There is an urgent need to encrypt data in databases to protect sensitive data especially in untrustworthy cloud.But operations on encrypted data could not be efficiently executed, such as range queries and sort operations.To resolve this problem, order-preserving encryption (OPE) scheme were suggested which preserved the order of plain text.mOPE was a mutable order-preserving encoding scheme based on binary search tree which worked on all data types with ideal security.The order-preserving codes might change by insertion or deletion, and the high frequency of codes mutation would bring additional computation overhead to the server in mOPE.This paper demonstrated a new scheme called cmOPE(custom and mutable order-preserving encoding) based on complete binary search tree encoding which reduced the overhead of codes mutation.This paper implemented cmOPE and compared the performance between mOPE and cmOPE.The result shows cmOPE is more efficient.%不可信云计算环境下的数据隐私保护问题逐渐成为研究重点,而保护隐私的主要方法之一就是对数据库中的记录加密,但对密文进行排序、范围查询等操作较为困难.保序加密能使密文的大小顺序与明文保持一致,支持上述对密文的操作,2013年提出的mOPE可变保序编码是一种基于二叉搜索树编码的保序加密方法,支持任意的数据类型,且除了明文顺序外不泄露其他任何信息.由于保序编码可能随着插入或删除记录而变更,服务器额外开销较大.对此作出改进并提出cmOPE(custom and mutable order-preserving encoding)方法,基于构造完全二叉搜索树来调整保序编码,降低了编码变更带来的额外开销.实验结果显示,修改了编码调整策略的cmOPE方法有效地降低了服务器的计算开销,提高了对密文增删改的效率.
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