Correlation properties of an improved summation generator with 2-bit memory (Q1603844)

Recently, \textit{H. J. Lee} and \textit{S. J. Moon} proposed an improved summation generator with 2-bit memory [ibid. 80, 211-217 (2000; Zbl 1037.94542)]. By adding an extra bit of memory in the nonlinear combining function, the proposed generator is claimed to be secure against correlation attacks that the original summation generator suffers. In this paper, we show that the improved generator is still vulnerable to correlation attacks since there exists a high correlation between the input sequences and the output sequences.