They have eavesdropped on the enemy for decades, tracking messages from Hitler's high command and the Soviet KGB and on to the murky, modern world of satellites and cyberspace. But a lowly and yet mysterious carrier pigeon may have them baffled.
Britain's code-breakers acknowledged Friday that an encrypted handwritten message from World War II, found on the leg of a long-dead carrier pigeon in a household chimney in southern England, has thwarted all their efforts to decode it since it was sent to them last month.
As the bird's story made headlines, pigeon specialists said they believed that it may have been flying home from British units in France around the time of the D-Day Normandy landings in 1944 when it somehow expired in the chimney at the home where it was found in the village of Bletchingley, south of London.
After sustained pressure from pigeon-fanciers, the Britain's GCHQ code-breaking and communications interception unit in Gloucestershire agreed to try to crack the code. But Friday, the secretive organization, whose initials stand for Government Communications Headquarters, acknowledged that it had been unable to do so.
"The sorts of code that were constructed during operations were designed only to be able to be read by the senders and the recipients," a GCHQ historian told the British Broadcasting Corp.
"Unless we get rather more idea than we have about who sent this message and who it was sent to, we are not going to be able to find out what the underlying code was," said the historian, identified only as Tony under GCHQ's secrecy protocols.
Code-breakers, he said, believed that there could be two possibilities about the message's encryption.
One possibility, he said, was that it was based on a so-called onetime pad that uses a random set of letters, known only to the sender and recipient, to convert plain text into code and is then destroyed. "If it's only used once, and it's properly random, and it's properly guarded by the sender and the recipient, it's unbreakable," the historian said.
Alternatively, if the message was based on a code book designed specifically for a single operation or mission, GCHQ code-breakers were "unlikely" to crack it, Tony said.
The pigeon's skeleton was initially found by David Martin at his home in Bletchingley, when he was cleaning out a chimney as part of a renovation. The message, identifying the pigeon by the code name 40TW194, had been folded into a small scarlet capsule attached to its leg.
Mr. Martin was skeptical of the idea that GCHQ had been unable to crack the code. "I think there's something about that message that is either sensitive or does not reflect well" on British special forces operating behind enemy lines in wartime France, he said in a phone interview. "I'm convinced that it's an important message and a secret message."