this post was submitted on 30 Jan 2024
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Privacy
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To illustrate op's point I'm going to spin up an instance, federate with everyone, and not tell anyone what that instance is.
Then I'm going to feed all that data into my new website, called Open Lemmy Stats, where anyone can query the user data ive accumulated. The homepage will be ripe with insights, leaderboards and all kinds of data on prolific users.
Additionally, I'll display a snapshot/profile of a random user by feeding that users data to GPT4 to make inferences about the user's political affiliations and display the results.
Worst of all, I'm not going to out my instance for everyone to know it as the one to defederate. In fact I'm spinning up a few instances that will host innocuous communities that I plan to mod and support to give my instances cover for their true purpose: redundant fediverse datastreams for my site, Open Lemmy Stats.
I'll also have a store where anyone can buy my collected fediverse data for a handsome sum.
Just kidding I'm not doing any of this. But someone absolutely will or already is.
How to work out what instance(s) if someone does this: A Lemmy instance doesn't have to send the same voting data to every instance, it could send different votes to different instances (stock Lemmy federates the same thing consistently, but there is no reason a modified Lemmy designed to catch someone doing this has to), encoding a signal into the voting pattern. Then, just check to see what signal shows up. If it averages several instances, with enough signal you could decompose a linear combination (e.g. average) of different patterns back out into its constituent parts.
A smarter system won't just take the mean of the votes from different instances but rather discard outliers as invalid input (flagging repeat offenders to be ignored in the future) and use the median or mode of the remainder. The results should also be quantitized to avoid leaking details about sources or internal algorithms; only the larger trends need to be reported.
Of course you could always just keep the collected data private and only provide it to customers willing to pay $$$ for access, which handily limits instance operators' ability to reverse-engineer the source of the data. And nothing prevents you from using separate instances for public and private data sets.