Godfather of encryption Phil Zimmerman has responded to attacks directed at him over the use of encryption software in the terrorist attacks on the US.

Zimmerman, who created the Pretty Good Privacy (PGP) encryption software a decade ago, apparently came under fire from some members of the internet community after it emerged that the US Government is investigating whether PGP, or a similar technology, was used by the hijackers to co-ordinate the attacks securely.

As a result of the investigation, the US authorities are calling for tougher restrictions on the recently relaxed encryption laws.

Some have argued that, although encryption is designed to protect people's privacy, in the wrong hands it can be used to protect malicious plans. One example of the hate mail received by Zimmerman read: "Phil - I hope you can sleep at night with the blood of 5000 people on your hands."

But Zimmerman said on Tuesday that he had "no regrets about developing PGP". He also attacked media articles from last week which expressed his view to the contrary, claiming that he was "overwhelmed with guilt".

Zimmerman also denied claims that, in light of the attacks, he had changed his principles on the importance of cryptography for protecting privacy and civil liberties.

In an open letter he wrote: "I had cried over the heartbreaking tragedy, as everyone else did. But the tears were not because of guilt over the fact that I developed PGP, they were over the human tragedy of it all.

"I felt bad about the possibility of terrorists using PGP, but I also felt that this was outweighed by the fact that PGP was a tool for human rights around the world, which was my original intent in developing it 10 years ago."

Zimmerman reminded us that "society's collective decision [over the FBI's objections] was that, on the whole, we would be better off with strong cryptography, unencumbered with government back doors".

Indeed, with government agreement, export controls on strong cryptography were lifted and no domestic controls were imposed on the technology.

In response to US Government calls for a review of encryption control, Zimmerman remains steadfast in his defence of the technology. "PGP users should rest assured that I would still not acquiesce to any back doors in PGP," he said.