WHEN you pay a call on a superpower, there's always something to spoil the party.

David Cameron got his cherished dinner with Barack Obama on Thursday night after the two leaders had issued fine words on terrorism and freedom of speech. It was all very ­convivial, no doubt, but it also involved a moment of prime ministerial throat-clearing.

Rendered in Cameron's language, what followed probably went ­something like: "Um. Some chap by the name of Shaker Aamer. Guantanamo. Apparently, he's the last of ours. Could you see your way clear to letting us, you know, have him back?"

For the president, who promised to shut down the Guantanamo facility as his first act in office, the request probably wasn't his choice of dessert. On the other hand, if Her Majesty's Government had been doing its job, the Shaker Aamer case would have soured relations long ago. As things stand, the ­prisoner grants us a neat parable on the "war on terror", freedom and what happens when politicians' words fail to accommodate reality.

It is likely, after Paris, that not many people in Britain will care much about Aamer. If our allies have him locked up in Guantanamo, many will say, he must be one of those terrorists, or at least inclined in that direction. But here's the thing: Aamer has been incarcerated for 13 years; locked away without trial - and without a single charge being brought.

The Americans admit they have no evidence they can use. They have a few tainted allegations extracted from one of Aamer's fellow detainees and lots of suspicions, but nothing that would survive even their built-to-order military ­tribunals. They are prepared to send the prisoner to Saudi Arabia, where he was born - but not back to his British children and wife. The UK is not trusted to provide the legal basis for monitoring an individual against whom no charge has ever been laid.

This is rank injustice, but there's a lot of that about. Few watched the great Paris march for freedom of thought and expression last weekend without a kind of aghast wonderment over those "world leaders" and their gall. It became a kind of game: how many of those hogging the limelight had not been involved in the jailing, killing, or intimidation of journalists? How many had not already begun to scent political opportunities in a series of bloody murders?

Cameron fell into that latter category. He had not even got out of Paris before he was granting interviews to promote the old, dire paradox: to safeguard liberty, we must sacrifice liberty. It was the kind of language rejected by Obama at his first inauguration in January 2009 when he dismissed "as false the choice between our safety and our ideals". The ­distinction should have been noticed at the time, just as it should be noticed in Cameron's recent reassuring words: our "ideals" are not our rights. Those are disappearing with each new piece of security legislation.

All of it is for our own peace of mind, of course. Doubt it and a ­politician will accuse you of being an apologist for terrorism. ­Sometimes just asking what is being done and why is more than a democratic government can tolerate. Security is its own justification. Bit by bit, we are being persuaded to forget this is the mad logic of the police state, even - perhaps especially - when it is disguised as "homeland security".

Reportedly, one part of Dave's dinner with Barack involved the supplicant requesting presidential help with US internet behemoths. Since Facebook, Twitter, Google and the rest of the for-profit domestic spies suck data from our lives, they are ideally placed - so Cameron believes - to co-operate with UK security services and deny a "safe space" to terrorists. A release for Aamer would help to make such a public-private partnership look a little less sinister.

Cameron's newest obsession is encryption technology. As the latest Counter-Terrorism and Security Bill was about to have its second reading in the House of Lords, he was fretting over the inability of government to eavesdrop at will on anyone it has reason to suspect of evil intent. There are - who knew? - "dark places" on the internet where plots can be hatched behind the veil of encryption. If re-elected, Cameron means to end these "no-go areas" legally.

Given that the bill going through Parliament already has plenty to say about data retention, and given that GCHQ and its American counterparts have been helping themselves to vast amounts of private information for years, encryption is one of the last bastions of privacy in the online world. It is also the reason why you can be fairly sure that your online banking or shopping is secure. It helps to keep a lot of messaging away from the prying eyes of crooks - and from government departments with few scruples and lamentable data protection records. Cameron thinks its loss a small price to pay.

He might have a majority on his side. What is chief among our liberties? After Paris, many would say the right not to be slaughtered while in the middle of an office meeting should rank highly. To shop, travel, work, or send the kids to school without the fear of mayhem and death is our modest request. If someone offers protection, they earn our gratitude and support.

There's an oddity, however, in the war on terrorism: no-one offers any such thing. They insist instead that no protection can ever be perfect, that the threat will always be with us, and that our only rational hope is to surrender just a little more liberty for the sake of just a little more protection. And so it goes on.

Security agencies never offer to surrender power when a crisis has passed. There is always a just-in-case reason to hang on to budgets, guns, gadgets and legal competence. In the case of Islamist terrorists, the securocrats and their masters have been careful indeed to describe a threat without end. Will this be over in 10 years? Twenty? One generation, two? They couldn't possibly speculate. What they mean is that they intend the new dispensation to be permanent.

That alone, you might have thought, would dent public confidence a little. For a sceptical few, it might even seem more than a bit suspicious. Just why was Afghanistan the longest war in centuries? Why do the governments of the West, endowed with so much military, economic and scientific power, always need a few more of your freedoms to win their battles in freedom's name?

Sometimes the doubting soul could wonder who the winners really are.

For now, the thought does not require encryption.