In this study a numerical cryptanalysis of a random number generator (RNG) which is built on a continuous-time chaotic oscillator is introduced. In order to scrutinize the security weaknesses of the RNG, a master-slave synchronization approach is presented as the basis of an attack scheme. By proposing this attack scheme, it is aimed to verify whether there exist any possibility to divulge the hidden knowledge about the RNG and to guess the subsequent number by knowing only the RNG structure and any scalar time series obtained from the output of the chaotic system. Simulation results of the attack scheme are presented in this paper, and it is shown that deterministic chaos itself cannot be identified as the source of randomness.