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Da code

When Judge Peter Smith inserted a coded message in his written judgment on The Da Vinci Code controversy, he was doing something that?s been around since ancient times, writes Prakash Chandra.

india Updated: May 15, 2006 01:29 IST

When Judge Peter Smith inserted a coded message in his written judgment on The Da Vinci Code controversy, he was doing something that’s been around since ancient times: steganography, or ‘covered writing’ in Greek. This practice of hiding a message within a larger one ranged from tattooing the shaven head of a trusted messenger in the 5th century (as Herodotus reports) to using invisible ink during the World Wars. This is, unlike cryptography, where you see the message, but only those who know how can read it. Ancient Spartans wrote messages on parchment wrapped around a cylinder at an angle. When unwound, it displayed a meaningless jumble of letters — till the recipient deciphered the secret by rewrapping the parchment at the same angle around a similar cylinder.

Modern cryptosystems use algorithms. These act as ‘keys’ to unlock codes. Thus, if you replace every A in a message with a D, every B by an E, and so on through the alphabet, only someone who knows this ‘shift by 3’ key can decipher the message. The Enigma machine of World War II used gears, keys and lights to write in cipher — characters intelligible only to those who had the ‘key’. By shifting the ciphers once every time a letter was encoded, Enigma minimised the frequency of certain letters appearing in the code and giving it away.

Modern computers make ciphers almost unbreakable. When you buy online, your credit card number is encrypted. The store’s computer generates a pair of ‘public’ and ‘private’ keys — for encryption and decryption — and sends the ‘public’ key to the buyer’s computer, which uses it to encrypt the message. That message is sent back to the store’s computer, which decrypts it using the ‘private’ key. Snoopers see only the ‘public’ key.

Steganography often uses digital media content as camouflage, hiding a file in multimedia like an image, an audio file or a video file. This makes it an important security tool to watermark images for copyright protection. But steganography can also be used for illegal purposes. No wonder it’s used for covert communication by terrorists, just as judges use it for fun.