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Comment by darken | Hacker News Reader

Salts are generally stored with the hash, and are only really intended to prevent "rainbow table" attacks. (I.e. use of precomputed hash tables.) Though a predictable and matching salt per entry does mean you can attack all the hashes for a timestamp per hash attempt.

That being said, the previous responder's point still stands that you can brute force the salted IPs at about a second per IP with the colocated salt. Using multiple hash iterations (e.g. 1000x; i.e. "stretching") is how you'd meaningfully increase computational complexity, but still not in a way that makes use of the general "can't be practically reversed" hash guarantees.

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As I said the key for hashing PII for telemetry is that the client does the hashing on the client side and the client never transmits the salt. This isn't a login system or similar. There is no "validation" of the hash. All the hash is is a unique marker for a user that doesn't contain any PII.

How does the client generate the same salt every time they visit the page, without using cookies?

Use localstorage!

Kidding, of course. I don't think there's a way to track users across sessions, without storing something and requiring a 'cookie notification'. Which is kind of the point of all these laws.

Storing a salt with 24h expiry would be the same thing as the solution in the article. It would be better from a privacy perspective because the IP would then not be transmitted in a reversible way.

If I hadn't asked for permission to send hash(ip + date) then I'd sure not ask permission if I instead stored a random salt for each new 24h and sent the hash(ip + todays_salt).

This is effectively a cookie and it's not strictly necessary if it's stats only. So I think on the server side I'd just invent some reason why it's necessary for the product itself too, and make the telemetry just an added bonus.

If you can use JS it's easy. For example localStorage.setItem("salt", Math.random()). Without JS it's hard I think. I don't know why this author wants to use JS, perhaps out of respect for his visitors, but then I think it's worse to send PII over the wire (And an IP hashed in the way he describes is PII).

EU's consent requirements don't distinguish between cookies and localStorage, as far as I understand. And a salt that is only used for analytics would not count as “strictly necessary” data, so I think you'd have to put up a consent popup. Which is precisely the kind of thing a solution of that is trying to avoid.

Indeed, but as I wrote in another reply: it doesn't matter. It's even worse to send PII over the wire. Using the date as the salt (as he does) just means it's reversible PII - a.k.a. PII!.

Presumable these are stored on the server side to identify returning visitors - so instead of storing a random number for 24 hours on the client, you now have PII stored on the server. So basically there is no way to do this that doesn't require consent.

The only way to do it is to make the information required for some necessary function, and then let the analytics piggyback on it

I think I agree with you there. But again, the idea of a "salt" is then overcomplicating things. It's exactly the same to have the client generate a GUUID and just send that up, no salting or hashing required.

Yup for only identifying a system that’s easier. If this is all the telemetry is ever planned to do then that’s all you need. The benefit of having a local hash function is when you want to transmit multiple ids for data. E.g in a word processor you might transmit hash(salt+username) on start and hash(salt+filename) when opening a document and so on. That way you can send identifiers for things that are sensitive or private like file names in a standardized way and you don’t need to keep track of N generated guids for N use cases.

On the telemetry server you get e.g

Function “print” used by user 123 document 345. Using that you can do things like answering how many times an average document is printed or how many times per year an average user uses the print function.

IP address is "non-sensitive PII"[0]. It's pretty hard to identify someone from an IP address. Hashing and then deleting every day is very reasonable.

[0] https://www.ibm.com/topics/pii

What's the point in hashing the IP + salt then, just let each client generate a random nonce and use that as the key

What you're describing is called "pepper". What's called "salt" is not varied between rows and therefore not stored with the hash (and best stored far away from the hash).

You have this exactly backwards.

You're right of course, that's what I get by posting late at night. Can't edit or delete anymore.

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