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The creator of PGP encryption software warned last week that proposals from the US and UK governments that encryption products should be fitted with back doors to enable law enforcement agencies to read private e-mails would damage commerce and do little to deter terrorists.
Following the attacks, government ministers in the UK are seeking to re-introduce key escrow legislation which would force businesses to lodge their encryption keys with trusted third parties, despite protests by business leaders and civil liberties campaigners.
In the US, senator Judd Gregg has called for a worldwide ban on encryption products, unless they contain software back doors to give police and the security services the ability to read encrypted messages.
But Zimmermann said there was no evidence that the terrorists had used encryption or that law enforcement agencies could have prevented the attacks if encryption had been outlawed.
"We have to demonstrate whether a policy of back doors would have any impact, what information was available to investigators before this event and whether they were stopped by encryption," he said.
Reports in the US suggest that investigators over-looked leads and information before the attacks for reasons that had nothing to do with encryption, said Zimmermann.
"The FBI had boxes of documents from other cases written in Arabic. They just did not have enough translators to read them," he said.
Zimmerman said his own experience in developing encryption technologies had taught him that back doors would make encryption software less secure, leaving messages more vulnerable to eavesdropping by hackers or criminals.
Back doors have already been extensively debated - and rejected - by government, law enforcement agencies, the computer industry and civil liberties groups.
"After many years of debating, we concluded that cryptography was good for security. That was a good decision and one of the areas of debate was whether terrorists would use strong encryption. To try to reverse that could be a terrible mistake," said Zimmermann.
"We should not be banning encryption. There are so many reasons why that is a bad idea. If you do that, the bad guys will simply use other products that don't have a back door."