How do you break into a bank, learn about your boss's private life or locate a Russian nuclear test site? With a home computer. All these illegal activities have actually happened. As industrial societies become increasingly dependent on computers and as more and more information is being stored and transmitted in electronic form, many of these systems have become vulnerable to attack.

"Horizon" looks not only at ways they can be broken into but at the efforts to prevent this and preserve our privacy in this brave new electronic world. One answer may lie in the computer-generated codes which are used to protect the messages of banks, businesses and governments and may one day be regularly available to us all.

And a new development in code making suggests that soon it may no longer be so easy for thieves, or the authorities, to keep tabs on us....