Comment by echoangle
1 year ago
I don’t really understand how the protocol can ensure that the server can’t identify the client.
As far as I understand, the client sends some information A to the server, the server applies some private key X and returns the output B to the client, which then generates tokens C from the output.
If the server uses a different X for every user and then when verifying just checks the X of every user to see which one is valid, couldn’t the server know who created the token?
Here's a resource I found that walks through the ideas of the protocol, starting with simple implementations that have a problem, and then solving the problem one by one: https://privacypass.github.io/protocol/
I think that's the best conceptual overview of a crypto protocol I've ever seen.
That is an excellent explanation of how the protocol works. Thank you for bringing it to the discussion!
I really love this style of explanation. There was another one I saw recently (OIDC, I think?) that I wish I'd bookmarked but I forgot to
See section 5.5 of the linked paper https://petsymposium.org/popets/2018/popets-2018-0026.php. I'm not sure if/how Kagi implemented this, but the idea is that Kagi's "public" component can be committed to publicly (e.g., in the browser extension itself).
[I implemented this at Kagi]
And you can validate this, if you try to issue a Privacy Pass search without a private token, you'll get a `WWW-Authenticate` header that kicks off the handshake, and that should be the same for all users for a given epoch (month). E.g.
But how do I validate that I’m actually getting the same value as everyone else? Is the value I should get published somewhere (in a verifiable and not editable way) so I can see that I’m not being tracked?
Or does the extension validate this and the correct value is hardcoded in the extension like stebalien suggested?
5 replies →
Thanks for looking it up, that makes sense.
In the simplest terms, the token generation process B->C is done with the user's private key. So even if the server knows A,X,B they can't link it to the token C.
But if the server is allowed to vary X, it can basically act like different servers to each client, and can then when given a token check for which server would have been valid. The solution I got from the other replies is to make sure that the server uses the same X for everyone by verifying it as a client.
It appears that the public side of X is sent as the first part of the handshake, without any login info yet, and can be verified as part of B, thus a varying X would be easy to detect... I think.