[kyboren]

> They could support this new scheme AND just store every thing in plain text.

Could they?

I am not clear that this is possible. I thought the entire point of "Alice provides a zero-knowledge proof to the server that she possesses an AuthCredential matching some particular entry" is that the server learns nothing about Alice other than her possession of a matching AuthCredential. Indeed, the paper says: "Because of the zero-knowledge property, the server has assurance that the user possesses such an auth credential without learning the UID certified by the credential, or other information that might link this use of the credential to other uses or to credential issuance."

It would be nice if someone more knowledgeable could confirm whether it is indeed possible for Signal to compromise user privacy while using this scheme. Is SheinhardtWigCo right when they write, "In contrast, I cannot verify this new claim that my group memberships are protected. I have to trust them."?

[tialaramex]

SheinhardtWigCo's central idea is that if someone receive a packet over the network it has an IP address in it, and that's the sender's "identity" and so the Signal servers can't avoid knowing Alice's "identity" when she does this, and then they can collect such data to try to re-assemble group membership in terms of IP address "identities".

For example let's say a packet arrives from 10.20.30.40 [[ all IPs used are from 10/8 as examples I am aware that Signal probably rejects packets claiming to be from an RFC1918 network ]] which contains proof that group #1 member #4 has authorised adding a new member #8

SheinhardtWigCo believes this tells us that this identity (10.20.30.40) is a member of this group, group #1 and they suppose that Signal's server could in fact store this, and then perhaps later tell some Spooks a list of such members of group #1 and it could do this on a vast scale, so that it would be able to say for any "identity" (IP address) the list of all identities (IP addresses) which seem to be members of groups which that identity is also a member of.

Now, I don't think Spooks would find that very useful, but there you go, that's what SheinhardtWigCo thinks is a big problem here.

[ Edited to clarify early paragraph ]

[kyboren]

I see, thanks. I was confusing "logging" for logging the association between AuthCredential and UID rather than logging IP addresses. For what it's worth, Signal does allow connecting over Tor.