This is a cryptography blog and I always feel the need to apologize for any post that isn’t “straight cryptography.” I’m actually getting a little tired of apologizing for it (though if you want some hard-core cryptography content, there’s plenty here and here.)

Sometimes I have to remind my colleagues that out in the real world, our job is not to solve exciting mathematical problems: it’s to help people communicate securely. And people, at this very moment, need a lot of help. Many of my friends are Federal employees or work for contractors, and they’re terrified that they’re going to lose their job based on speech they post online. Unfortunately, “online” to many people includes thoughts send over private text messages — so even private conversations are becoming chilled. And while this is hardly the worst thing happening to people in this world, it’s something that’s happening to my friends and neighbors.

(And just in case you think this is partisan: many of the folks I’m talking about are Republicans, and some are military veterans who work for the agencies that keep Americans safe. They’re afraid for their jobs and their kids, and that stuff transcends politics.)

So let me get to the point of this relatively short post.

Apple iMessage is encrypted, but is it “secure”?

A huge majority of my “normie” friends are iPhone users, and while some have started downloading Signal app (and you should too!) — many of them favor one communications protocol: Apple iMessage. This is mostly because iMessage is just the built-in messaging app used on Apple phones. If you don’t know the branding, all you need to know is that iMessage is “blue bubbles” you get when talking to other Apple users.

Apple boasts that the iMessage app encrypts your messages end-to-end, and that it has done so since 2011. This means that all your messages and attachments are encrypted under keys that Apple does not know. The company has been extremely consistent about this, discusses it in their platform security guide, and in recent years they’ve even extended their protocol to provide post-quantum security. (A few years ago my students and I found a bug in the protocol, but Apple fixed it quickly — so I am very personally confident in their encryption.)

This is nice. And it’s all true.

But encryption in transit is only one part of the story. After delivering your messages with its uncompromising post-quantum security, Apple allows two things that aren’t so great:

1. iMessages stick around your phone forever unless you manually delete them (a process that may need to happen on both sides, and is painfully annoying.)
2. iMessages are automatically backed up to Apple’s iCloud backup service, if you have that feature on — and since Apple sets this up as the default during iPhone setup, most ordinary people do.

The combination of these two features turn iMessage into a Star-Trek style Captain’s Log of your entire life. Searching around right now, I can find messages from 2015. Even though my brain tells me this was three years ago, I’m reliably informed that this is a whole decade in the past.

Now, while the messages above are harmless, I want to convince you that this is often a very bad thing. People want to have private conversations. They deserve to have private conversations. And their technology should make them feel safe doing so. That means they should know that their messaging software has their back and will make sure those embarrassing or political or risque text messages are not stored forever on someone’s phone or inside a backup.

We know exactly how to fix this, and every other messenger did so long ago

If you install WhatsApp, Facebook Messenger, Signal, Snap or even Telegram (please don’t!) you’ll encounter a simple feature that addresses this problem. It’s usually called “disappearing messages“, but sometimes goes by other names.

I’m almost embarrassed to explain what this feature does, since it’s like explaining how a steering wheel works. Nevertheless. When you start a chat, you can decide how long the messages should stick around for. If your answer is forever, you don’t need to do anything. However, if it’s a sensitive conversation and you want it to be ephemeral in the same way that a phone call is, you can pick a time, typically ranging from 5 minutes to 90 days. When that time expires, your messages just get erased — on both your phone and the phones of the people you’re talking to.

A separate feature of disappearing messages is that some platforms will omit these conversations from device backups, or at least they’ll make sure expired messages can’t be restored. This makes sense because those conversations are supposed to be ephemeral: people are clearly not expecting those text messages to be around in the future, so they’re not as angry if they