Today's computer networks surround us with many forms of 'hidden writing'. Take websites' 'meta tags' for instance. They're not talking directly to us humans, but to the 'bots' sent out by search engines. Webmasters use meta tags like an angler's brightly coloured lure, to hook traffic via these bots. Or take a website's 'robots.txt' file. This tells the bots in a firm, loud voice where not to look as they gallop around your site hoovering up its contents.

In recent years webpages and MS Word documents have become vast oceans of invisible ink. Less and less of the code is visible to human visitors as the 'pure' text message. Try the 'view source' option in your browser or call up those files in a text editor (Editpad/Textpad on a PC, or Simpletext/BBEdit on a Mac), and you'll stumble into a cacophony of non-human conversations behind the scenes.

Another form of invisible conversation in the digital age is 'steganography'. This literally means 'covered writing' - hiding messages in other messages so that casual observers won't notice anything unusual. It's as old as the ancient Greeks, when the recipe would go something like this... Take one messenger, shave his head, tattoo message onto it, wait for hair to grow back, dispatch messenger, reach destination, shave head again to reveal message.

Steganography hit the headlines shortly before the attacks in the US, with claims that terrorists are using steganographic software to hide messages within digital files (audio, video or still images). A few changed pixels in a jpeg image here, or a tiny bit of imperceptible 'noise' in an MP3 soundfile there, nothing too dramatic for human eyes or ears to detect.

USA Today reported in June 2001 that: "Through weeks of interviews with US law-enforcement officials and experts, USA Today has learned new details of how extremists hide maps and photographs of terrorist targets - and post instructions for terrorist activities - on sports chat rooms, pornographic bulletin boards and other popular Web sites." There may be several hidden agendas in all this. The experts and officials are usually nameless, and the media are served up an almost too-perfect combination (espionage + internet + terrorism + sport + porn!). But steganography isn't just a spy story. It's big business and a major research area - the fifth annual conference "on information hiding" will be held in Noordwijkerhout in October 2002, and among the chief concerns is copyright protection.

In the age of digital reproduction, as more audio, video and other products circulate as digital commodities, it's easy to make perfect copies. Here, steganography is about deliberately creating hidden imperfections. Not only 'watermarking' (adding a hidden copyright message) but also 'fingerprinting' (embedding a secret serial number or other set of characteristics to distinguish this particular copy from all other copies). Steganography could indeed become a key feature on tomorrow's media landscape, on anything from a Hollywood blockbuster DVD to a digital artwork near you.