People trying to communicate across oppressive national firewalls could be interested in ScatterChat, a secure IM (instant messaging) application developed by an international group of hackers, human rights activists, lawyers and security experts.

The application is based on the open-source Gain IM client and uses the anonymous Tor network to offer secure end-to-end encryption for both chat and file transfers, the developer group Hacktivisimo said.

Windows versions are available now, and a Mac OS X Fink version is in development, according to the web page offering links to download the software.

The software is designed for "non-technical human rights activists and political dissidents" but could also be also useful for corporate environments and other settings where privacy is important, according to the group. The software was launched at the Hackers On Planet Earth (HOPE) conference, which ended Sunday in New York City.

The anonymity and encryption provided by ScatterChat ensures that both the identities and messages of users "remain a mystery," lead developer J. Salvatore Testa said .

ScatterChat, which uses a technique called "onion routing" to hide the computers taking part in a secure conversation, is immune to replay attacks, supports Perfect Forward Secrecy, and features limited message deniability properties, according to its website.

The application uses 2048-bit ElGamal and 1024-bit DSA (Digital Signature Algorithm) for encryption and authentication. For each new conversation, it generates a new 256-bit AES (Advanced Encryption Standard) message key, a 256-bit SHA-1 (HMAC) Hashed Message Authentication Codekey, a 256-bit nonce and a 256-bit AES file transfer key.

Additional information is available here.