What does David Cameron want?

David Cameron appears to want to strengthen the laws that allow the security services to intercept communications so that no method or element of online communication is out of reach of the state, as long as they have a warrant personally signed by the home secretary. The security services complain that the growth of encryption of online data means there are already services available that are sold as guaranteeing privacy or are in some other way beyond the reach of the intelligence services.

It could mean that a new intercept law might outlaw services such as Snapchat, by which text, photos or video are shared for up to 10 seconds before they are deleted from the company’s servers. More than 700m photos and videos are shared each day using such services. It could also mean that companies that offer encrypted email services could be banned or required to hand over their encryption keys to the security services in specified circumstances such as terrorism or paedophile cases.

The prime minister also appears to want to future-proof any new measure. Traditionally the security services and the police have always had the authority to intercept and read any letter or listen in to any phone call as long as they have a warrant personally signed by the home secretary. Cameron’s comments suggest that he wants a blanket law that would cover not only existing forms of communication such as encrypted services or Snapchat-style services but also any that might develop into the future. This would amount to an extremely sweeping new power.

But the details are still unclear and Cameron’s aides are reluctant to spell out in any more detail what might be involved beyond saying that it is a matter for after the general election due in May.

Does this match with the new powers the security services in Britain want to tackle terrorism?

The demand for more powers for the security services made by Andrew Parker, the head of MI5, in the immediate aftermath of the Paris attacks was not the first time a security chief has tried to put the subject on the table. Parker warned last week that the pace of technological change meant the “dark places [on the net] from where those who wish us harm can plot or plan are increasing”, and that agencies’ capability to tackle them were decreasing. Previously, the first act of Robert Hannigan when he took over as head of GCHQ in November was to launch a public attack on the US technology giants, accusing them of being ““the command-and-control networks of choice for terrorists and criminals”.

The security agencies say the use of encryption for emails and chats – increasingly offered as standard by the internet companies – is making it harder for them to track terrorist suspects. They are also exercised by software such as Tor, which disguises the location of the person surfing the net, sending messages or using chat. Anyone using Tor immediately becomes suspect, even though they may be doing so only because they want to ensure privacy.

Neither Parker nor Hannigan explicitly mentioned the communications data bill – the snooper’s charter, as it is known by its critics – but everyone from Cameron downwards knows that is also on their minds. The snooper’s charter was blocked by Nick Clegg, the deputy prime minister, after a joint parliamentary committee – which included a former cabinet secretary and was chaired by a Tory ex-Home Office minister – concluded that its provisions were so sweeping that they amounted to “overkill”, and a better balance was needed between security and privacy.

The Liberal Democrats have made clear they will block any attempt to introduce the snooper’s charter before the election. Cameron’s focus on the issue in the immediate aftermath of the Paris attack suggests he wants to turn it into a clear dividing line with the Lib Dems in the election campaign.

What would the snooper’s charter do?

A revised communications data bill would require the internet and phone companies, particularly the big US tech firms such as Google and Facebook, to retain and store all communications data that tracks users’ web and phone use, including browsing histories, emails, texts and any other online activity. The legislation is also expected to introduce a “request filter” – where many argue the real privacy dangers lie – which would enable the police and security services to search the mountain of personal data held by the internet companies.

It would not cover the content of those emails or texts – the subject of Cameron’s intervention. But, crucially, warranted intercept evidence by which agencies snoop on the content of calls and emails is not admissible in UK courts. Communications data is. It is important to the security services to be able secure this data “through the front door” so it can be used in court to secure convictions.

What’s wrong with the snooper’s charter?

The committee of MPs and peers said the danger in this request filter was that it would enable large-scale data-mining or fishing expeditions to be carried out. A second key problem was that it required overseas companies, particularly the US tech companies, to hand over private data about their customers on an executive order signed by a British home secretary. Clegg’s veto was partly based on the reluctance of US companies to comply voluntarily with a request from a foreign security service to hand over their customers’ data rather than being forced to do so by a judicial warrant endorsed by a court, as US law requires.

Don’t the British security services already have the capacity to access all this data? Didn’t the disclosures made via Edward Snowden about the agencies’ secret surveillance capacities show that they do all this already?

Well, to borrow the MI5 chief’s phrase, coverage is “patchy”. GCHQ’s Tempora programme, as revealed by Snowden, does have the capacity to “slurp” up everyone’s communications data crossing the Atlantic, but it can hold it for only 30 days, not the 12 months proposed under the snooper’s charter.

But the real difficulty for the security services lies in the fact that the legal framework now lags far behind their technical capacity to carry out web surveillance. Since 2009 the agencies have legally been able to require internet and phone companies to store all the personal communications data they routinely hold on customers for billing purposes. This requirement was introduced in the aftermath of the 7/7 attacks in London in 2005. However, the security services argue that the amount of data companies retain for their own billing purposes is declining every year, so leaving much activity on the web undocumented.

What could be done instead of introducing sweeping extra internet surveillance powers?

One answer is the creation of more comprehensive databases that track travel within Europe. Advanced passenger information systems already require this on transatlantic flights, allowing no-fly lists to be implemented. Theresa May, the home secretary, wants this extended to all internal EU flights and train journeys to hamper the movement of foreign fighters from Europe to Syria. But coverage is patchy and the practical application of warnings indexes and no-fly lists variable.

The battery of new powers now going through parliament covering the managed return of those coming back from Syria and Iraq, the temporary seizure of passports to prevent terror suspects leaving the country, and a reinforced anti-radicalisation programme placing new duties on prisons, universities and local authorities to tackle those at risk of being drawn into terrorism could all help too.

How should terror suspects be tracked?

The image, made popular by television and the movies, is of three people tailing an individual, with the trackers ducking into shopfront doors if the target turns round. According to the intelligence agencies, this is a huge underestimation of the numbers involved. The figure they usually give is about 30 people to track one person. Court documents dating back to when the intelligence agencies were tracking IRA suspects suggest the numbers involved could be double or close to triple even that estimate.

Allocating even 30 people is a huge deployment and requires the intelligence agencies to make hard choices about whom they should be following. That partly explains why MI5 had only intermittent coverage of Lee Rigby’s killers, and its French counterpart the same with the Charlie Hebdo killers.

Thousands have gone to Iraq and Syria over the last few years, from journalists to doctors and nurses. But MI5 has cut this down to a list of about 600 people regarded as being of concern. Round-the-clock surveillance would be applied only to the “critical dozen or so” who would be regarded as being involved in attack planning.