Meet @cryptofax2tweet, a new Twitter account I run. So, what is so special about this account? As the name suggests, it can be used to tweet by sending an encrypted QR code using a fax, for example when your government decides to turn off the internet. In case you are not interested in the technical details on how it works and just want to use it, you can download the cryptofax.pdf file and open it in Adobe Reader. A one page user's guide is also available.
But I am digressing. One of the more interesting features of XFA is the possibility to create all kinds of barcodes, both one- and two-dimensional. The list of different types you can create in the specification is about five pages long(!). Also, the specification claims that the content of the barcode can be encrypted before creating it using an RC4/RSA hybrid encryption.
I had recently read about Google's @speak2tweet account and liked the idea but not the Medienbruch — the change from one medium (voice) to another (text). So I thought about implementing something using XFA which would allow people to send tweets via fax.
After that was solved, I looked into encrypting the content of the tweet. Note that the encryption just helps against an attacker who only monitors the phone lines and not the @cryptofax2tweet account. Still, it might help people who have printed out the fax and it gets intercepted before it has been faxed. Unluckily, it looks like this particular functionality from the specification has not been implemented in Reader (the fact that the LiveCycle® Designer ES Scripting Reference does not talk about it at all points in this direction, too).
The other end of the service needs to decode the QR code, decrypt the content and tweet it. This part was a lot easier than the PDF part, as it could be implemented in less than 50 lines using Ruby and the ZBar barcode library. A fax number was thankfully provided by AS250.net so that I only needed to deal with emails from the fax2mail gateway.