A5/1 is a stream cipher used in old GSM phones to ensure encryption during conversation. The algorithm was meant to be built directly in hardware to guarantee fast encryption of the information.

Nowadays, A5/1 could be recreated in software and ensuring the same speed of encryption.

This is what you will need to do: Recreating a software version of A5/1

Notes:

You can use any language you believe is suitable for this task (that is, any language you are familiar with)

Your code should contain a function that accepts as parameter a 64 bits key (it can also be a class constructor if the language allows it)