The inventor of the popular encryption program Pretty Good Privacy (PGP) claims he has received hate mail because his discovery was used by hijackers to organise the attack on the World Trade Center.

Computer programmer Phil Zimmermann said he had been sent emails claiming that PGP had become a weapon of war, levelling the playing field between powerful countries like the US and "zealots". One message began: "Phil - I hope you can sleep at night with the blood of 5000 people on your hands."

Zimmerman posted the first version of PGP on the internet as shareware 10 years ago. The program was the first key-based encryption system that could not easily be intercepted by governments.

The US Government is now investigating whether PGP was used by hijackers to organise the recent attacks, and US legislators are calling for new restrictions on the sale and distribution of the technology.