DEPENDING on what kind of spy you are, you either love technology or hate it. For intelligence-gatherers whose work is based on bugging and eavesdropping, life has never been better. Finicky miniature cameras and tape recorders have given way to pinhead-sized gadgets, powered remotely (a big problem in the old days used to be changing the batteries on bugs).

Encrypted electronic communications are a splendid target for the huge computers at places such as America's National Security Agency. Even a message that is impregnably encoded by today's standards may be cracked in the future. That gives security-conscious officials the shivers.

But the same advances are making life a lot harder for the kind of spy who deals with humans rather than bytes. The basis of spycraft is breaking the rules without being noticed. As with the Russians arrested last month in America and now deported, that involves moving around inconspicuously, usually under false identities, and handing over and receiving money by undetectable means. For those that get caught, the consequences can be catastrophic.

The biggest headache is mobile phones. For spycatchers, these are ideal bugging and tracking devices, which the target kindly keeps powered up. But that makes them a menace for spies (and for terrorists, who often operate under the same constraints). Removing the battery and putting the bits in a fridge or other metal container disables any bug, but instantly arouses suspicion. If two people being followed both take this unusual precaution near the same location at the same time, even the most dull-witted watcher may infer that a clandestine meeting is afoot.

Creating false identities used to be easy: an intelligence officer setting off on a job would take a scuffed passport, a wallet with a couple of credit cards, a driving licence and some family snaps. In a world based on atoms, cracking that was hard.

Thanks to electrons, it is easy to see if a suspicious visitor's “shadow” checks out. Visa stamps from other countries can be verified against records in their immigration computers. A credit reference instantly reveals when the credit cards were issued and how much they have been used. A claimed employment history can be googled. Mobile-phone billing records reveal past contacts (or lack of them).

Missing links, in fact, are almost as bad as mistakes. A pristine mobile phone number is suspicious (especially when coupled with new credit cards and a new e-mail address, but no Facebook account). An investigation that would have once tied up a team of counter-espionage officers for weeks now takes a few mouse clicks.

With enough effort, a few convincing identities can be kept alive—a minor industry in the spy world involves keeping the credit cards for clandestine work credibly active. But for serious spies these legends wear out faster than they can be created.

Dead on arrival

Biometric passports are making matters worse. If you have once entered the United States as a foreigner, your fingerprints and that name are linked for ever in the government's computers. The data can be checked by any of several dozen close American allies. Obtaining a passport with a dead child's birth certificate is increasingly risky as population registers are computerised. Stealing a tourist's passport and changing the photo (a tactic favoured by Israel's Mossad) is no longer easy: in future the biometric data on the chip will need to check out too. Only the most determined and resourceful countries can do that—and the cost is spiralling.

Technology creates other problems. Take the dead-letter drop, where an item can be left inconspicuously and securely for someone else to pick up. Intelligence officers are trained to spot these, in places that are easy to visit and hard to observe (cisterns and waste bins in public lavatories, or under a heating grating in a church pew, for example). Time was when monitoring a suspected dead-letter box involved laborious work by humans. Now it can be done invisibly, remotely and automatically. Next time you bury a beer bottle stuffed with money in a park, you should ponder what cameras and sensors may be hidden in the trees nearby.

The days of the “illegal”, living for many years in a foreign country under a near-foolproof false identity, are drawing to a close. Spymasters are increasingly using “real people” instead: globalisation makes it unremarkable for those such as Anna Chapman, one of the ten Russians deported from America (under her own, legally acquired, British name), to study, marry, work and live in a bunch of different countries. Like so many other once-solid professions, spying is becoming less of a career and more a job for freelancers.