Signal has started rolling out Signal Login, an optional registration method that allows users to create a Signal account without providing a phone number. The feature will be rolled out first on Android and will come to iOS later.
As we previously reported, Signal Login is not a free alternative registration method. Signal requires users who choose to register without a phone number to make a one-time payment. The price at launch is $2.99 / € 3.49, although this may vary by region and currency.
They’re using zero-knowledge proofs for the payments so they can’t link the account to the credit card used to pay it.
Google can, since it’s their OS and their payments.
But even if you’re not using Play Store to pay, the vast majority of people who are paying are going to be using the same device/IP/fingerprint/etc. where Signal is running, so it’s not impossible to put the two together to link an account to a specific payment, if say, a government forced them to do so in order to track someone. It’s just a bad idea for several different privacy reasons.
Google can know you’ve paid but they can’t link it to your Signal account thanks to zero-knowledge proofs.
Except for tracing IP addresses, timing etc… as you’ve mentioned, but that’s much more involved and cannot be done retroactively.
They can… as I said they control the entire OS. They can see you’re running Signal with account# XYZ, that you clicked the pay button, that Signal generated the ZKP (which they can read in memory or network packets), and all the key clicks of the payment details you entered for which ZKP, the entire process.
Zero-knowledge proofs as a concept sound sus to me. How do you prove that you don’t know something? The fact of being tested already means you know of it. What’s a good writeup n how Signal does it? And how can a user of Signal test or certify it?
Here is a basic explanation for how zero-knowledge proofs can work in a very similar context as Signal.