The book also discusses the role of symmetric ciphers and symmetric block cipher, and details the need for fast symmetric cipher for hardware (DES), software (RC5, Blowfish), and the implementation of universal (AES, RC6, Twofish, IDEA, GOST). In addition, this book covers the issues surrounding software development and design-oriented ciphers, including the cipher is based on algorithms that are formed depend on the secret key. Some specific issues related to the cryptographic strength of the cipher algorithm is considered flexible data transformation, and probabilistic model combinational advised to obtain a minimum evaluation of the cipher software under consideration.