Abstract
This paper addresses to a variation (modification) of a symmetric block encryption-decryption algorithm based on nonpositional polynomial notations (NPNs).The proposed encryption model includes two stages.The first stage is that a plaintext is split into bit blocks of a given length.Each plaintext block then undergoes an initial transformation, where a pseudorandom sequence (PRS or key sequence) is added bit by bit to the block through XOR operation.The key sequence is generated with a designed PRS generator.In the second stage, an encryption-decryption algorithm based on NPNs is applied to the transformed bit sequence.A computational model has been developed and implemented to test statistical properties of the sequences enciphered under the proposed modification of the encryption-decryption algorithm.