A Harvard computer science professor has announced he has discovered a way to make codes unbreakable.

Michael Rabin, Watson professor of computer science, working with doctoral student Yan Zong Ding, said he has found a way to send and receive messages that cannot be decoded, a discovery that could have profound implications for cryptology.

"All the currently existing encryption schemes are based on improvable assumptions," Rabin said. "As a consequence, it is in principle possible that people using a secret algorithm and a sufficiently powerful computer will be able to break a code."

Rabin said people often eavesdrop on conversations and capture and store the coded messages. They then read the messages once they have perfected their code-breaking methods--a technique that Rabin's system would thwart.

"This new encryption preserves the secrecy of messages indefinitely so that even an adversary with unlimited computing power and who is infinitely smart in code breaking cannot ever decode the message," he said.

Rabin's encryption method involves using an infinite stream of randomly generated, unstored characters to transmit messages. The sender and receiver of a message agree when to start reading signals carried in the stream and the key to decode them, but the stream itself just disappears, leaving hackers no way to store and decipher the message.

Richard Lipton, a computer science professor at Princeton University, said he supports Rabin's claims.

"Rabin's method is new and potentially changes the crypto landscape," he said. "His work is very practical and is a major breakthrough."

But there are those who doubt the value of Rabin's discovery.

Cipher Deavours of Kean University said Rabin's method would have no practical purpose because it will not work for very long codes, an assertion Rabin refutes.

"This is not true," Rabin said. "It's not a problem at all, and it works very efficiently for very long messages."

Other critics dismiss the importance of Rabin's work.

Robert Morris, a retired cryptographer who was chief scientist for the National Security Agency, said although Rabin's method does make code-breaking more difficult, code-breakers will simply find another way to eavesdrop on the message.

But Rabin stands by the importance of his discovery.

"In a very unique situation, there may still be success in breaking a code, but this does not compare with just being able to tap lines and break code with a computer like before," he said. "We raised the bar very high. I predict that if decryption becomes impossible, people will also raise walls against other forms of theft."

What Morris is saying is "as nave as saying let us not try to find a cure for AIDS because people may die of heart disease anyway," he added.

Others wonder if Rabin's discovery will make law enforcement more difficult, if criminals decide to use unbreakable codes for illicit purposes.

"Dangers always exist--this is a societal question, and if people want to pass laws against using this, they will," Rabin said. "The world is moving in a different direction--restrictions are being lowered, but there are important consideration of privacy and thus many benefits in cryptology."