It's not a backdoor, it's just safe browsing. And safe browsing is controversial since the beginning because this (but now most people have forgotten it).

But don't forget that cryptographic techniques are already used to protect the privacy of users, Mathew Green just wrote a good analysis.

* https://blog.cryptographyengineering.com/2019/10/13/dear-app...

> To address these concerns, Google quickly came up with a safer approach to, um, “safe browsing”. The new approach was called the “Update API”, and it works like this:

> * Google first computes the SHA256 hash of each unsafe URL in its database, and truncates each hash down to a 32-bit prefix to save space.

> * Google sends the database of truncated hashes down to your browser.

> * Each time you visit a URL, your browser hashes it and checks if its 32-bit prefix is contained in your local database.

> * If the prefix is found in the browser’s local copy, your browser now sends the prefix to Google’s servers, which ship back a list of all full 256-bit hashes of the matching URLs, so your browser can check for an exact match.

> At each of these requests, Google’s servers see your IP address, as well as other identifying information such as database state. It’s also possible that Google may drop a cookie into your browser during some of these requests. The Safe Browsing API doesn’t say much about this today, but Ashkan Soltani noted this was happening back in 2012.

> It goes without saying that Lookup API is a privacy disaster. The “Update API” is much more private: in principle, Google should only learn the 32-bit hashes of some browsing requests. Moreover, those truncated 32-bit hashes won’t precisely reveal the identity of the URL you’re accessing, since there are likely to be many collisions in such a short identifier. This provides a form of k-anonymity.

> The weakness in this approach is that it only provides some privacy. The typical user won’t just visit a single URL, they’ll browse thousands of URLs over time. This means a malicious provider will have many “bites at the apple” (no pun intended) in order to de-anonymize that user. A user who browses many related websites — say, these websites — will gradually leak details about their browsing history to the provider, assuming the provider is malicious and can link the requests. (Updated to add: There has been some academic research on such threats.)

> And this is why it’s so important to know who your provider actually is.

> What does this mean for Apple and Tencent?

> That’s ultimately the question we should all be asking.

> The problem is that Safe Browsing “update API” has never been exactly “safe”. Its purpose was never to provide total privacy to users, but rather to degrade the quality of browsing data that providers collect. Within the threat model of Google, we (as a privacy-focused community) largely concluded that protecting users from malicious sites was worth the risk. That’s because, while Google certainly has the brainpower to extract a signal from the noisy Safe Browsing results, it seemed unlikely that they would bother. (Or at least, we hoped that someone would blow the whistle if they tried.)

> But Tencent isn’t Google. While they may be just as trustworthy, we deserve to be informed about this kind of change and to make choices about it. At very least, users should learn about these changes before Apple pushes the feature into production, and thus asks millions of their customers to trust them.