The Iranian government has severed some citizens' connections to secure websites and services, including Twitter and Google's Gmail, according to reports by Iranians and network experts.

Starting yesterday, people who said they live in Iran began describing problems reaching websites secured with SSL (secure socket layer), which is used to encrypt traffic between computers and websites. SSL-secured sites begin their URLs with "https" rather than "http."

The Washington Post first reported on the new crackdown last week.

"Since Thursday, Iranian government has shutted [sic] down the https protocol which has caused almost all Google services (Gmail, and itself) to become inaccessible," said Hacker News user Sara70, who intimated that he lived in Iran.

Among the websites that automatically encrypt communications either for login or throughout a session are Gmail, Yahoo Mail and Microsoft's Hotmail, Twitter and Facebook, virtually every banking website and online retailers like Amazon.

Some who said they reside in Iran said that SSL-secured websites within the country were accessible, but those outside Iran were being blocked.

The Tor Project, an organisation that helps people connect to the web anonymously, often used in countries where governments monitor their citizens' online activities, provided details.

Tor said that it appears the Iranian government is "ramping up censorship" in three different ways: deep packet inspection of SSL traffic, selectively blocking some IP address and TCP port combinations, and filtering for select keywords.

"The blocks on SSL are not complete and not nationwide," said Tor early today, speaking from the experience of users who are trying to access the web from inside Iran via Tor. "Where blocking is in place, initial investigations show [the government is] identifying the beginning of the SSL handshake and simply interrupting the handshake."

In a follow-up message, Tor said that it "now appears to be that nearly all SSL/TLS traffic is blocked on a few major Iranian ISPs," and offered users a technically sophisticated workaround.

Iranian government officials have said that they were working on a firewall that would essentially create a "national Internet" which would block all external content from its citizens. In January, for example, state-controlled media said that the government-run Internet would become operational in the next few months.

Other countries, including North Korea and the People's Republic of China, regularly block access to outside websites on a permanent or partial basis.