Steganography is the art of hiding messages. It is different from cryptography in that the very existence of the message is hidden. Whereas an encrypted message may be plain for everyone to see (even if it isn't understood), steganography provides true secrecy. An international spy would rather cross borders with a hidden message, rather than with a briefcase full of ø¥2®Ę«!˜↑Ω™₤₪ﻖכֿאָ - the possession of which is as good as a one-way ticket to Guantanamo Bay.

The ways of hiding messages are limitless. For example, the background image of this webpage contains, hidden among the pixels, very important information for the benefit of MI5 (I've made it nice and easy for them to find). A message may be tattooed into your scalp so that your hair, if you have enough, will cover it. A message may be hidden within the gaps between letters in a sentence, disguised as white noise or concealed within strings of binary numbers.

Of course, with ever more advanced methods of encryption come more advanced ways to discover and untangle these messages. As soon as someone thinks of hiding messages within the RGB values on a digital image, someone else is writing a computer program to identify it. And, as computers get better at spotting these messages, someone thinks of another layer of encryption.

Some years ago I was given three pages of music manuscript, apparently from the same source, using the same key,[JPG] that had messages hidden within the staves. That's clever! I thought, but any admiration that I had soon vanished as I tried to play the music. The three pieces each had a different problem. The first [JPG] contained a consistent code but sounded terrible. I imagined the SS officer standing by the piano while the hapless spy played this cacophony. "Für you, zee war iss over! Not eeffen Schoenberg could compose such degenerate tripe." The second [MP3][Mid][JPG] is beautiful music (I wonder whether it is the work of Bach). However, although the message is consistent with some of the notes, there are apparently illogical gaps where the code doesn't apply. The third, at first glance a Coda to the second piece, was impossible to play because it made no musical sense. The dots were arranged without any reference to actual music. ( I'd be happy to hear of any more information about the history of these pieces.)

It seems to me that the first requirement, when trying to hide a message within music is that the music should be musical. And let me assure you - even those of you that sneer at twelve-note compositions - direct substitution of one letter for one note does not sound just like modern music. It sounds like a hidden message. Sometimes, by converting the letters to numbers, one can conceal a short message that may not draw too much attention to itself. Sometimes one is lucky and the notes seem to fall into place.

One such tune [MP3] [Mid] was derived from Pi to 31 decimal places. Numbers from 0 to 9 correspond with ten (non-consecutive) notes that I selected, not part of a recognised scale but chosen because of their likely compatibility. The resulting tune is slightly odd, but it is undeniably music.

Melody is irrelevant to the message if it has been concealed within MIDI signals [Mid], changes to which are indiscernable to the listener. MIDI is a system of controlling instruments from a computer. The MIDI file has no musical information, just a series of values ( Note, Volume, Velocity etc. Between 0 and 127) which the MIDI-compatible instrument - usually a keyboard - will recognise. Put this file into a music program (such as Cubase) and you can read the numbers. In this case I have added a further layer of coding to make the message harder to discover, and harder to decypher. There is important information of use to MI5 but, apparently, they have computers that make short work of this type of code.

This music [MP3][Mid] (I have convinced myself) sounds just like music but contains a hidden message. Unfortunately, the elaborate process involves quite a bit of musical composition and is so time consuming that it is quite impractical for use by international spies. On the other hand, I don't believe that the message would be detectable by any computer, unless it was especially programmed, and in order to program it you need to know what you are looking for.

My greatest leap forward has been to hide messages within World Music. [MP3] The numerous scales and microtones provide plenty of opportunities for the steganographer. Moreover, just imagine your enemies having to listen to hours of Arabic pop or Moldovan dance music all day long!

Undoubtedly, computers are becoming better at detecting patterns within digital data, but I wonder whether they can really be effective in a world with millions of digital images and audio files being exchanged each day. My message was clumsily hidden within a few hundred pixels, but how simple it would be to add twenty pixels to a thousand images. Even if these messages were detected, it requires time to decode them, and then there is no guarantee that the message would mean anything to anyone apart from the intended recipient.

In this way, computers can never replace human information. Finding your suspect and following him about town, on buses and trams; Chatting to his friends; Listening to his phonecalls; Going through his bins. These things are more likely to yield results than trying to filter a billion digital messages on the offchance that there might be something worth knowing within.

With all of these hi-tech methods being deployed to find hidden messages, it is the low-tech approach which might be more successful. Hide your messages in ways that require a human to discover them. They are unlikely to be discovered because it is more expensive to employ intelligent humans (a rare commodity) than to use a computer. Politicians the world over are easily impressed by technology because they know nothing about it. When the snake-oil salesman from the computer company tells them that his computer will solve all of their problems, they believe him. Our leaders rarely resist an opportunity to appear modern.

Technology in itself won't solve all problems. Computers are never going to understand rare dialects or private languages (backslang, for instance). Neither will they recognise metaphor or humour. In Japanese Noh theatre, a meaning - even a story - may be conveyed by the flutter of an eyelash or the movement of a little finger. Who, apart from Noh connoisseurs, would even register these movements? A conspiracy of Noh theatre actors is something that we should all fear. It may, indeed, be already upon us.

The internet is full of stories about how Al Qa'ida is hiding messages within digital pictures, which they transmit to their fanatical supporters via ebay (why ebay?). I believe, though, that with all the effort going into detecting secret messages, they might be better off not bothering to hide the message at all. When the detectives are digging up the patio and pulling up the manhole covers who's going to notice the dead body in the armchair? The paradox is well summed up by the Hungarian poet FRIGYES KARINTHY who wrote, "I am not allowed to tell anyone, so I tell everyone."

In that spirit, I am putting this picture up on the web. Either you can use whatever software you have to detect hidden messages [there is some free software at www.outguess.org] or you can try to detect a message by looking at it with your eyes. Let me know if you find anything like incitement to riot or violence. Alternatively, you might like to inform Westminster Council's Department for Street-Cleansing.