YSK: Your Lemmy activities (e.g. downvotes) are far from private

You Should Know@lemmy.world – 2747 points – 1 years ago

i.imgur.com

Edit: obligatory explanation (thanks mods for squaring me away)...

What you see via the UI isn't "all that exists". Unlike Reddit, where everything is a black box, there are a lot more eyeballs who can see "under the hood". Any instance admin, proper or rogue, gets a ton of information that users won't normally see. The attached example demonstrates that while users will only see upvote/downvote tallies, admins can see who actually performed those actions.

Edit: To clarify, not just YOUR instance admin gets this info. This is ANY instance admin across the Fediverse.

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Couldn’t we just use a hash for the usernames instead?

The hash function would still need to be public to share data between instances.

That's the point of a hash function. You have a public hash function, say SHA-256. It's easy to check a username against it's hash, but virtually impossible to reverse the hash back to the username.

Edit: Instead of storing, say, eddie, we'd store 3b9d8298f1b5086d012618feebb2da1a394357c1dab7523443c9f6a743c4c84d. Then when the instance gets a Like from eddie, it hashes his username to get 3b9d8298f1b5086d012618feebb2da1a394357c1dab7523443c9f6a743c4c84d, realizes there's a match, and doesn't update the count.

Note that when given 3b9d8298f1b5086d012618feebb2da1a394357c1dab7523443c9f6a743c4c84d, it would take millions of CPU years to compute the original username from it. Therefore, we can check for duplicates without actually checking the name itself (a similar method is used for checking passwords; Lemmy is open source, we know the hashing algorithm, but we can't unhash user passwords, only check them).

While there is an enormous amount of possible passwords, there is only a limited (and quite small) amount of users. Couldn't you just hash all the usernames one by one and map the hashes to the usernames? So you could still reverse engineer the usernames of those who voted on a post.

Edit: Salting with the post id would make this attacking process harder, but still realistic. Probably the only real solution is to hide the votes table from federated instances, I'm not sure if that brings technical problems.

That was what I was implying, yes.

Just hash each username and store it. Then just check the usernames hash to see if it matches.

I was more comnenting that you could still reverse engineer the users who voted on a post

Actually, you’re not really wrong.

All the more reason to give out limited data to all other instances. Why do these instances really need this data? Mastodon doesn’t need it, not quite sure why Lemmy does it.

Yeah I don't understand why every instance can't keep track of their own votes privately. Sure, voting manipulation is a thing, but it's possible regardless.

Honestly I really hope Lemmy does something to address this issue. Otherwise it's kind of a dealbreaker for me.

If anything, wouldn't that make vote abuse even easier? Just send 100 upvotes with 100 random hashes.