Abstract
Recently two encryption schemes were proposed by combining circular bit shift and XOR operations, under the control of a pseudorandom bit sequence (PRBS) generated from a chaotic system. This Letter studies the security of these two encryption schemes and reports the following findings: (1) there exist some security defects in both schemes; (2) the underlying chaotic PRBS can be reconstructed as an equivalent key by using only two chosen plaintexts; (3) most elements in the underlying chaotic PRBS can be obtained by a differential known-plaintext attack using only two known plaintexts. Experimental results are given to demonstrate the feasibility of the proposed attack.