A Japanese man has admitted in court to writing a data-destroying Trojan horse.

Masato Nakatsuji, a 24 year-old who in January became the first ever virus writer to be arrested in Japan, admitted in Kyoto District Court that he created a Trojan and used popular copyrighted animation footage to spread it via the internet.

The malware was designed ultimately to wipe music and movie files from users' computers.

The malicious code, believed to be the Pirlames Trojan, was spread via the controversial Winny file-sharing system in Japan in 2007.

Nakatsuji made the admission during the first day of the trial, where he answered charges of copyright infringement and defaming an acquaintance by embedding his photograph into the malicious code.

"Al Capone was charged with tax evasion rather than racketeering, and Nakatsuji is being charged with copyright infringement rather than for creating malware," said Graham Cluley, senior technology consultant for Sophos.

"If he is found guilty, the general public are unlikely to worry that it was his ill-advised choice of graphics which got him into legal trouble rather than virus writing.
"However, a clear message needs to be sent to the computer underground that they will not be shown a blind eye if they spread malicious code and damage innocent people's computers and data."

Prosecutors described how Nakatsuji is alleged to have created the Trojan horse, attached it to copyrighted animated pictures and planted links to it on internet message forums.

However, Nakatsuji's defence has argued that the malware was not seriously malignant, and that justice would not be served by punishing the graduate student for spreading the Trojan when there were no specific laws against doing so.

Isamu Kaneko, the author of the Winny file-sharing program, was fined by a Japanese court in December 2006 for assisting in copyright violation.