Looking for GSM A5 info

[Stephan Somogyi]

I'm looking for information about the A5 encryption algorithm used in
GSM phones. Specifically:

- How does the algorithm work and is its encryption methodology similar
to any other well-known algorithms?

- Is A5's implementation mandatory to produce an world-wide
interoperable GSM device?

- What are the variants of A5 (there was some discussion of less secure
versions), how do they differ, and where are they used?

- Are there any known weaknesses in or attacks on A5-encrypted GSM
conversations?

- Are there notable instances where GSM deployment was delayed or
halted due to A5? (I remember hearing that such a delay happened in
Australia, but I don't recall details.)

Any related information, or pointers to related information,
appreciated greatly.

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Stephan Somogyi                Mr Gyroscope                Digital Media