In a stream cipher, bits or bytes are encrypted individually and it is not necessary to store blocks of data before their processing, as typical block ciphers do because they cannot directly cipher blocks that are shorter than their block size. Usually, stream ciphers are used when a high encryption speed is required or when the amount of data is unknown. Due to the necessity of encrypting high amounts of data in real time, there has been an increasing interest in cryptography in the last decades.