The details might be a bit more complicated, but the core of end-to-end encryption is easy enough to understand. You encrypt a message for someone, and no-one else can read along. Say I send you a message. My phone encrypts that message using a unique key. I send that encrypted message to you. You are the only one who can decrypt (and read) that message, by using the unique key on your phone. Anyone who intercepts the message along the way will just see an incomprehensible mess. And no-one but you two has the key that decrypts that mess.
End to end encryption is one of the few technologies we can rely on to protect our conversations.
And that's the essence of end-to-end encryption: no-one but the sender and the intended receiver can read the message. Of course, you may haggle over the details. If your phone shows notifications, and you're in a bar with your phone on the table, someone else might read my message when it arrives. If you decide to publish my message on Instagram, the whole world can read it. And if my phone's been hacked, the message is compromised as well. But the essence remains: by using end-to-end encryption, you prevent anyone but the sender and the intended receiver from reading your message.
But what about the police?
One more thing that doesn't diminish that essence: the investigative powers of the police. The police may, in certain circumstances, seize and search your phone. When they do, they might be able to read your end-to-end encrypted messages. However, only a concrete and individualised suspicion against you allows them to do that. And even then, there's many conditions to be met. Under these circumstances, the police is also allowed to remotely hack into your phone. But because this is such a targeted power, it doesn't interfere with the essence of end-to-end encryption. In general, it holds: no-one but the sender and the intended receiver can read along.
When providers of instant messaging services, like Facebook's Messenger and WhatsApp, and Apple's iMessage, are forced to monitor all their users' messages, it does interfere with that essence. Such scanning can only be done on the phones of individual users, because the decrypted messages are available only there. That's why they call it 'client-side device scanning'. Under such a requirement, a third party would be reading along with end-to-end encrypted messages. That third party is the provider of the messaging service. But if the provider thinks they've found something criminal among your messages, they might forward your name and the message to the police. You can no longer be sure that only the sender and the intended receiver read the messsages.
The broad adoption of end-to-end encryption is a good thing, but many policy makers don’t seem to have noticed.
The European Commission has hinted at such measures before. They say that these measures are necessary to prevent the distribution of child sexual abuse material. Policy makers are only now proposing this, because service providers like Facebook and Apple are increasingly using end-to-end encryption in their services. This means they can't just read along with the messages we send to each other. That's a good thing, because then you, as a user, know that these companies (and who knows who else) can't read along. The companies have been showing off about this: they say that because they value privacy so much, they developed their technology in such a way that even they can't read along.
The broad adoption of end-to-end encryption is a good thing, but many policy makers don't seem to have noticed. End-to-end encryption is one of very few technologies that we (you individually, and we as a society) can trust to protect our most sensitive conversations (and photos and videos :)). That applies to the "I don't have anything to hide"-crowd, but also to managers who want to protect their corporate secrets from espionage, or indeed victims of sexual violence when they talk to a therapist or aid worker.
It's incredibly important to deal with child sexual abuse. But we shouldn't throw the baby out with the bathwater, by undermining the very technology that allows us to protect ourselves.
This article is translated from Dutch to English with the great help of Pieter and Amber Balhuizen.