A very simple PIR scheme on top of homomorphic encryption that supports multiplying with a plaintext and homomorphic addition, would look like this:
The client one-hot-encodes the query: Enc(0), Enc(1), Enc(0).
The server has 3 values: x, y, z.
Now the server computes: Enc(0) * x + Enc(1) * y + Enc(0) * z == Enc(y).
Client can decrypt Enc(y) and get the value y. Server received three ciphertexts, but does not know which one of them was encryption of zero or one, because the multiplications and additions that the server did, never leak the underlying value.
This gives some intuition on how PIR works, actual schemes are more efficient.
[Disclosure: I work on the team responsible for the feature]
In this PIR model the server has to read the whole database, otherwise it would be easy on the server to see, that these rows were not accessed and therefore they are not the one the client queried.
In this PIR model the server runtime is O(n) where n is the number of rows.
To keep it practical, we do support sharding the database. Client leaks a few bits of hashed query to pick the right shard, where we process the entire shard. There is a inherent privacy-performance tradeoff: less shards = less leakage vs more shards = better performance & less privacy.
Didt things got better lately? I had very limited success with crossover circa 2021. And that's from a point of view from someone who have no Windows actively running on personal devices. Comparing to what I get in my Linux AMD GPU machine crossover seems to be not as good.
> The Sensitive Content Analysis framework enables third-party developers to integrate Communication Safety in their apps. The framework allows developers to detect if content that children receive or attempt to send in their apps may contain nudity, and developers can add protections in their apps when sensitive content is detected.
Thanks. This feature is described as optional for 3rd party apps and a claim is made that the information is not sent off the device. Assuming this is true, I take back my criticism about the policy being invasive.
