A5/2
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A5/2 is a stream cipher used to provide voice privacy in the GSM cellular telephone protocol.
The cipher is based around a combination of four linear feedback shift registers with irregular clocking and a non-linear combiner.
In 1999, Ian Goldberg and David 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.