After the pornographic e-mails were sent, the law firm hired a security consultant, who put a packet-capture device on the lawyer's network and found evidence that Ardolf was logging in, according to court filings.

Both the WEP (Wired Equivalent Privacy) and older WPA (Wi-Fi Protected Access) systems suffer from known cryptographic weaknesses. By sniffing network traffic, tools such as Aircrack can quickly figure out passwords on WEP networks. They can also break encryption on WPA-PSK networks that use simple passwords. Security experts recommend that home users go with the newer WPA-2 encryption, but this can be tricky, because it isn't supported on older routers and wireless cards.

In interviews with law enforcement, Ardolf claimed to not know the difference between WEP and WPA. But he owned a copy of Aircrack and had hacking books in his house and an "ethical hacker" bumper sticker on his bathroom mirror, according to investigators.

Ardolf pleaded guilty on Friday -- two days into his trial -- in U.S. District Court for the District of Minnesota. He faces 40 years in prison on the charges. A sentencing date has not been set. Ardolf's lawyer could not immediately be reached for comment.

The IDG News Service