SECURING E-MAIL AND OTHER MESSAGES FROM PRYING eyes has become a big issue on the wide-open Internet. Now, Elementrix Inc. in New York is applying one of cryptography's oldest techniques--the so-called one-time pad--to the problem. As the name implies, the scheme calls for encoding every message with its own unique key, or password. That way, breaking into any single message doesn't help a spy break into the next one. Elementrix' Power One Time Pad software, which is priced at $495 and designed for IBM-compatible personal computers, automates the generation of those keys.

Here's how it works: First, two individuals decide on an initial key--a string of text characters, say. They then use that to encrypt the first message to pass between them. To calculate a key for the return message, the software uses a combination of the first key and the actual computer file that it helped move. The process repeats, with every new key getting derived from a combination of previous keys and messages. The result is an endless series of seemingly random keys. Another feature: Elementrix officials say they have overcome the problem of faulty communications lines and computer crashes, which might otherwise corrupt the process.

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