一个优化的分组密码的工作模式*

作为基本工作模式OFB具有流密码的特点，它允许明文的分组单位长度小于分组密码的长度，从而可适应用户数据格式的需要。但当分组单位长度远远小于分组密码的长度时，此模式使用分组密码的效率不高。因为不管加密多短的明文块，每加密一块都要使用一次分组密码。为了提高其效率，引进了计数器和缓冲器，使分组密码的输出得到全部使用。同时为了增强安全性，改进了OFB模式的反馈输入方式，使得在P. Rogaway等人给出的强安全性定义(priv)下是可证明安全的，并用M.Bellare和V.Shoup的玩游戏的方法给出了一个自然、通俗易懂的证明。

Optimized Block Cipher Mode of Operation

As a based mode of operation, OFB has the character of stream cipher that it allows transmission units shorter than the block-cipher length to be encrypted. So it satisfies various data mode need. However, when the transmission unit is shorter than the block-cipher length, OFB is inefficient, since for every transmission unit, regardless how short, a call to the block cipher is needed. In this paper, in order to raise its efficiency , we introduce a shiftcount and a bumper and make all the outputs of block cipher be used. On the other hand, in order to enforce its security, we improve its feedback input mode and make it be provable security in the sense of strong secure notion of privacy. Finally we provide its security proof using the playlng-game technique, the process of proof is natural and easy to understand.