He paused and glanced at Graves to see if the information was making sense. Graves nodded.

`The problem is basically one of access-line proliferation. A data bank is just a collection of information stored on magnetic tape drums. It can be anywhere in the country. To get information out of it, you need to hook into the main computer with an access substation. That can also be anywhere in the country. Every major data bank has a large number of access substations. For limited or special-purpose access - stations that need to draw out information once or twice a week, let's say - we employ commercial telephone lines; we don't have our own lines. To tie in to a peripheral computer substation, you telephone a call number and hook your phone up to the computer terminal. That's it. As long as you have a half-duplex or full-duplex telephone line, you're in business.'

Graves nodded. `How is the call number coded?'

`We'll come to that,' Decker said, looking at Venn. `For now, we'll concentrate on the system. Some of the major data banks, like the ones held by Defence, may have five hundred or a thousand access lines. A year ago, Wilkens' congressional committee started to worry about unauthorized tapping into those access lines. In theory, a bright boy who knew computers could tap into the system and call out any information he wanted from the data banks. He could get all sorts of classified information.'

Decker sighed. `So I was hired to install redundancy checks on the system. Echo checks, bit additions, that sort of thing. My job was to make sure we could verify which stations drew out information from the data banks, and what information they drew. I finished that work a month ago.'