Sweet is the spectacle of a home secretary bitten by her own snake. The outrage of Jacqui Smith's television expenses claim lies not in its content, lurid as it is, but in the way it was exposed. How many times must the home secretary have been assured in security briefings that her latest purchase of some data storage gizmo was "totally secure"?

"Don't worry," the briefers would have said, "the material will be protected by the finest firewalls, the most foolproof anti-hacking devices and the most savage legal defence. Nothing will be transferable and only the highest in the land will have access. Besides, home secretary, as you have so often said, the innocent have nothing to fear."

Yes, they do. They have the revelation of their husband's taste in movies, apparently leaked by contractors in receipt of easily copied discs, now on offer to anyone with £300,000.

Anyone who knows anything about computers knows computer security is a contradiction in terms. It is garbage. Yet this is not believed by gullible ministers and MPs who troop through the Commons lobby to vote through new measures for more state surveillance.

I am sure when they submitted invoiced expenses to the fee office last year MPs were told they would be secure. They may have believed it. They thought that what they told their constituents about the security of ID and NHS records also applied to them.

They thought their dodgy second homes, mildly sleazy fiddles and squalid movies would be a private matter between them and a silver disc somewhere in the Palace of Westminster. Every chit for a new bathplug or dining room suite or blue movie was invested with "adequate safeguards". No phrase is more beloved of a minister than that.

We now learn that Virgin Media lists every downloaded TV movie by title and distributes the list via the public post. If a customer hopes to reclaim the expense from an employer, the list is passed to the relevant accounts department. There it will be shown gleefully round the office before being shared with the world on YouTube.

How many times must we repeat this? There is no such thing as a secure computer. It does not exist. The secrets of the Pentagon have been penetrated by Chinese hackers and British nerds. The most carefully programmed firewalls in east Europe were crashed at the click of a mouse by Russian agents. Such is the magnetism of secrecy that poachers will always be a step ahead of gamekeepers.

In the last eight years, the same MPs who are howling at their data vulnerability have voted for the most extensive surveillance system in Europe, as well as the biggest data storage in the most expensive and inept computers. Britain under Labour has become the world capital of privacy intrusion.

A fifth of all closed-circuit cameras in existence are in Britain, despite the Home Office admitting they appear to make no difference to crime or drunkenness. Smith has legislated or approved an astonishing range of powers. She is contracting with private firms to set up a data storage device to record all emails and internet uses, costing £46m. This is a precursor to her £12bn "interception modernisation upgrade" also to record every text and phone call. This is ludicrous and illiberal extravagance.

Smith wants, under the coroners and justice bill, to "remove barriers to effective data sharing to support improved public services". Improve at what cost in liberty? She supports the Metropolitan police's evidence gatherer teams. These claim powers to "record identifiable details" of citizens at any gathering who might be "bordering on civil disobedience" (including journalists reporting them). As the Guardian has revealed, such filmed material is put on "spotter cards" and stored in a "corporate intelligence database", in gross breach of the European Convention on Human Rights.

ID cards and NHS computers promise to store the defining details and medical records of the entire population. As data sharing spreads, these records will be virtually open to public view. In 2000, just nine organisations were allowed warrants to access secure government records: the figure is now almost 800. For a small fee, anyone will be able to learn anything about anyone else. It may be illegal, but like computer downloads it will happen.

This means every patient's medical history will become available to insurance firms, rendering some uninsurable. Court and criminal records will end the privacy of a spent conviction and make many, including those who have committed no crime, unemployable for being on a police data system. It was reported last week that terrorism laws are more used for local government and crowd control than national security.

As she battles to extend detention without trial, traveller surveillance and electronic databases, Smith will incant the presence of safeguards. Like most ministers and Whitehall officials, she is putty in the hands of high-pressure computer salesmen. She believes what they say, against all the evidence of the liberty lobby and computer failure. Perhaps she now knows better.

One of the few home secretaries who dominated his department rather than be cowed by it was Lord Whitelaw in the 1980s. He boasted how after any security lapse, the police would come to beg for new and draconian powers. He laughed and sent them packing, saying only a bunch of softies would erode British liberty to give themselves an easier job. He said they laughed in return and remarked that "it was worth a try".

Now the try always works. What is extraordinary is the weakness of the liberty lobby in opposition. It almost never wins. The Liberal Democrat Chris Huhne is proposing an excellent "freedom bill", repealing repressive legislation in 20 areas, from pre-charge detention through DNA databases to children's records. But it stands no chance of enactment. Nor are the Tories any longer libertarians - witness Chris Grayling this week absurdly protesting inadequate security for the G20.

The Joseph Rowntree Reform Trust recently recorded just 15% of 50 government databases as "effective, proportionate or necessary". It concluded that 10 actually broke privacy law. Yet a staggering £100bn is to be spent on them in the next five years.

The only hope is that now MPs have been hoist by their own petard, they might be more mindful of the liberties - and privacies - of others. I would not hold my breath.

simon.jenkins@theguardian.com