I've just released (available for download) implementations of the A5/1 encryption cipher. This is the encryption algorithm that is used in Telstra GSM mobile phones to encrypt traffic.

Quoted from the wiki page (http://en.wikipedia.org/wiki/A5/1):

A5/1 is a stream cipher used to provide over-the-air communication privacy in the GSM cellular telephone standard. It was initially kept secret, but became public knowledge through leaks and reverse engineering. A number of serious weaknesses in the cipher have been identified.

I've written two implementations which are very much the same:

1. PHP Version
2. C# Version

Originally the PHP version was developed from a small script i created to demonstrate how the registers were to function. This later developed into a full implementation. At the time I required a C# version, so i re-wrote it.