Computers: Feedback: 'Clipper' charts a stormy course

The US government has been heavily promoting a gadget that allows everyone to have private conversations over the telephone, down fax connections and through electronic mail. The 'Clipper chip', works by encrypting the stream of information using advanced cyphers, which could not even in theory be broken by the most powerful supercomputers. - unless these are owned by the US government. For built into the Clipper chip is a mathematical 'trap door' which allows the government to decrypt any message encrypted with any chip.

The US government says this is essential to protect itself from criminal gangs who would otherwise be able to communicate without fear of their conversations being monitored.

The issue has stirred up a tremendous argument between civil libertarians, software companies and the US national security establishment. But now the Clipper chip has been cracked in a most damaging way. A researcher at Bell Laborotories has discovered how to jemmy shut the trap door which is the whole point of the chip.

Dr Matthew Blaze discovered that hinge of the trap door, so to say, was a number 'only' 16 bits long: that is to say, there are fewer than 65,000 possibilities. It is perfectly feasible, he found, to try these at random until one is found which satisfies the machine at the other end that the transmission is legitimate, but does not allow the government to listen in. The process can take 30 minutes or so, which makes it impractical for voice calls or for faxes. But it would allow the unbreakable encryption of E-mail which the Government is supposed to be able to read.

Opponents of Clipper point out that really unbreakable encryption which no government can crack is already available. One of the latest versions of this, known as PGP 2.61, has just been released by MIT and is available to American sites on the Internet. In theory, this should not be exported. In practice, it will be.

Mike Godwin, chief counsel for the Electronic Frontier Foundation, a Washington-based pressure group, said: 'Clipper as it is projected to be used is not affected by this problem. But it does cast doubt on the ability of the National Security Agency to come up with a standard that really meets commercial standards of acceptibility.'