Introduction of various cryptographic modes of operation was conditioned by noted imperfections of symmetrical block algorithms. Design of certain cryptographic modes of operation is very similar to streaming algorithms in such a way that these modes generate a stream of random bytes. That stream is merged with plain text using XOR function in order to create cipher text. Beside the fact that CFB, OFB and CTR modes of operations create streaming algorithm their common feature is that they need no decryption function in their operation. The said fact is exploited in this research through an experiment to demonstrate that minor modifications of AES algorithm and pre-processing (speculative encryption) can achieve parallelization of its execution. In this purpose a Java application was created in which a parallel solution was implemented using 2 and 4 threads in both 256 and 128-bit encryption and decryption, whereby acceleration up to 20% was achieved under certain conditions. This paper presents theoretical foundations and most important segments of the conducted experiment as well as accelerations achievable under certain conditions.