How Hash-Primarily based Protected Searching Works in Google Chrome

By Rohit Bhatia, Mollie Bates, Google Chrome Safety

There are numerous threats a person faces when looking the net. Customers could also be tricked into sharing delicate data like their passwords with a deceptive or pretend web site, additionally referred to as phishing. They could even be led into putting in malicious software program on their machines, referred to as malware, which might acquire private information and likewise 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 net with Protected Searching protections, Chrome makes use of the Protected Searching service from Google to determine and thrust back numerous threats.

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

This put up describes how Chrome implements the Replace API, with applicable tips that could the technical implementation and particulars in regards to the privacy-conscious features of the Replace API. This needs to be helpful for customers to know how Protected Searching protects them, and for builders to flick thru and perceive the implementation. We are going to 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 embody the construction of the webpage (HTML), the styling (CSS), dynamic conduct within the browser (Javascript), pictures, downloads initiated by the navigation, and different webpages embedded in the primary webpage. These objects, additionally referred to as assets, have an online deal with which is known as their URL (Uniform Useful resource Locator). Additional, URLs could redirect to different URLs when being loaded. Every of those URLs can doubtlessly host threats similar to phishing web sites, malware, undesirable downloads, malicious software program, unfair billing practices, and extra. Chrome with Protected Searching checks all URLs, redirects or included assets, to determine such threats and defend customers.

Protected Searching Lists

Protected Searching offers an inventory for every menace it protects customers towards on the web. A full catalog of lists which might be utilized in Chrome will be discovered by visiting chrome://safe-browsing/#tab-db-manager on desktop platforms.

An inventory doesn’t include unsafe internet addresses, additionally known as URLs, in entirety; it might be prohibitively costly to maintain all of them in a tool’s restricted reminiscence. As a substitute it maps a URL, which will be very lengthy, by way of a cryptographic hash operate (SHA-256), to a singular fastened dimension string. This distinct fastened dimension string, referred to as a hash, permits an inventory to be saved effectively in restricted reminiscence. The Replace API handles URLs solely within the type of hashes and can also be referred to as hash-based API on this put up.

Additional, an inventory doesn’t retailer hashes in entirety both, as even that might be too reminiscence intensive. As a substitute, barring a case the place information is just not shared with Google and the checklist is small, it incorporates prefixes of the hashes. We discuss 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 period set by the server within the checklist replace response.

For these concerned with looking related supply code, right here’s the place to look:

Supply Code

  1. GetListInfos() incorporates all of the lists, together with their related menace 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 based mostly lookups.

How is hash-based URL lookup performed

For instance of a Protected Searching checklist, for instance that we’ve one for malware, containing partial hashes of URLs recognized to host malware. These partial hashes are usually 4 bytes lengthy, however for illustrative functions, we present solely 2 bytes.

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

Each time Chrome must test the repute 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 Searching to carry out the lookup. As a substitute, Chrome makes use of full hashes of the URL (and a few combos) to lookup the partial hashes within the domestically maintained Protected Searching checklist. Chrome sends solely these matched partial hashes to the Protected Searching 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 doubtlessly unsafe assets by putting them on a Protected Searching checklist, the malicious actor can host the useful resource on a special URL. A malicious actor can cycle by way of numerous subdomains to generate new URLs. Protected Searching makes use of host suffixes to determine malicious domains that host malware of their subdomains. Equally, malicious actors may cycle by way of numerous subpaths to generate new URLs. So Protected Searching additionally makes use of path prefixes to determine web sites that host malware at numerous subpaths. This prevents malicious actors from biking by way of subdomains or paths for brand new malicious URLs, permitting strong 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 Searching API’s URLs and Hashing specification, Chrome computes the total hashes of URL combos 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 combos, 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 assorted URL combos for a URL, and computes their full hashes.


As an example, for instance {that a} person is making an attempt 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 combos 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 combos towards the domestically maintained Protected Searching lists. These lists, which include partial hashes, don’t present a decisive malicious verdict, however can rapidly determine 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 combos.


Chrome finds that three full hashes 1a02…28, bb90…9f, and bac8…fa match native partial hashes. We be aware 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 Searching 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 combos. If any match is discovered, it identifies the URL with numerous 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 Searching protects Chrome customers from numerous malicious threats on the web. Whereas offering these protections, Chrome faces challenges similar to constraints in reminiscence capability, community bandwidth utilization, and a dynamic menace panorama. Chrome can also be aware of the customers’ privateness decisions, and shares little information with Google.

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

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