Mi­chael Rish­er, the A.C.L.U. law­yer lead­ing the suit, said the law was un­con­sti­tu­tional be­cause it is too broad. The law, he said, ap­plies to all reg­is­tered sex of­fend­ers, in­clud­ing those whose con­vic­tions have lit­tle to do with on­line ac­tiv­i­ties, like sex work­ers or those con­vict­ed of in­de­cent ex­po­sure. What is more, he said, is that it makes no dis­tinc­tion be­tween on­line ac­tiv­i­ties, and it would cur­tail even In­ter­net us­age that could not be crim­i­nal.

“This is a prob­lem un­der the First Amend­ment,” Mr. Rish­er said. “Amer­i­cans have a right un­der the First Amend­ment to speak anony­mous­ly, and this evis­cer­at­ed that right. Peo­ple, for ex­am­ple, would have to turn over the screen names they use to com­ment on the New York Times Web site.”

“That’s not ac­tiv­ity that can be used to com­mit a crime in any way,” he con­tin­ued. “It is pure speech, of­ten pure speech about im­por­tant po­lit­i­cal is­sues of the day. It’s an area where there is no rea­son for the gov­ern­ment to be re­quir­ing peo­ple to iden­ti­fy them­selves to the po­lice.”