APIs used for Enhanced Safety customers in a future put up.

Threats on the Web

When a person navigates to a webpage on the web, their browser fetches objects hosted on the web. These objects embrace the construction of the webpage (HTML), the styling (CSS), dynamic conduct within the browser (Javascript), photos, downloads initiated by the navigation, and different webpages embedded in the principle webpage. These objects, additionally referred to as assets, have an internet tackle which is known as their URL (Uniform Useful resource Locator). Additional, URLs might redirect to different URLs when being loaded. Every of those URLs can probably host threats resembling phishing web sites, malware, undesirable downloads, malicious software program, unfair billing practices, and extra. Chrome with Protected Shopping checks all URLs, redirects or included assets, to establish such threats and shield customers.

Protected Shopping Lists

Protected Shopping offers a listing for every risk it protects customers in opposition to on the web. A full catalog of lists which might be utilized in Chrome could be discovered by visiting chrome://safe-browsing/#tab-db-manager on desktop platforms.

An inventory doesn’t comprise unsafe net addresses, additionally known as URLs, in entirety; it will be prohibitively costly to maintain all of them in a tool’s restricted reminiscence. As an alternative it maps a URL, which could be very lengthy, via a cryptographic hash perform (SHA-256), to a novel mounted dimension string. This distinct mounted dimension string, referred to as a hash, permits a listing to be saved effectively in restricted reminiscence. The Replace API handles URLs solely within the type of hashes and can be referred to as hash-based API on this put up.

Additional, a listing doesn’t retailer hashes in entirety both, as even that will be too reminiscence intensive. As an alternative, barring a case the place knowledge isn’t shared with Google and the checklist is small, it incorporates prefixes of the hashes. We consult with the unique hash as a full hash, and a hash prefix as a partial hash.

An inventory is up to date following the Replace API’s request frequency part. Chrome additionally follows a back-off mode in case of an unsuccessful response. These updates occur roughly each half-hour, following the minimal wait length set by the server within the checklist replace response.

For these fascinated by shopping related supply code, right here’s the place to look:

Supply Code

  1. GetListInfos() incorporates all of the lists, together with their related risk sorts, the platforms they’re used on, and their file names on disk.
  2. HashPrefixMap reveals how the lists are saved and maintained. They’re grouped by the scale of prefixes, and appended collectively to permit fast binary search primarily based lookups.

How is hash-based URL lookup performed

For instance of a Protected Shopping checklist, as an instance that we’ve got one for malware, containing partial hashes of URLs identified to host malware. These partial hashes are typically 4 bytes lengthy, however for illustrative functions, we present solely 2 bytes.

['036b', '1a02', 'bac8', 'bb90']

Every time Chrome must verify the fame of a useful resource with the Replace API, for instance when navigating to a URL, it doesn’t share the uncooked URL (or any piece of it) with Protected Shopping to carry out the lookup. As an alternative, Chrome makes use of full hashes of the URL (and a few mixtures) to lookup the partial hashes within the domestically maintained Protected Shopping checklist. Chrome sends solely these matched partial hashes to the Protected Shopping service. This ensures that Chrome offers these protections whereas respecting the person’s privateness. This hash-based lookup occurs in three steps in Chrome:

Step 1: Generate URL Mixtures and Full Hashes

When Google blocks URLs that host probably unsafe assets by inserting them on a Protected Shopping checklist, the malicious actor can host the useful resource on a distinct URL. A malicious actor can cycle via varied subdomains to generate new URLs. Protected Shopping makes use of host suffixes to establish malicious domains that host malware of their subdomains. Equally, malicious actors can even cycle via varied subpaths to generate new URLs. So Protected Shopping additionally makes use of path prefixes to establish web sites that host malware at varied subpaths. This prevents malicious actors from biking via subdomains or paths for brand spanking new malicious URLs, permitting sturdy and environment friendly identification of threats.

To include these host suffixes and path prefixes, Chrome first computes the total hashes of the URL and a few patterns derived from the URL. Following Protected Shopping API’s URLs and Hashing specification, Chrome computes the total hashes of URL mixtures by following these steps:

  1. First, Chrome converts the URL right into a canonical format, as outlined within the specification.
  2. Then, Chrome generates as much as 5 host suffixes/variants for the URL.
  3. Then, Chrome generates as much as 6 path prefixes/variants for the URL.
  4. Then, for the mixed 30 host suffixes and path prefixes mixtures, Chrome generates the total hash for every mixture.

Supply Code

  1. V4LocalDatabaseManager::CheckBrowseURL is an instance which performs a hash-based lookup.
  2. V4ProtocolManagerUtil::UrlToFullHashes creates the varied URL mixtures for a URL, and computes their full hashes.


As an illustration, as an instance {that a} person is attempting to go to The canonical url is The host suffixes to be tried are, and The trail prefixes are / and /blah. The 4 mixed URL mixtures are,,, and

url_combinations = ["", "","", ""]
full_hashes = ['1a02…28', 'bb90…9f', '7a9e…67', 'bac8…fa']

Step 2: Search Partial Hashes in Native Lists

Chrome then checks the total hashes of the URL mixtures in opposition to the domestically maintained Protected Shopping lists. These lists, which comprise partial hashes, don’t present a decisive malicious verdict, however can shortly establish if the URL is taken into account not malicious. If the total hash of the URL doesn’t match any of the partial hashes from the native lists, the URL is taken into account protected and Chrome proceeds to load it. This occurs for greater than 99% of the URLs checked.

Supply Code

  1. V4LocalDatabaseManager::GetPrefixMatches will get the matching partial hashes for the total hashes of the URL and its mixtures.


Chrome finds that three full hashes 1a02…28, bb90…9f, and bac8…fa match native partial hashes. We observe that that is for demonstration functions, and a match right here is uncommon.

Step 3: Fetch Matching Full Hashes

Subsequent, Chrome sends solely the matching partial hash (not the total URL or any explicit a part of the URL, and even their full hashes), to the Protected Shopping service’s technique. In response, it receives the total hashes of all malicious URLs for which the total hash begins with one of many partial hashes despatched by Chrome. Chrome checks the fetched full hashes with the generated full hashes of the URL mixtures. If any match is discovered, it identifies the URL with varied threats and their severities inferred from the matched full hashes.

Supply Code

  1. V4GetHashProtocolManager::GetFullHashes performs the lookup for the total hashes for the matched partial hashes.


Chrome sends the matched partial hashes 1a02, bb90, and bac8 to fetch the total hashes. The server returns full hashes that match these partial hashes, 1a02…28, bb90…ce, and bac8…01. Chrome finds that one of many full hashes matches with the total hash of the URL mixture being checked, and identifies the malicious URL as internet hosting malware.


Protected Shopping protects Chrome customers from varied malicious threats on the web. Whereas offering these protections, Chrome faces challenges resembling constraints in reminiscence capability, community bandwidth utilization, and a dynamic risk panorama. Chrome can be aware of the customers’ privateness decisions, and shares little knowledge with Google.

In a observe up put up, we are going to cowl the extra superior protections Chrome offers to its customers who’ve opted in to “Enhanced Safety”.


Please enter your comment!
Please enter your name here