On Monday David Cameron managed a rare political treble: he proposed a policy that is draconian, stupid and economically destructive.

The prime minister made comments widely interpreted as proposing a ban on end-to-end encryption in messages – the technology that protects online communications, shopping, banking, personal data and more.

“[I]n our country, do we want to allow a means of communication between people which we cannot read?”, the prime minister asked rhetorically.

To most people in a supposed liberal democracy, the answer would surely be “yes”: the right to privacy runs right in parallel to our right for free expression. If you can’t say something to a friend or family member without the fear the government, your neighbour or your boss will overhear, your free expression is deeply curtailed.

This means that even in principle Cameron’s approach is darkly paradoxical: the attack on Paris was an attack on free expression – but it’s the government that intends to land the killing blow.

Terrorists must not be allowed to disrupt our way of life, we’re often told in the wake of atrocities. We must leave that to governments to do in the wakes of these attacks.

But it’s in the practicalities that the prime minister’s approach slips from draconian to dull-witted. There is no such thing as “good guy encryption” and “bad guy encryption”. The same encryption that protects you and me protects companies, protects governments, and protects terrorists.

Facebook Twitter Pinterest There is no such thing as ‘good guy encryption’ and ‘bad guy encryption’. Photograph: Kacper Pempel/Reuters

Encryption is what protects your private details when you send your bank details to a server. It’s required for governments and companies when they store customer information, to protect it from hackers and others. And it’s built right in to whole hosts of messaging applications, including iMessage and WhatsApp.

If Cameron is proposing an end to encryption in the UK, then any information sent across the internet would be open for any company, government, or script kiddie with 10 minutes “hacking” experience to access. It would spell the end of e-commerce, private online communications and any hope of the UK having any cybersecurity whatsoever.

If instead the prime minister is proposing it is only encrypted messaging that’s banned, the picture becomes hardly any clearer: if my Amazon online shopping session includes an ability to message a seller, is that now banned? Will the government produce a list of people who are allowed to use encryption?

Most messaging apps are global, and not built in the UK. Apple’s chief executive, Tim Cook, is unlikely to wish to dismantle the central privacy and security feature of his company’s iMessage tool at the whim of the government of one small island. The choice he then faces is whether to operate in the UK or not. The prime minister who intended to build “digital Britain” instead risks building an isolated, analogue nation.

It is true that terrorists use encryption, much as in real life they use bank accounts, locks, money transfer services and public transport. If the presence of terrorists on a given service is reason enough to shut it down, we’ll find there’s really no form of civil society left to defend.

In his eagerness to look tough on terror, Cameron instead looks like a man flailing wildly and virtually at random. Charitably, it can be hoped the prime minister was seeking to propose an unworkable policy for a few days’ good headlines.

The fear is that he is serious, and understands so little of what he is legislating that he really believes it would be possible to somehow stop terrorists communicating privately without astonishing collateral damage to Britain’s economy, freedom, and security.

If that’s the circumstance, then the prime minister needs urgently to abdicate his responsibility in favour of someone with more digital nous. He could begin with a concussed kitten on a ketamine trip, and work up from there.