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Here we go again. UK Home Secretary Amber Rudd has said that end-to-end encryption in apps like WhatsApp is “completely unacceptable” and that there should be “no hiding place for terrorists”. Her comments resurface the debate over banning encryption – an idea that is realistically unworkable and potentially mathematically impossible.

Speaking on BBC One’s Andrew Marr Show, Rudd called for the intelligence services to have access to encrypted messages, following the revelation that British extremist Khalid Masood used WhatsApp a few minutes before killing four people in Westminster last week. But her comments are ill-informed at best.

“My impression was that primarily she doesn’t know what she is talking about,” says at the University of East Anglia, UK.

Not just for terrorists

Encryption underpins everything from messaging apps to online shopping to government websites. It works by jumbling up information sent across the internet in such a way that only the intended recipient can de-jumble it. Anyone who intercepts the signal would require a nearly impossible amount of computing power and time to work out the contents of the message.

An outright ban on end-to-end encryption would mean no more online banking, no more online shopping, and no more online privacy. “Encryption is not just for terrorists, it’s for everyone,” says at the Electronic Frontier Foundation.

Even if a government attempted such a ban, it could never legislate the mathematics of encryption out of existence. People who really wanted to would still be able to find ways to encrypt their messages.

Another way to get past encryption could be to insist that encrypted services add a “backdoor” to their product. Intelligence services would have access to this backdoor, meaning they could read encrypted messages between people of interest, while everyone else would still benefit from the security of encryption. But in reality, this is unworkable.

Once there is a back door in a service, it would be vulnerable to attack. Any sufficiently capable group could gain access to the encrypted information – which could pose a far bigger inconvenience to security services than not having sweeping access to WhatsApp messages. If you create a backdoor for the good guys, you create a backdoor for the bad guys too.

Hacking devices

In any case, it would not be necessary for security services to break encryption for everyone in order to read WhatsApp messages on a suspect’s phone. Last year, the government passed the Investigatory Powers Act – also known as the “Snooper’s Charter” – which allows security services to directly access people’s devices when they have a warrant to do so. “If they identify a person of interest, they can hack the device and read what’s on it,” says Bernal. This means they wouldn’t have to crack encryption to intercept the information.

We don’t know if Khalid Masood said anything on WhatsApp that might have revealed his intentions. But we do know that he was known to the police but was not under investigation. Had he been, the security services already had the powers needed to read his messages.

Bernal thinks that encryption is simply an easy target in an issue that is far more complex. “It’s much easier to blame the internet. Instead of talking about if he was radicalised in prison and what policies could have led to that, we’re focusing on encryption.”