GSM Encryption Algorithms

A partial source code implementation of the GSM A5 algorithm was leaked to the Internet in June, 1994. More recently there have been rumors that this implementation was an early design and bears little resemblance to the A5 algorithm currently deployed. Nevertheless, insight into the underlying design theory can be gained by analyzing the available information. The details of this implementation, as well as some documented facts about A5, are summarized below:
A5 is a stream cipher consisting of three clock-controlled LFSRs of degree 19, 22, and 23.
The clock control is a threshold function of the middle bits of each of the three shift registers.
The sum of the degrees of the three shift registers is 64. The 64-bit session key is used to initialize the contents of the shift registers.
The 22-bit TDMA frame number is fed into the shift registers.
Two 114-bit keystreams are produced for each TDMA frame, which are XOR-ed with the uplink and downlink traffic channels.
It is rumored that the A5 algorithm has an "effective" key length of 40 bits.