A5/1: Meaning (information, definition, explanation, facts)

A5/1 is a stream cipher used to provide voice privacy in the GSM cellular telephone protocol.

The cipher is based around a combination of three linear feedback shift registers with irregular clocking and a non-linear combiner. A5/1 is supposed to be a "strong" version for trustworthy countries,

In 2000, Alex Biryukov, Adi Shamir and David Wagner showed that A5/1 is quite weak; after a moderately difficult preparation phase, it can then be cryptanalysed in nearly real time.