Hiding messages in gene-modified microbes

"Forget invisible ink or lemon juice – spies can now send messages hidden in genetically engineered bacteria. The new method, dubbed steganography by printed arrays of microbes (SPAM), uses a collection of "Escherichia coli" strains modified with fluorescent proteins that glow in a range of seven colours.

Each character of the message is encoded using two colours, creating 49 possible combinations – enough for the alphabet, the figures 0 to 9 and a few other symbols. "You can think of all sorts of secret spy applications," saysDavid Walt, a chemist at Tufts University in Medford, Massachusetts, who led the research.

The new technique is not the first example of biological encryption – researchers have previously hidden messages in DNA
 – but Walt says his method is easier to use. "If you're out in the field and trying to send a message, you're not going to have access to a DNA synthesiser," he says, whereas you could carry vials of bacteria.

The bacterial code has a much lower information density than DNA, however, which would limit the size of a message. "You probably could send 500 to 1000 symbols on a regular piece of paper," says Walt – enough for a quick mission update, but it probably won't let you smuggle state secrets out of a country."

More: http://www.newscientist.com/mobile/article/dn20965

Hiding messages in digital images and audio files

"Steganography literally means 'covered writing.' A Greek term, it refers to hiding one message inside another. While the concept has been around for centuries, the marriage of computers and the Internet has brought it to fruition.

Both picture and audio files are ideal for containing hidden messages, he explained; they tend to be very large, and software that substitutes single bits of data at the end of an eight-bit packet creates differences so subtle they can't be detected by the human eye or ear.

In steganography, or stego for short, the visible or audible message is called the carrier, while the hidden message is the payload. Together they form the covert message." More: http://gcn.com/Articles/2005/05/13/Hidden-in-plain-sight.aspx?Page=2

If that isn't enough consider that the hidden text may contain an executable command...

Spies can use Google as a tool of espionage

Use of Google search engine in espionage:

'Google Suggest works by listing up to 10 suggestions each time a letter is added to a search term, based on the most popular searches made by other Google users that begin with the same letters. The words offered change as each new letter is added.

Some of the options that appear as your search term takes shape can often seem quite strange: "runescape", "rotten tomatoes" and "rock and chips" for "r", "ro" and "roc" in "rocket", for instance. This is the key to how Mazurczyk's team adds its own search suggestions to the list to encode secret messages. '

More:
http://www.newscientist.com/mobile/article/mg21028175.900-spies-can-send-messages-hidden-in-a-google-search.html

Espionage terms explained here.