Thread by @colmmacc

iCloud Private Relay is one of the coolest things to happen in privacy and network encryption in a while. I'm going to update this thread as I learn more about it. Early impression: Overall it looks great, but I can see a few gotchas too.

There are two WWDC21 talks online about it already. The first is a general overview, so it's not too detailed, but it's well crafted.
developer.apple.com/videos/play/ww…

The second has more detail, skip to 24:30 for the private relay section: developer.apple.com/videos/play/ww…

At a high-level: Private Relay is like a smaller version of Tor and it uses a simple kind of onion routing. Your iPhone sends an encrypted DNS request or connection to an oblivious ingress proxy. That ingress proxy can see your IP address, but not the details of your request.

Then your DNS request or connection is forwarded to an egress proxy. That proxy can see the website you're accessing, but it doesn't know your IP address. Neither proxy gets to see your actual HTTP requests or connection level data, that's still end-to-end.

Apple run one of the relays (the ingress proxy I think) and "a content provider" runs the egress proxy. Sounds like a CDN, but no announcement yet. Both of these proxies need huge and deeply-distributed global foot-prints to be effective.

Of the two relays, I think the egress proxy is more trusted because more tracking can be encoded into unique DNS names via server-side redirects and javascript tricks. It'll be interesting to see if anyone tries that.

Apple's explanations so far are about anonymizing IP location from websites. There's no mention of abuse or law enforcement. There's also no statement about not logging or not being able to recover the correlation. It's definitely not being pitched as a Tor, which is fine.

There's no mention of IPv6, but the example IP addresses in the more detailed talk are all documentation-and-example-prefix (nice touch!) IPv6 addresses, so good chance it uses IPv6 when possible, which gives a lot more space for address randomization.

Private Relay uses HTTP/3 QUIC over UDP/443 for comms between your device and the private relay network. I worry a bit that QUIC might not be entrenched enough to avoid getting blocked outright by networks who don't want this. It'll be interesting to watch the brinksmanship.

Today QUIC on UDP/443 is blocked on something like 5% to 7% of networks, which is a lot for something you want to work seamlessly. I'd gave gone for TCP/443 but that's me!

The biggest gotcha so far might be the bad neighborhood problem. Whenever you share an IP address with a broad bunch of other people, you're going to run into abuse processes triggered by some of those other people.

Simple example: someone else gets their password to a website wrong enough times and that website might treat them like a bot, trigger Captchas or rate-limiting, maybe even reset your password because you came from the same IP.

Obviously I work for a competitor of sorts, but if this is true I'll be turning this off. As much as I like some of the cool things CloudFlare do, I do not trust how they manage security and privacy issues.

https://twitter.com/VirtuallyNathan/status/1402279806662635520

I'm still chewing on this slide, there's more in it than it appears. The server side connection shows a 518-length TLS hello message. Was that inside one of those 1292-byte QUIC datagrams? If so, that's neat, and maybe there's length-obfuscating padding?

If there's no length obfuscation, private-relay will still be vulnerable to content-fingerprinting. It's no worse than regular connectivity, but in theory the relay providers or someone in the patch could still make really good guesses about what the website/content is.

The other gotcha, which is kind of the point actually, is that GeoIP lookups become more coarse, so a website's guess about where you are loses fidelity. Apple suggest using CoreLocation instead, but not everyone uses it.

Someone asked me about the blind signatures that are in the protocol: they are boring, which is good! Not enough detail to see how the chaining works, but I'd be surprised if it wasn't super well designed. Apple gets that stuff right.

Caught in the cross-fire of all this is that those fun articles you sometimes see about a congressperson or senator's wikipedia article being flatteringly edited from congress's network will all go away.