In this paper a modified discrete-time chaotic system is presented from the statistical point of view. This chaotic system is used in a cryptosystem and, for improving the presented method used in security data transmission, its structure is changed. The technique is implemented for a Rössler hyperchaotic system. The improvement consists in modifying the existing system in order to obtain a higher robustness for the cryptosystem; for this, a time varying delay is added in its structure.