End-to-end encryption: what it protects, and what it does not
End-to-end encryption has turned into a marketing checkbox, and the repetition has worn away most people's sense of what it actually covers. The good news: you do not need cryptography to understand it. You only need to look at the places where a message can be intercepted at all.
The three places a message is exposed
A message travels from you to the other person through three stretches, and each is protected by different means.
End-to-end encryption covers the second and third stretches: the keys live only on the participants' devices, so the server relays bytes it cannot interpret and cannot decrypt them even if it wanted to, or if it were asked to.
- Your device: the message exists in plain text here, because you are reading it.
- The network between you and the server: your provider, the Wi-Fi hotspot, any intermediary on the path.
- The service's server: where the message waits until it is delivered.
What end-to-end encryption does not do
This is where the awkward details live, the ones app descriptions rarely mention. None of them cancels the value of encryption, but each one limits it in real life.
Put another way, end-to-end encryption protects the pipe and the warehouse, but not the endpoints. And the endpoints, meaning the phones, are usually the weakest part.
- It does not protect the device. An unlocked or compromised phone exposes everything, encryption or not.
- It does not constrain the other person. They can screenshot the conversation or forward it onward.
- It does not cover cloud backups. A chat backup in the cloud may be stored in a form the cloud provider can read.
- It does not hide metadata. Who wrote to whom and when generally stays visible to the service.
- It does not stop key substitution unless you have verified safety numbers with the other person at least once.
How to check whether it is even on
The phrase “your data is protected with modern encryption” does not mean end-to-end: it usually describes ordinary TLS, which every website has. Beyond an explicit statement in the documentation, the practical sign of real end-to-end encryption is being able to compare a key or safety number with the other person inside the app.
Check separately whether it is on by default in every chat or only in a special mode you have to enable manually per conversation. Those are very different levels of protection, even though they sound identical in a feature list.
Where Chattr stands
Plainly: Chattr does not have end-to-end encryption. Messages are encrypted in transit and on the server, we do not use conversations for advertising and do not share them with third parties, but cryptographically that is weaker than Signal or WhatsApp, and we are not going to phrase it any other way.
Chattr solves a different problem: staying reachable where other messengers will not connect, and asking for no phone number and no personal data at sign-up. If you need the strictest possible cryptography for a particular conversation, it is reasonable to use Signal for it and keep Chattr for everyday messaging on networks where Signal does not work. We think that answer is more useful than an advertisement.