Cracking code

By Charles Seife in Washington DC ONE of the biggest challenges facing a secret agent is sending secure messages back to base. If the other side cracks the code they could throw you in jail—or change their plans to scupper your work. But there is a way of encoding a message in sequences of DNA which will be far more difficult to decode, according to scientists in New York. During the Second World War, German spies used a very fine photographic emulsion to turn typewritten text into tiny microdots—circular photographs roughly the size of the full stop at the end of this sentence. Now Carter Bancroft, a biophysicist at the Mount Sinai School of Medicine in New York, and his colleagues have come up with a similar technique to store a hidden message in a tiny smear of DNA (Nature, vol 399, p 533). They assigned each letter in the alphabet to a triplet of DNA bases; for instance, the letter `A’ was represented by CGA, while `Y’ was represented by AAA. In this way, they converted their message, “June 6 invasion: Normandy” into a strand of DNA. They then put a 10 base-pair tag at either end of the message strand. When strands of the message-carrying DNA are mixed with jumbled fragments of ordinary human DNA and smeared on a piece of paper, the message is all but impossible to read, as it is hidden by the “noise” of the ordinary DNA. But the person the message was intended for will know the sequence of the tags at either end of the message strand and can use the polymerase chain reaction to copy the message-carrying DNA strands over and over, until the molecules with the message greatly outnumbers the junk DNA and can be read. Bancroft says that he is not sure if spies will readily adopt the technique—because the laboratory equipment needed to code and decode the message would have to be miniaturised. But he suspects that it might have a future in marking everyday items that suddenly acquire an unexpected value. “It might turn out to be useful for tagging specific items of interest, such as baseballs that are valuable or other valuable items,
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