A5 is the encryption algorithm commonly used by GSM networks in Europe. It has a 64-bit key, although in practice at least ten of these bits are unused. A weaker version of A5 called A5/2 is exported to some countries, including Australia. It was originally a secret algorithm but was eventually leaked, which helped the case for using the open Kasumi algorithm as a replacement for A5. Comp128 is the more common GSM alternative to A5