Mr Scarfo used an encryption program to scramble his computer files so that when the FBI gained access to his hard disk in January 1999, it was unable to unscramble them without the password.

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The program uses the Advanced Encryption Standard (AES) to communicate with its control server whose address is 212.7.208.65 .

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"Skype is a nightmare for law-enforcement agencies" because of its encryption, says David Vincenzetti, chief executive of Milan-based HackingTeam, which sells a program called Remote Control System that works on computers, smartphones and Blackberries.

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Once launched, the virus is designed to install a Trojan horse or backdoor program, and allow investigators to record everything typed into that machine, thereby getting around encryption and passwords.

CNN: Top Technology Stories for 2001