对称密码体制的发展趋势将以分组密码为重点。分组密码算法通常由密钥扩展算法和加密（解密）算法两部分组成。密钥扩展算法将b字节用户主密钥扩展成r个子密钥。加密算法由一个密码学上的弱函数f与r个子密钥迭代r次组成。混乱和密钥扩散是分组密码算法设计的基本原则。抵御已知明文的差分和线性攻击，可变长密钥和分组是该体制的设计要点。-Symmetric cryptosystem trends will focus on block cipher. Block cipher algorithm is usually extended by the key algorithm and encryption (decryption) algorithm is composed of two parts. Key Expansion Algorithm will b-byte master key customers to expand into r sub-key. Encryption algorithm from a weak cryptography on the function f and r sub r th iteration key component. The spread of chaos and key block cipher algorithm is the basic principle of design. Explicitly known to resist differential and linear attacks, variable length keys and grouping of the system design features.