AirChat is a free open source program developed by the Anonymous hacking group to anonymously communicate with other people over the air waves. To be able to use it you will need a ham radio with the open source Fldigi modem controller connected to your laptop or desktop computer.
AirChat transmits data using a radio connection, there is no need for Internet infrastructure or mobile phone network coverage. Sending data over the air waves has been possible since the invention of radio, as the Morse code pulses over the airwaves proved. Amateur radio operators send each other data messages daily with just their radio equipment, the Anonymous collective is not devising any new technology, what they do is to add privacy and security to something that already existed.
The main problem of sending data packets over the airwaves is lack of bandwidth, that makes this technology slow and only suitable for low bandwidth voice, text chat and low resolution photos, the developers admit that they have traded bandwidth for greater security.
AirChat encodes data inside air waves with Anonymous own Lulzpacket protocol handling integrity and encryption. Due that in some countries encrypted airwaves over specific frequencies are banned, you are given the choice of sending the data unencrypted to avoid breaking the law. Other legal considerations are that ham radio operators must be licensed to operate on amateur radio frequencies, that will put you on a government list but this is not necessary if you only plan on listening in.
When you transmit data with AirChat there is the option to send it to nearby contacts unencrypted or broadcast it encrypted with a public key encryption that only the receiver will be able to decrypt with his personal private key.
The reason for Airchat is to stop a government switching off the the Internet to stop a protest group, like it has happened in the past during the Arab Spring revolution. An added benefit is that, as far as the top secret documents leaked by Snowden reveal, the NSA spying scheme only monitors the flow of data over the Internet and not the airwaves.
There are other similar projects that allow you to exchange data with other people without an Internet connection, like Commotion Wireless, but their data transmission range is limited. AirChat developers claim to have used their software to send photos 180 miles away through the airwaves without any Internet connection. And you don’t have to worry about hardware MAC addresses identification, that ID is not passed on to any access point like it happens when you use Wi-Fi.
Something to know about amateur radio (aka ham radio), is that it is illegal to broadcast over licensed frequencies, if you did you could interfere with commercial radio stations, airports and emergency services. Broadcasting on licensed frequencies will attract the authorities attention, they will track you down like they do with pirate radio stations and charge you. Only use AirChat over unlicensed frequencies.
Update 2016: Project appears dead, it has not been updated for the last 3 years.