Image caption Over recent years, the options for open and covert communication have multiplied

The list, say experts, is endless and limited only by individuals' imaginations.

Over the last two decades the number of available choices for terrorists, organised criminals and of course, ordinary, law-abiding citizens to communicate has proliferated alongside the growth in digital technology.

There are essentially two categories here: secret and public messages, both of which carry a risk of detection for the original sender.

Sophisticated terrorists are all too aware of the risks of leaving a "digital footprint" that can be traced and identified, hence why it took so long for US intelligence to track down Osama Bin Laden, who relied on couriers delivering messages and data by hand.

Counter-terrorism officials, like MI5's Director-General Andrew Parker, contend there should be no digital "oasis" where law-breakers or terrorist planners can hide messages and communicate freely without fear of surveillance or interception. His critics argue that government intrusion into private communications has already gone much too far.

Production houses

When it comes to disseminating information as widely as possible, the internet has long been the obvious choice.

Back in 2001, in the immediate aftermath of the 9/11 attacks, al-Qaeda's leadership posted a number of videos from their Pakistan hideouts to the Qatar-based TV station al-Jazeera. Frustrated by the channel's decision to broadcast only a small fraction of them, heavily edited, al-Qaeda then switched to uploading them to the internet.

Since then, al-Qaeda, the Taliban and Somalia's al-Shabab have all developed media production houses to churn out their online messages, some of which are produced to high production standards.

From Yemen, the local al-Qaeda franchise AQAP disseminates the online magazine Inspire, which famously carried an article aimed at recruits in America entitled "How to build a bomb in your mom's kitchen". Inspire has been cited as the inspiration behind a number of jihadist attacks in the US and Britain but British police warn that anyone caught downloading it will be arrested and prosecuted.

As to secret means of communication, there will doubtless be many obscure methods known only to practitioners, IT experts and those working in government Signals Intelligence (often contracted to Sigint).

Image caption A British MI6 agent was caught using a fake rock with a transmitter as a modern-day "dead drop" in Moscow in 2006

Loners leave a minimal trail - so, for example, the convicted Norwegian mass murderer Anders Breivik spent four years with almost no social contact while he planned his attacks of 2011. But here are some of the more commonly known options: