Theoretical cryptanalysis of the Klimov-Shamir number generator TF-1

The internal state of the Klimov-Shamir number generator TF-1 consists of four words of size w bits each, whereas its intended strength is 2 ^2w. We exploit an asymmetry in its output function to show that the internal state can be recovered after having 2 ^w outputs, using 2 ^1.5w operations. For w = 32 the attack is practical, but for their recommended w = 64 it is only of theoretical interest.