Well your key doesn't have much entropy to begin with.... On a more serious note, the code you written uses a 8bit key. you need to change the line: (key[pos_

Well your key doesn't have much entropy to begin with.... On a more serious note, the code you written uses a 8bit key. you need to change the line: (key[pos_c] << 31) | (key[pos_c] >> 1); to: (key[pos_c] << 7) | (key[pos_c] >> 1); The algorithm can obviously also be modified to use 64bit chunks that could be faster on 64 bit hardware.