“We will meet you at sunset.” It seems an unusual time for an interview, until I realise that Alex (we’re not allowed to know his surname) a 29-year-old cryptanalyst at Government Communications Headquarters (GCHQ), is talking in code. Or is it a cipher?

“If we were using a code, ‘sunset’ would stand for a particular time or place,” he explains. “In a cipher, the statement would be made up of a sequence of jumbled letters – an algorithm – and you’d need a key to unscramble it.”

Even at the highest levels of national intelligence, simple disguises such as these are still common, the maths and psychology graduate says. “People stick to what they know, so the basic codes are still used. If a terrorist is caught with a piece of crumpled paper in his pocket with a few letters scribbled on it, we’ll ask to have a look.” Most cryptanalysis, however, involves some form of technology. “We collect emissions from electronic devices – be it a BlackBerry or a computer, then apply statistical analysis. All technological encryptions can be broken down into binary code, ones and zeroes.”

So far, so mysterious. An adept talker, Alex’s words seem revealing, until you realise he’s giving nothing away. Descriptions are intriguingly nonspecific, composed of problems, targets and meaningful information. Deflecting questions is a practised art, he says, although the secrecy of the work takes getting used to.

“When I first started, I felt completely paranoid. Nothing was a coincidence.” GCHQ operates on a strictly need-to-know basis. “At first I felt like a five-year-old, scribbling ‘TOP SECRET’ on files.” Has he ever felt threatened because of what he does? “Not personally, although sometimes people in certain situations have difficult decisions to make.”

It’s still a bit Bletchley Park, he says. “We’re not all mathematicians squirreled away with blackboards, but some things are very similar.” Modern-day cryptanalysts write cutting-edge programmes to crack codes; they also turn to squared paper and highlighter pens when problems get difficult. “It’s a game of frustration and persistence. You think ‘I’ve tried everything’. Then you remember that there could be lives on the line, so you try something else,” he says. “Our targets don’t know that we’re watching them. They can change something for no particular reason and all your work becomes meaningless.”

Other problems have been around for years. When a new computer comes along, 100 or 1,000 times faster than the last model, the codebreakers try again. Even eureka moments are tempered, however. Cracking a code is the first step, then the hard work starts. “Making out words in a block of text is not enough. You have to draw something meaningful out of it and put it into a format that someone else can understand. We use linguists to help us.” So are there any secrets he is allowed to share? He thinks for a moment, then says: “I’m terrible at puzzles.”

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