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Progress in the Privacy Sandbox (October 2021)

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Welcome to the October edition of "Progress in the Privacy Sandbox," tracking the milestones on the path to phasing out third-party cookies in Chrome and working towards a more private web. Each month we'll share an overview of the updates to the Privacy Sandbox timeline along with news from across the project.


Chrome Dev Summit, agenda now live, attend virtually from November 3rd

From the 3rd of November we will be hosting the Chrome Developer Summit. You will be able to get an update on Privacy Sandbox in the keynote along with an opportunity to ask questions to the leadership team in the AMA, and time for more detailed questions with the engineering teams in the Office Hours. Sign up today and we hope to see you there!

This month also included the W3C's Annual Conference (commonly known as TPAC) where all the various groups across the W3C meet to discuss a variety of topics across the entirety of the web. You can view the minutes and videos for the breakout sessions and specific sessions including Privacy Sandbox topics are included below.

Continuing conference season, the IETF (Internet Engineering Task Force) is hosting their regular 112 online technical plenary. Similarly to TPAC there are a number of individual sessions where Privacy Sandbox topics are discussed such as the PRIV (Privacy Respecting Incorporation of Values), PEARG (Privacy Enhancements and Assessments Research Group) and the MASQUE (Multiplexed Application Substrate over QUIC Encryption) working groups. These are deep technical discussions on protocol designs—if you have the appropriate expertise and an interest in contributing to these discussions, please consider joining.

Strengthen cross-site privacy boundaries

Third-party cookies are a key mechanism that enables cross-site tracking. Being able to phase them out is a major milestone, but we also need to tackle other forms of cross-site storage or communication.

Federated Credentials Management API

The Federated Credentials Management (FedCM) proposal is the new, more meaningful name for WebID. Federated identity is a critical service for the web, but given that it's explicitly about sharing aspects of identity with other sites, there are implementation details which overlap with cross-site tracking.

The Federated Credentials Management proposal explores a range of options from simple migration paths for existing solutions to more private methods of connecting to services with the bare minimum of information shared.

This proposal is still at an early stage and discussion can be followed in the W3C's Federated Identity Community Group. The group also hosted a breakout session at TPAC which explored an overview of the proposal. There is also a very early prototype version of the API available behind a flag from Chrome 89, but this is purely for experimentation and will change as discussion progresses.

The Federated Credentials Management API overlaps with and builds on some of the functionality already present in the Credential Management API.


As the cookie-related proposals progress, you should be auditing your own SameSite=None or cross-site cookies and planning the action you will need to take on your site.


If you set cookies that are sent in cross-site contexts, but in 1:1 relationships—like iframe embeds, or API calls—you should follow the CHIPS proposal, or Cookies Having Independent Partitioned State. This allows you to mark cookies as "Partitioned" putting them in a separate cookie jar per top-level site.

Work is progressing on CHIPS and while the feature is available behind chrome://flags/#partitioned-cookies and the --partitioned-cookies CLI flag, it is not yet in a fully testable state. We will provide updated testing and debugging details once the implementation is more complete.

The top-level site, has an iframe to sets a cookie with the 'Partitioned' attribute. When the browser is on with an iframe to, no cookie is sent! CHIPS creates a partition per top-level site.

First-Party Sets

If you set cookies for cross-site contexts, but only across sites you own—for example, you host a service on your .com that's used by your—then you should follow First-Party Sets. This proposal defines a way of declaring which sites you want to form a set and then marking cookies as "SameParty" so that they are only sent for contexts inside of that set.

First-Party Sets are available for local developer testing behind the chrome://flags/#use-first-party-set and chrome://flags/#sameparty-cookies-considered-first-party flags, allowing you to specify your own set of related sites, and experiment with cookie behavior across them.

Storage Partitioning

The web platform includes other forms of storage that may enable cross-site tracking. The TPAC breakout session on the state of browser storage partitioning provides an overview of Chrome's progress along with discussion from other browser vendors.

There's no immediate need for developer action, but if you make use of SharedWorker, Web Storage, IndexedDB, CacheStorage, FileSystem API(s), BroadcastChannel, Web Locks API, Storage Buckets, or other form of storage or communication API where you rely on accessing that data across multiple sites then you should track this topic for future updates.

Preventing covert tracking

As we reduce the options for explicit cross-site tracking, we also need to address the areas of the web platform that expose identifying information that enables fingerprinting or covert tracking of users.

User-Agent string reduction and User-Agent Client Hints

We've expanded the origin trial for testing Chrome's reduced User-Agent format to include third-party embeds. If you primarily provide cross-site content for other services, you can enable the third-party option when registering for the origin trial to receive the reduced format on requests to your resources.

You can track the full timeline for reducing Chrome's user-agent, with further examples and details of rollout phases. You will also need to Migrate to User-Agent Client Hints (UA-CH) if you rely on the platform version, device, or full build version information in the current User-Agent format.

We're continuing to standardize existing names for Client Hints by adding the Sec-CH- header prefix where missing. Pending approval, we hope to expand the range of GREASE characters for UA-CH.

Show relevant content and ads

As we move towards phasing out third-party cookies, we need to introduce APIs that allow the use cases that depended on them but without allowing cross-site tracking.


FLoC is a proposal to enable interest-based advertising without the need for individual cross-site tracking. We've been evaluating the feedback from the earlier origin trial of FLoC before we advance to further ecosystem testing. While we continue to work on next steps and decisions for FLoC, you should see some exploratory code around the concept of topics (previously referenced) to show up in the Chromium code base soon. As all of Chrome's development happens in the open, this work will be visible, but there isn't anything immediately actionable for developer testing (nor is this applicable to users). We hope to continue sharing these discussions and updates in the new PATCG (Private Advertising Technology Community Group).

Measure digital ads

As the companion to displaying ads without cross-site tracking, we need privacy-preserving mechanisms to measure the effectiveness of those ads.

Attribution Reporting API

The Attribution Reporting API gives you the ability to measure events on one site, like clicking or viewing an ad, that lead to a conversion on another site—without enabling cross-site tracking.

We would like to continue testing the Attribution Reporting API and we plan on extending the origin trial through to Chrome 97. Current origin trial tokens expired on October 12th, so you will need to apply for updated tokens to continue testing.

Fight spam and fraud on the web

The other challenge as we reduce the surfaces available for cross-site tracking is that these same fingerprinting techniques are often used for spam and fraud protection. We need privacy-preserving alternatives here as well.

Trust Tokens

The Trust Token API is a proposal that allows one site to share a claim about a visitor—such as "I think they're human"—and allow other sites to verify that claim, again without identifying the individual.

Trust Tokens are one part of the overall strategy to tackle spam and fraud on the web. In the "Anti-fraud for the web" breakout at TPAC, representatives from across the ecosystem discussed some of the current challenges and approaches.


As we continue to publish these monthly updates and progress through the Privacy Sandbox as a whole, we want to make sure that you as a developer are getting the information and support that you need. Let us know on @ChromiumDev Twitter if there's anything that we could improve in this series. We'll use your input to continue improving the format.

We have also added a Privacy Sandbox FAQ which we will continue to expand based on the issues you submit to the developer support repo. If you have any questions around testing or implementation on any of the proposals, come talk to us there.

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