A5/2

A5/2 is a stream cipher used to provide voice privacy in the GSM cellular telephone protocol. It was used for export instead of the relatively stronger (but still weak) A5/1. It is one of seven A5 ciphering algorithms which have been defined for GSM use.

The cipher is based on a combination of four linear feedback shift registers with irregular clocking and a non-linear combiner.

In 1999, Ian Goldberg and David A. Wagner cryptanalyzed A5/2 in the same month it was published, and showed that it was extremely weak – so much so that low end equipment can probably break it in real time.

Since July 1, 2006, the GSMA (GSM Association) mandated that GSM Mobile Phones will not support the A5/2 Cipher any longer, due to its weakness, and the fact that A5/1 is deemed mandatory by the 3GPP association. In July 2007, the 3GPP has approved a change request to prohibit the implementation of A5/2 in any new mobile phones. If the network does not support A5/1, or any other A5 algorithm implemented by the phone, then an unencrypted connection can be used.