A5 is a stream cipher. A5 is based on a combination of three linear feedback shift registers (LFSRs) with irregular clocking. It uses 64 bit keys. The register lengths are 19, 22, and 23. All the feedback polynomials are sparse. The output is the XOR of the three LFSRs.A5 uses variable clock control. Each register is clocked based on its own middle bit, XORed with the inverse threshold function of the middle bits of all three registers. Usually, two of the LFSRs clock in each round. It is used in GSM networks.