This book provides the most complete description, analysis, and comparative studies of modern standardized and most common stream symmetric encryption algorithms, as well as stream modes of symmetric block ciphers. Stream ciphers provide an encryption in almost real-time regardless of the volume and stream bit depth of converted data, which makes them the most popular in modern real-time IT systems. In particular, we analyze the criteria and performance indicators of algorithms, as well as the principles and methods of designing stream ciphers. Nonlinear-feedback shift registers, which are one of the main elements of stream ciphers, have been studied in detail. The book is especially useful for scientists, developers, and experts in the field of cryptology and electronic trust services, as well as for the training of graduate students, masters, and bachelors in the field of information security.