“Personal Internet-based communications like e-mail, instant messaging and social networking are powerful tools,” said Deputy Treasury Secretary Neal Wolin. “This software will foster and support the free flow of information — a basic human right — for all Iranians.”

The U.S. action comes at a time when Iranian authorities have created cyber-intelligence units that are developing new methods to seek out and snare the opposition, including fake Facebook accounts. Authorities also are contemplating the creation of a national Internet that would approve which sites could be available in the Islamic republic. The government has also adopted a new law that threatens bloggers with jail time if they “defame sanctities” — a broad accusation in Iran — in their postings.

The new efforts mean that every time opposition bloggers in Iran fire up a laptop, they risk a knock at their door by the cyber-police.

“They have filtered my blog. After I changed the address, they filtered it again,” an opposition activist named Banafsheh said via e-mail. Like other opposition activists interviewed, she declined to allow her family name and her blog name to be used because of fear of prosecution. “The next step will be an attack on my site by their hacking squads or, in the worst case, my arrest. Many web administrators have been taken in,” she said.